pullin hard

i wouldn't say myth, but un edumacated!!!!
i pulled up to a chevy gaser the other day on the highway, he had a 2 wd drive ext cab pickup, maybe 92 ,93, somewhere around that, anyway, he had 2 very large pipes sticking out the back, looked bad, not in a good way.
then he must have though, i will show this guy how loud my truck can be and go nowhere fast!!!!
this thing got 2 time louder, sounded like thundering soup cans, and the truck made no head way, didn't move, it took him i bet 45 secs to just get infront of my bumper with his front bumper.
been there done that, had gas trucks, put stuff to them , made no real dif, got worst gas mileage, because always on the gsa for the sound of course.
anyway, for the people that think louder is faster, well alittle skinny pedal, and 6 sec later, i had got rid of this bad sounding truck , for good!!!!

Big Bob

Brian, It was Ted Nannetty who posted that on TDR. I remember it very clearly.

Last spring at BD's 2004 Spring Dyno Days, there was a stock 04.5 with a 48RE that dynoed 251 HP, another stock 04, 555 with the 48RE that dynoed 245 or 247 and I dynoed 250 with Ghost Rider. I had a AirAid intake and a 51" Nelson 86131M muffler, the only mods at that time. Ghost Rider is an 04, 555 with the 48RE.

Not much of a difference.

John Rodriguez

I dyno'd at 278 stock on a DynoJet 278C, lost my kitty, 4" magnaflow muffler, put down 281... took the hose completely off the turbo, so it was straight sucking in air (which looks cool and is **** loud, btw) and put down 282.

Armapoc

The Physics behind this seems reasonable. Faster moving air has a lower pressure than slower moving air. Therefore the turbo will spool faster because of the differential pressure before and after the turbo. The smaller tube after the turbo forces the air to move faster and therefore you get lower pressure post-turbo. This allows better spoolup. I've seen a few people put 6 inch exhaust on and the truck will lose a great amount of power, almost 20 HP. 5 inch exhaust doesn't seem to affect it too much, but a 6 will definently have an effect. The 6 inch exhaust has a lot more backpressure directly after the turbo.

I really don't think an intake will make much of a difference either way, except maybe a slight increase.

I'm gonna say

Reality.

6 inch exhausts can kill power...

DSLRammin

This all makes sense now. With my truck stock and an EZ, it felt supper strong on the highway, it passed like my T/A did. I put the 5 inch on and it seemed like a dog and a terrible drone. know that i'm stacked, well that all together different. Once again, my 97 chev, supper strong, put duals on it and dog city.
I personally think it's all about the right amount of back pressure.

doug

no, you have a contradiction here and a mis-application of bernoulli's principle. The reason that faster moving air has lower pressure is because its total energy level must remain the same. The bernoulli principle is just another way of saying that you can exhange kinetic energy (velocity) for pressure energy without altering the total energy in the gas. Thats an important physics point leading up to my point, which is the following:

what spins the turbo is the difference in exhaust gas total energies on each side of the compressor wheel. We know (from above) that a simple velocity/pressure exhange (due to different pipe size) cannot possibly result in an energy change, because the very law that describes this pressure change requires that the energy level remain the same!

So in this situation we dont' care about the velocity/pressure relationship as influenced by pipe size. We care about backpressure caused by friction. air molecules banging up against steel

Thus it is the larger pipe, not the smaller pipe, that reduces the pressure energy on the exhaust side of the pinwheel. Bigger pipe = less friction = reduced pressure = reduced energy level = bigger energy differential across the turbo = better spool up.


no, the 6 inch pipe presents less friction and therfore less backpressure as seen by the turbine looking down the pipe.

my measurements reveal an explanation for the loss. Ted Jannety had it right, even though he didn't explicitly measure the effect he predicted. I did.

There is a special situation where an otherwise stock truck is outfitted with an upgraded exhaust system. What you have done in this situation is alter the relationship between boost pressure and drive pressure that the ECM is calibrated for. The same boost pressure now requires less drive pressure to acheive -- by about 1 psi in my tests. This situation occurs because our engines operate with high drive pressures -- about 10-15 psi greater than boost pressure in my tests.

Anyway, the bigger pipe means that the maximum boost pressure allowed by the ECM (before it de-fuels) now occurs with less fuel and less energy output from the engine -- because the engine does not have to work as hard to produce this boost level. the ECM sees critical boost levels, backs off, and produces less power to keep itself happy. Of course, this does not happen with a performance enhancing box, due to boost fooling.

As for 5" pipes on an otherwise stock truck but with an EZ, I can only suggest that the wastegate may be causing the doggy performance. The wastegate set point is now acheived with a lower drive pressure (lower engine output with less fuel). The turbo caps boost at a lower engine output level.

its all about drive pressure and the built-in limits of the ECM and the turbocharger wastegate.