i have a ‘71 340 I bought a couple of years ago. Was told it was a “fresh”build. I pulled the valve covers and the oil pan, it was nice and clean inside so I put it in my car and ran it a bit. It always kind of annoyed me because I couldnt get it just right. I messed with timing, carb etc and still wasn’t real happy. I recently pulled the trans to overhaul it and decided I’d go a little further on investigating the engine. I figured I’d pull it apart and ck the bearings and was interested in the cam that was in it and if it had been installed correctly. The guy I bought it from was not positive or convincing on what cam was in it. The bearings all look real nice, unfortunately no sense in going any further with the cam as it has a lobe thats probably about to wipe out. Nice little divit on the bottom of the lifter. Anyway what I’m getting at is I’m gonna replace the cam, degree it correctly and put er back together. What I’m interested in doing is finding out the compression ratio. The pistons are still in the block, so if I know the bore and stroke and I cc my heads and decide which head gasket I’m gonna use is there an accurate way to determine compression ratio with an unknown piston manufacturer to get the specs from it.
340 compression ratio
- Thread starter ktcnn94
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rklein383
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I presume your pistons have valve reliefs. You could use clay to fill in the reliefs to determine the volume of the reliefs and add that to the cc's from the heads and the volume of the space in the cylinder caused by the piston being down in the bore slightly at max height.
What month 1971?
That’s a thought. I was thinking of cc’ing the pistons at TDC like the combustion chambers would be done but not sure where to go from there.
Not positive on that, I can look tomorrow, why do you ask?
That’s always a possibility. I’ll check it out.
If the engines out cant you pull one piston and rod? Get your compression height from piston and cc the valve reliefs while its out.
alot of pistons will have the part number stamp on the top of it....
but unless you know the deck height on the pistons..and cc on the heads...plus the head gasket cc ...you are guessing at the compression ratio.
but unless you know the deck height on the pistons..and cc on the heads...plus the head gasket cc ...you are guessing at the compression ratio.
yeah that’s what I was trying to figure. I was gonna cc the heads, choose a gasket or two to figure it but wasn’t sure what to do about how to determine the info I needed for the piston part of the equation since I don’t have the info on the piston. Wasn’t sure if I could cc the piston in the bore at TDC and get anywhere with that.
Get a bore scope and look through the spark plug hole to see which piston may be in the engine. There are some pretty cool scopes that use the USB on a phone and a small enough to go through a spark plug hole.
I cc'd my TRW 2332P with a cheap syringe from work. 2 valve reliefs were just shy of 8cc I believe. I'm sure you could do them in the block but you still need to figure compression height on piston anyway.
Glad ya caught it before it got worse.
Glad ya caught it before it got worse.
I pulled the heads off if it. I wanted to see what I had in there and figured it would be easier to degree the cam with the heads off.
Yeah I figured I was gonna have to pull a piston and try to measure it or ID it after I cc it in the bore to get the correct figures. I was just hoping someone had a way to figure it just using the piston cc along with the combustion cc, head gasket thickness and bore and stroke.
Post a pic or 2. Worth a shot.
I’ll get a couple of pics tomorrow if I can get down to the garage. Thanks
IF STOCK PISTONS, they will come up over the block. And stock CR on the early build 1971 340 is 10.25, where 1970 and earlier is 10.5? What I read somewhere...... My 340 is a late 1970, so I'm guessing it went into a 1971 car. Just guessing, when I get around to it I will find out for sure.
Yessir, I’ve read that before too. I was just trying to see how close I could figure it. I don’t think it’s stock/factory pistons in it. Wanted to be sure before I order my head gaskets to make sure I get the right ones. Plus I’d just kinda like to know what it is when I put it all back together. Thank you very much for the info.
Read this thread to figure out the compression for your parts combination....
Once you find out where you are, then you will know where you need to go...
How to Check Compression
340/360 heads usually have 70 - 72 cc combustion chambers on the ones that I've measured...
Cool, that’s exactly what I was looking for. I should be able to get the measurements but I’ll probably struggle with those equations but I’ll figure it out. Thank you very much.
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Once you get the numbers, I can help you with the math...
Sorry, I went on to bed last night. Thank you very much. I’ll be in touch once I get my readings. BTW I noticed 4spdragtop called you Karl (with a K), thats also my name and spelled with a K. Not real common, anyway thanks for the help.
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