HP Rating / Dyno


New Member
Aug 9, 2005
I have the 1969 351W 2V, that is the one with a HP rating of 250. Is that 250 at the rear wheels or the fly wheel? The original set up was a C-4 trans. and 3:00 gears. It now has a C-6 trans. and 3:73 gears. The questions is, does the C-6 hold back more HP to the rear wheels than a C-4? If so, about how much? I'm seeing poor number on the dyno, only 235 HP.
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If youre actually getting 325 at the wheels with a c6, youre doing pretty good. I think the drivetrain will typically soak up in the neighborhood of 20% of the power at the flywheel. The original numbers were flywheel, gross if I remember right, which doesnt take into account a number of factors that a real car has, but an engine in a dyno cell doesnt.
yup thats flywheel power. and i dont think they even put on all the accessories on the engine to make it drop down even more. my c code had a 200 hp rating. my dyno results with mods are in my sig. its actually really sad lol
Yeah 250 would be at the flywheel with no accessories with ideal conditions. Yes the C6 does rob more power than the C4. I am not sure exactly how much there is said to be a difference. In general expect about a 20% and sometimes as high as 30% loss in power for an automatic tranny. Looks like its time for you to change intakes and carb.
The drivetrain loss is about 20-22% with a C4.
I don't know about the C6, except that it soaks up quite a bit more power.
I have heard, don't know the accuracy, that if you change from C4 to C6, you will lose about 35 rwhp on the dyno. With no other changes of course.

Also have to look at weight...
100 lbs is equal to 10 rwhp or .10 sec in the quarter.
So once you start driving, you not only have the extra parasitic loss of the C6, but you have the equivalent of another 1 rwhp per 10 lbs of weight added by using the larger tranny.

250 fwhp with 20% parasitic loss, gives you 200 rwhp.
That said, I believe that Ford had a habit of underrating their engines in the late 60s-early 70s. Even the smaller V8s.
If they were going to get Uncle Sam and the insurance companies to believe the 428 SCJ only made 335 fwhp, then they couldn't rate the 351w 2v at 300 fwhp!