Doug Tschantz

I remember reading something a couple of years ago on this subject but never let it sink in. I do know that a square motor ar far as bore/stroke is better right? Please help me understand this before I start building my engine! Thanks

Doug
00 Pontiac

Colonel

Well, it's according to what you're trying to do. From a physics standpoint, a longer rod is always better. It makes for a more efficient engine by reducing side loading, piston speed, and by increasing time at TDC. But, we only have but so much room to work with when trying to get a certain amount of stroke out of an engine. So, a compromise between stroke and rod length has to be made.

Cal

iTrader: (1)

You want an over-square (bigger bore than stroke) for best high-rpm power and durability at high rpm. Under-square, long stroke engines are good in tractors and trucks where low rpm torque is most important. Somewhere between these two is a compromise for street applications.

LOnSLO

iTrader: (15)

Well, not an expert but seeing how stock B/S ratio is 1.076, I would try to keep it at least at that. Let a square motor be the max you want to go. Something else to throw in will be rod/stroke ratio. Again, stock R/S ratio is 1.68, so don't forget to consider this when picking a combo.

Bowtieman4life

iTrader: (1)

I have read that the Ford 5.0 and the old Chevy 327 have the best B/S. They both had a big 4 inch bore and a 3 inch or 3.25 stroke. Our motors are ok but only because the long rod helps. If we had 5.7 rods we would have a worse ratio than the LT1. It will put less load on the crank because of the better angle.

LOnSLO

iTrader: (15)

Inertial side loading is high with these motors, unless you destroke them. You need a 6.6 inch rod at least to make up for the stroke size. This is for a theoretically perfect LSx race motor though. That will never happen unfortunately.

Colonel

For racing, I'll take the biggest bore I can get, regardless of the stroke, and I'll take as much stroke as I can get while retaining a descent rod/stroke ratio. This would depend largely on piston velocity (rpms have to be taken into consideration here) and side loading of the piston.

robertbartsch

...for a street engine I like more torque in the low end RPM. So that means more stroke than bore...

ChrisB

A longer rod isn't actually always better - if you have a very low restriction intake side, etc, then a shorter r/s ratio can lead to a higher instantaneous piston veloctiy, giving a higher delta-P and a greater intake pulse. - it really depends on your setup.


But that said, R/S ratio within the LS1 context is pretty much irrelevant as it is dictated effectively by other factors. For example:

1) Strength - you need to determine what your expected power, rpm, longevity, etc. goals are and ensure that you aren't compromising any of those. This will pretty much fix the minimum piston height (ring package/land concerns), maximum stroke/bore (block strength), etc.

2) Budget - the more you do the more expensive things become - you need to determine what you are willing to spend (then triple it )

3) Size - if strength and budget allow, go for as big of a motor as possible - r/s ratio, etc. are all much less important than this.



For rod and stroke there is a max stroke you will be able to effectively clearance on a block, and with a minimum piston crown height the rod size will be effectively fixed (since 1/2stroke + rod + piston crown has to = deck height).

The side loading isn't that big of an issue - heck, honda's run down in the 1.4 range, which is getting down there.

I would say run the longest rod that your stroke and piston will allow for, and don't worry about the rest (unless you are really trying to make a 300k motor - but then, performance isn't your concern anyway)