ZO6 hp claims underrated?
 

As with many performance GM engines over the years, I've heard rumors that my '03 Z might have closer to 430 hp as opposed to 405 hp? Any truth to this rumor?

Cheers.

Not likely, I believe that 2002 ZO6s run about 350-360 HP at rear wheels on dynos.

Don't know, but they did under estimate the LT4 engine, so I guess it's possible.

Nope. 405bhp w/12% drivetrain loss is about 357-360rwhp.

The crank horsepower numbers have been underrated, but not by much.
Normally 02-04 Z06 Corvettes dyno in the 347-360 rwhp range (stock).
For example: My stock 03' Z06 dynoed at 356 rwhp stock.

There are always exceptions though.

that must be fairly accurate, my FRC dyno'd at 307. which is right at 12% loss.

Years ago, they sometimes overrated engine horsepower (actually it was due to the method of rating horsepower, called gross horsepower). For example the 1967 427 cu-in 435 horsepower was rated without accessories (alternator, power steering pump, water pump, etc.) and exhaust.

Our 345-350 HP Coupes and Convertibles would rate about 415 HP using this system. Which would explain why in conjunction with better tires our C5s have slightly better performance than these monsters.

However, they did underrate some engines, the 1969 427 cu-in 430 HP L88 "racing engine" was actually about 500-520 hp (using 103 octane gas).

Every car can be a little different on the actual hp numbers that are achieved out of the car, but I doubt there would be that much of a difference. Only way to tell is get the car dyno'd and then work back to the crank HP.

Remember the DZ302...

Chevrolet produced a special 302 cu in (4.9 L) (referred to as 5.0 L) engine for Trans Am racing from 1967–1969. It was the product of placing the 3-inch stroke crankshaft into a 4-inch bore block. The 302 used the same crank as the 283.[5] When the journal size increased to the standard medium journal size, the crankshaft for the 302 was specially built of forged steel.[5] This engine was used only in the first-generation Camaro Z28. Conservatively rated at 290 hp (216 kW), actual output was around 472 hp (352 kW). This block is one of 3 displacements that underwent a transformation for the 1968/1969 period when the main bearing size was increased from 2.30 in to 2.45 in.

472 HP in Traco prepared race trim maybe. Sorry but the 302 as purchased was no wheres near the 472 HP from the factory.

I have a chevy 302 sitting in a firebird, I blew the head gaskets and its been sitting.
Its a fun high revving engine.

I think the only BS that GM slung with the LS engine was with respect to the F-Bodys. They chassis dyno the same as a C5, yet were rated less.

358rwhp when my Z06 was stock.
I think GM was on target with the 405 rating.

I have seen in person a 370rwhp dyno on a stock 2002 Z06 though. I think its rare, but it happens to see one above the average.

Toque

Depends on the parameters set within the dyno. All dyno's don't read the same, so that's more responsible for the variation than anything.