Dr. Evil

Guys,

What hp level is a 4" good for?? When is it necessary to step up to 5"??

Any idea on the "quietest" 4" system (just curious on that one )

dodgediesel4x4

I'd say 4" is good up to 450hp. Anything after that, it wouldn't hurt to upgrade to 5". It also depends on if you're w/ a single or twin turbo setup. If you have twins, then I'd say 5" is the way to go. Just a thought to ponder.

As for "quietest" system, I'd say they are all about the same, as long as the muffler and resinator are both on there. No actual evidence to back this up, also a thought to ponder.

RowJ

I've been thinking a lot about this question lately.

The purpose of going to a 5" exhaust is to increase flow (across the turbine fan blades in the turbo) and reduce chances of back pressure slowing this flow...right?

If so...than two thoughts come to mind.
1-Faster flow throught the turbo is probably more important to a big single...to get it moving....than it is to a smaller top turbo on a set of twins.
2-The need for better flow is really a function of higher HP...and it's larger quantity of exhaust...more so than it is single vs twins.

The other important factor is when the back pressure effects the engine itself...causing an increase in cylinder pressures. This engine back pressure (TIP - Turbine Inlet Pressure) can be measured by replacing the boost gauge probe with an air pressure gauge and reading it a WOT.

It seems to me the time to switch to 5" exhaust is when this measured back pressure exceeds 1:1 at WOT....assuming you have enough air to handle your fueling set up.

Is there a set HP for the CTD needing 5"? Probable around 550 hp....if my reasoning makes sense.

For Example - I'm slightly overfueled (under aired) right now at 500 hp +/-....have dynoed from 489 to 522 hp.
My measured back pressure is right at 1:1 at WOT (45 psi boost) with a 4" exhaust. If I go to 50 psi boost I get 65 psi backpressure....well over 1:1. With a bigger turbo, I believe I can get more HP and still maintain 1:1 backpressure...but don't know for sure. Will find out shortly.

RJ

apwatson50

Your TIP has more to do with your exhaust housing. now measure your TIP and your TOP(turbine outlet pressure) if those two are 1:1 or close than you need bigger exhaust.

Mark Craig

Dr. Evil,

A good system like the MBRP we sell that has a packed muffler (un packed muffler are actually adds a small bit of restriction) is good up to 500 RWHP, after that a 5.0 inch system starts to make sense for keeping EGT down and the correct CFM of flow up, remember that you also have to get the correct air CFM in (intake and tuebo) to make it work correctly on the exhaust side too!

Mark @ DPPI

Bart Timothy

Doc, you just hit on one of my pet peeves.

Here is something to think about. The turbo is the most restictive part of the whole exhaust system. A 12cm exhaust housing is equivalent to a 3/4" hole.

I don't know where the point to move from a 4" to 5" is, but I've seen so many 600+ hp trucks using a 4" that I can't count them. My guess is at 600 hp with a 4" system, there is going to be so little back pressure, after the turbo, that it would be very difficult to measure. TST performed back pressure tests on a stock 2nd gen exhaust and couldn't measure ANY at 375 hp.

Plus there are times when moving up in size, actually costs power.

Before twins came along, 5" exhausts were far and few between. I think most of the early twins went to 5" because the bottom turbo was plumbed for it.

The biggest thing larger exhaust systems buy is lower EGTs. If money is no object, and more power is the goal, then when to move up in size is an entriely different question from when money is in the equation. How big a deal is it if you put a 5" exhaust on a 600 hp truck and pickup 6 hp? That's only a 1% increase and not to good of a deal on a limited budget.

In my mind, the question of when to move up in size is more complicated - what is my budget, where is my targeted hp goal, what do I want my eventual setup to be, and how do I plan to get from here to there? There are so many young people here, on limited budgets, looking for more performance, who buy a bigger exhaust and new intake system, when their money would have bought so much more performance with an EZ box, a #10 plate, or the safety of guages. I call it the "rice rocket mentality", and I think a lot of them must be doing it more for how it looks or how it sounds whitout regard to how it performs.

The quietest 4' system is the one with the most muffler.

Gotlift01

IMO, I think the right time to move to a 5" exhaust is when you are knocking on the 500HP + mark. I don't think it is necessary to do it unless you are having problems with your EGT's getting high because of not enough flow. Also pushing that much air out of a turbo above 60psi, the 4" will probably be a little more restrictive due to some added backpressure therefore not allowing the turbo to spool as quickly and raising your EGT's. Turbo's need only a tiny amount of back pressure to function properly and putting a 5" exhaust on a truck with less than a minimum of 450HP is pointless and a waist of money, and in fact it will probably hurt your performance some due to the massive size of the pipe and not enough boost flow to match it.

And I think Bart is right about people only installing it because of the looks and the fact that most people think bigger is better and that's hardly the case. Bigger is never better unless you have the right equipment to back it up!!!

That's just my opinion on it so I could be completely full of it I'm sure somebody will correct me if I am

RowJ

Nice simple explanation that makes sense on the surface.

I agree TIP is greatly affected by size of turbine housing.

My one question is... why wouldn't raising back pressures on the outlet side (TOP) balance out and back up through the turbo...and automatically raise TIP. I can't see TOP ever being close to TIP. Both may be very high with a tiny exhaust but won't TIP always be greater by a fair amount? Am I missing something?

Bart - I agree with gaining 6 hp by going to a 5" exhaust is a waste...but doing it and lowering EGT's......or seriously slowing the climb to a pegged EGT gauge is worth a lot. At least to me. Allows continued use of present turbo.
I don't think we are disagreeing here....but it adds importance to the question of when to go to 5"?

RJ

apwatson50

You want a pressure drop accross the exhaust side of the turbo so that the exhaust gasses want to go through it. If the pressure is 1:1 accross the turbine than the only thing making the exhaust gasses go through the turbo is the pistons forcing it through. However if there is a pressure drop than the gasses want to go through making the turbo spool faster and taking less energy to make is spool and keep it spooled.

Look at the pulling trucks/drag car, they usually have a pipe going straight into the air, how much back pressure do you think those have? not much besides how cool it looks i'm sure there are other reasons, i.e. no backpressure.

Gotlift01

I've talked to a couple guys that have upgraded from 4" to 5" on stock trucks just because they had the money and were bored, and they said they actually thought it had less power than it did with the 4" and they didn't really notice a drop in EGT's either. Once again it all comes down to preference and your bank roll.

Dr. Evil

Excellent discussion guys...just what I was looking for. Thanks.

Dr. Evil

Now, the question is:

Rip's "Fair Shake" OR "Warpspeed Performance"

Anyone heard anything bad about either??

CTD NUT

I would tend to question 5" exhaust at even 500 hp.....the turbo that is used will be a very critical factor in egt over any sized exhaust system. People here are suggesting that over 450 - 500 hp you should have 5" exhaust. From what I have seen, I can't agree with that statement as a generalization. An example of what I mean : I have seen a few twins trucks now that are a bit over 500 RWHP and are using 4" exhaust and only barely able to hit 1100* egt maxed out at WOT. If the egt's were to drop much below 1100*, there will most certainly be a power loss. How could 5" exhaust possibly help these particular trucks? Cooling off exhaust gases excessively will drop the velocity. When the goal is to have absolute max exhaust gas velocity at the turbine outlet with the minimum back pressure, 5" exhaust will potentially hurt performance rather than help. I can see if TOP becomes too high and there is an egt rise as a result, there could be a benefit to 5" exhaust.....but if that is not the case, how would it help?

Gotlift01

I would have to agree with that on the fact that 5" is sometimes just TO BIG. Personally I would never put 5" on my truck because I hate the looks of that massive pipe hanging below my truck. It's amazing the difference in looks with only 1". I think that 4" is the better way to go for all around performance, 5" could either benefit you ( in high high HP trucks ) or hurt you so why chance it? IMHO

And in response to DR. EVIL, I have heard good things about warpspeed exhaust and haven't really heard much about the other one. I personally haven't heard either one on a truck, just heard what people have told me!

Dr. Evil

Dude, thats like the best part