engine builders....got a question

dragnazz5.0 · Feb 13, 2006

well today in class we were talking about engines. my teacher was talking about how a longer rod is better because it causes the piston to pause or dwell at the top of its travel longer than a shorter rod. so i got to thinking. what is the longest rod you can put in a stock 302 bottom end without notching the cylinders and how much do they generally run for a good set with pistons that have a shorter compression distance? also, since the 351 has a longer stroke...would it work in a stock 302 block? and what would the outcome be? thanks

bluevenom867 · Feb 13, 2006

dragnazz5.0 said:
my teacher was talking about how a longer rod is better because it causes the piston to pause or dwell at the top of its travel longer than a shorter rod.

Thats not always true.

In high compression motors,a shorter rod helps because it pulls the piston away from the combustion chamber faster (helps with detonation).And at low rpm its not as good because of the longer dwell time at BDC can cause intake revisions.

Its usualy true though that the rod angle is more resistant to side loading and will revv with less "stress" I guess you could say.And at high rpm,the idea is that the longer dwell time let it have a more effienct burn.As it can put more pressure acrosst the top of the piston evenly.

So basicly...dont worry to much about it.Unless your building a Pro Stock or one of them Engine Masters Challenge motor (whitch btw,no one theroy seems to win... ) just dont bother.Go with whats easiest to get a hold of,witch for a 302 thats the 5.09" stock rod.

Black1987Stang · Feb 13, 2006

Sorry cant help u with the 351 rod question, i think the biggest u can take a 302 is a 347.

but theres some disadvantages to it too...Its kinda like the debate from stroking a 302 to a 331 or to a 347...347 would prob make more power but they're not as reliable (not saying it wont last or anything)...My friend told me a whole bunch of disadvantages from the 347 to 331, I think some were something with the wristpins, more pressure on the cylinder walls, lack of lubrication (i forget where), and a few more...if u do a search about 331 VS. 347 im sure youll see some disadvantages.

90mustangGT · Feb 13, 2006

There is such thing as a long rod 302, using 5.4 rods with shorter pistons. DSS sell them I know.

-add- With a longer rod, you can use a shorter, lighter piston which helps when revving the engine at high speeds.

BaXTeR3221 · Feb 13, 2006

I know they have fixed the wrist pin issue on the 347's. Any good builder can build you a 347 that will last just as long as any other motor.

bluevenom867 · Feb 13, 2006

90mustangGT said:
-add- With a longer rod, you can use a shorter, lighter piston which helps when revving the engine at high speeds.

The shorter piston is also less stable in the bore and doesnt disapate heat as fast.

See?

It can be debated for ever,you'll never know unless you can build two EXACTLY IDENTICAL motors with the only differance in rods and piston hiegth.And run them on the PERFECT dyno,with CONSTANT conditions.

Basicly,pic your theroy and build it

88Stangboy · Feb 14, 2006

ive heard that "theory" about long rods to.
But unless your building a extremely high hp engine for racing your not going to see a difference. In my mind, i may be wrong, but why would piston dwell time even matter? The valve still stays open the same amount of time, so unless you increase duration, how does that help with filling the chamber more or whatever? I wouldn't waste my time with longer rods, there is no point in my mind.

dragnazz5.0 · Feb 14, 2006

does anyone know anything about using a 351 crank in a 302 block?

Rick 91GT · Feb 14, 2006

90mustangGT said:
There is such thing as a long rod 302, using 5.4 rods with shorter pistons. DSS sell them I know.

-add- With a longer rod, you can use a shorter, lighter piston which helps when revving the engine at high speeds.

Yep you can build a long rod 302 or 306 using a 5.4" rod, still may need to clearance the walls a hair depending on the rod bolt design.

Rick 91GT · Feb 14, 2006

dragnazz5.0 said:
does anyone know anything about using a 351 crank in a 302 block?

Not possible, the 351W has a 3.00" main, the 302W has a 2.249" main...

dragnazz5.0 · Feb 14, 2006

oh ok...i must have misread the info or it was wrong on the website i looked at

dragnazz5.0 · Feb 14, 2006

Rick 91GT said:
Not possible, the 351W has a 3.00" main, the 302W has a 2.249" main...

couldnt you just cut the main down to 2.249? i dont see why it would be a problem since the 302 has that size main

90mustangGT · Feb 14, 2006

Rick 91GT said:
Yep you can build a long rod 302 or 306 using a 5.4" rod, still may need to clearance the walls a hair depending on the rod bolt design.

I know, and all that work and your just one crank away from a 347. Unless your class limited, it just sounds stupid to me.

bluevenom867 said:
The shorter piston is also less stable in the bore and doesnt disapate heat as fast.

It will still have more piston than a 347, wouldn't it?

Rick 91GT · Feb 15, 2006

dragnazz5.0 said:
couldnt you just cut the main down to 2.249? i dont see why it would be a problem since the 302 has that size main

You could cut the crank, pay to have it rehardened etc... But why? You could buy a aftermarket crank at half of that price. Plus with a 351 it has a 3.5" stroke, you would need to come up with a weird rod and piston combo to try to make it work.

dragnazz5.0 · Feb 15, 2006

well than you would have to get a taller deck block. im just trying to figure out if it would make a decent motor or not. its only 1/10th of an inch longer stroke than a 347 stroker so you could use a stock 5.09" rod with the 3.5" stroke instead of the 3.4" stroke and 5.4" rod

Rick 91GT · Feb 15, 2006

dragnazz5.0 said:
well than you would have to get a taller deck block. im just trying to figure out if it would make a decent motor or not. its only 1/10th of an inch longer stroke than a 347 stroker so you could use a stock 5.09" rod with the 3.5" stroke instead of the 3.4" stroke and 5.4" rod

It isn't that easy... First a 302W block only comes in a 8.2 deck or if you go with a race block you can go 8.7". You can not just put a 5.09" rod and a 1.35" CH piston in that 8.2" motor since you would have interference problems with the piston contacting the crank. You may be able to come up with a crazy custom piston and a rod to work but it would not be worth it. If I was building a 8.7 block I would use something different as well. You would be surprised what .100" does to a rotating assembly. Plus like I already posted turning it down and reheat treating that crank really isn't the best, and it isn't cheap. I really do not think you can turn down a 3" main to a 2.25" main, it is hard enough to get a billet piece changed from a 3" main to a 2.749" Clevland main, and that's a good quality crank.

For all the money and custom parts it would take it is much easier and cost efficent to stay with a 3.25" stroke or 3.4" stroke set-up.

dragnazz5.0 · Feb 15, 2006

ok...well i guess you made a better point that i ever could have hoped to make. thanks for the info rick

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