checking compression ratio

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Budgetmoparman

Budgetmoparman

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Can the compression ratio be figured out with the motor assembled? Maybe I am missing something but couldn't you pull the spark plug out of #1 cylinder, make sure it is at tdc, use a syringe and fill the combustion chamber with fluid and figure out the cc volume? I would like to figure out the actual compression ratio of the 340 in my car.
 
sure could
and there is a tool than NHRA and others use
easier on a SBM than a BBM due to plug location, just roll the car over on its side
If you know the year of your 340 and if the motor has not been rebuilt you can figure it out pretty close- cc of the heads are known (and don't use the blueprint mimimums use the "as cast" J or X or most any 360 heads are about the same
If rebuilt, if you did not do it, you do not know what pistons are really in it
do a compression test- if high figure high compression pistons- if low figure the late pistons
do the math you will be close
 
Wyrmrider said:
sure could
and there is a tool than NHRA and others use
easier on a SBM than a BBM due to plug location, just roll the car over on its side
If you know the year of your 340 and if the motor has not been rebuilt you can figure it out pretty close- cc of the heads are known (and don't use the blueprint mimimums use the "as cast" J or X or most any 360 heads are about the same
If rebuilt, if you did not do it, you do not know what pistons are really in it
do a compression test- if high figure high compression pistons- if low figure the late pistons
do the math you will be close
Click to expand...
It is a 69 340, .030 over, not sure what pistons, they were in it when I bought it and I just cleaned them up when I rebuilt it. I do know that they stick above the deck .018". They are old cast slugs from back in the day. The heads are X heads. I don't remember what the gasket thickness is, I would have to look back at my receipts.
 
If they are only .018 out you'll be about 9.8:1. You can fill the chamber at TDC but you have to be dam sure there is no air bubbles in the chamber.
 
A lot can happen to an engine, pistons, and heads in 40 years.............
 
Thanks for the replies. It isn't that big of a deal, I just thought I would try to figure it out. I was just curious.
 
Budgetmoparman said:
Thanks for the replies. It isn't that big of a deal, I just thought I would try to figure it out. I was just curious.
Click to expand...

If you want to know the true accurate compression of your combination, you can follow the procedure in this how-to thread:

How to Check Compression

You will have to remove the head, but it can be done even with the engine in the car....
 
This is simple. Static compression ratio is "somewhat" irrelevant. Do a compression test on it and see what your average cylinder pressure reading is. That will tell you what you need to know in terms or running on pump gas.
 
you also need to know what cam is in it and adjust if you are going to make a change
do post up a compression test, throottle open squirt in some oil and do it again if low
 
krazykuda said:
If you want to know the true accurate compression of your combination, you can follow the procedure in this how-to thread:

How to Check Compression

You will have to remove the head, but it can be done even with the engine in the car....
Click to expand...
I did read that article before and it is a very good article with lots of info. I was just wondering if there was a way to do it without tearing the head off. It might have to wait and be a winter project. That would be a good time to swap the cam then also. It has a comp XE268H now but I think I should go bigger. Not sure what cam yet but I will worry about that when the time comes. Thanks for the reply though.
 
BTW, OP, is this on the engine stand? Seems like you could get a decent ballpark number if you:
- find true TDC on the compression stroke
- flip the engine so the spark plug hole is 'up' and so that the spark plug hole's axis is at a certain angle to vertical. You have to make sure there are no air pockets for accuracy. You'd have to look at a head off the engine to see what angle on the spark plug hole (relative to vertical) would put the hole right at the top of the chamber. (I have one buried somewhere...)
- and carefully fill 'til the fluid is a few threads up in the spark plug hole. (How far up is another important parameter.)

Your measured volume plus the computed displacement for bore & stroke can be used for computing SCR.
SCR = [Fill volume + cylinder displacement]/[Fill volume]

FWIW, OP, I get 9.2 to 9.7 for the parts you describe; the variation depends on the head gasket thickness (.028" or .039" or .051"). Sounds right for stock pistons with a later head gasket.
 
nm9stheham said:
BTW, OP, is this on the engine stand? Seems like you could get a decent ballpark number if you:
- find true TDC on the compression stroke
- flip the engine so the spark plug hole is 'up' and so that the spark plug hole's axis is at a certain angle to vertical. You have to make sure there are no air pockets for accuracy. You'd have to look at a head off the engine to see what angle on the spark plug hole (relative to vertical) would put the hole right at the top of the chamber. (I have one buried somewhere...)
- and carefully fill 'til the fluid is a few threads up in the spark plug hole. (How far up is another important parameter.)

Your measured volume plus the computed displacement for bore & stroke can be used for computing SCR.
SCR = [Fill volume + cylinder displacement]/[Fill volume]

FWIW, OP, I get 9.2 to 9.7 for the parts you describe; the variation depends on the head gasket thickness (.028" or .039" or .051"). Sounds right for stock pistons with a later head gasket.
Click to expand...
The engine is in the car. Guess I will have to wait until winter to figure it out.
 
Yes, that would be a bit tricky to do. Getting the air out of the pocket at the top is the problem.

If you have that head gasket PN, that would help zero in the numbers. The computed numbers are pretty close... close enough to reliably pick a cam for instance.

But if you have many mods to the head, like milled or chambers polished or some valve work, then it can run the computed numbers off several tenths; in your setup, the SCR varies a bit over 1 tenth of a point for each cc.
 
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