Compresstion Help
 

Hello. I recendly had to replace my short block do to a cracked CID. I bought a New shotr block from Heartland Engine builders. would have done the rebuild myself but no good block and got my New short block for a fair price, My concern is he said the motor would have 10.1 Compresstion with my parts but when i put the motor back togather the pistons was not dome pistins like i had before they just had really deep fly cuts in them, So i called him and he asured me that the motor would have 10.1 comp.I know theres no way to tell now what comp. i have,The track is closed for the season, Can someone tell me how much HP i would lose if i did go from 10.5.1 to 9.5.1? Or is there a flat top piston that could make 10.5.1 comp. with my 60 cc edelbrock RPM heads. The motor feels good but cant tell on the street, If you want to see what all i have in my motor go to this link. Thanks for any Advise, Ronnie MY PAGE

Re: Compresstion Help
 

At 60cc you're not going to get to 10.5:1 with flat tops and big valve reliefs. Even 9.5 is going to be tough; its looking more like 9.2-9.4:1 depending on where the piston is relative to the deck (http://crt-performance.com/compression.htm). Generally speaking, a full point of compression (say from 9.5 to 10.5) is worth about 15% net power at the crank.

Re: Compresstion Help
 

When I ran 351W heads, 59.5 cc's, pistons down 0-.005" in the bore with 2.7cc valve reliefs and .049" compressed gasket we figured 9.5/1 CR. That's with 4.030" bore and a 302 crank. Hope that helps a little.

Rev

Re: Compresstion Help
 

Thanks for the reply, I figured you was going to say that!!! Well i guest i will have to wait and see what the car does when the track opens, Thanks Again, Ronnie

Re: Compresstion Help
 

Thanks for that info. it does help, Ronnie

Re: Compresstion Help
 

I'm sorry Jeff but I'm not buying 15%. I mean maybe I'm completely wrong cause I know you have quite a background in this but I figure more like 3-4%. So a 400 motor would gain about 12-16 hp with a full point raise in compression based on 3-4% or 60 hp based on 15%. I can't remember all the specs on my DSS 306 but with the P heads I'm making about 9.8, 10:1 based on factory tolerance.

Re: Compresstion Help
 

You're right, I must have been on dope when I posted that. Generally speaking its 3-5% for a full point of compression. Thanks for correcting me! All I had to do was read the page I referenced!

Re: Compresstion Help
 

Not a problem man. I forgot to add that the DSS 306 has flat top pistons with valve reliefs and if I remeber right the P heads have about a 60cc combustion chamber I used to know the formula for CR but I can't remember it right now.

Re: Compresstion Help
 

I actually got my P-heads down to less than 42cc's! I've got my TW heads down to less than 40cc right now and looking for more. The SBF responds like gangbusters to increased compression.

Re: Compresstion Help
 

AMEN brother!!! Even on pump gas. ;)

Re: Compresstion Help
 

SO what your saying is if my motor with 10.5.1 compression had 400 HP and with 9.5.1 i will loss 60 HP, And if so is there anything i can do to raise the compression with just taking the heads off. I really didnt want to pull the motor right now, Thanks Ronnie

Re: Compresstion Help
 

No what we're saying is if you did drop your CR from 10.5 to 9.5 on a 400hp motor you would lose about 15hp or so. The easiest way to make that back up it to take the heads and have them milled. Infact if you have enough piston to valve clearance you could take them down quite a bit and get your CR above 10.5:1.

Re: Compresstion Help
 

Thanks Alot thats good news, The block and the heads have never beem milled so i think iam going to check on getting the heads milled down, Thanks Again, CAR9FAN

Re: Compresstion Help
 

One more thing, when you have the heads milled, check the gasket alignment where the intake meets the cylinder heads. If the heads are milled too much the intake won't match up correctly. You then have to have the intake milled to match the cylinder heads. If not the intake gaskets will leak vacume/oil/coolant, any or all.

Re: Compresstion Help
 

Ok Thanks For The Info. Car9fan

Re: Compresstion Help
 

Don't forget about your pushord lengths. I think they will change too, will milling.

Re: Compresstion Help
 

Thanks For The Info, Car9fan