I know the small block "block deck height" specification is 9.60 inches. But where can I find the specification for the piston height (above or below deck) for 68-71 340 motors? I've looked everywhere in my 1971 factory service manual and found nothing. Isn't there a resource on this forum for blueprinting a motor? You know, recommended bearing clearances, piston to wall clearance, head gasket thickness, and the like for a street/strip car?

The main problem I am having is working on getting 10:! compression on a 305 Mopar motor that I plan to build. Using the online "Eagle rod calculator" and the 2.96 crankshaft, the Trans AM size 6.25 rods (versus factory 6.123), and the factory pistons, I only come out with 8.15:1 compression and the piston ends up 0.029 below deck. I wish someone could tell me what pistons were used in those Trans Am 305 motors. Anyone have a clue?

The early higher compression 340s had pistons that come out of the bore about .020"

I have not worked on a trans am 305 but I can say with certainty that it will be difficult to get a reasonably high compression without a piston that is above deck. Head milling will likely be required also.

OP, you may want to look at NHRA web site for the 1968 to 1971 340 piston deck hieghts and cylinder head combuston chamber sizes and use those for figuring the original true compression ratios. I built several 1971 340 NHRA legal "stocker" motors for a customer back in the mid 1990s, using NHRA specs and doing the math those motors had a true 11,5 to 1 compression ratio in bluepring conditions I don't remember the exact specs. now on how high the deck of the pistons where above the cylinder deck, the head gasket thickness came into play so you could raise the pistons up with a thicker head gasket or lower it with a thinner gaskest On your deal your going to have to have a custom set of pistons made to get the exact compression ratio you want. Which is common and normal when building destroke or increase stroked motors, regardless of brand and size IHTHs
BTW, Are you aware that when yuo destroke a short stroke motor to start with you will loose all the bottom end power that it had? If not maybe increasing the stroke for your deal might be better for a pump gas street and srip car The lack of bottom end power was probally one of the main reasons Mopar came out with the 360 C.I. motors in 1971 for a lot of thier cars and trucks, more C.I., more power and torque

I would suggest keep Googling with different approaches/questions especially regarding that class of racing. there's a ton of info out there, most good some bad. Good luck in your quest!

If you are fixed on a 305... 273 block +.060 bore and a 360 crank will give you 305ci. Not sure it will fit tho.
http://victorylibrary.com/mopar/a-bs.htm

Head cc 62
domes +10cc
bore 4.04"
deck clearance 0
stroke 2.96"
Piston Compression Height: 1.987

That'll get you 11.22:1 cr. So find a piston that's close or a custom set.

The height above/blow the deck is calculated.
Add 1/2 Stroke + Rod Center to Center + Piston pin center to top of piston height, then subtract the block crank center to top of cylinder height.

Definetily not 305 C.I. with a 4.100 bore wih a stock 360 stroke 372 C.I. maybe

On a 273 bore.. but where the heck do you find a 273 lol.

The number that I remember for the piston above the deck in the HC 340 is 0.018". It could be 0.018" to 0.020".

The racing series limited engines to 5 liters. A Mopar 4.04 bore and 2.96 stroke comes out to 303.55. The sanctioning body allowed destroking to get the displacement. I remember sitting in the infield at Brainerd watching the cars, including the 1964 LeMans that some very smart crackpot was racing. That was racing! Cars that looked like cars you could buy the next day lumbering around the track (compared to the purpose-built racecars they were not graceful). And there was a turbo BMW 2002 that let out a huge blast of flame every time the driver shifted.

Remember that those engines used steel shim head gaskets. When Cab says to NHRA specs he isn't saying what he really means, that every dimension in the engine had to be changed to get to that compression ratio. That's the difference between the blueprint specs and the way Chrysler actually built them. On a 340 cubic inch engine to get 11.5:1 compression you need no more than 66.4cc above the piston. When the stock heads came in at somewhere between 68 and 73cc something had to give.

R.

I did not see the data for the rod length included in the above, 305 rods were 6.25 and stock rods are 6.123. Which rods were used in the above calculations? Thank you.

OK, I see the problem I am facing, it was so basic it flew right over my head. I am trying to squeeze a smaller volume 38.125 cubic inch cylinder of a 305 into the same large 64 cc space (combustion chamber) as the stock 340 42.5 cubic inch cylinder. Naturally, since I start out with a smaller volume, I am not squeezing it down as much, hence the compression works out to about 8.5 to 1 rather than the factory 10.5 to 1.

Now I see in place of the factory flat top piston with valve reliefs, which displaces a negative value of -5cc or -7.5cc, what I will need is a piston with a dome displacement of around 10cc to make up the difference. The only problem with that (in my own mind anyway) is I can only picture the piston smacking the combustion chamber OR, worse yet, the valves smacking the pistons on the way up. The only other alternative I can think of (which is impossible, of course) is a 49cc combustion chamber volume.

I suppose what I really need to see/find is a Mopar 305 Trans AM piston so I know what I am dealing with. I wonder who made/makes those? Maybe what I need is a 340 12.5 to 1 piston, which in my 305 might figure out to be 10.5 to 1 piston?

Most likely Diamond Racing Pistons would be able to make you a set.

6.123. Becouse your not going to find the 45 year old 305 rods.

Be aware that the blocks are usually .015" or so higher than what they quote, and the piston height above deck [blueprint spec] as I remember it was .045". So in the real world the early 340's used to have around .030" positive deck height allowing for the extra .015" in block height. That's what I have found when checking piston heights in factory engines as well.

This is correct, but that is with a shim gasket. You must ass the extra gasket thickness to the positive deck height.

I've run them over .060 out with no problems.

You could sleeve every cylinder, notches at top for valves...