Revelation222

How does the cam in a turbocharged car affect the power output in relation to it's LSA, duration, and lift? I know the basics for what makes a good turbo cam,but im trying to find out what you look for in regards to moving HP and torque curves as well as max rpms in a given combo.

From what ive seen from most of the turbo kits on a stock 346ci the curves are a bit peaky and/or fall off a little early. Also the torque is much higher the the HP output. Now, while I love torque, im thinking more about street applications and the lack of traction. With high gears and a bigger turbo it would help, but i guess im looking for more control in how the power comes on.
Can your cam specs change these features? Also, does it help to have stiffer than normal springs for an given cam because of the added pressure of the boost?

Thanks for any help you guys can give.

Excal

I Know exactly what u mean

but um the Twin set-ups don't act quite that way..some also say the massive torque and lack of power is because of the log design..who knows.

I know turbo cams don't need as much lift and don't need to be as aggressive.

definetly high gears if u want any traction at all..

I suppose springs can only help but don't know if they're a must for mild applications.

MIGHTYMOUSE

iTrader: (59)

good points. lets figure this one out.. i too see twin kits graphs looking just like stock, only higher, while single kits seem to all be done by 5500 if they are lucky with the same cam and valvesprings.

on a boosted application you DO want to upgrade the springs if you are running a lot of boost, because you have to hold the valve shut against the extra air pressure (force)on the area of the valve

MIGHTYMOUSE

iTrader: (59)

i forgot to add, i believe what is popularly done to single kits is to oversize the exhaust turbine for a given hp application.

JordonMusser

iTrader: (1)

I think the reason why you see lay overs on the top end of single kits is poor design/poor turbo choice.

my LT1 pulls to 6500rpm or so, with inferior heads to yours.

Pro Stock John

iTrader: (34)

Revelation, what is that you want to do?

Peak hp/tq numbers for turbo setups have to do a lot with the turbo size and the size of the exhaust housing you use.

A cam can help build power and fatten up the curves. Might extend the peak up but if you had a T4 series single turbo I have seen data that showed that cam swaps won't really significantly change WHERE it peaks but how much it makes at PEAK.

Revelation222

Essentially, I am looking at a few kits that use the T-76 single. On larger CI engines the 76 seems to work fine as far as the curve and peaks are concerned. However, the same kits on stock inches are too peaky for my tastes.

Would the choice in turbo make that much of a difference? Common sense would tell me that the bigger the turbo the more possible power, abeit the slower spool time, but is there something in the different designs that can affect the curve directly? Maybe the newer turbos can make the curve different. Like for instance a smaller more effecient turbo.

I guess thats why I am looking for info on how the cam works in relation to the turbo. By using the t-76 equivalent or better, and trying to select a cam that will broaden the curves, even out the HP & TQ, and let it peak higher. Maybe Harlan or Rob can chime in and help us out on this one.

Earl H

iTrader: (1)

I have a Single Turbo on an '03Z06. It makes 580/580. It pulls cleanly to 6000rpm and my torque curve is more like a table top. Over 500 ft-lbs over a 2500 rpm range. Not peaky at all.

I have to say that the tune can make a difference. Before my last tune, my torque curve was a little peaky. After my tune at DSW, the car behaves much better.

Revelation222

What are the specs on your cam? What did the dyno charts look like before or after the cam if you happend to dyno between the instal? What kit are you using? Maybe those numbers will help place a flag on what does what in different turbo applications.

Earl H

iTrader: (1)

I am currently running the MTI B1 cam. Specs: 114 LSA, 221\221 duration, .558/.558 lift. I haven't run any other cam with my current turbo setup. I will be running Cam Motion Reverse Split spec on my turbo'd 422. I'll let you guys know how it turns out.

MYTURBOT/A

I read in maximum boost that a higher A/R turbine will produce higher max HP. On another note look at the new redline motorsports turbo kit, that dyno graph looks better then some N/A graphs i`ve seen.

Pro Stock John

iTrader: (34)

I had a very knowledgeable friend walk me thru a lot of Turbo 101 fundamentals.

The bigger the exhaust housing the more horsepower you will make, but conversely the turbo will spool up slower. The difference between different turbo housings can equate to a difference of 50rwhp on the dyno.

When you see some dynos where the tq is monstrous and the hp is no where near the torque, that can point to restriction with either the exhaust housing, the down pipe or BOTH. With a T4 series turbo you can probably go up or down 50-75rwhp by changing exhaust housings.

With respect to camshafts, I wish I could point to a person or an online article that really detailed how to pick a camshaft for a turbo setup. From what I have seen a lot of overlap at .050 is probably a bad thing. Can I personally say that zero overlap is ideal for a turbo? Based on what I have seen little to no overlap is the better way to go. I think that overlap might be a lot more important than LSA when spec'ing a camshaft but I only have three datapoints for that.

To much overlap in general for FI might be a step in the wrong direction because of how inefficient the motor might get with all of the charge blowing out the exhaust.

Warbird

Most kits that fall off at mid-RPM have poor design elements built in. I had this explained in detail to me on exactly why this happens. Too much back pressure in the exhaust can begin to stall the turbo as pressure and RPM increase, the exhaust begins to back up into the engine and the turbo begins to fight against its own pressure slowing down the turbine. The symptoms of this problem can be seen and heard in dropping boost, boost spikes, overheating and harmonic resonance in the engine. You can only throw a bigger turbo at the problem for so long. Based on my conversations with Garret for instance, a properly designed kit with a T68 single would be well into the efficiency range for our motors and should make big power. The problem is that a turbo is only as good as the plumbing feeding it. In my case my bigger displacement engine manifested the issue much faster as my exhaust velocity and pressure are much higher than a stock displacement motor.

A log design turbo has severe limits built in because of its inability to move exhaust, most notable on a single turbo. Look at it this way if you have a single 3" log style manifold running into the turbo, it could remain efficient up to say 5000 rpm (stock displacement engine) after that all 8 cylinders begin pumping far too much exhaust into the manifold and backpressure builds, that exhaust has nowhere to go so it will find an exit, either back into the engine or splitting the manifold. You can see it on the dyno when HP begins to fall of early.

A better exhaust setup for a single turbo would employ either full dual headers to allow more volume or a log with longer runners that face forward. We shall see if the proof is in the proverbial pudding with my new set up in the spring. My car will retain the exact same engine combo and turbo with a new exhaust setup and I will overlay the dyno graphs on the old setup. It should be a great comparison since nothing else will change except the efficiency.

Revelation222

Sweet, Warbird I'll be looking for that post from you. It might help us with those issues by looking at the comparison between your set ups. Your logic seems spot on for the cause of the problems. I hope that someone can chime in on how we can slightly relieve some of these problems with our camshaft selection. The Turbo setup even if poorly designed sould leave room for improvment given other variables.

It seems as if the cam you choose could play a deciding factor between an "OK" turbo set up that is too peaky, fall off early, etc., and a "Good" turbo set up with which most of us could live with. Yes, I wish there was a better designed kit out there for our cars, but I'm just trying to work with what I see as viable options for a "daily driver". Hell, I wish I could copy Harlans set up, but I gotta drive my car regularly in the California heat, so it narrows down my choices for Turbo set ups at the moment.

MYTRUBOT/A---I have been following thr Redline set up for some time now. It does look as if it has major potential. Im basically waiting to see if they can get the A/C back on without any issues. It also has the same problem as many of the kits by peaking too soon and dropping off too early. I'm hoping springs and possibly a cam choice could remedy this.

Thanks for all your input guys, lets hope we can get some more info on this issue. I figured for sure Rob or Harlan would have made a comment or two by now. Maybe cmotorsports will read this a give us some info seeing as they make custom turbo cams.

Pro Stock John

iTrader: (34)

What do you mean by this?

I've been looking into how turbo systems work and from the sounds of things, we need to define some terms or something....

Small= T3, smaller, less efficient, 3 bolt exhaust housings
Big= T4,bigger, more efficient 4 bolt exhaust housings
Huge= Mid-frame, aka thumpers

A T76 would be a T4 series turbo. Since most of the folks on here want big power most folks will want to focus on bigger turbos.

Power peak is dictated by motor specs.. turbo size, turbo exhaust housing size, exhaust size (downpipe etc).

I have seen big singles (T4's) peak at no higher than 6100. Anyone have an LS1 with a big single that peaks higher than that?

The fact is that a different cam might increase power and torque, AND AVERAGE power and torque by quite a bit. Given the duration and LSA of our stock cams, I am guessing that there is very little overlap in a stock cam?

Revelation222

Ok, no problem, lets look at the t-76 on stock cubes for our discussion. Let's try to discuss how to change its power curve and peak by using different components such as a camshaft or other variables