Pontiac 1973 Trans Am HP to the wheels on dyno to compare to improved 455 rebuild

[jp455]

Hi all...anyone happen to know what hp a standard 73 Trans Am would make at the wheels on a dyno?

Im asking because I just recently had a full engine rebuild with the following... Holley Sniper (A/F ratio is spot on), 6X4 heads, Edelbrock intake (Torquer I think, that I had added over 20 years ago so I dont recall), Comp roller cam (276/282 224/230 lift with 1.5 rocker .502/.510 110 L.S.), 1.6 roller rockers, electric water pump, flat top pistons, and stock Pontiac cast iron headers, and the power numbers are...well...pitiful. What would you guys estimate this combo should give?

So Im curious what a standard Trans Am is making on a dyno to compare. Mine needless to say Im thinking is way below a stock car...

[73ta]

I would guess 450 hp at the crank, 300 at the wheels for your combo. Stock 250 at the wheels
.

[Alponcho]

I believe the 250HP figure for stock/non SD '73 455 referred to HP at the crank (net HP), which considers losses due to standard accessories, air cleaner, exhaust, and emissions stuff. If so, HP to the rear wheels is lower than that because it takes into account losses due to the transmission, rear end, brake drag, etc. In stock form, I suspect HP to the rear wheels would be just north of 200, maybe 215 with a manual transmission. If those 6x4 heads are stock/unported, I would be surprised if you saw more than 225 HP at the rear wheels, and that's assuming the tune is spot on everywhere. With some light to moderate porting, maybe 300HP to the rear wheels. Those are my best guesses anyways. Can you share your results?

[yellow1098Greg]

Very interesting......what are your numbers? I'm planning a similar build with a 461 stroker....don't want to be disappointed...but I have had this kind of thing in the back on my mind...wonder what's holding you back

[Alponcho]

HP is a function of airflow. You can get a very rough ballpark idea of gross HP for a well sorted combo by multiplying airflow at your max lift by 2. I have a 462 in my GTO and going from heads (670) with minor porting that flowed 200cfm to 250cfm gave me a bit over 100hp with no other changes except carb tuning (stock q-jet). Although that additional 100hp doesn't all get to the rear wheels, the difference is quite noticeable when I push it.

[yellow1098Greg]

Makes sense...so your 462 gave 100 hp over stock essentially? 8-10k for 100 hp is what it looks like these days

[jp455]

Well mine ended up way below what I would have thought...peak at the wheels was 182hp and 255tq. Seriously? 182?? Disappointed is a bit of an understatement.

I can attach the dyno graph but not sure how to attach a pdf...

[jp455]

BTW...this was in a Mustang dyno. They’re notoriously under rating...but still, looks awfully low to me with this combination.

[Alponcho]

I was underwhelmed by my 462 until I had my heads ported. That really woke it up. That said, I think you should have a little more in that engine of yours, so maybe something isn't quite right. Do you have enough timing in it or did you have to pull some out to prevent detonation? You said flat top pistons and 6x4 heads, so maybe around 9.5 cr? If you're near or above that, your cam (I'm guessing XR276?) might not be the best choice.

[jp455]

That’s right...it’s the XR276 hydraulic roller. The timing is good, no detonation, A/F ratio is spot on with a Holley EFI, and the engine does run well. Not sure what the compression ratio is as these pistons have been in the engine a while before it got refreshed. Still...even if the cam isn’t ideal I’d certainly expect to see more than 182 at the wheels. Or am I just nuts?