What to do with a 343?

WesternRed wrote: Ok, well Wiseco makes a forged flat top piston for the 360, might as well say that it is a 343 piston.   Been around for a couple of years at least I reckon.   Here you go:   http://www.campbellenterprises.com/amc-parts/amc-pistons-360-pistons-401-pistons-wiseco.php   $553 including rings is pretty damn cheap in my books, not a lot more than cast really.   And its actually 030 and 045 oversize.

They look like they would drop compression alot compared to a correct 343 piston?

WesternRed wrote: Why?   11.6:1 in a 360 with 50cc heads would be near enough to 10.2:1 a bit further down the bore in a 343.

Than they must be taller than an OE 360 piston. I have read that OE 360 pistons in a 343 drop compression a LOT lower than 10.02 :01 more like 9 :01 so in that case I guess it would be a good option for a forged piston assuming the quench doesnt get too affected, how far down in the bore are these in a 343 when installed?

Edited by 348AMX - Jul/08/2012 at 11:18pm

The 343 is an excellent engine to start with and can be a real screamer with not too much trouble. Throw a set of dog leg heads and some freeflow exhaust manifolds on it with a good cam and intake and you will be shocked at the results.

stock 360 high compression pistons in a 343 are the same 1.60 compression height as stock 343 pistons. The difference is that the 4bl 343 pistons has a -3cc valve relief whereas the high compression 360 pistons have an -18cc valve relief thus the drop in compression. Both are .060 in the hole.

Wiseco 360 pistons are .010 shorter than stock to allow for decking the block so you would end up with the pistons being .070 in the hole PLUS they have a -21cc valve reliefs rather than the stock -18cc valve reliefs. The resulting compression for high compression replacement Wiseco 360 pistons in 343 at .030 over bore with nothing taken off the block would yeild a compression ratio of 7.9 with 50.6cc heads and 7.44 with 58cc heads.

They also make a true flattop piston with no valve reliefs and at .030 over bore with nothing taken off the block the compression would be 10.35 with 50.6cc heads and 9.5 with 58cc heads. With any of the obove, there will still be .070 deck above the top of the pistons which is not an optimal burn configuration and will likely result in pinging on the higher compression ratio if not both.

For my money I would get a set of Bulltear custom pistons to minimize deck clearance and render the compression ratio you can run it on pump gas with. The Wiseco 360 pistons are excellent pistons but designed for the taller 360 block so are not all that great in the 343.

If you keep the 10.2:1 compression being 0.070 down in the hole will require premium fuel. Even then it will likely ping under a hard load, but shouldn't for normal driving (just hard acceleration, gear down if pulling a hill). If you cut it back to 9:1 you won't ping on premium, will on regular under hard pulling. Mid grade might be okay though. If this is going to mainly be a street car I'd use the 360 pistons as is and let it ride at 9:1 compression. Just polish the edges of the pistons to slightly round them to reduce detonation. Sharp edges heat up faster and stay hot, that's the main cause of detonation.

Sticking 0.010" up out of the block isn't a problem. Stock head gaskets compress to about 0.040", leaving 0.030" clearance. That should be okay, but definitely check the valve reliefs -- they may need to be cut a little deeper.  Racing Head Service recommends 0.080" clearance between the intake valve and piston, 0.100" on the exhaust valve. Ford recommends the same for their small blocks, up to 6000 rpm. Those measurements seem to be industry standard.