Lower HP numbers

So I have a 09 GT500 it has all the usual Bolton’s with a TVS Gen2R and a new shortblock from ModMax Racing. So last year on the old motor it put down 710. This year on the new motor it only put down 670 I was told it was having belt slippage so I added a American Racing Solutions belt tensioner,they put in back on the dyno and couldn’t get any more out of it safely. What could it be? Is it possible the extra power lost will be regained again when the new motor is broke in? Only 700 miles on it so far. Any opinions? I’m actually fine with the power since that is more than enough for the street but I am curious to know why the change? Thanks!


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Same tune and same dyno using the same correction factor?

Did the compression ratio of the new motor change? If it dropped a little that could case the 40whp drop.

Could also be a degree or two less timing and std vs sae numbers or uncorrected vs corrected numbers.

It was 8:9:1 for the new motor and yes same tune and dyno


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Did you happen to change rear gears when you redid the motor? Lower gears will sap your dyno numbers a bit...

I feel like I see about 5-10rwhp from when a GT500 is new to when it has 50-80k+ mi, but that's based on dynoing a ton of cars over the years and feeling like the higher mileage ones make more. I don't think break-in is your issue. I would look carefully at dyno conditions, even when corrected, if the atmospheric data is bad or out of the ordinary, you can have swings in power. That's someone of a stretch though, but we're kind of grasping here as it should make the same power unless ring seal is really bad or compression is very different. If ring seal was poor then you would see smoke out the tail pipe or excessive blow-by.

I don’t think it’s a major difference for compression from both motors, only thing I can think of is the tuner pushed the timing a little more on the last motor when it went and went more conservative on the new engine


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Really can’t answer until you know the datalog specifics, most importantly boost and timing.
If it’s truly same tune, same boost but down 40rwhp I would be concerned.
I’d still bet on what you said, more conservative tune on a new motor.
Rings are set by now, let it eat.
-J