Muffler size for Turbo Installation

lr172 · 09-17-2009, 07:31 PM

I recently installed a turbo on my 3.2 Carrera. I am not flowing as much as much air as I would like / hope at higher RPMs. I put a pressure gauge in the pre-turbo exhaust circuit and am getting ~14PSI while on 6 PSI boost. I believe that I am supposed to be closer to or under a 2:1 ratio. I believe my turbine is a bit too small (Garrett Stage III wheel with .63 AR housing) and will likely address that some day.

However, I also used 2.5" pipe and muffler (straight through design/Magnaflow) post turbo. Further research seems to indicate the 3" is recommend in my CID / HP range. I am wondering if this is contributing to my backpressure. I am willing to replace it if it helps, but wanted to hear from the experts to see if you guys think this is part of the problem.

Any thoughts?

Thanks,

Lary

RarlyL8 · RarlyL8's Garage

Yes, definitely. Turbo performance exhaust theory is simple - they don't like any. That's not practical so the compromise is sound level vs power. For up to 400HP you can use a 3"ID straight through system. This is important. A baffled system robs power. Bends are OK as long as the ID remains consistant. My dyno work shows no appreciable difference between my free flowing 3"ID straight through muffler vs a straight pipe or Zork tube at that power level. Going larger than 3" will not add significant power at this level but will add a bunch of sound. Deminishing returns in other words.

ben parrish · 09-18-2009, 05:16 AM

Intresting. Brian, I am also adding a turbo to my 3.2...will begin shortly. My s/u will use all factory parts. K27-7200 and stock 930 muffler. Will I also experience too much back pressure?

911st · 09-18-2009, 07:23 AM

It could be ether or both.

Can you put your pressure gage after the turbo and before the muffler. This will tell you if it is the muffler or the small turbine you are running or both.

2.5" can support around 300hp thought that is near its suggested limit.

A small hot side can induce back pressure on its own.

Also, the ratio of hot side to compressor wheel size can induce back pressure.

Thus, could be one, the other, or more likely both are costing you some efficiency. I suspect a lot of people with the current quick spool big turbos are near the same level as you.

For that turbo can you buy a different hot housing for under $200 and install it yourself? I think you want more like a .82 A/R or such.

RarlyL8 · RarlyL8's Garage

Turbo looks like an issue but you know that already.
Dyno runs on my 300WHP test engine show a 12HP increase using my muffler over stock. That doesn't sound like much but is only a small part of the story. Proper muffler will:
- Allow all the power out that the engine can produce
- Lower the boost threshold
- Increase throttle response
- Allow you to hear some really cool sounds such as turbo whistle
- Save a bunch of weight in the right spot. Mine is 30 pounds lighter than stock '86-on.

lr172 · 09-18-2009, 09:19 PM

Thanks for the assist guys. Yes, I know/suspect the turbo is a large source of the backpressure. My research points me to either a Garrett GT35R or a replacing my hot side With a Garret Stage V turbine. Neither are cheap and I am not ready for the investment yet.

It sounds like my 2.5" is too small. I will replace it with a 3" round straight through. I went 2.5" because that was the only size Magnaflow that had the same side entry / exit that allowed an easy fab for driver side exit. I just don't have the budget for the nice stuff that Brian and Ben make.

I am still struggling with Proto chip AFRs. I believe the high RPM richness is because my turbo is more restrictive than the one Todd uses on his kit. However, I am also too lean in my part throttle map as welll. I recently gave up on the idea of going through tune cycles with his chip and started an aftermarket EFI installation. My Megasquirt arrived today. That will allow me to control all the variables, including the boost level.

911st · 09-19-2009, 07:43 AM

You would probably never find this one because exhaust is mis-spelled but check this link out.

Turbo Exaust Theory

911st · 09-19-2009, 07:46 AM

Here is some of the best info on turbo exhaust I have found on line.

http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

DonE · 09-21-2009, 04:20 PM

Interesting stuff. Ironically, when I was building my motor and designing the exhaust, I spoke to an engineer at Garrett about what diameter exhaust to use out of the GT35r with a 3.4L motor producing over 500hp. He said there is no HP difference between a 3" and 4" pipe for the CFM my 3.4L motor produces. So, who to believe - who has good data?

My stock 05 Powerstroke uses 3.5" pipe the length of the truck and it is a 6.0L motor.

911st · 09-21-2009, 04:39 PM

How much HP dose your truck make?

Length might have somthig to do with it to.

TurboKraft · 09-23-2009, 08:38 PM

4" is not necessary.
3" is plenty and will support lots of power.

We just used magnaflow's largest 3" muffler with a GT35R on a turbocharged 964 and pulled a repeatable 520rwhp/550rwtq. Backpressure is not an issue. (And the car was funny quiet with that huge muffler.)

2.5" flows better than many suggest, too. Case in point:
- 2.8L 951 making 400rwhp on a 2.5" system, gained <24hp when switching to a 3" system -- less than a 7% gain when others were saying this car was choking on the 2.5L
- recent 3.8L Twin Turbo -- 657awhp through 2.5" tubing & mufflers after the turbos

When the system uses all mandrel bends and free-flow straight-through style mufflers (Magnaflow, Borla XR1) then you reduce backpressure, boost kicks in sooner, you get the cool whistling sound through the exhaust, etc.

lr172 · 09-25-2009, 06:15 AM

Thanks for the good insight here. I only have a couple of inches of pipe to the muffler and 20" before it bends and exits. All Mandrel bent and a Magnaflow free flow.

Sounds like there is not too much I am leaving on the table, as I am currently probably only 325 HP or so at 6#'s of boost.

However, when I get in there to upgrad the turbo I'll swap out a 3". This will become more of an issue when I dump the W/G exhaust into the muffler this winter. I opened it up next to a cop the other day and he pulled me over for excessive exhaust. I need to tame the sound of this somewhat to stay off the radar, so to say.

Larry

DonE · 09-25-2009, 05:26 PM

Quote:

Originally Posted by 911st

How much HP dose your truck make?

Length might have somthig to do with it to.

325 hp, 570 tq, 18.0:1 CR - stock numbers. It has a chip for torque, not HP as I pull a horse trailer with it.

911st · 09-26-2009, 06:51 AM

Cool Don.

325hp with 6 lbs boost sounds about right.

TurboKraft makes some interesting points of reference.

-658hp through two 2.5" mufflers is 328hp per which is about where you are.

-More importantly, a change from 2.5 to 3" on a 400hp motor made a 7% increase in HP.

Some thoughts.

2.5" is about 4.9 sq in,
3" is about 7.0 sq in (apx 29% more flow)
3.5" is about 9.6 sq in. (apx 33% more flow)

325 hp w 2.5 is 66hp per sq in
400hp w 2.5" is 100hp per sq in
424hp w 3" is 60hp per sq in.
525 hp w 3" is 75hp per sq in
525 hp w 3.5 would be about 54hp per sq in.
325hp w a 3.5" is 34hp per sq in (power-stroke)

At 66hp per nominal sq in, it dose not look like the 2.5" is a big issue.

However, from the example we do knot know if and even further increase in exhaust tubeing size would further continue to make improvement in efficiencies.

The general rule is a turbo dose not benefit from back pressure and the less one can get the better. Thus, bigger is better.