340 compression test

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LA340

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Picked up a 340 with X heads, completely unknown build.
Did a compression test on my 340 and got what I think are very high results.
The engine was on the floor of the shed with a 904 and starter bolted up so I could crank it over.
Has a tunnel ram with no carb on top.
I can see domed pistons through the spark plug holes so was thinking that this may be around 10.5:1 compression but just don't really know.
All figures from the test were obviously done cold and with no oil squirted in the spark plug holes.
220psi to 240psi on all cylinders.

This seems a bit high to me.
Any ideas what a 340 with 10.5:1 compression in good condition reads on a compression test?
Is there any way to calculate the compression ratio from the psi readings I made knowing that this is a 340?

Comments welcome!
 
LA340 said:
Picked up a 340 with X heads, completely unknown build.
Did a compression test on my 340 and got what I think are very high results.
The engine was on the floor of the shed with a 904 and starter bolted up so I could crank it over.
Has a tunnel ram with no carb on top.
I can see domed pistons through the spark plug holes so was thinking that this may be around 10.5:1 compression but just don't really know.
All figures from the test were obviously done cold and with no oil squirted in the spark plug holes.
220psi to 240psi on all cylinders.

This seems a bit high to me.
Any ideas what a 340 with 10.5:1 compression in good condition reads on a compression test?
Is there any way to calculate the compression ratio from the psi readings I made knowing that this is a 340?

Comments welcome!
Click to expand...
Not to be a smart ***,but check your gauge.It seams really high.I thought 150-180 psi.
 
No offence taken, all good ideas/thoughts are gratefully accepted.
This was a new gauge in January, I have used it only once before and got expected results from a 318 (130 to 155psi). it has just hung on the wall in its box until this test.
 
...............my 10.5 motors r usually 160 + or - depending on the head gasket thickness and bore size........kim.........
 
I had a 340 with domed Ross pistons. Compression was 12.5:1 and it was .040 over. My compression when checked cold with no oil squirted in cylinders was 195-220 psi.
Ben
 
10.5 pistons should be a flat top so my guess is 12 to 13 for that high the cam will matter a bunch what the actual pressure will be also
 
my stock 69 340 was 150 average with a 509 cam it went to 110 average
 
Stock 1970 10.2:1 340 w mopar .284/.484 cam is running 150-155psi. Domed pistons are not stock and would be higher then 10.5:1 Mabe you need a cam w/ more duration and or thicker head gaskets to lower your cylinder pressure. 220 is aweful high. If you figure out your static compression mabe you can get a cam to get your dynamic compression down where you need it, depending on how much of a dome you have.
 
as said...10.5 to 1 is a flat top piston...if you see a dome...it is not a stock piston...
 
Thanks for all the great replies.

Thinking out loud....
To avoid a rebuild. Are there any spacers on the market in the US that could be fitted under the heads that would reduce compression (may have to get longer push rods if I run out of adjustment on the rockers)...
And maybe cam swap?
 
NO spacers, but you can go thicker on the head gaskets for a few more $$. But, I'd take it apart and research the cam. Assuming the rings and valve job seal the cylinder well two things comprise cylinder pressure: static compression ratio, and the intake valve closing point. If the cam is small, and the static is higher (11.5-12:1 which is easy with the domed old school pistons) you will have high cylinder pressures. I'd say that was woefully undercammed too. But, see what you have first, before you worry about what to buy to fix it.
 
Got the cam specs with some direction from 'crackedback'. THANKS!

I would have thought the late inlet valve closing would have kept the DCR down and result in lower psi readings than I obtained.
Is it possible that the lifters have drained of oil after a period of several months sitting then inversion on the engine stand for a few hours (while I pulled the sump off for a look) before the compression test was performed. Resulting in them compressing and not opening the valves for the advertised duration of the cam?
This 340 must have strong valve springs, they are dual springs (spring, damper, spring)

Waiting on a cam degreeing kit to see exactly how it is installed but the '0' marks on the crank and cam sprockets seem to be lined up and from what I can tell the keys look to be 0deg on both cam and crank.

Will update when I know more, but if anyone can suggest other things to look for I would appreciate it.
 

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With a cam that size, I bet it's 13:1. My 360 in my old Valiant was a blueprinted 12.5:1 and with the mopar 509 cam has 210 PSI to give you some comparison.
 
Sounds like hp to me! That is about the same cranking compression that i have on my 340. Pistons are 13-1 trw's . Had a 11-1 360 with flat tops that had 185-200.What cam and where it is installed can make a big difference also.
 
How did you make out on this ? I am having the same issue with my 372 stroker. It is a "toned down" high compression engine that I have to run racing fuel in on the street. I want to make it more streetable, but had @220lbs on comp test so not sure what way to go with it. It's a 30 over 340 w/ domed pistons, 3.58 stroke, has 549 lift cam, 70 cc open heads and 90 head gaskets. I have to back the timing way off to make it run at all on the street and the fuel costs are killing me! Any help would be great. Feel free to PM if you have ideas! Thanks
 
Hi cudatom.

Young family takes priority at the moment so slow going.
I plan to put the 340 into my daily driver in place of a 318 so am focusing on the exhaust that needs replacing anyway.
2.5" with X and block huggers.

Anyway, without pulling heads off mine and from what I can see down the spark plug holes I think I have pistons like in this pic.
http://www.forabodiesonly.com/mopar/showthread.php?t=33448

So I have been doing some reading and guessing/calculating in the hope that I can run the 340 on the high octane unleaded here in the UK.

Have been reading about Dynamic Compression Ratio and the dramatic affect that the cam I have will have on lowering this from that of the Static Compression Ratio (with pistons like these - circa 12.5:1)
I think (hope) that the high psi readings on my compression test will be as a result of the lifters having bled down and causing the intake valve closing event much earlier than the cam actually is closing when the engine is running and oil is fed to the lifters to keep them up when under load.
http://www.empirenet.com/pkelley2/DynamicCR.html

There is one calculator on the internet that I can not find just now that I was fiddling around with that when I had it setup with known values such as bore and stroke and a few guesses like the cc of the dome pistons so it was reading 12.5:1 as a SCR then switched to the values that the cam provides to calculate the stroke length after the intake valve closes I was getting a DCR of around 8:1.

I'll look for the calculator and post when I find it.

I guess your 10.72:1 is going to be your SCR.

Good luck and let me know how you get on please.
 
I have that piston in my 340 and get about 230 psi.
 

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cudatom said:
How did you make out on this ? I am having the same issue with my 372 stroker. It is a "toned down" high compression engine that I have to run racing fuel in on the street. I want to make it more streetable, but had @220lbs on comp test so not sure what way to go with it. It's a 30 over 340 w/ domed pistons, 3.58 stroke, has 549 lift cam, 70 cc open heads and 90 head gaskets. I have to back the timing way off to make it run at all on the street and the fuel costs are killing me! Any help would be great. Feel free to PM if you have ideas! Thanks
Click to expand...
Have you thought about running E85 has much higher octane than pump gas and alot cheaper tha race fuel.
 
I have but problem here in WNY is E85 is hard to find. There are not many stations around betweeen cruise ins that sell it. May as well go to airport w/ cans and get av gas!
 
LA340 said:
Hi LETSRACE.
Out of interest, what cam and fuel are you using?
Thanks
Click to expand...

i use a fuel additive call boost juice 1 gal to 15 of gas its about 6or 7 $ a gal on the internet .... i run a 11.5 to one small block in my dart on the street no problems 528 / 284 purple shaft , 391 gears ,850 holly, 3000 stall,904 trans.....want to fast mash on the gas
 
I use race gas VP MotorSport 105L due to running 150 shot of nitrous. The cam is Comp cams Magnum 20-244-4 Hydraulic.
 
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