niphilli

Hey all,
Finally got to take the car out for a good long drive this afternoon. For a change I actually had some sunshine and clear roads. Man I forget how much I love these cars sometimes.


Well getting to the point....I have always noticed how these cars 'plateau' in power above 6,000 RPM. They pull like a raped ape up to 6K rpms then noticeably level out. Finally got a chance to strap my car to the rollers a few weeks ago and got the following results:



The car reached 99% of its HP peak by 5,600 RPMs and pretty much flat-lines between 5600-6600. I have learned bunches of great technical info over the years off of these message boards and usually a 'plateau' like this is because of an intake or exhaust restriction. I think these cars have plenty in the Head and cam department from the factory to keep pulling to 63-6400 RPM.

For this pull there was no significant Knock retard, and the A/F stays pretty constant at 11.5:1 (rich, but not causing the 'plateau' I dont think).

I am just wondering if you guys with intake and exhaust mods see the same behavior in the HP curve...Reason I am asking is because it looks like these cars are significantly starved for air, or bloated (cant get air out) above 6K RPM. I have seen people dyno well over 370RWHP with stock manifolds and cats (internally stock '02+Z) so I assumed that was not the restriction.

Can anyone post up a graph of a stock car with an intake to compare? What do you guys think?

TIA,
-Nick

black_knight

iTrader: (1)

From what I hear this is the intake runner length. Since it is fixed and not variable, the power peak will pretty much always be there, excepting N20 or FI...

Or unless you get some kind of custom intake, but then you have to worry about heat soak...

I wonder what the runner length is on the lsx...

niphilli

Hey Knight, thanks for the input. I think the intake manifold is definately a restriction at some point, but I dont think it is causing the problem in this case. I think the intake manifold should be fine to 63-6400 RPM in this case.

I am thinking it is the intake box, or exhaust system that is causing the problems....I guess it could be due to the fact that it is soo rich (11.5:1), but I am not sure...

Just trying to find out where my money is best spent next. From what I have seen with the Zs headers are a waste of time and money if you are going to stay internally stock. Full bolt on cars (without headers) can go 380 RWHP where LTs usually chime in around 390-395 (not worth it to me). My plan is to go with an air box and a tune...maybe a ported TB while I am at it.

02RedHawk

iTrader: (17)

Where was the wideband A/F measured at - before or after the cats?

niphilli

Wideband Was measured after the cats at the tailpipe.

Slowhawk

iTrader: (12)

It's your Cam that is dictating the hp curve,actually that is a perfect curve for a daily driver/performance driver.Tuning will bring your HP to atleast 370rwhp.Headers could bring it past 390hp with a nice curve.
I just tuned an 03 Z06 yesterday that was running Kooks headers with Cats and the stock Titanium exhaust.Car sounded great,no interior noise at all.

jimmyblue

iTrader: (11)

If you look at the MAP reading (logger) as you run up,
the degree to which your intake tract is reducing output
can be seen there; if it has dropped a few tenths PSI
(redline)-(low-RPM WOT) you are seeing flow-
dependent pressure drop. 0.1PSI ~ 2-3HP, 2-3 lb-ft
as torque goes with cylinder pressure goes with input
charge.

With oversized intake components I see only -0.1PSI
though I don't spin as high as a tweaked-up Z06 'Vette.

Anyway, if you were to see -0.2PSI (drop) then I
would look to the intake (filter, bridge, MAF, TB)
while less drop would indicate better gains to be
found in the exhaust & cam.

niphilli

Hey Jimmy, thanks man. Can I log MAP readings with ATAP? I will take a look next time I make some logs.

Old SStroker

No flame intended, but that's exactly how the engine was designed to run. A hp curve that flattens out rather than drops like a rock indicates the engine is breathing well. If you look at the area under the torque curve you'll see it's very flat and above 300 from about 2900 to 6100, the area where it's meant to run. That's 85%+ of maximum torque throughout the useful operating range. About the only other auto engines that do this are some turbo engines (eg: Audi), and some DOHC engines with VVT.

You could probably free up a few high end ponies with a less restrictive catback exhaust, and a few with an intake. Remember that noise of both intake and exhaust is a concern for the OEM, as well as water, snow and debris ingestion. Don't expect huge gains for your bucks, however. If those are the only changes, expect the torque curve to fatten slightly with rpm and probably never drop below your stock baseline.

Obviously you can change the shape of the curve with a cam change, but there are other compromises.

If you look at the rpm drop from gear to gear you'll notice the LS6 stays in the fattest part of the torque curve. That, too is how it was designed. You probably have tried a 30 mph pull in 6th. I was impressed how smoothly the Z06 handles that. If you haven't, try it.

My $.02

niphilli

You are right Hawk, I have done some digging and it is indeed a characteristic of the camshaft.....I just expected the cam to peak quite a bit higher than it actually does....My '99 T/A peaked at like 5700 RPMS. I searched around and found a bunch of dyno graphs of Zs with LT headers and intake modifications and the curve looks EXACTLY the same just shifted up a few ponies....

The first time I rolled into this car at 50MPH in 6th I was amazed it pulls 3X as hard as my T/A in 6th.

I knew there was not anything wrong with the curve, and that it was a good characteristic to have a flatline instead of a 'peak', it just feels strange when winding out between 5800-6600...

Thanks again for the input guys

Old SStroker

That's the downside(?) to a hp curve that flattens out on top...you want to rev it forever.

If you have any ricer buddies, take them for a ride. Do the 50 punch in 6th. gear. Personally I like the 30 punch in 6th. Have them watch the tach. Lastly, take them thru the gears (or at least the first 4!)