[tkinion]

So I'm watching Overhaulin on TV when they remake Lance Armstrong's GTO. They put the car on a dyno and it put out a mere 175 horses (give or take). It looked like the factory 400 in it, but I can't say for sure. If so, can somebody explain to me how a motor can go from 350hp factory to 175hp on down the road, especially if the motor seems to run smooth? Second, does a rebuild restore the horsepower to full potential? :confused

 

[freeyayo]

hmm. good question, my guess would be that the engine would return to its potential after restoring.. but i'll ask a few of my friends.

 

[69GTO]

You gotta remember that the 400 was rated at 350 gross HP. Thats with all of the accesories like alternator, WP, PS, etc disconnected. It is really about 290-300 NET HP. Then you figure in a 20% + HP loss to the drivetrain, and your at 230-240 HP to the wheels........175 sounds to be about 50 or 75 HP shy of what it should be running. Maybe due to poor tuning?

And YES, a full rebuild using OEM quality or better parts, would restore the engine to its original HP (assuming its tuned properly).

 

[II-Savy]

Guys an old tired engine, with all worn parts, decades old will TOTALY put down lower numbers....every time and twice on Sundays. After the engine is restored and all the clearences are perfect, the motor should put out what it did way back then.

 

[freeyayo]

Then you figure in a 20% + HP loss to the drivetrain, and your at 230-240 HP to the wheels

but wasn't the Horsepower back then already calculated from the rear wheels and not from the engine ?

 

[lars]

but wasn't the Horsepower back then already calculated from the rear wheels and not from the engine ?

No. It was gross horsepower at the flywheel. Gross at the flywheel means that the engine was run on an engine dyno - not chassis dyno - with tubular headers, a bell-mouthed carb inlet with no air cleaner, no accessories, and the engine was run at its "blueprint" specifications. Production engines did not meet those same specs, and never achieved the 350 gross rating. (For example, the actual cylinder head cc volume of the GTO heads put the real compression ratio in production form at about 9.75:1. Advertised comp ratio was 10.75:1. The only way to get the comp ratio up to the advertised ratio was to run the minimum allowable chamber volume, which is what Royal Pontiac did with the Royal GTOs: They set the engine up to it's "blueprint" specs the way the factory did when they tested the engine at 350. Only this way could they get the engines to put out the factory rating). Other screwey things Pontiac did with the ratings is that the standard 400 was rated at 350 horse for both auto and manual cars. The manual cars had the "068" cam in them, but the auto cars had the little "067." Go figure how the two could possibly put out the same hp numbers when the difference between the two cams alone is about 10-20 horse...

The drop in horsepower from Gross to Net at the flywheel is about 20% (install cast iron manifolds, air cleaner, and all accessories). The driveline loss in the chassis (with mufflers and transmission) to the rear wheels drops horsepower about another 20%. If you figure that production engines put out an actual gross rating of about 300, you can get about 240 net at the flywheel. Then drop that another 20% for driveline loss and you get 192 at the rear wheels if everything is new, fresh, and the way the factory built it. If they got 175 on OverHaulin' , they're doing pretty good... sounds like the car runs about like a normal stock 400 GTO.

You can use some basic race math to figure this out and verify it. Most bone stock 400 "350 horse" GTOs run about a mid-15-second quarter mile if the car is tuned right (and you see a lot of them run quite a bit slower...). A 3700-pound car running a mid-15-second quarter is putting out 190 horsepower to the rear wheels. And that's exactly the same number as we arrived at in the discussion above....

A 3700-pound car putting out 350 to the rear wheels will run about a 12.8.