peaky vs flat curves how to make it flat?

[Alex_V]

(Disclaimer, never owned a turbo car, just have an L28et waiting to go into something)

 

After years of researching turbo setups, Ive seen many that look like a "pyramid" and other that looks like a trapezoid (sp?) but w/o the right side (a slope up, then mainly flat line the rest of the rpms).

 

So whats the difference? The only thing Ive seen (only been paying attention to that detail the last few dynos I've looked at) is that bigger turbos seem to be smoother like that. I don't see any reason why a smaller turbo wouldn't do the same, unless they are just restrictive. But I still don't see why some one couldn't mod around it.

 

So whats your guys experiance? Whats the widest power band you've gotten?

 

I need to dig up some good examples, Ill post them later.

 

~Alex

[JMortensen]

The bigger the cam, the more peaky the hp and torque curves will be, and the more hp you'll get (within reason). Stock cam is very small, has very flat wide torque curve, but is limited in ultimate power output. Still, people are getting pretty respectable numbers out of stock cammed engines. For my money if I were building a turbo, I'd get a larger turbo and a larger cam and rev it higher than the 5500 rpm or so that the stocker is good for.

[OlderThanMe]

I am looking to build a motor with a flat power curve so I can make use of the turbo. Let's roleplay:

Lets say you have an engine that has 1000 horsepower for 500rpm of the power range. 6500-7000rpm power range and boost hits really late. Will a car with 500 horsepower and a 4500 rpm long power range be faster or slower?

If you have a sweetly setup automatic tranny and drag race the car then the first setup would be fine. If you are road racing the car with a manual tranny the second motor would probably be faster around the track.

Just my $.02

[Guest Mike]

It makes sense to me that a car with a really wide torque curve... let's say fairly flat from 1-7K RPM will be faster than any car with a very "peaky" curve (huge peak from 6-7K RPM)... given similar peak HP/torque. I suppose if both cars had CV transmissions turning their respective motors at peak HP/torque things would equal out... but how many of us have CV transmissions?

[OlderThanMe]

but how many of us have CV transmissions?

Exactly my point.

[JMortensen]

Back to reality and the L series, the thing that I see that bugs me about a lot of turbo builds is when guys put a bigger turbo on but leave the stock cam. The bigger turbo can build more pressure at higher rpms, but the stock cam can't turn the higher rpms effectively. So the mismatched parts leave you with less hp than is EASILY obtainable. It's like putting triples on an engine with a stock cam. I have no doubt that the engine would make more power, but all of these parts work better, and the total power output is so much higher when they're matched to each other. There is a happy medium to be found for sure. But the L series is good for 7K rpm with no problem at all, and to run that stock cam is giving up usable rpm for no good reason in my opinion. Torque x rpm/5252 = hp, right? So I would think you'd really want those extra 1500 rpm...

[Guest Mike]

"Back to reality"... was I off topic? If so, I'm sorry and I won't post on this thread again.

[JMortensen]

Why so sensitive Mike? Geez... "Reality" being that there is no CV transmission for an L series...

[Guest Mike]

Me? Sensative? NEVER:icon54:

 

The CV scenario was just to make a point about peeky torque curves only being "equal" in one tiny RPM range. But, of course, everyone already knows that. It wasn't intended to be off-topic. It's just my slanted ADD way of explaining why when Mars turns green the cost of tea in China bakes watermelon cakes with firey torque:mrgreen:

[OlderThanMe]

http://forums.hybridz.org/showthread.php?t=111606

here is the sequential turbo thread I have been posting in. I have had this idea about sequential turbos since I saw it on a truck engine. There are still a bunch of bugs with this system idea that I am trying to work out. This should give some low end boost and then revert to the bigger turbo for higher boost in the upper RPM range. This should give the HP benefits of a big turbo but also give low end boost as well. It will have good power and have it for 5k+ rpms of the 7k RPM lmit.

[OlderThanMe]

Back to reality and the L series, the thing that I see that bugs me about a lot of turbo builds is when guys put a bigger turbo on but leave the stock cam. The bigger turbo can build more pressure at higher rpms, but the stock cam can't turn the higher rpms effectively. So the mismatched parts leave you with less hp than is EASILY obtainable. It's like putting triples on an engine with a stock cam. I have no doubt that the engine would make more power, but all of these parts work better, and the total power output is so much higher when they're matched to each other. There is a happy medium to be found for sure. But the L series is good for 7K rpm with no problem at all, and to run that stock cam is giving up usable rpm for no good reason in my opinion. Torque x rpm/5252 = hp, right? So I would think you'd really want those extra 1500 rpm...

 

Good point. Garrett is running like 26 psi and 460 hp with a stock head and cam on a 3.0 bottom end...He could probably get another 75-100 ponies with a mild turbo cam and some head work.

[Tony D]

But the L series is good for 7K rpm with no problem at all, and to run that stock cam is giving up usable rpm for no good reason in my opinion. Torque x rpm/5252 = hp, right? So I would think you'd really want those extra 1500 rpm...

 

This is what JeffP is experiencing right now. The engine has very driveable torque right off idle, and can pull foll boost at 3400rpm. With the cam now easily pulling strong to 7300+, he is finding that Horsepower Curve really[/] starts climbing at 5000 rpms, about where most "stock cammed highly boosted" engines are peaking horsepower wise.

 

The difference between 15 psi and 20 psi was far more dramatic than his previous build.

 

A good indication of what proper cam and RPM utilization can be illustrated on his prior setup, and the current build.

Previously he had full spool at 3K rpm on the small turbo, and with a larger turbo at 3500rpm. Running something like 415HP to the rear wheels at 23psi and peaking around 5800rpm. Stout...

 

Currently the car makes in the area of 523hp to the rear wheels at 15psi, with full boost available at 3400, and pulling the peak to more near 6900rpm. (Mustang -v- Dynojet conversions come into play to make apples and apples in this case...we noticed 15% when doing identical testing, so the dyno run below should read a peak of 525 on a dynojet, which is what the earlier runs on his page were completed with.)

 

Of course there was some head work involved on the last build, but the numbers were only nominally changed the largest difference was the cam profile, and a turbo to accomodate the higher flow dictated by the higher rev capability.

 

Torque is flat and impressive, he has a photo of one of the pulls on his webpage.

 

 

Would you classify that as "Peaky" or "Flat", just so we are all on the same page. This run cut short due to detonation, that's the dyno racking back and forth above 6300 rpms because he lifted and the BOV kicked in...

[OlderThanMe]

Boy that looks nice and flat!!! I have seen dyno charts that were very steep very late. It averaged right below 200hp! that is really cool!

160 ponies right at 2500... That is some pretty good power!

[ZR8ED]

 

Here is my last dyno run (last fall) I don't have gobs of hp, but it sure is nice and flat. It is very easy to drive, but it also doesn't feel very fast because the car pulls as hard at 3500 as it does at 5000. (butt dyno)

 

Stock cams,

t3/t4 hybrid turbo,

huge i/c

3" ss exhaust

2.5" i/c piping.

60mm tb

stock heads.

[Roostmonkey]

 

Mine is similar, very smooth delivery. This was on a Dynopac