Free Parts Cost - Performance Q&A

Kevin McClelland Mar 1, 2010 0 Comment(s)

Rather Be Blown
Q: I have a 327 small-block Chevy that was built by my local speed shop with a 9:1 CR. I was thinking of installing a blower to the engine, but I don't like the 9:1 CR and would rather go with a 7.5 or 8:1. Other than changing the heads or pistons, what would be a good way to achieve this? The heads are camel-hump 64cc. The shop said the engine would handle the blower, but I have always seen blown engines built with a low compression. The blower I have is a 6-71 with a fuel-injection setup. Before I get creative and do my own thing I would like a little help from you guys.

Freddy Reyes Jr.
Santa Ana, CA

A: First, you didn't tell us what you're trying to do. If this engine is just going into a street rod to impress your friends and have a little fun, the 9:1 compression won't be an issue. Simply underdrive the blower and keep the boost under control. You could easily get away with somewhere around 5 psi of boost with 9:1.

Now, if you're looking to really work the engine, you will want to lower the compression. Yes, dished pistons or larger chambers are the correct way to go. We would go with aluminum heads with 76cc combustion chambers. You will get the added power that a current aluminum head design will give you, plus get your compression in line. This will drop the compression right into the 7.5 to 7.8:1 range. This will allow you to run the boost up and stuff more atmospheres into your little Mouse.

A more affordable way to lower the squeeze would be to go with thicker head gaskets. Cometic will build you custom head gaskets to the thickness you wish. If you used 0.090-inch-thick, 4.100-inch-bore head gaskets, you would pick up 11 cc from the current head gaskets you're probably running. This would be a much cheaper and easier way to lower the compression, but not the most efficient.

The choice is yours. Are you looking for power or pizzazz? If we were building a blown small-block, we'd want our Mouse to roar!

Tale Of Two Temps
Q: My '68 Camaro's 327 engine has a mild build-up of '69 302 heads, a Comp Cams 292 cam, an Edelbrock Performer RPM intake, flat-top pistons, a block deck, balanced, a four-speed, and 4.10:1 gears out back. What is the ideal temperature for the engine for performance and durability? Thank you.

Charles Creed
Mount Airy, NC

A: Great question. For performance you want the engine temps to be low. For longevity and wear you want high temperatures. Go figure. Let's find a compromise.

When you're looking for maximum performance you need to have low engine temps because the heat that is transferred from the cylinder heads and intake manifold into the fresh fuel and air charge lowers the power potential of every cylinder firing. Also, the cooler the combustion chambers, the more spark advance the engine will tolerate before reaching detonation issues.

As for wear and durability, higher engine temps are favorable to boil all the moisture out of the engine oil. When you run lower engine temps and short-duration engine run time, the engine oil doesn't get over 212 degrees F. When the moisture isn't boiled out of the oil it creates acids that attack the metal of the engine and destroy the oil's lubricity. A nice engine oil temp is around 230 degrees F. This is a very safe operating temp for the oil and it can take away the heat from the rotating assembly and pistons. Most of us don't want to know how hot our engine oil is running down the road. A normal passenger car running down the freeway at 70 mph will commonly see a 250-degree oil temp.

For a nice compromise on street driven engines, I like 180-degree thermostats. This is a good balance between engine wear and performance potential. Hope this gives you a little insight into engine temps.


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