Performance Q&A Questions - Performance Q&A

Kevin McClelland Feb 1, 2009 0 Comment(s)

Octane Dependency
Back in the '60s I raced a gasser and have played with a '66 Nova for about 20 years. As you can guess, I am pretty low-tech. However, I have a project now that I really want to get right, and I could use your help.

The project is a '57 Chevy primarily for show, but I want good street performance too. The engine is already built. It has a 3970010 block with factory 11:1 pistons. The heads are 340292s with 2.02/1.60-inch valves. I thought I could lower the compression to about 10:1 for use with 93-octane by using a 0.070-inch head gasket. After reading one of your articles, I learned that this gasket might increase the quench area and affect performance. Will this be significant? I'm sure you will recommend aluminum heads. Are there any that will give me 10:1 compression?
Jim Jackson
Murfreesboro, TN

We sure wish we had 93-octane gas out here in California! 91 is a weak blend, and those couple of points would keep us out of our knock sensors on late-model cars. For your early iron, knocking down the squeeze a point or two is the right direction. Increasing the piston-to-head (quench) clearance is a simple way to reduce the compression ratio.

How much will it kill the efficiency or power of your engine? I don't think you need to worry about it. If you're trying to stick with the early-theme LT-1 style of small-block, stay with your iron heads. You didn't state how thick your current head gaskets are. If you're running Fel-Pro performance gaskets, they displace 8.9 cc. If you step these up to 0.070-inch gaskets, you'll displace around 15.6 cc. Finally, if you built your old-school engine true to form and used the original 0.016-inch steel shim gaskets, they only displace 3.5 cc! As a rule of thumb, for every 1 cc of change you will see 0.1 compression ratio changes on a 350 small-block. So if you have the super-thin steel shim and open it up to the 0.070-inch gasket, you should see a change in compression of about 1.2 points. At most, you will lose 3-6 percent with the point of compression change and a couple more points for the increased quench. We'd have to guess that you would see around 5 percent less power.

Since you said we were going to recommend switching to aluminum heads, take a look at the Air Flow Research 180cc Eliminator Street heads. These heads will outperform production steel heads to the tune of 70-plus horsepower! They're also available in 65cc and 75cc combustion chamber volumes. Using the 75cc chambers will allow you to run the thin (correct) head gasket to retain your quench and lower the compression by a little over a point. The production 292 iron heads have a 64cc combustion chamber. The AFR heads feature 3/4-inch deck thickness for ultimate durability and lightweight 2.02/1.60-inch valves with 8mm valve stems. They come with either straight-plug (PN 0911) or angle-plug (PN 0917) configurations. These heads will truly wake up your old-school '57 buildup. If you wish, paint the aluminum heads to match in Chevy Orange, and not many people will be the wiser.


Slight Disconnect
I have an '85 Monte Carlo SS 305 and I installed a vacuum-control distributor, a Mallory PN 8548201, and the throttle response does not respond when first accelerating. It has a pause, as if the power shuts off, then after about 1,000 rpm the car takes off and runs well throughout the rpm range. Can you give me some insight on what my problem might be? Also, it still has the factory-equipped carburetor. Could this be my problem?
William Breedlove
Via e-mail

From your information, we keyed into a couple problems. The Mallory distributor you installed is an HEI replacement-type distributor with some very nice upgrades. It features improved electronics, a High Dielectric strength cap and rotor, stainless steel mechanical advance, and an adjustable vacuum-advance can. This is a perfect upgrade to a non-computer-controlled vehicle. Your '85 Monte SS, with its 305, is computer-controlled. The computer controls the spark advance, fuel mixture, and transmission functions. You must have picked up a "check engine" lamp when you installed this distributor. With this change, you're giving the computer-controlled carburetor fits. It's probably running full rich, which is way too much fuel at idle and affecting the way the car responds to throttle changes. At wide open throttle it should run just fine.

Depending on emissions regulations in your state, you could stick with your new distributor and replace the computer-controlled Q-jet with an earlier 650 nonfeedback Q-jet, or install a Holley or Edelbrock carb of your choice. For your 305, we'd stick with a carb under 650 cfm. You could use an adapter plate to go aftermarket or change to an Edelbrock Performer manifold. The proper Edelbrock Performer series carburetor would be PN 1406, a nonemissions, electric-choke, 600-cfm carb. To mount this to your stock manifold, you'll need a square-bore-to-stock-Q-jet manifold adapter (PN 2696).

Even though Q-jets are out of production you can still get them from selected rebuilders. If you wish to stick with an original Rochester Q-jet, check with JET Performance for complete performance rebuilt Q-jets for any application, and JET can even build a custom tailored Q-jet setup for your Monte.

Finally, we didn't talk about the spark timing you may be running. If you followed the initial spark advance on the emissions label under your hood, you set the initial timing at zero, or top dead center. With the Mallory distributor you installed, you should set your timing around 10-12 degrees before TDC. Make sure that when you're setting the timing, you disconnect the vacuum advance can. This will at least make your car run the best until you can swap out for the correct carburetor. Good luck.



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