Here's an interesting question...I never thought I'd ask it cuz I thought I had my facts straight, but a fellow at a local shop mentioned something today that I thought sounded odd. So here I am bringing it to the experts (that's you guys, in case you didn't know...)

If you are using a supercharger on an LS1 (specifically an 02), can you expect power gains or power loss from the install of long-tube headers?

The guy at the shop said that you should use the stock exhaust manifolds to maintain more backpressure, which would improve the output of a s/c motor. He said using headers will decrease backpressure and actually cause a loss in power.

???

As I had understood it, the supercharger and the exhaust backpressure have nothing to do with one another. Why would adding headers decrease the overall power output? Seems to me that the supercharger's job is done at the end of the intake stroke, and the headers just help to make a more efficient pump by more quickly evacuating the spent gases. If anything, I would think that headers should improve the torque that was not gained by installing an s/c (cuz s/c's improve HP more than torque, typically).

Thoughts?

Thanks!
-Smokey

 

Both cases are probably true....just depends on how much boost you are putting out.

 

Smokey Tirez,
A way that you can think of the situation is to think in terms of absolute pressure.
In your manifold is say 31 " of mercury at idle, that would be vacuum or no pressure, i.e. approx 0 psi. As you open the throttle you let air pressure in and the pressure goes up to say 14.7 psi, or approx 15 psi.
Now if you add a blower, the pressure will increase to 22 psi (at 7 psi boost).
So what's happening in a blown engine is inlet pressure is going from 0 to 22 psi.
The cylinders fire the denser charge (@ 22 psi) and make a denser exhaust flow-probably higher pressure and temperature.
The phenomenon of the reflections of the pressure waves that result in superior cylinder scavenging is basically the same but slightly modified by the now different density of the exhaust gas.
As the basic situation is still the same you would expect that LT headers would be effective at 7 psi boost (22 psi absolute), and say 12 psi boost (27 psi absolute), as they would be at 0 psi boost i.e. (full throttle NA or 15 psi absolute).
So your thinking is correct that LT headers would be just as effective.
With regards to "back pressure" now possibly less with LT headers, that will fall out in the process of proper tuning after the blower/header installation.
Roy

 

Excellent post Roy. I'm usually the one getting called out as a data/tech nerd, but that one takes the cake. That's the kind of detail I like to see man!

So let me ask you this, would the dynamics change if I'm using a roots blower vs a centrifugal one? Ie., having a Magna spin right into the top of the motor vs. having the forced air come through the TB and routed through the intake? That's the only missing variable I can think of that would make this guy infer that headers would cause a loss of power in the overall combination.

But from your description it would seem that improved scavenging would occur with headers regardless of the type of s/c. A pump is a pump. If you can't evacuate spent gases from the chamber then you're robbing your intake stroke's efficiency.

So let's take this from a more direct angle: Given proper tuning, is this guy just wrong, or is maybe approaching the combination from a different school of thought (one that I am unaware of).

And here's yet another question: Do you feel that with the proper tuning that the same net hp gains could be achieved without the use of headers? Ie., is it all about the tuning? Since physics is involved I would assume not. I would assume that you may be able to tune around the combination for optimum hp, but you would never realize the true potential for the torque output unless you were using headers.

One final question, perhaps this guy meant that there was a loss of overall hp but an increase in torque?

I'd also like to see a reply in this thread from folks who have dyno tests on a s/c motor with and without headers. That will likely be tough since most folks will do everything at one time to save costs. But has anyone experienced a loss in power from their combination once they upgraded their s/c'd motor to headers?

Thanks!
-Smokey

 

The mechanic is wrong.
The incremental increase in power component from the headers will be roughly proportional to the increase fron the power adder, whether roots/centrifugal(belt or exhaust driven). Whether power is accentuated at high rpm (more power) or lower rpm (torque) is a function of exhaust primary tube length/diameter primarily.
Do all the proven things to get the most 02 through the machine and then tune it-you will have the most HP.
Roy

 

I'll be he's confused by the loss in boost on a gauge. If you are pulling 7.5lbs of boost pre-headers, putting in longtubes might reduce that to 7lbs of boost being displayed. While you "think" that means a loss in power, it really isn't. You are still pushing the same, higher volume of air there is just less "push back" up where the MAF sensor is because there is less backpressure.

Besides, headers help bleed heat out of the cylinder...means you can tune better. Horsepower rules!

 

That's what I thought. Glad to see I'm not crazy.

Thanks folks!

-Smokey

 

Exhaust combined with a cam with too much overlap (both valves open at the same time) can reduce power. The intake has pressurized air/fuel, a good exhaust should have some vacuum. If both valves are open for a short period, you leaked some of the boost out with the exhaust and you'll have less power.

That means less air/fuel stuffed into the cylinder and a lower power level.

Come to think of it, I should find a higher lift lower overlap cam myself.

 

Exactly As said,your gauge might read less boost but you will have more RWHP.

Here is something I did years ago on my Mustang.Stock motor with D1 pushing 16lbs of boost made 460hp.Put in a lower compression H/C 306 motor with the same pulley and hit 10lbs of boost but made 540hp

 

So I spoke with these guys again and asked them about the headers and boost issue. The answer was basically what was said here, that if you have headers you will read less boost. They said though that it was "a wash", meaning that in the end you have the same power output but are just making it with less boost when you use headers.

Finally, the guy at this shop said that the way you would recover the power you expected to get was to use a smaller diameter pulley.

I guess that makes sense to me. But I know #$#@ about it cuz I never played with FI before. What do you guys think about these recent comments?

-Smokey

 

Think of an exhaust restriction as fighting AGAINST the piston when it's trying to push the exhaust gasses out of the cylinder.

Lets look at this another way... using their (flawed) argument, if you stuck a potato into the exhaust, it should be a wash too, since backpressure will go up, but so will boost.

Bottom line, let those mechanics fix your car, but don't listen to them regarding the peformance of your car.

 

I hear ya. It all sounds odd to me.

Thing is, this is coming from a VERY reputable shop in my area (I'm not naming names). And by reputable I mean reputable on this forum. So that's why I wanted to check with you guys.

Thanks,
-Smokey

 

If you install headers,you WILL NOT make the same power.You will make more With the same blower pulley.

 

Oh I beg to differ.

The facts are if you put headers on a positive displacement blown engine such as a Magnacharger, you will lose power. Bin there, done that.

To recover the lost boost you will have to turn the blower faster. It is the amount of Mass Air that remains in the chamber for combustion that counts for power. Headers very purpose is to reduce backpressure, which allows significant air through the valve over lap event; this is a loss of Mass Air for combustion.

Everyone here agrees that flow is improved at the exhaust port, what is not understood is that a roots blower dose not care about pressure behind the blower, it pass a fixed amount of Mass Air Flow relative to RPM. Therefore given that the roots blower fixes Mass Air Flow and the Headers increase flow out, do the math and there is less Mass Air in the combustion chamber, this is just fundamental. The real world experience confirms this.

But then again I could be wrong.

I sometimes think that we rationalize our expectations with wishful thinking and ignore reality. This is profitable for the installers but leads to disappointment. Without first hand knowledge I would trust good people like Lingenfelter who’s sole purpose is to make you happy than use information from this forum.

 

I could very well be wrong since I HAVE NOT done a Header swap on a Magnacharger car but have always gained power on Procharged cars by installing headers

 

Ahh..now this is getting interesting. Our first dissenting opinion on the subject.

What you are saying auctiondepot is exactly what I'm hearing from this shop. So it will be interesting for me to hear this discussion go further.

I've always been part of the camp that believes headers improve flow no matter what. Since an engine is essentially a pump you get more power by getting more mix in and then doing what you can to get that mix back out. But superchargers are foreign to me because I've never used them and only read about them and their benefits.

To be honest I'd love to hear the "final word" (aka "Truth" with the capital "T", which as we all know is elusivve). I'd love for this final word to be that headers aren't necessary on such a setup because that means I have more money for other mods.

Keep it rolling
-Smokey