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post #1 of 19 (permalink) Old 06-16-03, 06:33 AM Thread Starter
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LPG Compression

I've just pulled my 2v open chamber heads off my 351. The compression with these heads was 150psi, my cam is, I think standard.

I have some 2v closed chamber heads that I have ported and cleaned up, what will the likely compression be? Will it be too high? bearing in mind I am using LPG. I have flat top pistons .30

If the compression is too high what is a suitable LPG cam that has overlap to reduce compression ( I don't want a ping!)

Also how much extra power should I gain with these heads?


Clevo
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post #2 of 19 (permalink) Old 06-16-03, 07:36 AM
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i have closed chamber heads and a small cam in my 351, and i run straight gas and the comp is too high for gas.

if i were to do this again, i would run standed 351 heads next time and that way you wont get pinging up high, and you could run heaps of advance = more power

jim
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post #3 of 19 (permalink) Old 06-16-03, 10:39 PM
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general rule is closed chamber heads on a factory deck height 351 will bring comp upto around 11 to one u need 10.5 max and that will require a recurved dizzy and a really good tune

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post #4 of 19 (permalink) Old 06-22-03, 07:20 AM
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Re: LPG Compression

You have to take out about 10cc by laying back the chambers with a die grinder and you'll end up with approx 10.1:1 (like mine with Gas Research straight LPG) which will be perfect
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post #5 of 19 (permalink) Old 06-22-03, 07:37 AM
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I have heads with layed back chambers and my engine was a little lacking, after being told you need a zero deck for it to work good, i put new pistons in and cam with almost 50% power gain
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post #6 of 19 (permalink) Old 06-23-03, 04:42 AM Thread Starter
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Where abouts do I do the layback on the heads, near the spark plug side or the other side? Any pics availble? How much metal is 10cc

120why I currently have 0 deck clearence with pistons, if I put these 2v head on I'm assuming compression will be at least 11 to 1.


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post #7 of 19 (permalink) Old 06-27-03, 06:28 AM
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I;ve got less than 10.1, can't remeber maybe 9.6 or 9.7.Get the dissy done i run 16 degrees initial and 32 total with gas research, it works great
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post #8 of 19 (permalink) Old 07-04-03, 06:37 PM
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Re: LPG Compression

The ACL website (Tech-fax article 613), indicates that flat top pistons on a 351 using small chamber 302 heads (57cc) will give about 11.3 to 1 comp ratio. I've used this combo running straight gas for over 2 years with no detonation. Your main consideration should be the quality of the LPG that you are using. They are NOT all the same. Contact your suppliers and ask them what their LPG consists of. You want to use the blend that has the highest percentage of propane. The main cause of detonation is running LPG with a non-modified dissy advance curve. View the following link for more details.

http://www.gasresearch.com.au/epg.html
also see ACL (Tech-fax article 905)
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post #9 of 19 (permalink) Old 07-04-03, 07:21 PM
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Re: LPG Compression

Quote:
Originally Posted by clevo
I've just pulled my 2v open chamber heads off my 351. The compression with these heads was 150psi, my cam is, I think standard.

I have some 2v closed chamber heads that I have ported and cleaned up, what will the likely compression be? Will it be too high? bearing in mind I am using LPG. I have flat top pistons .30

If the compression is too high what is a suitable LPG cam that has overlap to reduce compression ( I don't want a ping!)

Also how much extra power should I gain with these heads?


Clevo
It'd be at least 11:1 but if your running zero deck height (piston level with the top of the block)? You'd run around 13:1.

From the factory clevo's don't run zero deck height.

I threw some 302 closed chamber heads on a stock 351. Had no issues running pump fuel.

150psi is about 8.9:1. 185psi = 9.5:1 etc So 11:1 would be about 200-210psi.

Remember cam affectes the compression. You can have 150psi stock cam put in a large cam and have the compression drop by 20psi!

Brenden

XB Fairmont (street car) 11.07@123.02mph.
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post #10 of 19 (permalink) Old 07-05-03, 05:17 AM Thread Starter
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When the pistons are are TDC they are extremely close to the top of the block, could be between 1-2mm below but not a lot. What did factory clevos come with, a lot less?

The gasket thickness must also be a consideration with compression. How thick are they, I have never measured one. I'm assuming Felpro gaskets are thinner than a standard one. I have heard you can use two gaskets if you want to lower compression by 1 (ie from 11:1 down to 10:1) Not willing to try unless I know it works!!

I think LPG varies from 98-105 Ron so if your 351/302 heads went OK on pump fuel (96-98Ron) I should be fine with LPG and Premium unleaded when I run out of gas.

What cam do you think would be suitable with this setup and reduce compression, I am looking at 270 inlet duration and 280 exhaust duration.

Paul
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