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spitnl 09-17-2008 05:31 AM

Tolerable compression without squish
I was wondering how much compression could you safely run on an engine without effective squish using 94 octane gasoline?


techinspector1 09-17-2008 03:05 PM

Just a wild-*** guess, but maybe 9.0-9.5 with iron heads?????
Depends on the cam also. A hotter cam with a later intake valve closing event will allow more compression before detonation. Have you done a compression test on the motor to determine cranking pressure?

k-star 09-17-2008 03:27 PM

How about some details??

Engine type, CI

What is the static compression??

What heads?? iron or alum??

heavy car high gear, light car low gear???

towing 10,000 lbs????



spitnl 09-18-2008 01:29 AM

Sorry Keith,

Chrysler 383 +0.030
440source aluminium heads 84cc
0.088 squish (pistons are 0.040 in the hole, 0.044 head gasket)
4000 lbs car
no towing
3.31 diff
4 speed manual transmission
Lunati 60302 cam
Advertised duration: 262
Duration @0.050: 220
Lift: 0.475"
Advertised duration: 268
Duration @0.050: 226
Lift: 0.494"
Stock 1.5 stamped rockers
600 cfm Edelbrock #1405
Weiand stealth aluminium intake
1.75 headers
Static Compression is currently 8.5:1 but I would like to raise it to 9.5:1 by milling the heads.
I know it is best to raise the compression by buying custom pistons to lose the 0.040 in the hole, but engine is in the car now and the pistons are $$.
I cannot do a compression test because the heads are not yet on.


k-star 09-19-2008 04:53 AM

You will be fine..

Something to think about,,

Make sure you calculate the amount of mateial needed to reduce the chamber 14cc to get your compression of 9.5. The intake side of the head will also need cut, you will probably need another set of pushrods because of the shaft mount rockers. You will probably need to work the intake holes also. When you add all this up it may be a better deal to get the correct pistons.

If it's a fresh engine i would buy the pistons before i choped the heads/intake JMO.


spitnl 09-19-2008 10:01 AM

Thanks for your reply Keith.

I have checked for pistons, the one I found are $700-$800 from Ross Pistons.
Having the heads and intake milled is about $200.
I will need custom pushrods anyway, with the original ones lifter-preload is 0.080 at this moment.

If you've got another source for the pistons I would be glad to hear.
I don't think it is possible to change the pistons with the engine in the car?


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