Just curious...

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

 

shifting...they started the dyno too low...didn't account for the shift

 

seeing how it had 400 ft-lbs at the low rpms and never went below that, I wouldnt be complaining too much

see how the graph on the torque scale was less than 100 and the hp was from 200-550. yeah, I wouldnt be complaining

 

But the HP stays... It drops slightly, but it's like that on every dyno graph on the LS2 head shoot out. I can't find my dyno sheet right now.. Oh, They (ESP) started my dyno pull @2400 rpm.

 

These are engine dyno pulls. No trans so no shifting.

The dip is pretty normal for engine dyno graphs. It's got to do with how the dyno ramps up the resistance to match the power output. When the graph is started the motor is at WOT partial load. Then the computer in the dyno cell over loads the motor so that is bogs and lets it increase rpm at a predetermined rate.

 

That would make sense. I can't find info in the article that tells when they applied the load. I'll get back when/if I find it.

On a side note... I wonder how a stoked LS1 would perform. -Comparison of TQ curve, not the dip but just a similar cam w/ the TFS heads. Seeing as I cannot bore the stock LS1... Ehh... Thinking out loud.

 

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!



My $.02

 

I'd like to get the TFS heads, but they're about the same price range as the AFR's.

With just headers and not a full exhaust system, the 'hole' probably might not be as bad w/ a full exhaust system with cats. A bit more back pressure might have an effect on that.

The cam wasn't too crazy. 232/232 .600 114. Maybe slightly less exhaust duration? 230 maybe? Another thing I noticed.. .. They didn't mention what valve springs they used. A good set of beehives can control valve float up to insayne RPM #'s. But the hP carried and leveled strong up to 7000rpm. But TQ fell at 5500. Normal for TQ not to carry high into the RPM band. I really didn't see too many signs of valve float. ie, drastic power dropoff.

 

the problem is all in communication of the data. if those graphs started at 0 on the y-axis, they would look perfectly normal. but you are looking at plots with no reference to 0 so it looks skewed. you're only seeing 80/480*100 = 17% of the total vertical data.

here is that data in a differently scaled plot:



doesn't look so severe now, does it?