engine rpms

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oldsman41

Senior Member
need some advice here. what is the sweet spot on rpms for a 255 merc flatty?motor is a stocker. od trans. the engine purrs along at 2000 rpm but i hear a lot of guys say its better to run them at 2400 or more. i know they make peak horse at 3600 so what do you guys run your flattys at? thanks
 
Depends a lot on the vehicle it's in. My truck is like pushing a barn door thru the air, need some rev's to do that. A 32 roadster weighs a lot less and has less aero load, so can spin a lot slower at cruise. I'm geared for 2300 at 55 with a stock 3-sp. If I go much lower, 2nd gear slow speeds become a problem. You can always kick it out of OD.
 
When dynoing these engines (stock). Max torque is around 1800-2200. Haven;t none many but JWL in his book flathead facts has made several 100 dyno rund on stock and modified engines. I have found that when running the L-100 cam the sweet spot is about 2400. This shows up when you floor in in Hi gear at around 2k
and accelerate. When the engine gets to 2400, things begin to happen very fast. Aldo best gas mileage is around 24oo in cruise.
 
thanks guys gee at 2400 sounds pretty busy under the hood but at least i wont be hurting anything when i get in it once in awhile. i still will probably run at the 2grand mark but it is good to know i wont hurt it much at 22 to 2400 when needed.
 
When you think about it, the stroke (therefore piston speed) isn't much different than a modern SBC; they have an ultra-light valve train (lighter than modern OHC) ; and in the big trucks they ran them north of 3,000 all day.
 
The sweet spot for the 255 with late stock cam is right at 2000 RPM. Reving higher produces less torque/efficiency. So, it will push the vehicle best at about 2000 RPM whether it is a Roadster or a Tank.
 
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