FullAuto

The 347 never had an oil buring problem. The way the engine was assembled led to oil burning problems. The wrist pin intersecting the ring was only an issue for engine "assemblers". The new kits changed the rod length to keep the pin out of the top ring to target the inept "assemblers". They needed something to allow the average idiot to "assemble" a larger motor. The 5.4" rod kits were being used by professional "builders" without any issues. They are still used today. The entire oil burning thing is an internet myth started to make people feel better about their lack of capabilities.

83gtragtop

+1, On modern 347's the problem has all but been eliminated.

FullAuto

That logic is INSANE!

There is no scientific data to support any of this in relation to rod/stroke ratio. Cylinder head design and cam profiles will play a bigger role than rod ratio ever dreamed when it comes to power between the two motors.

As far as a rod ratio arguement, there is a greater r/s ratio difference when comparing a 289/302 than comparing a 331/347. When has anyone ever heard a 289 guy say a 302 will not last as long because of r/s ratio?

At what r/s ratio is the boundary crossed into a measureable difference between the two? Ever heard of a 400 SBC or 454 BBC? They both have a worse r/s ratio than the 347 and somehow it's never brought up with them. Do Ford guys know something the Chevy guys don't?

The fact that the 347 piston puts more side load on the cylinder wall than the 331 piston is simple physics, but how do you put a value on it and who determines whether or not the value means anything in the big picture.

Twister

the logic is based on a 351 and a 302 with the same heads and cam with published numbers from FRPP. The rest comes from engine theory with knowlege of different rod ratios and bore and stroke combos.
Obviously, you know that in theory, a square engine like a Ford 400 (equal stroke and bore) is the best possible engine for both torque and HP. Almost all successful race engines are "over- square" meaning that bore is larger than stroke. A short stroke relative to bore size is beneficial because of less piston drag. A large bore relative to stroke allows larger valves to be used with less cylinder shrouding. the BS ratio (hehe) for a 302 is about 1.33, while a 331 is about 1.24, and the RS ratio is also important

88stangaruu

In my opinion, 331. You can do the almost exact same thing with a 331 as you can with a 347, maybe a few ft lbs of torque difference...like 5-10 at the most. I prefer the shorter stroke of the 331, shorter strokes put less strain on that POS stock 302 block most people throw them into, so go with the reliability. Thats just my opinion though.

FullAuto

The problem with the engine comparision from FRPP can be simply be described as "302 parts will make 302 power".

I don't believe there are any performance gains from less piston drag that would out weigh the fact you'd be giving up cubic inches by shortening the stroke. Whatever you do to a smaller engine you can do the same to a larger engine. The larger engine will always make more power.

Twister

yes, but unless you have HUGE port heads and a moster custom cam....the heads in question are FRPP's X heads

88stangaruu

I agree somewhat and i disagree somewhat...if thats possible

But i mean, piston drag? This is a street motor were talking about isnt it? Who brought that up? Just throw some low tension rings on there and be done with it, if your machine work is done correctly, there shouldnt be to much drag.

But the problem in my mind for the reliability issue, isnt so much the oil ring/piston pin interferance, its the cylinder skirt in the block, the 347 makes the piston come so so low in the bore that its pretty much on the bottom of the skirt that sticks out into the crank case...thats a fragile little piece of the block. Its about 1/8th inch thick or so. I mean, even a stock 302's stroke is pulling the piston down onto that skirt...ive never worked on a 347, so this is purly speculation..but its just a bad idea to me. Id be willing to give up a few cubes, i mean your only giving up 16ci between the two kits...thats nothing, if your a full out racer, then hell yea...347 all the way, thats common sense right there....but heres what im getting at...for the price of building a 347...a motor thats going to last into the low 100,000 range....why not get a 351W and throw in there? I mean after all... Its more cubic inches...