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Author Topic:   Boosting Compression
ryan
Dirt Roller

Total posts: 18
posted August 19, 2001 11:17 AM
What is the best way to raise my compression to 11 to 1. No domed pistons, Bowtie's are the only preformance head allowed besides stock. Right now I have flat top pistons My heads are: 1.94 1.5 76cc. This is my first year in Street Stock and I could use all the help I can get. Also what kind of oil do you think is best to run that I can buy at my local parts store? Thanks

CUSTOMPERFORMANCE
Dirt Freak

Total posts: 213
posted August 19, 2001 01:23 PM
use 2 valve relief flattop pistons, have the block decked to zero deck, which puts the piston right at the top of the bore. use the minimum thickness headgasket , make sure you have plenty of valve clearence. this will get you close, if it isnt you can have the heads angle milled also.

sdhnc29
Dirt Full Roller

Total posts: 74
posted August 19, 2001 11:41 PM

Why are you using 76cc heads ??

As Custom said :"use 2 valve relief flattop pistons, have the block decked to zero deck, which puts the piston right at the top of the bore. use the minimum thickness head gasket ". Also , get your self some 64cc heads !! Even if you do all of this , you will still have to get the heads milled if they are on a 350 . I'd shoot for around 12:1 - 12.5:1 if it's in your budget !! Of course this will involve angle milling , valve relief notching , and correcting your intake face of your heads . How many $$ do you want to spend ?? It directly relates to how fast you want to go !

As for oil , we normally recommend Mobil 1 15w-50 synthetic .

ryan
Dirt Roller

Total posts: 18
posted August 20, 2001 02:00 PM
I am running those heads this year because it was what I had and after building the car I wasn't in my budget last year. The most we can run is 11:1 Max compression its in the rules. I would like to get right at 11 to 1.

RangeRover
Dirt Full Roller

Total posts: 37
posted August 21, 2001 06:20 PM
Be sure to check how much quench distance you will end up with after decking the block and compressing the head gasket. This is the space between the flat part of the piston that would hit the flat part of the head if the quench was ".000. You want this tolerance to be no less than ".038, but shoot for ".040. A closer distance will negate the power benefits of the quench head design; the rush of air from the compression of the air/fuel charge in the quench area helps cool the combustion chamber. Plus, you want to allow enough safety tolerance for parts "growing" at high rpm. A standard Fel Pro gasket will compress to about ".040, conveniently, so you just have to worry about the decking of the block. Here's another tip that can save you time and money; If your piston is a few thousndths from the deck at TDC, just measure exactly by how much and purchase a set of gaskets that are thinner than stock to come up with that ".040 between the piston and the head. One more thing I can say about compression is that if you're allowed to run a 400, the larger bore will raise the compression ratio slightly with all other things remaining constant.

#23
Dirt Roller

Total posts: 9
posted August 21, 2001 10:41 PM
This may not be a very popular comment but I am the cheapest racer I know. Decking the block and 64cc heads cost money (you would not believe how much in Canada) What I've done the last few years is run 305 4 bbl heads. O.K. I know that the intake valves are smaller (1.85 vs 1.94) but unless your twisting thay motor past 5500 rpm the smaller vavle won't hurt too much. I get these heads at the local pic a part. have them checked for cracks, treat them to a good vavle job and go. These heads have as small as 58cc chambers and with steel shim head gaskets your compression should be 10.7 give or take a .1 Just a suggestion!


Good Luck!!

ryan
Dirt Roller

Total posts: 18
posted August 30, 2001 12:33 AM
Would using a longer rod work instead of decking the block. What are the pros and cons of using a 6.0 rod?

RangeRover
Dirt Full Roller

Total posts: 37
posted August 30, 2001 12:42 PM
Apples and oranges, dude. The 350 rods are 5.700" long- that's .300" shorter than the 6 inchers. You can imagine the pistons popping way out of the deck by only swapping rods. To use the 6" rods you need to get pistons that make up the distance difference by locating the wrist pin higher up in the piston. This will put the piston to deck clearance back where you started with the only difference being having longer rods. There are some advantages to running longer rods like: reduced side loading on the cylinder walls and more cylinder pressure by leaving the piston at TDC longer. Generally though, the longer rods tend to move the engine's power higher on the rpm range. Some experts simplify the decision by using 5.7 rods on short tracks and 6.0 rods on longer speedways. You should be fine with 5.7 rods for most applications, however.

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