Supercoupe w/ M90 : 12PSI = near double atmospheric pressure, why only 210HP?

DMX

Registered User
I know its not 100% efficient, but at nearly double the atmospheric pressure of 14.7PSI why are supercoupe engines making only about 50% more power than a n/a 3.8? 210 vs 140.

Same goes for torque, about 315ft/lbs when n/a is 215ft/lbs.


Isnt 210HP/315TQ what you should be seeing at about 7PSI boost? Think about it, @ 7PSI pressure is approx 1.5 times that of n/a.

Whats up with that?

edit : typo
 
im not totally positive so id need help on this one, but i think that supercoupes have a lower compression rate or something, good question though (again im not sure, id need some backing up on this one)
 
YES lower compression is one 8.2:1 instead of 9.0:1 and also the fact that the Eaton M90 Sucks 40-50 HP at around 5000 engine RPM (you can look at the chart on eatons website, it show how much HP it takes to spin it at certian RPMS). ALSO, the stock exhaust SUCKS on the SC but is decent on the N/A
 
Hey, Not only does a supercharger make heat, but it takes power to drive it. Relativley you can lose around 10% of hp for the energy it takes to drive a charger. It is like any other accessory that is belt driven. Take notice real serious racers remove power steering, a/c , and smog pumps because they are pulley driven. Others go to electric water pumps to lose another pulley driven acc. Also don't rule out the torque these superchargers help create.
 
Also, don't fall into the trap of "boost = power" I could put a potato in my exhaust pipe and get all kinds of boost, but I guarantee I wouldn't be making much power. There is no simple linear relationship between n/a horsepower and power created under boost at x percent above atmospheric. The most important aspect you need to look at is air flow. You can make more power with a wide open exhaust and maybe only 7 lbs of boost than a stock exhaust and 12 lbs of boost.
 
Yeah, the exhaust on these cars suck. Some feel they were deliberately sabotaged in this respect.

But another thing to watch on power ratings is the RPM at which the rating occurs. SC's won't post big HP numbers no matter what because the peak occurs so low in the RPM range when compared to some other engines. Similar in concept to a diesel, godsawful torque but pathetic HP numbers.
 
The 3.8L that is supercharged is not a 140HP n/a 3.8L with a supercharger.

It's probably more like a 120HP 3.8L w/ the supercharger boosting it up to 210.

Plus, if you look at the era, a 70HP increase compared to it's n/a counterpart is pretty good. Compare that to some of the turbo mopars and such (turbo version against the n/a version).

And finally, if Ford hadn't of tried to pull the numbers under it's beloved Mustang via choked up exhaust, it would be breathing closer to 225HP stock.

Micah
 
That may be true. But call summit, jegs, or anyone else. The back pressure that is so prevalent in these t-birds and 5.0's is one reason that the torque is so high. You may get some top end horse power, but you lose torque, which is your low end grunt. Needless to say 3 in exhaust isn't for every engine. I had a 5.0 whicj was stock and the first thing I did was 1 5/8 in equal length headers, off-road h-pipe and very free flowing mufflers 2 1/2 in all teh way back from the h-pipe and while from 2500 rpm's it would roar and pull you to the seat, my low end suffered quite a bit. It isn't so bad on a five speed because you can rev up your engine, but sucks for automatics. So don't forget that bigger isn't always best, and that backpressure is where some of that beloved torque is from.
 
I know for a fact you can have too free of an exhaust system. I had it. It sucked. I had to be making 3psi to even get the car to feel remotely responsive.

And the top end was not overwhelming. It was definitely NOT worth it.

And for what it's worth....my boost did not drop much....only about 1 psi from a good catback system to pretty much open headers.

Micah

*EDIT* Now that I think about it. I believe Stan W dynoed his XR7 with the exhaust and then with just ported manifolds/downtubes. He picked up less than 5HP.
 
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