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Author Topic:   COMPRESSION
bigcityracer
Dirt Freak

Total posts: 431
posted June 12, 2004 11:40 PM  
I know there is no one answer and many other things influence the compression. I run a 355 with 305 heads. compression Must be under 190psi. Can someone give a approximate compression ratio of pistons to run. Say 10.5:1, 12.2:1 or 13:1
Thanks

Racing7500
Dirt Roller

Total posts: 12
posted June 13, 2004 05:25 AM  
I ran that engine combination a few years ago,355 with 305 closed chamber heads, we ran 10/1 flat tops and had aroung 175 compression, although there was no rule for max comp we checked it when we had a skip all was around 175. only problem was those heads died at 5000 rpm not enough air flow.

UNVRNO
Dirt Maniac

Total posts: 136
posted June 13, 2004 06:55 AM  
the psi compression your talking about measures your ring leakage, guides etc...

it has nothing to do with engine compression like 11 or 12- 1.

bigcityracer
Dirt Freak

Total posts: 431
posted June 13, 2004 08:54 AM  
UNVRNO I would have to disagree with that. If you have a 355 engine and use 9.4:1 pistons and switched to 13:1 I'm sure your cylinder pressure would jump way up.

stockcar5
Dirt Forum Champ
Total posts: 578
posted June 13, 2004 09:08 AM  
the 2 things that infuance cylinder pressure the most are mechanical compression and cam selection.

i had a 383 with 12.1 compression once..1 swapped cams and my cylinder psi dropped from 225psi down to 190. the kicker is it made better power at 190psi!

so if i were you i'd ask a cam company if they have a cam with enough overlap to get what your looking for.

luke

------------------
www.geocities.com/dirtstockcar5

o5racer
Dirt Freak

Total posts: 402
posted June 13, 2004 09:48 AM  
I did a comp check on a 95 isuzu rodeo with a v6 last week and it pumped 195. all stock. my 12 to 1 358 pumped 190 but made twice the HP. cranking compression wont tell you anything about the actual compression ratio. the cam dictates the cranking compression.

ryan
Dirt Forum Champ
Total posts: 776
posted June 13, 2004 11:33 AM  
12 to 1 huh. Well I know who I'm going to protest whenever you show up lol. My legal 11 to 1 pumps around there also.

[This message has been edited by ryan (edited June 13, 2004).]

popwillis
Dirt Full Roller

Total posts: 30
posted June 13, 2004 01:00 PM  
The compression ratio of any engine is a comparison of the cylinder and combustion chamber at top dead center -vs- bottom dead center. A piston rated at 12.5:1 with 64 cc heads may only be 11.2:1 with 72 cc heads.

UNVRNO
Dirt Maniac

Total posts: 136
posted June 13, 2004 04:02 PM  
disagree if you want.

but the psi compression gauge doesnt know how much air is in the cylinder being compressed, it only knows to what pressure it gets to based on rings sealing etc.


crc1124
Dirt Maniac

Total posts: 143
posted June 13, 2004 09:30 PM  
I agree with you popwillis. Change pistons and the psi compression gauge will read different. The cam isn't the only that moves the air into the cylinder. The amount of air compressed also is influenced by carburator cfm, intake design and runner size, and head runner size. Compression ratio is measured on the cylinder and combustion chamber.

bigcityracer
Dirt Freak

Total posts: 431
posted June 16, 2004 09:14 PM  
quote:
Originally posted by UNVRNO:
disagree if you want.

but the psi compression gauge doesnt know how much air is in the cylinder being compressed, it only knows to what pressure it gets to based on rings sealing etc.


UNVRNO
I agree with your statement above. What I was meaning if you have a 355 say 10.5:1 and the only thing you change is the pistons to 12.5:1 I'm sure the cylinder psi will go up.

    

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