Big strokes break drives ??????
 

Guys i am in the process of building big power for a 32 foot off shore racer and my builder who is one of the best tells me to use 4.6 bore and no more than a 4.00 stroke so the torque dosnt get to high and smash the drive. He tells me a 535 inch with a tunnel ram can make 800hp which i agree but if we use a 4.250 stroke it will break drives. Bear in mind we run in rough water. What will a #5 speedmaster handle. I want to make a 600 inch with a 4.250 stroke. Maybe i need a #6 but big money. ?????? Thanks. Jason

at a 4.6 bore, wont you need more than 4.5" stroke to get 600 inches?

Contrary to what most people think, torque is proportional to displacement, rather than stroke alone. Of course, increasing the stroke, also increases the displacement, and torque. You're builder is probably right. I don't know what drive you have, but if you have a Bravo XR, you'll be good to about 600 ft-lbs., Bravo X to about 450 ft-lbs, and Bravo to about 400 ft-lbs. Of course, people routinely exceed these limits, but some of them also blow up drives.

Michael

If you are under 800 HP and are serious about an upgrade, let me know. Konrad will warranty their product up to 800 HP depending on boat, use, etc...

If I can help, please don't hesitate to let me know. Eat two Bravo XR's and you'd be in the price range of the Konrad anyway!!

Kirk
Elite Marine, Inc.

[email protected]

I've been racing a Top Gun with 900's and #5s for three years with no problems. Anyone who knows my team knows we run HARD. As Michael1 stated, torque is proportional to displacement, not stroke alone. We run 581s with a 8-71 blower pushing 5 lbs. The #5s are fine with the power.

Isn't that going way too far out of square??? Seems so to me, just a thought....I've allways built as close to square as possible ie ,4.5 bore to 4.5 stroke or as close as possible.

There is nothing that says you have to build an engine square. The only reason it is done with marine engines is they want to keep the rpm down to about 5000 tops anyway. If you don't mind running a higher rpm, for a given displacement, you'll get more power out of an oversquare engine, because the large bore will allow for bigger valves and lower inertial loads, for higher rpm running.

Michael

Yes the stroke=torque isn't pure reality. Stroke yields more cubes so more torque but two engines with same cubes will have similar torque curves even if one is big stroke and one is big bore. Read June 2005 Hot Rod. Great test of this exact theory.

I have always been told a square motor will last longer under abuse. Over square puts pressure on cylinder walls. Its funny people believe they need huge cubic inch to create power. A blown 540 can see as much power as 600 ci blower motor on pump gas.

Its NO WAY a 535 will make 800HP on pump gas without a blower. Period