I just tore down the 66 340 motor I aquired for the new ride. Maybe this will help someone doing a google on the 340. This appears to be an original 4 barrel motor, its been under a work bench since the 80s. The pistons are .055 deep The piston dish is 23 cc Head CC is 54. Even with a stock steel shim head gasket the compression ratio was no where near 10.25:1.. in fact barely just under 9.0:1. Even stranger with a zero deck the compression would only be 9.77:1.. Buick was full of BS with this one. Based on NOS piston pictures it appears as though the proper high comp 4 barrel pistons are what I pulled out of it. Anyone have the engine code breakdowns and locations? Thanks
Maybe they are 2 barrel pistons, they may have put the wrong pistons in during a rebuild. Aftermarket pistons are usually below the deck, they assume the block will be decked during the rebuild. .055" is more than usual.
That's all 100% typical of what the big 3 auto manufacturers did in the 60s. Stating higher compression then as built sold more cars to those looking for the largest hp numbers in there price range, and to the drag racers having to deal with building there motors with no more then the stated factory and Chiltons manual numbers they could go for those compression numbers and not be afoul of the NHRA rules.
I know the factory compression ratios are a joke. I thought it was more because of sloppy deck heights. What is interesting here is even with a perfectly blueprinted block 10.25:1 isn't possible. The pistons would need to be above the deck for that number!
I don't think so, you might want to go over those numbers again. It seems like there could be an error there somewhere. I built a 340 using the 300 aluminum heads but they have the same large chambers as the iron heads. Zero decked the pistons with a 22cc dish, 52cc chamber and .050" head gaskets for a 10.5: CR. Your results don't sound quite right to me. I don't doubt that the -.055" deck height would drop the compression but not all that much. Roughly equivalent to the difference between a composition and a steel shim head gasket and that's usually worth about .5 or so I believe. Jim
i did a thread in may of 2012. called 340 tear down specs. 66 340 4 barrel. 68-70 in the hole, head was 57cc's piston dish was -8.5 cc's came out to 9.55 compression. it was a factory 4-barrel with stock pistons
Here it is: https://v8buick.com/index.php?threads/340-tear-down-specs.248512/ >>>Here are some specs on a 1966 340 if it helps anyone. pin height 1.84,, rod length 6.387,, block deck lenght is 10.22,, head cc 57-58,, the dish on the pistons were 8.5 cc,, the pistons were .068 -.070 thousands in the hole. the stock compression with the stock steel head gasket is 9.55:1, with the thick fel-pro its 9.09:1 <<<
Yes, its stock. I saw a picture of an NOS 4 barrel piston- the dish looked the same and had the same two identifying notches cut in them. The NOS two barrel pistons only had a single notch. Any idea where and what the engine codes would be? I didn't see anything on the deck.
In terms of a round number a stock bore and stroke 340 would need a total CC volume of 75 to attain a 10.25 stated factory compression. 10.25 - 1 = 9.25 340 | 8 = 42.5 CiD 42.5 CID x 16.387 = 696.44 CCs 696.44 CCs / 9.25 = 75.29 CCs. Using the first set of numbers shown in this string I get a total stock CC volume above the piston of 88.14.
Huh. Been awhile since I've run the numbers, and the piston manufacturer also ran them. (Venolia) I think that much of a difference would have shown up. I did an actual test of the chamber cc. Something seems to be missing here. 52 + 22 is 74cc so that just leaves the gasket thickness. Jim
Your leaving out the 8.7CCs from the stock .050" deck height. What's the compression height of those Veniola Pistons?
The area above the piston needs to be added to the 696.44 number for the math to be proper. Unswept + swept รท unswept= compression ratio.
Actually those Venolias came out a few thou above the deck which varied a smidge depending on other clearances and factors like piston rock (very slight but still there). Figure an average of .004 above the deck though and that should be reasonably accurate. The deck was surfaced just enough to square up the block with the crank and 0-ringed. At the time I was more concerned about adequate clearance to the head. But here's the kicker: Since the engine is down for some crank work I pulled a piston out of the box and cc'd the dish. In my notes I have run across references to a 13cc dish and the actual measurement was 12cc. So that right there should account for the discrepancy I believe. With a flat top you should be over 11:1 at zero deck. Incidentally, while we are on this subject, these pistons were for blower service and so they have a crown that is .320" thick. Despite that, as a slipper style they are fairly light at 454.6 grams. Now, the TA-Rover heads, which fit the 340 block have a 32cc chamber. Mine measured 31 +-1 but they have had a very light clean up cut. The late Rover alloy heads (4.0 and 4.6) have a chamber of 28-30cc. With big valves and some light porting these make acceptable flow numbers. I'm considering cutting a deeper dish in the pistons and scaring up a set of Rover heads for this engine instead of the 300 aluminum heads I had on it. The advantage of course is that I can swap on the TA heads if I want. To do this I need to add another 20cc to the piston dish which is easily enough done on the lathe. The existing dish is .090" deep @ 12cc. That works out to .0075"/cc so I would have to go .150" deeper, leaving .170" in the piston crown which is plenty with a forging. Probably have to re-balance the crank but that shouldn't be a problem. Jim
I'm sure thats the difference in your ratio numbers. Funny how all these numbers get blurry after building a couple motors! I am interested in your 300 heads if you decide to go the rover head route.. PM me if you want to discuss.
What I posted here is the math to find out how many total CCs are needed above the top of the piston to REACH a given compression ratio, not what the ratio actually is from the factors involved in the motor as it is.
