J-Rho's '67 Camaro Z28 STX build Jason Rhoades builds a 1967 Camaro Z28 clone for SCCA STX autocross


Front suspension continued

A while back I left off a post with this picture:

The angle, a bit over 8 degrees (90-81.7=8.3) is the "kingpin inclination" (KPI) or "steering axis inclination" (SAI) angle.  It is basically, the angle from vertical about which the knuckle rotates when steered.

There are pros and cons to this angle being big or small.  One pro, is it ends to reduce the "scrub radius" that some people say is a bad thing.  It also provides a centering force akin to positive caster.

A con though, is that it produces positive camber in the outside wheel - i.e., if you turn right, the left front wheel will get positive camber just from turning.  The higher the KPI, the more positive goes the camber.  This is a Bad Thing.

Can't change KPI in this class, really requires a whole new knuckle.  But we can fight this Bad Thing with a Good Thing, and in our case that Good Thing is positive caster.

Caster is the same sort of angle as above, except as viewed from the side.  Positive caster produces negative camber on the outside wheel when turned.   The more positive the caster, the more negative camber gain.

Autocross has some fairly tight maneuvers, ones that often require large amounts of steering angle input.  Long wheelbase cars like the Camaro have to use even more steering input around given radius corners (see  CVD 4 - Skidpad for some explanation), making it all the more important that, to the maximum extent of our rules-bound ability, we minimize any unfavorable consequences of turning the wheel.

Many modern cars use a lot of caster.  The Porsche 911 GT3 uses upwards of 8.5 degrees; some Mercedes-Benz come from the factory with over 11 degrees.   Heck, even the latest Mustang uses 7 or more.  Lots of sporty cars made in the last 20-30 years provide 5-7 degrees.

Most people don't believe me when I tell them what the Camaro's base caster setting is.  Here's photographic proof, from my Chevrolet Chassis Service Manual, (far right column):

That's right, the factory caster setting is positive 1/2 degree, plus-or-minus 1/2 degree.  Meaning potentially zero degrees positive caster!  So with over 8 degrees of KPI/SAI, there is almost no positive caster to counteract that positive camber gain on the outside front tire.

Just another facet of how bad the first-gen Camaro's front suspension is from the factory.  :)

So, to try to nullify this Bad Thing we'll add some positive caster to the equation.  Common thinking amongst most of the first-gen Camaro guys is that "4 or 5 degrees is enough"...which of course means that I'll be aiming for at least twice that much!  :)  The thing is, even at about 8.5 degrees of positive caster, its beneficial effects are only canceling out the detrimental effects from the KPI.  A first-gen Camaro with 12 degrees of positive caster will only see about the same positive effect from caster, that a "normal" car would get with 6-7 degrees.

Adding all this caster isn't a trivial endeavor either.   There's a few side effects that must be managed.

One is an increase in steering effort.  I'm a big believer in providing the driver with a stable, precise, sensitive, and consistent platform upon which to operate.   If we require the driver to use big muscles to dance the car through the course, we can't expect the driver to have a lot of precision touch in his inputs.  To help enable this, I plan to run power steering.  An excellent race seat will hold the driver in, without any stabilizing forces through the wheel.  This should enable "fingertip" steering wheel forces, enabling the driver to maintain a much better feel for the front tires.  I believe the power steering ratio on the Z28 was also a bit quicker, which I like - it's also harder to be precise when you have to produce huge turns of the wheel in a small amount of time.

The other side effect is it can change your wheelbase.   The traditional (and FSM-specified) way of adjusting camber and caster on these cars is to use shims on the upper control arm's inner attachment point.  Use of shims at both ends produces negative camber, and their use at one end or the other changes caster.  The threaded upper bolts are long enough to produce large amounts of change.  This method of caster adjustment moves the upper ball joint "back", and if the hub is centered between the upper and lower ball joints, moves the wheel centerline back half that distance.

The problem here, is the whole wheel is moved back in the wheelwell, and with the amount of caster I plan to add, doing it all with the upper arm, would move the wheel back more than 1/2".  I want to be able to run very very low, and do everything I can to reduce or eliminate rubbing with the big 265-section front tire.  The Camaro's inner fender has the most vertical room right at the stock front wheel centerline, which means I need to keep the wheel centered to maximize how low I can go.

To keep the wheel centered, but still produce large caster values, means I need to also move the lower ball joint forward, while moving the upper ball joint rearward.  If I move each equal amounts, the wheel stays centered in the wheelwell, and I get to enjoy the yummy KPI-canceling positive caster.

Since I'm spending my custom control arm allowance on replacement upper control arms, I have to stick with the stock lower arms.  However, there is an allowance for replacement offset bushings.  So long as these offset bushings don't contain any more metal than stock, I can use them to produce some of this same caster gain.  You can see in the pic below, how the mount axis goes through the bushing cylinders at an angle - this produces a clockwise rotation (in top view) of the driver's side lower control arm, moving the lower ball joint forward.

John is just about completed with these custom Hydlar lower control arm offset bushings for the Camaro.   There's a good amount of "meat" to the stock bushings, allowing for a satisfying amount of longitudinal forward offset of the lower ball joint.  It probably isn't enough to produce entirely half of the caster I'm after, but it'll be close, and should make a big difference in keeping the front wheels centered in the wheelwell.

A lot of the first-gen guys add a bunch of positive caster to their cars all with the upper arm, and run into bad rubbing problems, because of this rearward movement.

Anyway, these offset bushings are a relatively small detail.  The thing many people overlook, is  lots of small details start to add up.  An ounce of weight savings here, a smidge of improved alignment there, maybe another 1/2 hp freed up, all make a difference.  Those who remember their Calculus know, that even when you're dealing with really really tiny quantities, so long as you find enough of them to add up, you can eventually end up with a substantial (Riemann ;)) sum.

This "sum of many tiny parts" thing was made most clear to me not in Calc class, but at the 2005 Solo National Championships, where I was codriving with Gary Thomason in his insane SM2 C5 Corvette.  Anybody who knows Gary knows how meticulous he is in the care of his cars - not just their finish and appearance, but in their overall state of tune and function.  Gary was constantly tweaking things with the car - 1/32" rear toe changes between runs, fraction of a PSI difference in the tires, etc.  I'll never forget the morning of day 2, him looking to find a way to get the running lamps to stay off.

The car's AFR-headed LS-based 408 was making 600+hp, but the real story was the insane low-end torque, combined with the stock short Z06 gearing.  The thing would obliterate the tires with a hair's too much throttle.  It took a perfect alignment of the planets before you'd even consider full throttle, and on most courses you never got there.  Yet here we were, out on the beat up old airbase at Forbes Field, trying to get the running lamps to stay off, which would reduce the load on the alternator and free up some tiny bit of HP.

We found a way to keep them off, and on his last run that day Gary blazed an unbelievably fast time to pull ahead of Andy McKee in his superior RX7, and take the championship.  While obviously you need the Big Stuff like top driving skill and the right tires in place to be contending for a National Championship, I now know that what many perceived as alien ability, or good luck on an unusually fast run, was actually the synchronous coming together of an uncountable number of tiny, carefully orchestrated, details... including that 'Vette's last 1/3 of a horsepower.

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Rear axle lateral locator complete

So you'd think with nearly complete freedom to design and implement a top-notch lateral locating device for this live axle car, one would choose a Watts link right?  After all, a Watts offers perfect symmetry when turning left and right, and doesn't move the axle laterally as the suspension moves through its range of travel.

Along those lines, the first big question for anybody implementing a Watts link, is whether to make the main pivot point chassis mounted, or axle mounted.  Axle mounted (image courtesy Griggs Racing):

Chassis mounted (image courtesy Fays2):

There's a couple imperfections with the Watts however.  The first pertains to the above choice.  If you choose the axle-mounted center pivot, then you get the "advantage" of a consistent roll center height.  But half the world will tell you you're an idiot, because the sprung mass of the car moves in squat and pitch, so the "lever arm" from the roll center to the center of gravity, changes during squat and pitch, dynamically altering the rear roll couple.  Plus it's harder to adjust.

So you can instead choose the chassis-mounted pivot.  This keeps the roll couple more consistent as the car pitches and squats.  But now you've got to build a big heavy structure to support the center pivot, plus...wait a second, maybe the roll couple changes from the diff-mounted center pivot (loose on entry, tight on exit) could be beneficial?  By now the other half of the world thinks you're an idiot for having done a chassis-mount Watts.

We won't even talk about a Mumford link, which might be cool if you're scratch designing a live-axle chassis, but on something like the Camaro it would take 40+lbs. of structure welded into the rear to support it.

In keeping with the style of a person building a 44 year old car for a Street Touring class, I've made the choice that lets everybody (not just half) think I'm an idiot - a panhard rod.

Now, we all know the downside of the panhard - it moves the axle laterally as the suspension moves.  The shorter the panhard, the more movement.  The panhard bar on the car now is about 34" long, which is as long as it could be made when considering the packaging restraints of the leaf springs.  Another mitigating factor is, as with the front, by keeping the rear suspension stiff, I'll be able to  minimize the vertical (and thus lateral) movement of the rear axle.  At 2" of travel (pretty much all it will get), the axle will shift laterally by only .06", about 1/16".  This is not a completely negligible amount, but I plan to run tall-ish sidewall rear tires, which will mask much of this, and we see tires move around on their wheels by 1/2" or more while under heavy load.

The other maligned characteristic of a panhard rods vs. Watts, is their asymmetry in handling.  The roll center, usually defined as the bar's midpoint,  rises or falls based on which way you're turning, and which way you have it mounted.  This will always create a difference in the way the car behaves turning left vs. right.  Even if you start with the bar horizontal at resting height, as the car rolls, the lateral forces fed through the now-non-horizontal bar will serve to load the rear axle differently depending on which way you turn.

On the surface this may seem like a bad thing, but it can be used to counteract other asymmetries.  For instance, in a live axle rear car, on hard acceleration, driveshaft torque is going to tend to weight the driver's rear tire, and unload the passenger rear tire - that's why you always see drag racers mounting their battery in the right rear, and why live-axle cars usually have such a bad time putting down power out of right-hand turns.  This car is going to be making good power with good gearing so when good grip is available, this asymmetrical loading can become very significant.

We can use the panhard to counteract this asymmetry somewhat, by mounting it to the left half of the axle, and to the chassis on the right.  This puts the panhard in compression, when in a right-hand corner.  That compression is going to help better load the rear axle in right-hand turns, counteracting some of the axle's natural asymmetry.

And actually, there's a lot that's asymmetrical about any race track, and any Solo2 course - both are either clockwise or counter-clockwise (ProSolo, with its mirrored courses, is the exception).   The panhard rod is unique in that it allows one to adjust-in some inherent asymmetry, that may help if a particular event features a strong abundance of key turns in one direction or the other.  It's also lighter and less complex than the other options.

My panhard bar is simple but should be very effective.  The sawtooth axle-end adjuster and screw-type chassis-side adjuster supply an essentially infinite number of discrete adjustment postions, as opposed to the 4-5 "holes" most solutions offer.

The solution overall is only about 16 pounds, a good bit lighter than the Watts options I've seen out there.  Only about 6 pounds of it is unsprung weight, and the axle-side mount (all unsprung) will be lightened a bit - it's designed to be bolt-on to the axle, but by welding it on, can eliminate the weight of the bolts and some other unneeded structure, while actually making it stronger.

The rear suspension still isn't  done from a fabrication perspective, though it's getting close.  Where's my rear sway bar?  Still that to do, soon thereafter should see some progress on the front.  There's one or two goodies visible in the pics above, I'll let people chew on them for a while before discussing them further...


Subframe connectors completed

Well these puppies are finally done!  Not content with the method of attachment as provided, I had John modify them to add a third attachment point, about halfway down their length.  John found a place that would work on the chassis, then drilled a hole and welded in a tube to hold the bolt:

Since the connectors don't sit flush with the floor of the car at that point, he had to make custom delrin spacers between the car and the connector.

Here they are installed.  They look like they're different heights because they are.  Uniformity of 60's chassis aren't quite what we see in more modern stuff-

My hope is this additional attachment point, on a different longitudunal axis (the as-delivered front and rear attachment points are in-line longitudinally) will provide a meaningful increase in torsional rigidity above-and-beyond the parts as they came.  We can't do weld in and are allowed 3 attachment points so by golly I'm going to make use of all three!

Every part on the car that doesn't absolutely have to be there, is going to have to work hard to prove to me it's worth its cost in pounds.  I may do some testing down the road with the connectors removed, to see if, in measured time around a course, I'm getting 24 pounds' worth of rigidity out of them.  If their benefit is not worth their weight in time, off they go.  These things are about as good as I think you could make 3-point bolted-in subframe connectors so if they aren't worth it, nothing I do legally with the allowance would be.

Third attachment point comes through in proximity of rocker and pan reinforcement for seat mounts, should be a good spot to take some loads.

Front connections all snugged up

And the view from front to back.

Another benefit they should provide a good safe place to jack up the car from the side.  The traditional front crossmember jack point will be inaccessible at the ride height I'm likely to end up with, especially since it looks now like we'll be able to use factory-only spoilers - a rule change which happens to suit me PERFECTLY, as I'd only ever wanted to run the factory front air dam and rear lip spoiler.  I don't know if I'll be able to use the traditional under-diff jack point in the rear either, once all the workings of the rear suspension are together.  These things may end up providing the only good place to jack up the car.  So hmmm, maybe I will have to keep them regardless...driving a car up ramps just to get a jack underneath is a tremendous pain - been there, done that.

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CVD Series – Chapter 3 started

Made a couple updates to the CVD series, chapter 2 and 3a.  If you mouse-over the  box at top, it should provide a drop-down to all chapters.

Chapter 2 is a boring yet somewhat gruesome dissection of a classic autocross lap.

Chapter 3a is the longest yet, explaining the process for evaluating and comparing the rolling acceleration capability of two cars.  This should be a good resource if you ever find yourself embroiled in a "horsepower vs. torque" debate.

Started work on 3b, then on to cornering!  Car got back-burnered for a couple weeks while John works a little magic on an A-Mod autocross car.

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Little more fab – leaf spring shackles

John made some more progress last week on the car.  I'm actually very excited about the most recent thing completed - he was able to bring to life in steel, a part I'd only been able to visualize.

What part?  Well, let's just say this car will have the most kick-butt leaf spring shackles ever.  :-)

I don't quite yet want to show the part, I'd like to wait until the car's had its underside cleaned and painted so it's all uniform.  This is something of a hint though:

For the next couple weeks John's been pulled into some other projects to the car's on his back burner for a bit.

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Staying sharp in the off-season – sim racing

The "sunshine tax" we Southern Californians pay affords us year-round opportunities for autocross and track days.  Still, since I won't have a car for a while, and there's some free time while the car is out, decided to do a little more sim racing this winter.

There are several sims out there, the two I'm most familiar with are the SimBin GTR series of games (GTR2, GTR Evolution, Race07), and iracing.com.

The SimBin games are fun and a tremendous value - for $20-30, you've got unlimited access to 100+ awesome cars and dozens of neat tracks.  Within a few minutes of installing, I was driving a Team Oreca Viper around the Nurburgring.  Their Porsche 935 is by far the most fun sim car I've driven.  You can download even more cars and tracks from sites like nogripracing.com.   They also have a "Hot Laps"  section, where people can post up their fastest laps, so you can see essentially the world record laps to compare to.

iracing.com is a little more hardcore.  When you sign up you only get access to a small handful of cars, on an even smaller handful of tracks.   The only real car you get when you start is a crappy Pontiac Solstice.  You've got to keep paying a monthly fee, and you have to pay more every time you want to try a new car or a new track.  Still, the simulation aspect of iracing is even better than the GTR games, which makes the subscription-based iracing a worthwhile endeavor for a few months in the off season.  From within the iracing website, you can look up the world record laptimes for a given car on a given track, to compare to.  No excuses here - with an average PC and wheel/pedal combo, you should be able to run with the very fastest guys in the world, once you've had some practice.

The sims always feel a bit cartoony and obviously the sensations are missing, but anecdotally it seems helpful.  It helps you remember how to stay focused, how to stay smooth on your inputs, and if you race against other people, it even feels like a bit of exciting pressure at times.  Some of my best driving in 2010 was the year's first event - the San Diego Tour, when I hadn't driven the Viper for 5 months, but had played a bunch of iracing after Santa brought a Logitech G27 wheel for Christmas.

Also, I look at a guy like Bryan Heitkotter.  Bryan was already a really good autocrosser, then he got into iracing and rose to be one of the gods amongst its ranks.  Meanwhile out in the real world, the guy is now just plain unstoppable, winning every race he enters, in a tough class against some really great drivers.

This isn't an advertisement for any sim racer (though if you sign up for iracing, I wouldn't mind if you told them I referred you ;-) ), just an observation that these programs have come far enough along now, to be valid training tools for use in the off season.

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More fabrication work complete

Despite having to simultaneously deal with a cold and waves of  "Camaro groupies" coming by the shop to check things out, John has made  a bit more progress on the car.

Unitrax completed the rear end with the ring & pinion and diff all installed right.  The passenger-side leaf spring mount was bent, rusty, crooked, and had been re-welded or something by a caveman with a blowtorch; it needed some fixin'-

The sawzall, saws all:

Removed, and readied for the new perch:

This is one place the Camaro's popularity helps.  Replacement spring perch from Summit was just a few bucks, they had it in stock, etc.  Probably not many cars where replacement welded-on pieces like this are so easy to come by-

All prettied up and rust protected:

I'd been thinking about buying a lathe and making some of my own bushings, but there are really only a few this car needs, and I don't have a ton of spare workbench space...so I'm having John make the custom bushings the car needs also.  He completed the first couple pairs, for the rear leaf spring shackles-

These Hydlar bushings should offer much improved response over the worn-out stock bushings, which in an informal durometer test, measured just above "marshmallow"

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CVD Series underway

I've put together the introductory chapter in a series I'll be writing entitled "Comparative Vehicle Dynamics - A guide to choosing your next competitive autocross or track car".

These will appear as "Pages" links from this blog - up above you should see a link to "CVD 1 - Intro".  Although the chapters will describe the process I generally follow in analyzing the capability of a given car for a given class, there isn't really anything in there specific to the Camaro, so I won't be delivering them as Posts to the blog.  Perhaps by the end however, the process will illustrate how the Camaro maybe wasn't as horrible a choice as most think it is.

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Fab work underway – subframe connectors

Haven't done much on this project myself over the past 2 weeks.  Cleaned the garage which was getting a bit funky from the disassembly phase, despite continuous cleanup.  Sometimes old cars are like leaky faucets of dirt and grime.  Feels like you could saturate the world's supply of Simple Green and shop towels, and still you'd be instantly dirty, knocking dirt all over the floor, the instant you touched it.  Hopefully by the end of the body/paint phase the car will be to the sanitary state I seek.  One thing I have been doing is cleaning up the side glass, to remove 40+ years of gunk and some recent overspray.

So while I've been slacking, John Coffey of Beta Motorsports got started on the car this past week, and was kind enough to snap a few photos along the way.

The rear axle was removed and taken to Unitrax, they've checked it for straightness and given it a clean bill of health.  They'll be setting it up with the new ring & pinion I located.  There's a nice variety of final drive ratio choices available for the car: http://camaros.org/drivetrain.shtml#AxleCodes

My initially chosen ratio will provide a bit over 66mph at my initially chosen redline, with a rear tire that does 820 revs/mile.  There are options to add more top speed in second (besides raising the limiter to unsafe levels), but I think this will be a good start.  At Nationals in 2010 I only hit 68-69mph top speed in the Viper, and with its R-comps, the Viper had a higher speed at the apex and could come off the corner harder than the nose-heavy street-tired Camaro will be able to.  At Nats 2009 we hit 72, and I did hit 74 and 72 at the SD Tour and El Toro Pro in 2010, so we'll see.  A taller final drive would make it a bit easier to drive, but I'm worried it might chug a bit (by V8 standards) off a really slow corner.  At Nats 2010 there were a couple 30mph corners in the Viper, and I've seen National-level events with 25-26mph corners.  Sure, there's always first gear, but after having done a bunch of shifting for 3 years in the Viper, I'm looking forward to a more traditional second-gear-everywhere approach in this car.  Not having ABS is going to keep my feeble and already-overloaded mind busy enough when braking into a slow corner.

***Along the above lines - I have an extensive process I use in evaluating cars (like the 240sx, Viper, or this thing) for a class, a part of which I've skimmed in the paragraph above, and lightly in other posts.  If you're interested in seeing this process more thoroughly described, email me (jason@rhoadescamaro.com) to let me know.  It's going to be a lot of work to explain everything so I don't want to start on that road unless I know people are interested.  I'd do so in a parallel blog as it's not really Camaro specific.  Analysis would broken down into the following seven chapters, which cover everything your car does in an autocross or track lap:

  1. Straight-line acceleration (which answers the eternal HP vs. TQ question, how to choose gearing, etc.)
  2. Skidpad (including an explanation of tires to the depth you need to know)
  3. Transitions/slaloms
  4. Straight-line braking
  5. The launch
  6. Corner entry
  7. Corner exit


A while back I bought a pair of subframe connectors: ( http://www.rhoadescamaro.com/build/?p=263 ) and John set about modifying them.  We're allowed three mounting points:

H. Longitudinal (fore-aft) subframe connectors (“SFCs”) are permitted
with the following restrictions:
1. They must only connect previously unconnected boxed frame rails on unibody vehicles.
2. Each SFC must attach at no more than three points on the uni-
body (e.g. front, rear, and one point in between such as a seat
mount brace or rocker box brace).
3. SFCs must be bolted in place and not welded.
4. No cutting of OE subframes or floorpan stampings is permitted.
Drilling is permitted for mounting bolts only.
5. No cross-car/lateral/triangulated connections directly between the
driver’s side and passenger’s side SFCs are permitted. Connec-
tions to OE components such as tunnel braces or closure panels
via bolts are allowed and count as the third point of attachment.
No alteration to the OE components is permitted.
6. SFCs may not be used to attach other components (including but
not limited to torque arm front mounts or driveshaft loops) and
may serve no other purpose.

But these only have two attachment points, at the very front and rear.  If I'm adding 24 pounds of steelitis, I'm going to get every last bit of value out of these boat anchors!  John will be adding an intermediate attachment point to bolt them to the rockers, but he needs to get them in position first.  Along with that is the installation of the (stock height) replacement subframe bushings.  Would probably use aluminum here if the rules allowed, but since they don't, delrin it is.

So apart come the subframe bushings, last torqued down about 44 years ago by some fellow in Van Nuys, CA:

John cleans things up a bit before proceeding:

And loosely put together with the new bushing-

Similar process for the rearward body mounts:

And together with new bushing and subframe connector-

Need to ensure the subframe is square to the body before anything is tightened down, so out come the plumb bobs:

I suspect it'll come out of John's shop with a more accurate subframe alignment than it had when it rolled out of the factory :)  It'll have to come apart again later when it's time for paint, so John will put some things in place to aid in more rapid future alignments.

Driver side connector initial install, no problems:

Passenger side, BMR, we have a problem:

The passenger side connector has a bit of counterclockwise-when-viewed-from-rear "twist" in it, and the rear backets are not square.  This part ain't goin' on the car!  I imagine BMR will ship me another one to make this right, so it's not the end of the world.  I consider myself lucky to even have a decent part available that's legal for this ruleset - pretty much every other subframe connector involves cutting the floorpan (you can see how a straight path would require this), and welding things to the car.  Who knows, maybe they were doing their website's photo shoot the day my parts were made, which distracted the welder-

A lesson here for everybody fixing up your car, is to not assume any part you get is correctly made or configured, even if brand new, and even when it comes from a highly regarded vendor.

Filed under: Chassis 2 Comments

Ahh the 302


For us car people there's a lot of different ways to find automotive pleasure.  For some it's all about the driving experience on a nice day.  For serious competitors, it's the thrill of  a close and tight race.  Plenty of folks enjoy "car porn", a collection of shiny TIG-welded go-fast parts laid before a car like in a preview for the new "Fast and the Furious" movie, and the romance of bolting those parts on the car.

For me, a lot of the fun is the brainstorming and bench-racing that goes on around the question of "How would car X do in class Y?".  In this case, that question, is "How would a '67 Z28 clone do in STX?".

There's a lot you can find out on the Internet, and with that information, you can start to form a halfway decent picture of how the car is going to perform.  Here are some things you can find out pretty easily, to help in the evaluation process:

Dimensions - Length, width, wheelbase, height, ground clearance

Engine - Displacement, peak power, peak torque, redline

Gearing - 1st, 2nd, 3rd, final drive ratio(s) available

Wheel/tires - sizes, width, diameter

Weight - total and front/rear distribution

Suspension - type at front and rear, adjustability

These are the things usually comprising the "paper" looked at when people say "Car Z looks good on paper ".  However, there's a lot more to find out, and this information can take some digging to find:

Weight of the car in a trim like yours - both the total weight, and the contribution of your weight-reducing efforts to both front and rear axle weights.  For Street Touring for instance, your front seats and mounts should weigh 25 pounds each - how much weight will be saved there?  How much in headers, exhaust, lightweight battery?  Are there any options you can remove?   How low can the car be run on fuel?  You have to look at published weights, or weights from people that have weighed their cars, then offset from those figures to account for the modifications you'd make.

How much wheel/tire can the car fit within your rules?  How much of it can go inboard from stock?

Dyno pull of the engine with modifications made for your class.  This may just be a cat-back exhaust, or that plus a CAI, headers, and a tune, or maybe even more extensive mods.  You also have to take into account the different types of dyno (Mustang, Dynojet, Dynapack, direct engine dyno) and build in some correction factors amongst the dyno types and operators.  Some massaging to do here, but a dyno is MUCH more valuable to evaluating autocross acceleration, than peak numbers.

In a future post I'll maybe go into some detail into the thinking used to take all that information and try to apply it to autocross, to see what cars will be faster than others, where on course.  But for now, that last bullet - engine dyno - is something I was missing somewhat in the pre-purchase planning stages of the build.  I figured an 11:1 compression 5 litre V8 should be able to make more power than a 2.8 litre BMW or normally-aspirated RX8 rotary, but how much more, and where in the powerband?  There's all kinds of legend out there on the 302's real power output, but not a lot of good hard data.  You end up reading through a lot of threads like this:


And not really figuring much out.  Like stock Z28 quarter mile times, you get numbers all over the place, but most are either wrong or completely unsubstantiated.  I did find this dyno:

I happen to know the DTS4000 is an engine dyno, so this was done out of the car, likely without accessories and definitely without any drivetrain loss.  That's about 345hp - certainly more than the factory's 290hp rating, substantiating many's claims of the factory having underrated the engine.  However we still don't know the details of the build - whether the motor was fresh or high mileage, if it had headers or stock manifolds, whether it was tuned at all, etc.  Assuming the reading is correct the air/fuel ratio seems on the lean side, over 14:1, not quite ideal for power.

So for me, the engine's power was a really big question mark when it fired up for the first time this past Friday, 11/26.

J-Rho's 302 is born, first startup

The smells coming off this thing were fantastic, it brought me back to the days as a young teen watching the cars go down the 1/4 mile at the now-defunct Carlsbad Raceway.  I attribute the smell largely to the fuel used - we didn't have any of the Sunoco GT100 fuel I plan to run on hand, so we made a mix of 91 octane pump gas and 110 octane leaded fuel.  No fancy widgets here to foul up from the lead, just my brain to intoxicate with carcinogens, yummy!  :-)

There's a lot of myth and legend about this motor out there, perhaps more than about any other small-block Chevy.  The components used to make the engine changed from 1967 to '68, and again in '69.  They made way more '69 302's than the other years, but in some ways, the '67 engine is easier to reproduce.  This is because there really isn't anything special about the 302 motor in '67, it was all parts-bin stuff - the same 4" bore, small-journal block used by the 327 and 350; the forged crank from a 283; "fuelie" heads with 2.02/1.60 valves from the Corvette, along with its "30-30" cam.  The carb is the same used on the SS396.  There really wasn't anything unique or special in the '67 Z28 engine, which made piecing together a reproduction, possible.  It took a few months, but my chosen engine builder (some may recognize the engine dyno room from these videos) has excellent connections in the vintage and A-Sedan racing circles, having built many smallblock Chevys under varying, but always strict, rulesets.

Here's a video of a full pull, the only one where we went over 7 grand:


Those awful red boxes are covering the dyno operator's readout showing instantaneous torque and horsepower figures, it also showed oil and water temps.  Hopefully Randy Chase will forgive me, I can't imagine he ever envisioned his Dashware software being used in this manner, especially in so crude a fashion.

In case you're wondering, I won't be publishing the numbers it made.  They'll go to the car's next owner, whenever that is, but won't otherwise be released until then.  Same goes for some final details of the suspension (spring rates, alignment, shock settings) once I have it sorted.  It's not like I expect a dozen other Z28 clones to get built next week, where I'd be giving up some kind of competitive advantage.  Perhaps to some extent, a goal in providing all the information in this blog isn't to provide all the answers I find on a silver e-platter; rather, it is to help make some of my fellow racers more informed in the critical thinking of competition car building/tuning, and empower them to figure more things out on their own.  Perhaps by doing so, some will be inspired to try new things or come up with some new ideas of their own, from which I can learn something in return.  By making my competition better, it forces me to get better, which ups the overall ability levels of everyone involved.  IMO there's not enough innovation and outside-the-box thinking in autocross, which I hope to try to improve.  And you don't have to have an oddball car, or spend a ton of $ to innovate - I see innovations pretty much every year in the ST Civics, even though their rules have been essentially stagnant for over five years.  It just takes effort.

So I don't come across as a total tease, I will share this - I went in with a pretty well formed set of expectations of what the motor would do based on all the research I'd done.  It didn't quite meet my expectations at the top end - I'd read enough stories of people revving these motors to 7500rpm or higher, as delivered from the factory, to believe this sort of thing would be possible.    After the dyno session, I plan to set the rev limiter at 7000rpm, well below the 7500+ I thought it would be good for.  Anybody revving one of these things much past 7k rpm successfully, is either floating the valves something huge, or isn't doing it with stock/ST-legal valvesprings.

At the same time, I was pleased by what it made in the midrange.  We dyno'd with 1 5/8" primary 4-1 headers, with a Y pipe into a single big 3.5" muffler.  I may retain a similar exhaust config on the car to reduce weight, or go with a dual 2.5" exhaust.  Bigger primaries or a larger OD header-back (note I didn't say cat-back, ha!) might help top-end power, but the midrange does tend to be more important in autocross.

One advantage of a dual system, is it should be able to package more muffler longitudinally underneath the car.  There's also a 93db sound limitation at my local lot in San Diego, which I'd like to meet.  Not sure yet if it'll be possible to run the exhaust all the way out the rear of the car or not, with the rear suspension still being worked on.

So, it's good to have the motor built, and home.  It will be tuned again next year, after I have it in the car, with the air cleaner and header/exhaust system in place.  There really isn't much tuning to do anyway, just adjusting the distributor advance.  Fortunately the air/fuel ratio was okay with the carb freshly rebuilt to stock specs- there's definitely more power to be had with some tuning there, but I have no allowances to do so.  Something I'll have to keep working on with the rulesmakers - the fuel injected guys can adjust their ECUs to produce whatever sort of fueling they'd like, would be nice if the old-school carb'd cars were given the same freedom.