loch

I installed a 4 inch on a friends 94, ours would be close. He wanted a baseline so he dynoed before and after, with no other mods the 4 inch gave him 19 hp. Now thats not a lot but on a truck has 160 hp or so like ours its not bad and it helps with egt. And if you allow the gas to esxape faster a by product should be faster spool up.

90firstgen

IMO, the 3" straight pipe sounds great! I loved mine. I just love my 4" more I would use a piece of flex pipe for connecting your offset pipe together. Relatively cheap and any truck parts store should have it. I know my truck not only ran better after I cut that darn muffler out, but it also ran a lot cooler down the highway. The temp guage didn't climb so high

Big Rich

I saw a big difference in EGT's when I went to the 4" (complete)...

At least 300* difference when running flat out. I tow alot, and now don't worry like I did before.

Rich

90firstgen

Yes, Rich is on the ball.... WOT EGT's go down a lot!

johnny5.9

Something we have not discussed yet is the effect of cam profile on exhaust scavenging. Exhaust scavenging is caused when the exhaust pulse exiting the combustion chamber accelerates the incoming air charge. This is only possible with a valve overlap event. This scavenging makes it possible to build much higher intake port velocity. If enough velocity is developed it's possible to build higher velocity through the piston actuated induction event too. This increased velocity then drives the filling of the cylinder above atmospheric before the valve shuts. Basically the engine can breath like it's bigger than it actually is. Us cummins drivers don't need to be bothered with this. First we have turbochargers, turbocharging already accelerates the incoming air making scavenging less important. Second 99% of us don't run a cam aggresive enough to accomplish this. Third diesels don't spin fast enough for any real positive effects from scavenging to be realised. The bottom line? As little exhaust restiction as possible to allow greater turbine speeds and as little intake restiction as possible to reduce pumping losses. The best article I've found(although not diesel specific) is 5/05 Popular Hot Rodding

flashgordon

Thanks Johnny, appreciated your in put!!!

BC847

My two cents . . . .

I've always understood exhaust scavenging as a function similar to tuned intake stacks whereby one takes advantage of the pulse-waves.

In the exhaust setting ~

As mentioned earlier, when the exhaust valve opens, there's a pulse-wave that emerges. With a specific sized primary exhaust pipe (ID and length) combined with the timing presented at a given engine RPM, said pulse-wave . . . . or more specifically the low pressure area immediately behind the pulse, can be presented to the other primary pipes at the collector. Said low pressure area drops the pressure (pulls) on the exhaust side of the engine.
That way, when an exhaust valve opens, there's a relatively very low pressure, if not vacuum for the cylinder exhaust to flow to. It's pulse furthers the process.
This increase in flow can be tuned to a specific RPM as needed.
Such is the difference between true extractor type equal-length headers versus the cosmetic headers


In our case it's whacked with the turbo. Those who've toyed with twins fast become aware of drive pressures. That being the pressure that spins the turbine (proper). Ideally, one would have it at a 1/1 ratio. Those that run the smaller exhaust housings run with a higher drive pressure relative to those with a larger (the larger housing offers less resistance to flow compared).

Perhaps there is some scavenging effect during the valve overlap event, but I'd think it would be rather small compared to a naturally aspirated engine.


IMHO, what we can do is increase the pressure difference across the turbine. That being accomplished by reducing any resistance to flow on the outlet of the turbine.

A larger DP is better. Perhaps not as profound in the OEM stock tune as compared to otherwise.

whereswaldo250

[QUOTE]IMO, the 3" straight pipe sounds great! I loved mine. I just love my 4" more I would use a piece of flex pipe for connecting your offset pipe together. Relatively cheap and any truck parts store should have it. I know my truck not only ran better after I cut that darn muffler out, but it also ran a lot cooler down the highway. The temp guage didn't climb so high

Thanks again 90firstgen
Wouldn't flex pipe make it sound raspy? It is definatly the cheapest
Thanks Ben

First time Iv used quotes hope I did it right

flashgordon

thanks BC847 thats how i have under stood naturaly asperated engs and the turbo just puts a stop to scavage!
there just isn't and low pressure to cause the afect!

But hay, i'm still up, to being educated on the theary of it. there is some sort of resurch out there that could make me under stand how it could work!

Like Bc847 said, turbo scavageing is when we add more boost or make a smoother path to and from the eng!

90firstgen

[QUOTE=whereswaldo250;1170070][QUOTE]
Wouldn't flex pipe make it sound raspy? It is definatly the cheapest
QUOTE]

I have a friend that used it on his 89 4x4 and it sounds great. Its not raspy at all. You should definately go for it.. you won't regret it

apwatson50

Not only the pressure difference but the temperature difference as it is the hot gasses not pressure that is really driving the turbine.

the other thing to think about is with a 4in downpipe how the transition from the 3in outlet to the 4inch downpipe is accomplished. Obviously we do not have unlimited amount of space to utilized so we basically just do whatever we can to get a 4inch downpipe installed. I have to experimentation or anything to back this up but how much turbulence is there in that transition right after the turbo? If there is significant turbulence than theoretically it could create more pressure than just having a 3inch downpipe. However from what everyones experience is a 4in downpipe helps everywhere so it probably doesn't create that much turbulence. However what if there was a downpipe design that gradually increased from 3in to 4in over a 12in length, would this promote laminar flow? Maybe, maybe not, I don't know? Maybe there isn't a better design than what we are all using, or maybe we should just all get turbos with 4in outlets!! lol

boy that ended up kindof long.

Tuckerdee

AP, I also have wondered if there is a turbulence issue with a "quick" transition. But...I once saw a video of a turbo diesel engine running on a shop stand. There was no pipe after the turbine. What surprised me is how the exhaust came out of the turbine. It spread out laterally from the exit at some like a 45* angle. I sort of just "spun" out of the turbo. I think the turbine imparts a lot of "spin" to the exhaust flow. So...it seems maybe a 3-4" quick transition wouldn't affect the flow that much since the exhaust is maybe spinning down the pipe with a tornado affect. I could be all wrong though.

johnny5.9

I was just wondering if a quick transition really matters. First of all, the gases are very hot meaning they want to expand rapidly. IMO this expanding principle would cause the gases to take up any available pipe volume quickly with little if any turbulence at the change in diameter.

apwatson50

If you look at the us gear exhaust brake site, of course they are trying to sell you their remote brake, they say that it is spinning and the turbo mount brakes restrict the flow by stopping the spinning. Which makes sense. But it still seems to me that the rapid transition would still cause turbulence, and a transision like a air horn for example would be best.

However from everyones experience including mine the rapid transition 4inch downpipe is better than a 3inch downpipe!!

whereswaldo250

Well I went to my local autozone and they have 3'' pipe but the biggest flex pipe they have is 2 1/2''. Any suggestions on where to get 3'' flex pipe?

Thanks Ben