cylinder preshure and cam relation

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cudaman51

cudaman51

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i have a 340 that i put together with trw supposed to be 10.5-1 j heads and purple cam 268 .455 cam. i wasn't smart enough to measure how much the piston stuck up above the deck. cylinder preshure is only 150-155 psi.that seems pretty low to me. could it be cam related or is the cr realy that low. I did have to have the intake cut to lower it for port alignment with tells me the block wasent that tall. what would the cr actually bee with those numbers.another question i hear comp cams 268h mentioned a lot my moter doesn't have mutch low end would it be worth a change
 
I've got a 270 .468 lift solid in my 273 with a calculated 8.9 to 1. Its cranking pressure is 150.
So you do not have 10.5 to 1.
Closer to 9 I'd say.
 
cudaman51 said:
i have a 340 that i put together with trw supposed to be 10.5-1 j heads and purple cam 268 .455 cam. i wasn't smart enough to measure how much the piston stuck up above the deck. cylinder preshure is only 150-155 psi.that seems pretty low to me. could it be cam related or is the cr realy that low. I did have to have the intake cut to lower it for port alignment with tells me the block wasent that tall. what would the cr actually bee with those numbers.another question i hear comp cams 268h mentioned a lot my moter doesn't have mutch low end would it be worth a change
Click to expand...

I measured and cc every thing on my 340 as i was building it! With a thin head gasket, my aka factory 10.5 pistons made a scr of 9.4...........was real Pisst about that fact, but all the machine work was done:angry7:

In blue print spec's, the heads are shaved down to 64 cc.......My heads had 75-76 cc in them.

My compression is around 135 psi(altitude make a difference in compression as well...........I live at 7000 feet) with a mpp .474/280 cam
 
The measured compression PSI doesn't really tell you that much about compression ratio, or even about cylinder pressure when actually running.

Compression ratio is a calculation, based on direct measurement of the cylinder dimensions, stroke, and combustion chamber volume.

Valve overlap and ring sealing will affect the measured PSI, but have nothing to do with compression ratio.

So don't get all worried based on just that. How does it run?

cudaman51 said:
i have a 340 that i put together with trw supposed to be 10.5-1 j heads and purple cam 268 .455 cam. i wasn't smart enough to measure how much the piston stuck up above the deck. cylinder preshure is only 150-155 psi.that seems pretty low to me. could it be cam related or is the cr realy that low. I did have to have the intake cut to lower it for port alignment with tells me the block wasent that tall. what would the cr actually bee with those numbers.another question i hear comp cams 268h mentioned a lot my moter doesn't have mutch low end would it be worth a change
Click to expand...
 
cudaman51 said:
cudafever do you know how much the pistons stuck up above the block. I guess i miaswell try regular gas and see what happens.cam is [email protected]
Click to expand...

Had to dig up my notes on that one. its only been 15 years:cheers:

.016" above deck.

When i went to rmr to race(4200? feet) i would have to mix one gal. of 108 octane fuel, to a half a tank of 91 or it would develop a ping at the top of 2nt gear, right before i shifted, in the heat of the summer.

If i new then, what i know now.......I would have done more prep to the top of the piston, and the combustion chamber.
Then i wouldn't need any racing fuel for my measly 9.4:1 compression.

Mine is [email protected]
 
The measured compression PSI doesn't really tell you that much about compression ratio, or even about cylinder pressure when actually running.
Click to expand...

Agreed, to compare two engs, both would have to have the same compression ratio, same cam(aka same intake valve open*) and be check at the same altitude.

Any variation of them three spec's and the comparison is invalid!
 
your compression ratio will change with the barometric pressure.....
 
mvh it runs fine. im just thinking to much. wondering why my cylinder pressure is so much lower than some of the other numbers i have heard. it really doesn't mater for it sounds good looks good and runs on pump gas
 
As they said. Simple poor execution on the factory's part to get the chambers where they were supposed to be and the pistons where they were supposed to be. This is why we always ask what the real compression is when someone posts "it's got 10.5:1 pistons..." Also if it's a purple shaft... was the cam degreed? Because if it wasn't, and it's as bad as the last one I used, it would cause low cylinder pressure when installed by lining up the dots.
 
Is it common to set up a new rebuild with new pistons and bring the pistons out of the hole .019. I know the factory pistons came out anywhere from .01-.023. Seems to me you would want to go zero deck and if you wanted more C.R., do it in the purchase of pistons or lower CC heads. I rebuilt a 340 some years ago with B.M.E. pistons and went .022 out and a 528' lift cam and was pretty unhappy with performance a best of13.02 car with 4:56 gears. i know that motor should of did better, but the cam had alot of duration (300?) as well. Just putting that out there for conversation.
 
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