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Compression ratio

Posted: Tue Sep 07, 2021 6:58 pm
by JimmyC
It seems like the stock compression ratio is pretty low, I guess to allow for for low octane fuel. Being that the lowest octane available here is 87 I would think the compression ratio could be raised without much issue. It seems like it would be easy to do, just shorten the jugs a bit. Has anybody done this? If so, how did it work out?

Re: Compression ratio

Posted: Tue Sep 07, 2021 9:40 pm
by rmel
What are you measuring for Compression ratio? On the 1'st cycle you should see in the
ballpark of 120 psi, then the 3'rd cycle about 130 psi. The Pinz engine is fine with
Octane ratings above 87, back in that day gas was leaded and "regular" was higher.

Back to your question w.r.t higher compression ratio, it's possible. Easier with a
piston change than touching the shoulder of the jugs. But, the question is for
what purpose? Higher HP or low end Torque. Going for the later is what the
engine was design for, so an improvement on the base design than a departure
is alyways best. That would mean Bore, Pistons, and Cam timing. That's kind'a
what the 2.7L upgrade kit is tuned for, not to mention EFI on top of that.

Re: Compression ratio

Posted: Wed Sep 08, 2021 9:52 am
by rmel
I should have added something to my prior comments.

If there's good reason to tear down the top end, e.g.
low compression due to worn rings, head issues etc...
Then that opens the door to make modifications. Or use
a standard Cylinder/Head refresh. Going the standard
route would restore full stock compression, just over
135psi. Or....
Replacement of the pistons from Stock to aftermarket,
e.g. Ross or Arias can yield higher than stock compression
and a larger bore e.g. 93+mm. So...higher compression
and more displacement.

One certainly could relieve the cylinder shoulder for increased
compression and stay with stock pistons -- but now your cylinders
are unique. And not the first thing someone would measure for
a future rebuild or jug replacement..whereas Pistons are
stamped, bores are measured easily so a repair is done with
consistency.

Re: Compression ratio

Posted: Wed Sep 08, 2021 11:47 am
by ChickenPinz
JimmyC wrote: Tue Sep 07, 2021 6:58 pm It seems like the stock compression ratio is pretty low, I guess to allow for for low octane fuel. Being that the lowest octane available here is 87 I would think the compression ratio could be raised without much issue. It seems like it would be easy to do, just shorten the jugs a bit. Has anybody done this? If so, how did it work out?
The manual calls out stock compression ratio as 7.5:1 and the RON min as 87.
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Re: Compression ratio

Posted: Wed Sep 08, 2021 12:07 pm
by ChickenPinz
And here's a useful page that discusses the impact of compression ratio on performance:
https://dsportmag.com/the-tech/educatio ... 101-part2/

As you increase the compression ratio, you increase the thermodynamic efficiency of the engine.

Re: Compression ratio

Posted: Wed Sep 08, 2021 1:02 pm
by rmel
I don't know about others experiences, but I find that when I run with a wee
bit more advance and 89 Octane, my 712 just runs a bit better than with regular 87.
Just a bit smoother at low RPM, and no hesitation. I do run 91 in the 710 but that's
a different setup with 2.7L + EFI.

Re: Compression ratio

Posted: Wed Sep 08, 2021 11:48 pm
by Jim LaGuardia
If you look at the parts book, there’s 128mm, 129mm , 130mm cylinders. The 712 AMB and WB had the short cylinders for higher compression. Going beyond 8.6:1 can cause Conrods to self eject. They can leave amazing holes in the block. I have both parts of a broken rod on my work bench from a poorly modified engine as a conversation piece.

Re: Compression ratio

Posted: Thu Sep 09, 2021 12:57 am
by Joeri
I had stock driven 7.5 bar engine in my 710m and 710k pinz and had a 8.5 bar engine in the orange pinz buggy....i had the cyl.heads machiened so that they fit a little tighter on the cylinders so smaller compressionchamber thus higher compression....but...on the other hand a low compression engine like this will handle a turbo quit nice :roll: :mrgreen:

Re: Compression ratio

Posted: Thu Sep 09, 2021 9:21 am
by rmel
I keep learning something new about these trucks over and over again :shock: Another good
example of the value this forum brings.

I didn't realize there were 3 cylinder lengths. And up to 2mm in difference! That's actually a lot.
SAV does not indicate length on their NEW 92's or re-bored 93's I presume new 92's are 130mm?

I never noticed a stamping on the Cylinders for length when I rebuilt the top end of the 712. I
have a couple of spares I picked up and will go take a look and measure them just for yuks.

Now this one is a definite "Add to the list" of possible improvements if one is in need of a top
end overhaul. And something to double check to make sure you know what lengths you have.

Re: Compression ratio

Posted: Thu Sep 09, 2021 10:11 am
by rmel
Just measured the spare cylinders, measures at 129.0 mm. I don't know what model Pinz
these were pulled from. But this now adds a wee bit of a wrinkle in using them as a spare.
Darn good to know !!

Re: Compression ratio

Posted: Thu Sep 16, 2021 6:42 pm
by JimmyC
I think our 87 AKI fuel is about 92 RON. It should be good for about 8:1 rather than the standard 7.5:1. It sounds like the easy way to do it is Jim's kit.