Re: Looking for some DOHC horsepower
Posted by shinyribs on Apr 25, 2012; 5:06am
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/Looking-for-some-DOHC-horsepower-tp3928486p3937354.html
I was reading "that page" and came across this:
"The massive stroke on 900 explains why they blow up so easily. Stroke is WAY too long for a performance motor, Honda still was stuck in the '60s."
"Both 750 and 900 rod virtually the same on pin end, you are correct, the narrow spot right under pin boss is the problem. 900 breaks faster because of the huge stroke which is way out of line for a motor that runs that kind of rpm. Rod angle increases which makes it easier to break at high speeds. 750 has less load on it, with less stroke the piston speed in foot per minute drops off, piston speed is one of the first things one looks at when thinking about making big power. Once 4000 fpm is crossed you are asking for motor blowup. Long stroke really shoves that number up. That's why you always see high perf motors that are oversquare, meaning bore much bigger than stroke. Short stroke lowers piston speed and big bore allows for bigger valves and better breathing into cylinder at higher speeds. 750 is exactly square at 62 x 62 mm, 3897 fpm. 900 is undersquare at 64.5 x 69 mm., 4293 fpm (?!!), not the hot ticket for a revving motor that LASTS. Fine as a stocker, but press on it and it will crack."
Now I'm no bike mechanic,but concerning the motors we used to build for our drag cars...it sounds in line with the same reasoning we used to use. Anytime we ever built a stroker motor it was because we were trying to shove a heavy car down the track. Build a lot of torque and gear it high so it won't spit parts at the topend...and DO NOT miss a gear!A longer stroke causes the connecting rods to whip around like mad...after awhile they end up shooting out of the block and on the track if you over do it.LOL! I saw a 383 sbc(350 chevy with a 400 crank in it) puke its guts out in the waterbox one day. It is an odd sight to see a car still doing a burnout,still hearing the motor run, but watch rods,pins,pistons and coolant go flying out of the block all at the same time.Wish I woulda had a camera.
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/Looking-for-some-DOHC-horsepower-tp3928486p3937354.html
I was reading "that page" and came across this:
"The massive stroke on 900 explains why they blow up so easily. Stroke is WAY too long for a performance motor, Honda still was stuck in the '60s."
"Both 750 and 900 rod virtually the same on pin end, you are correct, the narrow spot right under pin boss is the problem. 900 breaks faster because of the huge stroke which is way out of line for a motor that runs that kind of rpm. Rod angle increases which makes it easier to break at high speeds. 750 has less load on it, with less stroke the piston speed in foot per minute drops off, piston speed is one of the first things one looks at when thinking about making big power. Once 4000 fpm is crossed you are asking for motor blowup. Long stroke really shoves that number up. That's why you always see high perf motors that are oversquare, meaning bore much bigger than stroke. Short stroke lowers piston speed and big bore allows for bigger valves and better breathing into cylinder at higher speeds. 750 is exactly square at 62 x 62 mm, 3897 fpm. 900 is undersquare at 64.5 x 69 mm., 4293 fpm (?!!), not the hot ticket for a revving motor that LASTS. Fine as a stocker, but press on it and it will crack."
Now I'm no bike mechanic,but concerning the motors we used to build for our drag cars...it sounds in line with the same reasoning we used to use. Anytime we ever built a stroker motor it was because we were trying to shove a heavy car down the track. Build a lot of torque and gear it high so it won't spit parts at the topend...and DO NOT miss a gear!A longer stroke causes the connecting rods to whip around like mad...after awhile they end up shooting out of the block and on the track if you over do it.LOL! I saw a 383 sbc(350 chevy with a 400 crank in it) puke its guts out in the waterbox one day. It is an odd sight to see a car still doing a burnout,still hearing the motor run, but watch rods,pins,pistons and coolant go flying out of the block all at the same time.Wish I woulda had a camera.
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