October 2011 Chevy High Performance Q & A

Scales are for Fish

Kevin McClelland Sep 1, 2011 0 Comment(s)
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Several months back in How it Works, we had notable suspension industry experts chime in on the proper way to set your ride up with four-wheel scales. There was a ton of great information about finding the front-to-rear weight distribution, crossweights, and sway bars, and the effects on the static balance of the vehicle. With any vehicle that turns left and right it’s the goal to load all four tires equally and achieve 50-50 weight distribution front to rear. Many of us have swapped in the biggest, nastiest engine we could for power, ignoring the vehicle’s weight balance. You’ve seen the all-iron big-block Chevy stuffed into Vegas or Monzas. I’m not talking about race cars—these are street cars. Those things probably had 60 percent of the weight on the front end and 40 percent on the rear. They would burn the tires until you lifted. Also, they wouldn’t turn if they had to. They would plow through the turns, melting the outside edge of the tires no matter how much positive camber we screwed into the alignment.

1110chp 02 O  October 2011 Chevy High Performance Q And A Gear 2/2

After reading it, I hope you can appreciate the importance of equal tire loading and setting up suspension for different disciplines of racing. What I noticed from the story was that there was very little information about my favorite application of power: drag racing! As you all know, a while back I completely rebuilt my Davis roadster. During this rebuild I fabricated all new four-link suspension bars, replaced all the rod ends, rebuilt my Koni coilovers, and had the rear springs tested for pressures. The rear suspension needed to be completely set up for pinion angle and suspension preload to (hopefully) have the car launch straight when applying 800 lb-ft of torque. One final variable to throw into the mix was that I had never run the car with this much power. I had a really good handle on the setup for 600 lb-ft of torque, but never this much twisting force.

With four-link suspensions, you want to set a minimal amount of preload on the right rear tire. When the torque hits the rear suspension, the twisting moment of the driveshaft on the rearend pinion lifts the right rear tire off the track and plants the left rear. Weight transfer from the launch helps plant the load back on the right rear, but you may need to preload an extra few pounds on the right rear tire at rest to make the car go straight.

Well, I wanted to set my car up neutral, which means the weight on the rear tires is equal with the driver in place and all the fluids you’d have to race the car. Unfortunately, I don’t own a nice set of scales to measure this accurately. The old-school way to set this baseline is to set all four of the bars to the same length and, with the car race ready, adjust the upper right four-link bar until you find that it is free to turn, and no load is on the rod ends. From there, if you wish to increase the load on the right rear tire you would shorten the top right bar several flats (1 flat equals 1/6 turn).

We rolled into Fontana for the first pass, set the starting line chip at 4,800 rpm, set the transbrake, and let her rip! As I expected, the car drove to the right with the front end in the air. Coming back after the run, we applied two flats of preload by shorting the top right bar. This was just about enough preload with it still slightly going to the right. After one more flat the car left straight and flat on these full-throttle blasts. It’s my belief that everyone should set their car up to leave at full power. Then, if you’re going to throttle-stop race like in the Super classes, the car will leave correctly. I see enough cars racing Super classes that hit the tires with full power for a few tenths of a second before they come back to about 300 hp. They turn right, go left, or all over the place. If we take the time to work out the suspension in the first place we could win a couple more rounds. If I’d have had a set of scales in the first place I could have set the car with 50-75 pounds on the right rear tire. After I had the car worked out I had the chance to scale the car on my good friend Larry Scarth’s scales. What I found was that every flat of preload on my car was worth 35 pounds on the tires. With the three flats of preload I have 95 pounds of static load on the right rear to compensate for the unloading of that tire on the launch. I’m now in the market for a set of scales. Know of a good deal out there?

Vette Therapy

Q: I am a 27-year-old mechanical engineering student at Old Dominion University. I have been reading this magazine cover to cover since I was 15; what I have learned from years of your PQA is priceless to me! I’m putting together an ’81 Corvette that I bought four years ago when I was working as a pipe fitter. The car ran fine, (67,000 original miles) but the factory 190 hp just wasn’t cutting it for me. I yanked the choked-up 350 out, punched it out 0.040 inch over, and replaced the measly 8.5:1 slugs with some 10:1 hypers. I then turned to the valvetrain; I chose a Lunati Voodoo bumpstick, 0.510-/0.490-inch max lift, with a set of matching 1.5:1 roller rockers and hydro lifters.

The heads are now aluminum 58cc 2.02-/1.60-inch valves with 210cc runners from Procomp with a matching intake. I picked up a Mallory Pro Billet vacuum-advance HEI distributor and MSD 6AL box to handle the ignition duties because the CCC unit and smog system had to go. The exhaust gases are passed through a set of 15/8-inch primary full-tube headers and 4-inch unbaffled side pipes. The rotating assembly is the stock cast stuff; I figure it will hold up fine for what I’m throwing at it.

I have three questions that could help me wrap this Vette up and get her on the road for some much-needed right foot therapy. What carb should I run on this setup? I have a brand-new Edelbrock 650 with vacuum secondaries. Most of my gearhead friends say this will choke the engine, but some say it just needs bigger jets. Will this carb have good driveability and power output?

My next two questions concern the tranny I picked up for the car. The Vette came with a TH350, but I personally found it boring just slapping a Corvette into Drive. I sourced a Muncie M20 wide-ratio four-speed-equipped ’73 Vette that was being parted out. I got everything from the driveshaft to the pedals for a great price. What clutch should I run with the aforementioned engine combo, and what rear gear should I run to get the most out of this tranny and a stock tire size or 255/65 R15? From a little research on the car I believe it has a stock 2.90:1 rear gear. This seems a bit short for the four-speed to me. Any help is greatly appreciated! Great mag! Great column!

Michael W.
Via email

A: Good luck with your schooling, and congrats on a great field. We need more young people in engineering fields.

As for your Vette, you’ve really taken your ’81 to a new level. From your list of components, the 650-cfm carburetor is slightly on the small side. However, this will give you outstanding driveability and will only lose a few ponies right at the top of the rpm range. We’d start with your carburetor and the factory-installed jets. You may need to slightly richen the jets after you get the car running and can determine if it needs more fuel.

The clutch you use to mate your power to the transmission can really affect the car’s personality. If your clutch is too aggressive it can operate like a light switch—either it’s disengaged or it’s locked. This can be very annoying for street driving but just what you need for track days. We’ve had very good luck with a compromise; the Centerforce Dual Friction clutch will give you the best of both organic clutch application and metallic segmented torque-holding capacity. The Dual Friction technology pairs organic lining on the pressure plate side of the friction disc and segmented carbon composite pucks on the flywheel side. This clutch will give you the ultimate in street/strip holding power and performance without sacrificing pedal effort and driver control. We’d go with a 10.4-inch disc clutch, which will allow you to run the smaller and lighter 153-tooth flywheel. The clutch is offered in 10-spline disc (PN DF271675) and 26-spline disc (PN DF161675). The 10-spline is for the earlier Muncies, and the 26-spline should be for the ’71-and-up transmissions. The matching Steel Billet Centerforce flywheel (PN 700100) is a 153-tooth, 30-pound wheel. This clutch will give you years of trouble-free service and will take whatever your 350 will throw at it.

A common theme going through the car community is “save the manuals.” If you go to buy a new car today, it is virtually impossible to find a manual trans in anything except what manufacturers consider a “performance” car. Swapping out your lethargic TH350 trans that was equipped with a torque converter will completely change the manners of your Vette. Great find picking up all the swap parts from that ’73. Your Vette is equipped with a very unique Dana 44 centersection. It is an aluminum case with an aluminum posi carrier, really trick parts for a ’81 passenger car. The factory gears for an automatic trans-equipped, federal-emissions Vette was 2.87:1. If it was equipped with a manual trans or sold with California emission requirements it would have 2.72:1 gearing. Luckily, Dana 44 gears will swap out into this specialized Corvette carrier. Check with Randy’s Ring and Pinion for all your parts needs. Randy’s offers gearing for this differential from 2.73 all the way up to 5.89 gears. We’d go with 3.54-3.73:1 gears based on your 2.52:1 First in your wide-ratio Muncie. This will give you great acceleration without killing the streetability and highway cruising.

No matter how much your projects pull you away from school, finish your degree. You will have that for life. Cars come and go, but your education will be in your toolbox forever!

Sources: centerforce.com, ringpinion.com

Blown Rod

Q: I’m building a 372-inch aluminum block with AFR 195cc aluminum heads, a 6-71 blower (8 pounds boost, 9:1 initial compression), and electronic fuel injection. Do I use straight or angle plugs, or does it matter? I am thinking 75cc chambers to keep compression down. I am having custom headers made; what length primary header tubes should I run and what diameter? Finally, I will be running a built 200-4R to a 9-inch rear with 3.70 gears. What stall converter should I use? This is strictly a street engine, and all this goes into a ’31 (gulp) Ford weighing approximately 2,700 pounds. Thank you for your help—you’re the man!

Rick Carpenter
Via email

A: We’ve driven some crazy things in our days, but this one is going to be a handful. We assume you’re upgrading the chassis to handle the power and speed that this little Mouse is going to produce. Going over your specs, you need to focus on the street aspects of your build and let the power fall where it may. You could easily produce close to 600 hp with the above components without trying hard.

As for angle- versus straight-plug cylinder heads, we’d go with the angle plugs. In the late ’60s, Chevrolet found that by angling the spark plugs toward the exhaust side of the chamber, they would get better combustion efficiency and power. Since you’re having custom headers made, the header fabricator can build clearance around the spark plugs. Where angle plugs can give you fits is on street headers that have been built around straight-plug heads. It’s usually tough to get the plugs in and out, and you burn off plug wires with regularity.

Back to the street theme, we would stick with a 13/4-inch primary tube diameter around 32 inches long. This, matched with a nice 3-inch collector, will allow you to build a street-friendly exhaust system. Blown engines will produce a very aggressive exhaust note at WOT. Controlling that roar can be a challenge!

When you build your engine you’ll need to work out the compression with all your engine components. The cylinder head combustion chambers are just one of the factors. We ran some quick numbers and with a flat-top piston with two valve reliefs displacing 6.0cc, a 0.041-inch-thick head gasket taking 9.1, 1.5cc ring land volume, and the piston at zero deck you have 9.3:1 compression. This, with 8 pounds of boost, can cause issues for street driving on pump gas. You may need to look toward a mild dish piston to bring the squeeze into the 8.5-9 range.

Finally, the stall speed of your torque converter will depend on the camshaft you plan on running. Since you’re not trying to set any national records, go with a mild blower camshaft. Keep the intake duration in the low 220 range, the exhaust with 10-12 degrees more, and at least 112 separation angle. You don’t want to lose too much boost on overlap. This will give you a mildly aggressive idle that will not drive your EFI crazy. With this type of camshaft specs, you can keep the converter on the tighter side around 2,800 rpm. This will allow your engine to get right into the torque curve and the boost will take over from there.

When you get this beast together, make sure you take a picture and send it in. We know it’s a Ford, but it will have a killer Chevy underhood. Be careful and make sure that you hold on tight when you hit the loud pedal!

Big-Block Chevy Horsepower

Q: I have a ’69 Chevelle SS with a ’72 454 Chevy big-block engine that has been bored 0.030 inch over. The stock crank has been turned 10/10 on the journals, and I have reconditioned stock rods with ARP bolts. The pistons are TRW L2465F forged domed. All of this has been balanced for smooth operation. The cast-iron heads were ported and gasket-matched, and I had stainless steel 2.19 intake and 1.88 exhaust valves installed. The block has been decked and the heads have been shaved to 115cc to achieve a compression ratio of 10.25:1. The camshaft is a hydraulic flat-tappet Lunati special purpose grind PN H225-235 with stock rocker arms. The intake is a Weiand Stealth with a Barry Grant 750-cfm Speed Demon carburetor. The distributor is an ACCEL 52000 series with the 300+ ACCEL digital box and matching coil. Hooker Super Comp headers reduced down to 2.50-inch exhaust flowing through Flowmaster 40-series mufflers and 2 1/4-inch tail pipes in stock location with chrome tips. The Muncie four-speed transmission has a 3.55 posi 12-bolt rearend. It’s on 26-inch tires, and the engine has about 10,000-15,000 miles on it. This combination has a lot of power and I’m assuming it produces over 400 hp, but you know how it is when the horsepower bug bites you; I want more power. This car is a weekend warrior and usually gets 110-octane race gas.

I now have the funds for aluminum heads, roller camshaft, lifters, and roller rocker arms. I was thinking of purchasing the Edelbrock RPM Performer (PN 60459 or GM PN 12363390) heads and a COMP Cams roller camshaft and lifters along with COMP Ultra Pro Magnum roller rocker arms. Will I have any problems with the Edelbrock or GM heads and the pistons that I currently have in the engine? Would you recommend different heads than the ones I have chosen? What camshaft would you recommend? Would a hydraulic roller be better than a solid roller? What kind of distributor gear should I get for my ACCEL distributor? I would like to break the 500hp mark with a naturally aspirated engine, if that is possible with what I have. This is not a daily driver, and fuel economy is not a concern. I have subscribed to your magazine for more than 10 years and enjoy reading all of the tech tips and articles about making more horsepower. Any help you can provide me will be greatly appreciated. You have an excellent magazine; keep up the great work!

John Lubinsky
Via email

A: Isn’t this horsepower thing a real addiction? If it makes you feel any better, you’re not alone!

Your plan does have a few issues. Both the GM Performance Parts and Edelbrock cylinder heads are a semi-open chamber design, which will crash into your full-open-chamber TRW piston domes. If you wish to switch to aftermarket aluminum heads, you’ll need to go with an open-chamber design. We really like the Dart CNC Pro-1s, but this may be a little too much cylinder head for your build. Dart also offers a new Pro-1 oval-port 275cc inlet port aluminum head. These feature a 121cc open combustion chamber that can be milled down to a minimum 108cc combustion chamber volume. If you clip the heads 0.030 inch you’ll be right back to your 115cc pump-gas friendly 10.2:1 CR. The heads (PN 19000171) come completely assembled with 2.19-/1.88-inch stainless valves and 1.550-inch single valvesprings with 145 pounds on the seat and a max lift of 0.660 inch.

Also, the TRW pistons in your build don’t have dedicated wristpin oilers from the oil ring of the piston. This can cause issues for extended wide-open throttle runs. Also, they have limited valve pocket depth in the piston crown. This limits the size and duration of the camshaft you can run. From what we’ve seen, the max duration you could probably run with these pistons without cutting the pockets for clearance is the mid-240 duration range at 0.050 inch tappet lift. For sure, whatever package you put together, you will need to check the piston-to-valve clearance to ensure that you won’t have contact between the valves and pistons. Most builders will tell you to run 0.080-inch clearance on the intake and 0.100 inch on the exhaust. We’ve run them much tighter and would say that you could safely run them at least 0.020-inch tighter than these numbers.

Finally, for a camshaft, go with a mechanical flat-tappet. The hydraulic rollers are very rpm limited because of the total valvetrain weight on a big-block. To get one to rev to 6,000-rpm clean without valve stability issues is very tough. Mechanical rollers on the street are a ticking time bomb just waiting for the needle bearings to shatter into hardened metal fragments that will destroy your engine. With the cylinder head and a mid 240-duration camshaft, your engine will want to run into the 6,500-rpm range. Hydraulic flat-tappets are good, but the mechanical will give you the best of both worlds between a mechanical roller and a flat-tappet design. Since you are looking to run COMP Cams products, look into an Xtreme Energy XS282S mechanical that specs out at 244/252 degrees duration at 0.050-inch tappet lift, 0.590-/0.598-inch max lift, and is ground on 110 centers. With this camshaft your ACCEL distributor will be just fine with the gear on it. This camshaft, in conjunction with the Dart oval-ports, will exceed your 500 hp requirements.

Hope this gives you a direction for your Chevelle. Enjoy it and cook some tires for us!

Sources: compcams.com dartheads.com

Shifty Business

Q: I have questions about rebuilding a TH350-C. Who can help me? I have everything done except the valvebody. I bought a shift kit, but things don’t line up. I think it’s for a TH350, but without the C part. I sincerely need help with this, it’s driving me nuts. Thanks for any help you can offer.

Dustin Cross
Via email

AThe TH350-C was released in 1980 and ran through 1986, when GM redesigned the TH350 with a lockup torque converter. They were used to increase fuel economy. The shift kit you have is for a standard TH350 and doesn’t accommodate the converter solenoid and the appropriate passages. You need to pick up a B&M TransPak PN 30235. This is the next step up from a standard shift improver kit. The TransPak gives you two stages to choose from. First, a standard shift improver, and the second stage gives you full control of the trans. You can hold the trans manually in any gear for as long as you wish. The trans functions normally with the shifter in the Drive position. Check with B&M for more information, or just get one on order. Don’t forget where all those little balls go!

Source: bmracing.com

We love letters, especially technical questions. Submit your tech questions to Kevin McClelland at chevyhi@sorc.com. Regular shout-outs and good tidings are also always welcome.

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