Difference between a 318 and 340?

[440+6]

What is the difference between a 318 and a 340, other than the bore size? i ask b/c i'm buying a Sealed Power overhaul kit for my '84 318, but they only list them w/ 8.5:1 pistons. Since i'm boring the engine anyway, i figured i'd take it to a 20 or 30 over 340 size so i could get the higher comp. pistons, but i'm not sure if anything else in the kit wouldn't be compatible.

[crisser]

Unless you have a REAL thick cyl. wall, you won`t be able to bore that 318 out to a 340 much less a 20 to 30 over 340.

[kllrbee69]

Im not sure I really understand what you are saying.

Are you saying that you want to bore a 318 .30 over and try to fit the 340 pistons in there?
I dont think you are saying that you want to bore a 318 so far that it would be a 340, are you? you would have to bore it .130 to reach the stock 340 bore size.
Either way, I think they are both scientifically impossible as a .030 over on a 318 would make it something like 324 and 3.94 bore.
I do belive that you can make a pretty stout 318 with 8.5 CR though. You just need the right combo of parts.
If you decide to go that route, Im sure we can help.

[JCFcuda]

You could have the block sonic ckecked, it would tell you if you can bore it that much. Hensleys Performance, did some 318, and punched them out to 340 size a few years ago. I beleave they had to go through quite a few to get ones that would accept that big of a bore.
All that being said, it wouldn't be cost affective.

Jim

[Ray Bell]

We've done a lot of sonic testing of the Poly 318 blocks and haven't yet found one we'd be happy taking to 4.0"... much less 4.030"...

Mind you, the view Ben is taking is that he wants a certain cylinder wall thickness and won't do it without both that and keeping the cylinders in line. One block, IIRC, could be bored and get the wall thickness, but a couple of cylinders would have had to be moved to fit in with casting core shift.

The Poly, of course, came before the LA. 'LA' stands for 'lightweight A'... in other words the block of the 318 LA is a lighter version of that earlier block. Guess where one of the areas they saved weight was?

Right again... the cylinder walls.

Sure, some have done it, but I think you'll find that they've gone to marginal wall thickness and may even be getting cylinder wall flexing.

Guess what that does to piston ring seal?

[mini1]

Just go out and buy pistons separately. or get the 8.5cr and zero deck the block.

[DartGT66]

Yes, we've done it. Long before we knew it couldn't be done. The cylinder walls were paer thin, we actually turned a too long timng cover bolt through the cylinderwall and only noticed it when we started to wonder how much water it takes. Anyway, we tore it aparts and filled the hole with a brass plug and cut the excessive out (it was beneath the ring travel) and run the engine for several years. Amazingly, it worked just fine and even run decently, low 12's at best in a light car with just a basic performer intake, 1 5/8" headers and a juice cam. However, I wouldn't recommend doing the same to anyone.

[passing you]

The blocks are different between 318 & 340
340's have taller beefier main caps.

The 318's failure is usually the main caps cracking in race situations.

I'd be happy with a .060 or .070 over 318 and skip trying to cram 340 pistons into it.

[5th Ave Sleeper]

If you want a 340, just go find a 360 and build it. you can do anything to a 360 that one could do to a 340 and get just as much power and more tourqe. By the time you get done with the expense of sleeves and expensive machine work, you can have a reliable engine with just a .030 overbore with a 360
5th

[TeamDart]

I beleive the counterweights on a 340 crank are beefire than a 318 as well, but I've been wrong before.

If you build a 340, start with a 340 block, and use a forged 340 crank.... problem solved

[440+6]

Sorry guys, i read the bore specs on the 2 engines wrong......yea, i'm definitely not tryin to take that much material out. No big deal though. i appreciate all your input

[gwaii]

Lots of people have looked at the trying to bore a 318 to 340 specs with the same results as mentioned by others posting on this thread.This was years ago but Ohio piston used to make a 9.25 to1 cast piston for the 318 used in bus applications,They were reasonably priced and a quality product,but i dont know if they make them anymore or if Ohio piston is even still around.If i remember correctly the part number for them was p950 or something close to that.Hope this helps for those that are trying to find a bit more CR in a 318.

[5th Ave Sleeper]

You can get new high comp pistons from Mancini Racing or just about any mopar parts supplier. Heck, you can get a stroker kit and stretch that little 318 out to 390ci
5th

[rumblefish360]

Originally Posted by 440+6

What is the difference between a 318 and a 340, other than the bore size? i ask b/c i'm buying a Sealed Power overhaul kit for my '84 318, but they only list them w/ 8.5:1 pistons. Since i'm boring the engine anyway, i figured i'd take it to a 20 or 30 over 340 size so i could get the higher comp. pistons, but i'm not sure if anything else in the kit wouldn't be compatible.

As said above and the driver side engine mount is different, but I don't think your intrested in that area.

Look, if you want to do up a 318 with some punch to it, save the bore as much as possible. The reason is a thicker bore will stay striaghter and rounder longer and the boring, if need be, should be done with a torque plate installed.

KB has a zero deck piston for the street and light race as well as a domed version for really putting the nuts to the engines power output. You can ask Summit to swap out the pistons offered to the KB units. They'll be no problem with that and they have done that before.
I think they are actually happy to do so.. (They sounded it, LOL.)

More often than not (Believe it or not) the 318 does best with a 318 head on top. Of course, this is a power output level dependent thing. But, for the most part, the little bore does like a small and narrow head port for the street and cams approx. up to about 230 @ .050. From about that duration, you kind of hit a cross road and you could possibly use a 360 head.