bayer-z28

iTrader: (14)

I've been doing a lot of head research. I noticed in one of the latest issues of GMHTP that it seems all the LS2 Dyno graphs had a TQ dropoff at low RPM's.

What causes this?

I got these graphs here... http://www.gmhightechperformance.com.../photo_25.html

67 Rally Sport

I am a little curious about this myself. My guess is that it has something to do with the tune, but it's only a guess.

Greg

66deuce

iTrader: (1)

i think it's how the dyno loads the motor.....

Old SStroker

This is probably not the reason. The engine is loaded fully slightly below the lowest recorded rpm (2700 in this case).

The 3000-4000 "hole" may be caused by the intake and perhaps exhaust tuning lengths as well as valve timing. You could "fill" in this hole to get more average torque/power but you might sacrifice a little peak hp. I guess it would depend on what your goals were. My guess is that most magazine tests look for the biggest peak numbers.

Note that the 30 lb-ft max "hole" was different for the different heads, but was still about the same rpm. Same intake, exhaust and cam if I recall the test.

In the 2006 Engine Masters Challenge, which is scored by (average torque + average hp) from 2500-6500, the winner was as much as 37 peak hp and only a few peak lb-ft down on the runner-up, but the averages were better. The runner-up had a big "hole" in the lower rpm torque curve.

If you replot the torque curve with the y-axis (torque) compressed to what you normally see on a Dynojet graph, you might not see so much of the "hole". Horsepower is calculated from torque and rpm, so the hp curve also dips, but less radically because the rpm is below 5252 where torque and hp numbers are the same. Note that little torque blips have a bigger effect on hp above 6000. Both engines had about 83-85% of peak torque from 2700 to 7000, which would make a killer street driver!

FWIW, this is the same reply to the same thread on another forum, which shall remain nameless.


My $.02

bayer-z28

iTrader: (14)

Ah... GOtcha... I figured I'd see if responses would vary, this board is a little more active then the other one.

Patrick G

iTrader: (14)

That dropoff is very common with drive-by-wire LS1s and LS2s. There is a tuning fix. There's an entire thread about this in the dyno section from a month or two ago. Slowhawk knows how to fix the dip.

66deuce

iTrader: (1)

learn something every day.....

509TURBOFIREHAWK

Alot of times when ive seen this is with very powerfull racing heads, SB2.2's, splayed valve heads and so on, I would just about consider all LSX heads as production racing heads. I dont know why it happens. If you look at some of old school 23* stuff you dont see it as much. And alot of times you wont see it on an all out racing head as they dont test at that low of an rpm. just my .02 cents

Billiumss

iTrader: (11)

We never did get a final answer on this:

https://ls1tech.com/forums/dynamometer-results-comparisons/576635-most-c5-dyno-graphs-have.html