Expert: Dan Liddy - 12/26/2009

Question
hello,
i race a vintage british car, which is governed by an organisation which requires i stay with its original displacement. i recently broke its crankshaft, adn am in the process of drawing a new one, and it occured to me this may be the time to change the bore and stroke. my goal is to allow the engine to rev higher, and to reduce the stroke and increase the bore. i sonic tested the block, and it has enough material to go large enough with sleeves, to allow me to reduce the stroke to 3 inches, from 3.5. my questions is: is there an inherently better bore to stroke ratio to produce power? my current figures are long stroke and small bore.(taxable horsepower)
thanx,
charlie  

Answer
Hi Charlie:
"Square" is a term that relates to bore & stroke, with the big Chevy's they frequently build them with a 4 X 4 bore and stroke.  This gives both good torque and good rev-making. If you want something that spins up really quickly, "over square" is what you're after , that would be larger bore than stroke. However- - - -

I'm going to assume you will be road racing, almost all vintage associations do.  In that case I would go back to the "square" arrangement where bore and stroke are equal. This should give you an adequate amount of torque to get you out of the corners, and enough horsepower to get you down the straights.  If that's not possible, err on the side of a larger bore.

With a square arrangement, balance also figures in. Square motors are able to tolerate more revs, hence you can spin them a bit higher.  If it's overhead cam without a lot of silly hardware, you can safely spin it to 9000 RPM.  If it's a pushrod motor , only the truly venturesome would carry it beyond 7000.  Your cam grind will ultimately govern the rev limit anyway.  
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By the way, no matter what you do. switch to synthetic oils.  All the pros use them.  I personally use Amzoil, but Red Line and Royal Purple are good too, Avoid Mobil One.  

Hope this is a help, write back with other questions, and results.

Thanks for a challenging question!

Dan Liddy
Sarastota, Florida