So, this is for theoretical purposes. I know I asked on here and/or moparts before about boring a 273 to 340 specs and people said it shouldn't be done due to I believe the webbing between the cylinder walls. They said the 273 and 318 are similar and the 340 and 360 are similar. That being said, if the main difference between the 273 and 318 are the bore sizes, couldn't I bore out a 273 and buy a 318 stroker rotating assembly to achieve a 390 ci engine? If so, then I'd need a 318 compatible heads, intake, and carb, correct?
273 and 318 the same block?
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replicaracer43
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No, get a 318 block if you wanna do that, there can usally be had for free.
replicaracer43
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Remember one VERY important thing about making horsepower, it requires STRAIGHT and ROUND bores, that stay that way under LOAD. Anytime you max bore a production block, your doing it the wrong way IMO.
It was a theoretical question. I'm not going to stroke a 273. Thanks for the info.
grumpuscreature
Resident Curmudgeon
Nice idea but there is one fly in the ointment. Ma Mopar in her infinite wisdom used different water jacket cores in each block in her efforts to make each one as light as possible. The cylinder walls in each block does not have enough thickness to allow much more than .060" overbore. The one exception to this is the '71 360 blocks which were cast using 340 water jacket cores and can go out to 4.1" safely. Hope this helps...
Interesting idea..........now here's one to chew on Call up Darton Sleeves, see if they could come up with a max bore sleeve kit similar to what they have for the 3rd gen Hemi then ANY LA block could go out to max bore......and with a good poker face, say it's just a good running 273, 318.....show them the casting number on the side of the block for proof.
The 273 was the first LA engine and had many core shift problems because of the new design.
Quote from Allpar: The LA, introduced in 1964, had the same basic design but was very heavily modified. The lightweight ("L" A) or thin-wall casting and heads saved around 50 pounds; the wedge-shaped combustion chamber was much smaller than the A engines poly heads, greatly reducing the size and allowing the first LA engine (273 V8) to fit into the small Valiant.
Any overbore more than .030 and I would recommend the block be sonic checked. A thin cylinder wall could ruin more than your day. tmm
cuda65vpt
cuda65vpt
Would there be any advantage to using 273 heads on a 318? Or will it even work?
They work but because the 318 has a bigger bore you can put big valve heads on for more performance. A 318 with X or J heads and the right parts to go with can get you to 400 hp. without too many tricks. The 273 is a fun little engine but it lacks bore size and doesn't work well with large valves or large combustion chambers. A friend had a 318 from a Notchback Barracuda from the 70's. It had J heads, Crane Fireball cam, headers, and intake and carb. It screamed. He rebuilt it last year with 318 heads with 340 valves and the same combination. It screams too. tmm
seabee
Well-Known Member
Interesting subject. After checking piston prices I have considered boring out my 273 to a 318 for a cruiser. Know a guy on here that made a 340 out of a 273 and it has worked fine but this is not on a high performance build. I definitely don't want to toss my 273 heads that had so much work put into them.
Yup. I've got a '74 318 that I pulled from a Dart. It was running when pulled but in need of a rebuild, a good core. Was complete carb to pan, tried selling it for a $100 and didn't get any offers. Still laying around taking up space.
Yes, it will work.
Any advantage? No.
They put the same head on the 318 and 273 when they were made the same years (67-69). The 273 and 318 heads are completely interchangeable as they both have the smaller ports and 1.7/1.5 valves.
The 340/360 heads have bigger valves and will work on the 318/273, but will require a notch in the bore for valve clearance for the 273 because of it's smaller bore.
The early heads have smaller combustion chambers and may help raise compression on the later 318's.
This is not entirely correct. For 2.02 intake valves on a standard bore 273 you should notch the bores .070 max. For 1.88 intake valves lets do some math. 2.02 - 1.88 = .14. since we are talking about a radial distance, not a diameter .14 / 2 = .07. Or when using 1.88 intake valves on a 273, no notch required. Any over bore increases the clearance. That said, things are getting close so you should check before you slam everything together. 318's do not require a notch, ever.
As for over boring blocks, I hunted and searched for standard bore blocks and paid a premium for them. I always wanted the cylinder walls to be as stiff and strong as possible. You can get more power from using torque plates and good rings than boring a block .060. I only bore the minimum to get straight and round cylinders after everything is bolted together.
Like said earlier, get a 318 and swap your heads, or just use J heads with the right pistons.
Justin (onewildandcrazyguy) bored a 318 to 340 size for a member on here, he used a 318 truck block.
Dartswinger70
Well-Known Member
That is interesting. the old rule of thumb used to be no more than .040". I guess you never know until you try. That block is done though, no rebuild on that one.
You can always sleeve it...
Just like a loose woman... Stuff a 12 lb ham in there and pull out the bone..... Good as "new"....
A 273 can not be bored to a 340. You also can not make a 273 into a 318. In order to make a 318 or a 340 from a 273, 1 of 2 things must be done. Stroke it or resleeve it.
The blocks are all the same at a basic level. Internal water passages may differ somewhat.
It doesn't matter what or where the block came from. All blocks that seek a more than normal .030 size overbore should be sonic checked.
The only issue to making use of 273 heads in any other small block is the eay heads have different bolt angles and sizes.
If you want to build a stroker I would start with 72BNB's 318 block.
Or start with a 360 block. 340 blocks are to pricey IMO.
318's got to 390+
360's go to 400+ easy
The blocks are all the same at a basic level. Internal water passages may differ somewhat.
It doesn't matter what or where the block came from. All blocks that seek a more than normal .030 size overbore should be sonic checked.
The only issue to making use of 273 heads in any other small block is the eay heads have different bolt angles and sizes.
If you want to build a stroker I would start with 72BNB's 318 block.
Or start with a 360 block. 340 blocks are to pricey IMO.
318's got to 390+
360's go to 400+ easy
67Dart273
Well-Known Member
Stroking them is not the problem. Boring them is. You can't bore many / most? 318s to 340, either
Dartswinger70
Well-Known Member
LOL! I have to say I laughed out loud at this.
273 and 318 especially when both were in production at the same time are essentially identical to each other than bore size. 273 is know being a revving engine I feel it mainly do to the decent head flow per cubic inch
(probably the highest for stock la engine) but for a 318 it
273/318 heads limit the power to around 300-325 hp unless ported.
...this makes no sense because if you had the bore of a 318, you would want 318 heads to take advantage of the larger valve sizes. You;;d be wasting the work you put into the 273 heads (valves too small for your cylinder displacement), plus the work you put into the block. But, as previously noted, you cannot bore a 273 to 318. So it's a moot point.
I wouldn't build a 390 except in countries were 340/360 are in short supply. Not that there a particularly a bad engine, just power comes from in the most part is by air flow, breather to exhaust tip and that includes bore size. Bore size is probably one of the ultimate factors that limit power potential of an engine. Cause of valve shrouding limits valve size and valve size limits head flow potential.
So since all parts pretty much can be swapped between engines with a little work except pistons that would make bore size the only real difference between engines.
So 340/360/318/273 would be the order of bore size from largest to smallest 4.04"/4"/3.91"/3.625", 340 would be the best choice if they were as plentiful and cheap as a 360, to me on a ground up build still worth trying to get one for the few extra hundreds they go for, more air flow and Cid potential especially if your trying to go with heads with bigger than 2.02 valves. 360 there's really no reason to build anything smaller and cause of cost and being the largest stock engine it can be argued that there's no reason to start with any other small block. It does come with the best compromise of bore stroke and rod ratios. 318 if you got a good running one it's worth to do the basic hot rodding hop ups but for a full build a 360 would be better especially if shooting for over 400hp, but under that the smaller bore of the 318 ain't that big of a deal but your gonna need more gear and stall to keep up with the 340/360. 273, with some raw head castings and a Nascar budget probably could make some serious power from the small bore, but theres probably not very many 273 running around with more than 300 hp, and gonna need a LOT more STALL and GEAR to keep up with a similar hp 340/360.
Stroking makes sense on a street engine since it will move the power band down in rpm, needing less stall and gear especially since most seem only want to run 3.55 or less. And in a race engine where power level would need to turn a stock stroke engine north of 7500 rpm. Even though you gonna give up some hp and life to increased friction.
Depending on desired hp this wouldn't be a bad combo
still would have great rod ratios and still on the over square side ( bore stroke ratio) and with the extra cubic inch it would bring the power band down a little. But without a good head porter it would be hard to get more than 350hp, it probably make a good 1 hp per Cid gas mileage engine but you would be further ahead going larger bore engine.
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