260Velocity

GPM, that is the 572 combo i was thinking of. my current bore is 4.500 and isnt too bad. maybe just go to a 4.530 bore x 4.375 = 564 or 4.530 x 4.25 =548.

Lets throw another wrench on 548 combo, i have seen some builders using the 6.535 with the 4.25 crank on short decks. i hear that it gives more torque to the motor and better Rod Ratio. But this effects the CH of the piston. is this worth considering or not? It looks as if there are not any shelf pistond for this combo either, so that is one issue.

Any thoughts on that?

Does anyone know from dyno experience what 1 point of compression makes or losses in HP?

Knot 4 Me

Funny how builders differ on this subject. About 4 or 5 years ago I was at LPG speaking with Keith Eickert on building a 540 and he absolutely refused to build one on a 9.8" block Others build them and bigger without batting an eye.

Black Baja

iTrader: (2)

Leaving the marine side out of the build for a second and talking in terms of an all out max effort N/a motor. You ask what makes the most power? Shortest deck possible - the shorter the deck the better path you will have to the intake valve which equates to more power. Its all about air speed and velocity. The shortest Rod you can stick in the motor makes the most power. Why Because a shorter Rod does not dwell at top dead center as long as a long Rod. Which makes the piston accelerate faster from top dead center. This equates to less "pumping" loss. This is referencing extreme builds. Moving from a 6.385 Rod to a 6.535 is a difference in .150 which is nothing. I doubt there will be any difference in power between the two. The only time I look at using a longer Rod is not so much for having a better ratio but to try and have a lighter piston. On the forced induction side of things sometimes the shorter Rod is desired so we can have a dished piston and still have room for material in the crown. There are a bunch of 572 short deck marine motors running around these days with great results. If I rember correctly Brian 41 has built a few. Maybe try pm'ing him. If I had it to do all over again instead of building a 632 I would build a 572 short deck and spin it 7400 rpm but that's me.

FIXX

iTrader: (2)

smart man..if it was safe and reliable the merc would have used a 9.800 block on their 540 bulldog instead of a 10.200 tall deck block..also a longer rod gives the engine more time to stuff the cylinder full of air and more time for it to be pushed out..more air and fuel equals more power..

Black Baja

iTrader: (2)

Actually a longer Rod decreases air velocity. Velocity makes horsepower. Every form of racing has gone to short connecting rods. It's not a hidden thing anymore. Today's oem engine have less and less Rod ratio.

260Velocity

This is interesting info, Bob Teague has a 540 listed but is a Tall Deck block. Just an FYI. BUT, if we are concerned with the 4.25 stroke then why is or was the 496 so popular ? it has the 4.25 stroke with a smaller bore. i;m not sure of the rod length so dont know what the rod ratio is on this motor..

Black Baja

iTrader: (2)

496 uses a 6.385 Rod. They make an external balance 4.250 crank for use with stock 6.135 rods but they are junk. You need the 6.385 Rod to clear the counterweights for internal balance.

hogie roll

Rod ratios are almost irrelevant. Just go with the combo that keeps a reasonable piston deck height. You aren't spinning it fast and the odds you put over 500hrs on it are extremely low.

Cube for cube, two different stroke motors will make virtually identical power. It's been tested. It isn't worth debating anymore.

GPM

[QUOTE=260Velocity;4315598]GPM, that is the 572 combo i was thinking of. my current bore is 4.500 and isnt too bad. maybe just go to a 4.530 bore x 4.375 = 564 or 4.530 x 4.25 =548.

Lets throw another wrench on 548 combo, i have seen some builders using the 6.535 with the 4.25 crank on short decks. i hear that it gives more torque to the motor and better Rod Ratio. But this effects the CH of the piston. is this worth considering or not? It looks as if there are not any shelf pistond for this combo either, so that is one issue.

I think you would be happier with the 4.375 crank motor, the larger the bore the better the air flow. I think you're into a custom piston whether you use a 6.385 or 6.405 rod. I don't think the 6.535 rod leaves much of a piston with the 4.375 crank. Just my opinion.