Compression ratio

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Apr 3, 2011 | 11:28 AM

rolape2

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From: Portage Des Sioux, Mo

Compression ratio

I know it has been answered but can't find. What are the compression ratios for the 6.9 and 7.3? And was it mostly difference in the Pre-chamber? I am asking because I am building a 6.9 to replace a melted 7.3 IDI. I am swapping on the turbo from the 7.3 which was a rigged Pstroke non-waste gated unit (got it this way). Wanting to know if I should swap Pre chambers from 7.3 heads to lower compression for turbo? Or is that a waste of time? Pre chambers don't want to come out of 7.3 heads.
Please help and thanks

Apr 3, 2011 | 05:28 PM

94 smokin stroker

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From: Salisbury NC

I am not positive but i dont think the prechamber was what lowered it i think the 7.3 was 20:1 and 6.9 21:1 but am not sure i know dave s had his compression lowered by shaving some off the 6.9 pistons and regularly runs 15+psi of boost

Apr 3, 2011 | 08:09 PM

BlueOvalBud

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From: Massachusetts

I'm pretty sure the pre-cups will be the same between the 6.9 and 7.3 heads. I don't see why they would change them.
I don't think that will lower compression either.
The way to lower compression is to have a machine shop mill a certain measurement off the top of each piston. This creates more volume in the cylinder when the piston is at TDC...which lowers compression.

Dave S is running 25 psi of boost daily. HOWEVER! His engine is VERY modified, and a stocker won't be able to handle that kind of boost.

What kidn of boost numbers are you shooting for?

I'm VERY curious about the turbo setup you've got there! Any pictures?

Apr 3, 2011 | 11:00 PM

catfish101

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From: KY

The turbulence chamber is figured in on compression ratio because it holds 95% of the compressed intake air on these engines.

Apr 4, 2011 | 05:55 AM

tjc transport

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From: Marlboro Mental Hospital.

compression ratio for the 6.9 and 7.3 is almost the same, at either 21.5:1 or 22.5:1
if i remember correctly the 6.9 is a little higher, but only by 1 point or less

Apr 4, 2011 | 05:50 PM

rolape2

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From: Portage Des Sioux, Mo

according to the Haynes manual (bought this morning) 6.9 is 20.7 to 1, 7.3 is 21.5 to 1. I saw some one has International manual says same. However Dave Sponaugle shows 83 6.9 at 20.7 and 84-87 as 22.5. Dave knows a lot more about these than I and would assume he is right. Also 7.3 Pre-chambers are marked with and 88 on the face (visable while still in head) 6.9 prechambers depending on which of my heads (rebuilds not same) pre-chambers are either 61 N or 50CI N marked on side. 61 definatly has more volume than 50CI N. Have not been able to get a 7.3 pre-chamber out to compare volume. I am going to check with International tomorrow and see if I can get some 61N's and atleast match them up.