Performance Q&A Questions - Performance Q&A

Kevin McClelland Jan 1, 2009 0 Comment(s)
0901chp_01_z Performance_qa_questions Illustration_by_tom_truty 1/2

The General Makes Them Right
OK, I'm probably going to hear about this one. For many years I've been preaching the good word that GM's parts are designed and tested for 100,000 miles and over. I use factory components, like starters, on my race cars and have never had a failure. One other benefit from using factory-designed and -manufactured parts is that they fit! Many of the generic auto parts you buy today are a hodgepodge of designs to make one part fit as many vehicles and engines as possible. This will lower the cost and make the parts house very happy, since it only has to carry one part for many applications.

Several months ago I wrote about my buddy Phil Doyle's vacation to Disneyland, when his serpentine belt broke, wrapped itself around the water pump shaft, and pulled the water pump apart, driving the cooling fan through the radiator core. All this 450 miles from home.

Since then Phil's been ordering parts for the Suburban. Yes, a '93 2500 4x4 Suburban is 15 years old, but you'd think most parts would be OK. He started with our good friend Ken Casey at Burt Chevy for the water pump, clutch fan, fan blade, and the serpentine belt. Another local pal here by the name of Mike Eling had a great contact for radiators. It was a beautiful plastic-tank/aluminum-core radiator. We buttoned everything up on the engine and dropped in the radiator. This is where the trouble started. Phil's 'Burb was equipped with heavy-duty cooling and a tow package. This option gives his truck an integral engine-oil cooler in the return tank of the radiator. Everything was going fine; the trans cooler bolted right up, but when he went to connect the oil lines from the engine to the rad, the upper hose was 2 inches too short! After trying to reroute the line to gain a couple of inches, there was no way. For now, Phil bypassed the cooler by removing the oil lines and plugging the block. So he's not planning to use the truck for towing anytime soon.

After checking back with the radiator manufacturer, this was the proper radiator for Phil's application ... and about 10 other trucks. Maybe some of the other truck models or a different year had a longer hose, but not his. This is back to the proper-factory-designed part for the vehicle. Now, I'm not trying to get you guys to spend more money, but when the parts don't work, they're not really a bargain.

What Do You Want From a Mouse?

I've built up a 283 with a 270 Magnum Comp Cam with matching valvetrain, springs, retainers, locks, and roller-tip rockers. Heads are Camel Backs with 1.94/1.50-inch valves, ported. It has an RPM intake with a 600-cfm Edelbrock Performer carb and is fired by a Mallory Unilite ignition with an ACCEL Super coil. I was told to use the 8-inch harmonic damper over the smaller one. It just doesn't seem to have the high rpm like it should. I have a BorgWarner Super T-10 trans with the 2.88 First gear and a 3.42:1 posi with big 31-inch Pro Trac tires. I have a 3.70:1 9-inch rearend that I'll be putting into it, but I still wonder if the balancer is right for this engine.

I also have a 350 on the stand with a Comp Cams High Energy 268 cam and valvetrain with World Products 76cc heads and the same intake and carb. I know the 350 puts out more torque, but I like the whine from the short stroke in the 283. Both engines have the forged flat-top pistons and the 8-inch damper. Should I switch the one on the 283 to a smaller one or just drop in the 350? The 283 is better on gas, or so it seems, since it has a smaller bore. I'm retired and on a fixed income now, so anything to save dollars will help. I also have a 400 small-block to rebuild, but not sure which way I want to go yet. Thanks for your help.Daryl Brooks
Via e-mail

We were all for the 283 until you said you were pulling around 31-inch Pro Trac tires. Those tires probably weigh more than the rotating assembly of your engine! The 270 Comp Cams Magnum camshaft is a pretty big stick for a 283. With your buildup, you're probably only making around 300 lb-ft of torque out of your little Mouse. Gears are the only answer. With that tall a tire and 3.70:1 gears, it's like having 3.08s with a normal 26-inch performance tire. You're going to need more like 4.30-4.56 gears to get that thing going.

Yes, the 8-inch damper is larger and heavier; however, I don't think it's affecting the way your motor revs. The larger damper will slow the acceleration rate slightly but won't hurt the horsepower at steady state, nor will it limit rpm potential.

Finally, working with a fixed income, the 283 is going to get the best mileage out of all the engines you mentioned. If you wish to kick the mileage up a little more and pick up some torque to help move better (truck, car, or whatever you have those 31s on!), you could swap out the camshaft for a smaller one. Your 270 Magnum comes in at 270 advertised duration, 224 degrees duration at 0.050 inch tappet lift, 0.470 inch max lift, and is ground on 110 centers. We'd drop the camshaft down to an Xtreme Energy 256. This cam specs out at 256/268 degrees advertised, 212/218 degrees duration at 0.050 inch tappet lift, 0.447/0.454 inch max lift, and is ground on 110 centers. Yes, this is a much smaller camshaft, but with your ported Camel Backs on the 283, it will still make good power upstairs and you'll like the whine! Good luck with your projects and enjoy retirement!


Riggin' The Secondaries

I read your article about vacuum-secondaries not completely opening at WOT. Could I put a small bolt and nut in the slot on the secondary shaft to open them all the way, sort of mechanically, or would this cause some problem? The carb is a Holley 4150 850-cfm on a GM 502 Deluxe Crate engine. Thanks.
Paul Bourke
Lockport, IL

We used to do this back in high school to make the 780-cfm Holleys a mechanical-secondary. Several aftermarket companies would sell add-on linkage to convert vacuum carbs to mechanical secondaries. The linkage you're referring to has a specific purpose. It is to close the secondaries if the spring in the vacuum secondary diaphragm isn't strong enough to close the secondaries. This is a very important function and shouldn't be taken lightly when you're modifying the carb. There's nothing worse that a runaway car with a hung throttle!

We searched around for quite some time but couldn't find any of the companies we remembered selling these components. The main problem with converting a vacuum-secondary carb over to a mechanical-secondary is that you don't have a secondary accelerator pump. In the dyno cell, when you can open the secondaries slowly at full engine speed, this isn't a problem. However, from an idle, if you try to open all four barrels without a pump shot in the secondaries, you will have a tremendous bog. You could try to cover this up with a larger accelerator pump diaphragm (50 cc) and a larger accelerator pump nozzle, but this causes its own set of problems. Trying to drive the car on the street with that much shot going in every time you step on the gas would be way too rich.

Why don't you get a secondary spring tuning kit from Holley and tune in your secondary opening? Holley's Secondary Diaphragm Spring Kit (PN 20-13) gives you a selection of seven springs to tailor the secondary opening right to your engine. Check out the spring that was delivered in your 850-cfm Holley and look at the chart supplied with the springs. Move to one spring lighter until you pick up a slight bog from a hard launch. Then back it off one. To make the job very quick and easy, the Quickchange Vacuum Secondary Housing Cover (PN 20-59) makes it a snap to change out the springs. By tuning in your secondary opening you can get very close to double-pumper performance. Work this out first before you rig your secondaries.


Stovebold Six

I'm running a 250 six-cylinder and a four-speed with 3.08:1 gears in my '70 Camaro. I just bought the car, so I haven't driven it yet. My plan was to pull it and install a big-block, but with the price of gas I was thinking of just fixing the six so I can drive it as a commuter rather than buying a freakin' Kia or something. So, before I go and spend some money on fixing the six (it leaks oil and has a flat cam), I wanted to know what kind of mileage it would get rather than putting a mild small-block in it. I have most of the parts to small-block it, but if it will get better mileage with a six I want to keep the six. What do you suggest?
Glen Yotter
Via e-mail

Back in the day, we built many 230, 250, and 292 six-cylinder Chevys. They were great, reliable engines, but they weren't the best in mileage. We even built a 0.060-over 292 that was our "302" in a '63 Chevy truck. That thing was a kick to drive with a Muncie four-speed, 3.73 gears, and long 41/4-inch stroke. It didn't like anything north of 5,000 rpm, but it would haul the mail to that point.

The main problem with this Chevy engine design was the cylinder head. It used a Siamese, or common, inlet port for two cylinders. With this you didn't have much tuning going on in the intake tract. On our engine, we even went to the extreme of modifying the Offenhauser single four-barrel manifold with a divider wall that traveled down the center inlet port to feed cylinders No. 3 and No. 4. This was done in an attempt to divert some fuel into the center cylinders at higher engine speeds. At slow speed the center cylinders ran rich and the outboard cylinders ran lean. Dividing the center ports to about 3/4 inch from the floor of the port was a decent Band-Aid. We came up with this solution by watching the header tubes glow at different engine speeds.

Back to the sharing of inlet ports. Without any tuning, the engine doesn't get any benefit from a properly sized inlet port and manifold runner. When building torque for mileage you need everything you can get. If we were to guess, you should be able to knock down 15-18 mpg with a fresh 250 six. With a conservatively built small-block, this is also easily attainable and a whole lot more fun to drive.

Back to the "freakin' Kia." If gas keeps going the way it is, we may all be looking to smaller cars and keeping our toys to entertain us only on the weekends. Let's hope it doesn't come to this, but with GM closing four truck plants, they may have gotten the message. America, at least, needs a choice, and The General is stepping up to the plate. They will have small performance/high-efficiency cars for us all soon.

Don't Blame The Stall

I have had my '72 Chevelle for about 15 years and had a new engine put in about a year ago. It is a 355 small-block with forged flat-top pistons, 2.02/1.60-inch-valve 461 castings, a new cam with 0.474 inch max lift, and a 650 Holley double-pumper. A little better than the original 307! I was told by the guy who built it that it should push anywhere from 375 to 425 hp. It is not dyno-tested, so I really have no clue. I have a 4,000-stall torque converter. After I light up the tires and come to a stop, the car seems to want to stall. Is the stall too high for this engine, or is there another problem? This car is used only for the street right now. Any help would be much appreciated.
Kenneth Ahern
Fort Erie, Ontario, Canada

Having a 4,000-stall converter makes the engine's life very easy at idle. The higher you push the stall speed of a converter, the less load is placed on the engine at idle. This isn't the problem you're running into.

First, take a good look at your 650 Holley. You must start at the float levels. If the float level is too high, fuel can weep out of the secondary boosters on deceleration. Extremely high float levels can actually slosh fuel out of the secondary bowl vent, which dumps raw gas right down the secondaries. We could go into a long explanation of how to adjust the float level, but if you go to Holley's website and click through the Tech Service tab, then Common Questions, you'll find a great explanation of how to adjust the float levels. Holley also explains accelerator pump adjustments and vacuum-secondary spring tuning. It is a great resource for Holley carb tuning. After you've ensured that the floats are adjusted properly and the idle mixture has been set, this should take care of your stalling problem.

One other thing you may want to look for is that some builders will run full manifold vacuum to the vacuum advance on the distributor. This works well to clean up a choppy idle, but when you are decelerating to a stop, you have high manifold vacuum and high advance. When you come to a stop and the idle comes down and the load goes up, the vacuum drops and so does the advance. Make sure you are running ported vacuum to your vacuum advance can.

These few simple adjustments should help your stalling problem. As for the power of your little small-block, you're probably in the 375hp range. Without all the camshaft specs and the inlet manifold, it's tough to give you a complete answer, but 375 isn't out of the question. Good luck.


Moving Malibu

I was wondering if you could help me with my project I'm building. It's a 0.030-over 454 with 11:1 closed-chamber pistons and a steel 1053 crank. I have two choices of cylinder heads. The first is a pair of cast-iron oval-ports, casting number 336781. These heads have been cut to fit 2.30/1.88-inch valves with 113cc combustion chambers. The other set is a pair of iron rectangular-port heads, casting number 3994026, which have also been cut for 2.30/1.88 inch valves and have 118cc chambers. I'm building the engine with a Comp Cams Xtreme Energy solid lifter cam PN 11-678-5. This camshaft specs out at 282/290 advertised duration, 244/252 degrees duration at 0.050 inch tappet lift, 0.590/0.598 inch max lift, and is ground on 110 centers. I'm looking to get into the high 10s on motor alone. This '80 Malibu will be street driven also, so what would be your choice of heads, and is there a big difference in the power? Thanks.
Fred Reed
Houston, TX

Man, you want to have your cake and eat it too! We sure hope this '80 Malibu is ready to take the power of that big-block you're building. They have been known to twist pretty good when you put power to them. And you better step up to a 9-inch Ford rear or a Strange Dana 60.

With your 10-second goal, you'll need to go with the rectangular-port heads and do some work to them. Hopefully, whoever put the larger valves into the cylinder heads at least ported the bowls to match the larger valves. The benefit of installing larger valves on the inlet side of iron big-block cylinder heads is that you can finally have some type of short side radius. This is the turn from the port floor into the bowl of the port at the valve opening. Also, since you already have closed-chamber pistons that spec out at 11:1 compression, you'll need to mill the heads down to a chamber volume between 110 and 113 cc. This will give you a measured compression ratio in the high 9:1 to low 10:1 range. With the camshaft you selected, you'll need all this compression.

Rounding out your package, you'll need a good single-plane inlet manifold and a Holley 4781 850-cfm carb. The Edelbrock Victor Jr. PN 2902 is a good choice for the rpm range you'll be in. And on the exhaust side, you will need at least a 17/8-inch primary pipe with 31/2-inch collector. Check out Hedman Hedders engine swap headers PN 65216. They fit the bill and will bolt right in.

Your 10-second goal is right up there for a street-driven car, so remember: Keep as much weight out of it as possible. Adding an iron-headed big-block is going to strap down the front end with at least 250 pounds more than a small-block-equipped Malibu. Have fun with your toy and look out for the police!


Lack Of Oxygen

My '73 Camaro has a freshly rebuilt 350 small-block Chevy. It's bored 0.030 over, with 9.73:1 forged Speed-Pro pistons, Edelbrock RPM aluminum heads and intake manifold, an Edelbrock 750-cfm carb and hydraulic flat-tappet RPM camshaft (234/244 duration at 0.050 inch tappet lift, 0.488/0.510 inch max lift), Summit lightweight roller rocker arms (1.5:1 ratio), hardened pushrods, and a windage tray. The ignition is handled by an MSD 6AL box, distributor, and plug wires and a Flame-Thrower coil. Exhaust runs through 15/8-inch full-length headers and 21/2-inch Delta Flow 50-series Flowmaster mufflers. This is all going through a TH400 with a Stage 1 shift kit, a 12-inch B&M Torque Master converter, and a 10-bolt posi rearend with 3.73 gears.

My problem is with all of these parts, my car is too slow. My best time at the track is 14.9 seconds. My best 60-foot time is a 2.41seconds with a top speed of around 93 mph. Even though I race at 5,000 feet elevation, I feel this car should be quicker. I have even had it at a local auto shop with a chassis dyno. After three hours on the dyno and numerous timing and carb jet changes, I didn't gain anything!

Also, I am using a drop air cleaner with a 2-inch element versus a 3-inch element. When I do this, it leaves me with a quarter inch between the top of the air cleaner and the hood (I know I should get a cowl hood). When you look at the air cleaner with 2-inch element, there is roughly 5/8 inch between the top of the air cleaner and the bottom. While at the track, I even took the air cleaner off and still saw no change. Could my problem be the wrong stall converter? If so, what would you recommend? A friend has a B&M Hole Shot 2400 that he'd let go for $50. I was also recommended to try a Hughes 3,000 converter. Also suggested was a bigger full roller camshaft, like a Thumpr by Comp Cams. Any thoughts would be greatly appreciated.David JohnsonAlbuquerque, NM

The engine package you've put together is a dyno-proven package by Edelbrock. You should be right in the power range if you were at sea level. The lack of barometric pressure is not feeding the engine with the oxygen it needs. Since you spent the dollars on the chassis dyno, and we will assume that the operator/tuner knew what he was doing, you have all the power you're going to produce. When you rebuilt the engine, did you degree the camshaft in to ensure the cam was ground correctly and that the timing chain set was proper?

Looking at your package, you have honed right in on the problem. The torque converter is way too tight. The Torque Master converter from B&M is a great RV torque converter for a truck or very stock engine. The Edelbrock RPM Performance Package would need a 3,000 stall at sea level. This same 3,000-stall converter at 5,000 feet would probably only stall to 2,500 rpm because of the lack of torque output from your engine. To wake up your Camaro, try a Nitrous Hole Shot 10 converter from B&M (PN 20482). This is a great converter, which specs out by B&M to stall around 3,600 rpm behind a small-block. Again, this is at sea level. You'd probably see this converter stall around 3,000 rpm in your hometown.

Finally, you really need to do something about your air cleaner. Putting the lid of the air cleaner that close to the float bowl vents and making the air squeeze between the base and lid and turn down into the carb is really tough. Yes, we understand it ran the same at the track with the filter on or off. We assume you have a 14-inch open element air cleaner assembly. If this is the case, why don't you go with a K&N XStream Top Air Filter, PN 66-1401? This air filter lid gives you a large filter area built right into the lid of the air cleaner housing. This would double the airflow of the filter assembly you have now.

The last thing you'd want to do is install a larger camshaft. This would reduce your cylinder pressure even more and kill what little torque you have. If you were to install a larger-roller camshaft, you'd need to lower your rearend gears significantly and go even higher on the stall speed.

Try the things listed above and compare your performance to other vehicles at the track on a given day. That will be your best gauge of performance. Don't expect the sea-level performance you see in magazines.


Technical questions for Kevin McClelland can be sent to him at

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