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E31 head onto L28 block. What pistions to use?


Guest Anonymous

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Guest Anonymous

I recentely acquired an E31 head. I have an L28 block with triple DCOE 40 webers. I have been reading that the compression is 10.8 with flat pistions and 8.6 for dished pistions. I want to run the flat pistions but i still want the car to be able to run on pump gas. Does anyone know what the highest compression that i can run with pump gas is? Or has any one run this setup with pump gas?

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I have that combo with flat top pistons. My block is standard size, so theoretically I should have 10.2:1 compression. In actuality I think it is higher, because I've done some testing (mixing race gas with pump gas) and it seems like I need about 95 octane to run a decent amount of advance and not ping. I also took quite a bit of metal out of my combustion chambers unshrouding the valves and polishing the chambers.

 

I did have the current motor running on 92 and not pinging, but I had to retard the timing so far that it wouldn't rev and I was worried about burning up exhaust valves.

 

When I originally built my first Z motor I had the same head with dished pistons. It ran great, and had decent power, but nothing like what I've got now.

 

If I had it to do over again I'd probably get a P90 and mill it to get the compression and use cam tower shims to fix the timing chain.

 

Jon

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Guest Anonymous

I want to run the higest compression possible on pump gas. There are a lot of things that i can do to change the compression. Is the highest you can run on 91 octane 10:1? I dont want to have to mix special gas for this car.

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ok in order to figure this out you need to learn about dynamic compression. the max static compression (10:1 kinda deal) you can run on pump gas depends on the duration of the cam. this is where the dynamic compression forumla comes into play. I was very curious about all this about a year ago and I learned a ton from this message board. let me try to point you in the right direction. first read these two articles.

 

http://cochise.uia.net/pkelley2/DynamicCR.html

http://victorylibrary.com/mopar/cam-tech-c.htm

 

if you dont know what they are talking about when it comes to the cam stuff or are just new to how cams work considering all the specs then read this as well. it will clear things up alot. you might have to read it a few times to fully understand it though haha (same with the stuff posted above) read all 8 pages of this...

 

http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm

 

also what will help A TON! is to do the following search on this site...

search for "dynamic and compression" (the and makes sure the results have both words in them instead of one or the other) and for the author put "grumpyvette" and do the search in the chevy v8 forum.

grumpyvette is a retired engineer and he knows everything there is to know about how engines work and the physics of it. plus he is a really really nice guy and posts tons of info on it all the time. he will also answer any tough question you can throw at him.

 

so that should get you started and busy for the next few weeks heh. hope that helps although it doesent directly answer your question.

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