Larger exhaust = more power?

[CharlieK]

Hey folks!

Attached should be the engine tag from my 1980 Mack RD. It currently has 4" exhaust on it, and I have to re-do the exhaust. My question... Is there a power gain going from 4" to 6" exhaust? IF I was to do it... It would have to stay 4" just over 3 feet past the turbo (no room to squeeze anything bigger in there), then it could go to 6".. It would be 6" for another 2 feet, and then go to dual 6" if I was to do it. If all that makes sense....

Half the truck guys I talk to say I wouldn't notice the difference... The other half say that the first half is nuts, that I will notice the difference for sure.

If I was to do it, now is the time... I would do it if I know for sure that there would be any gain at all, no matter how little. Question is, would there be? My gut tells me I would not notice a difference, but I'm thinking if I could do it closer to the turbo I might...

Whatcha think?

Thanks,

Charlie

[doubleclutchinweasel]

ANY back-pressure down-stream from a turbo hurts it. The less back-pressure, the better. The more free-flowing the exhaust, the more the turbo spins.

Whether or not you would actually FEEL the difference is unknown. You might. You might not. It depends largely on whether the current 4" pipe creates any back-pressure.

And, still having a 4" section might defeat some of the gains the 6" would give. Or, it might not. Again, it depends on whether the current pipes cause any restriction or not.

6" does look cool, though!

[Phase 1]

" I would do it if I know for sure that there would be any gain at all, no matter how little."

-Then you should do it - there should be some amount of gain. Just for kicks, time it and check speed on a long hill where you don't have to shift, before and after.

[farmer52]

A larger exhaust is not going to hurt but doubt if you will see any appreciable gain in horsepower. You are restricted at the turbo and the 4" pipe under the cab. The only gain will be the "curb horsepower" = visual appeal. Post some pics.

[doubleclutchinweasel]

..."curb horsepower"...

I like that! Can't argue with that!

Even John Force said that flames on a race car made it go faster.

[b61mack]

I wouldn't go and bigger than 5", just my opinion anything bigger than 5" does not look good on an R model

[j_martell]

A few TDI nerds have been reporting over speed of the turbo and something known as "boost creep" as a result of gutting the exhaust systems in favor of free flowing straight pipes....

Basically with nothing to hold the turbo back, an over boost condition occurs, and extremely high (dangerously) turbine speeds.....

I know, little motors vs big motors, but food for thought.

I'd imagine however, the 4" down pipe would choke it back a little anyway

[ws721]

A good thing to do is install a boost gage. Then any modification you do will show. If you dont see any gain then you'll know, try something else. Im not sure on a Mack engine but a B.C. Cummins for every 2 lbs of manifold pressure amounts to around 22 h.p. So you can see what your adding for power, also mileage

[Timmyb]

I am thinking along the same lines... I was gonna go 4.5inch (114mm) from turbo. According to my math it should be a 20% increase in volume over 4inch. And it should still fit between engine and chassis. Then I was gonna go to 5 inch just for increased flow, more muffler options and unleash the sound.

[mrsmackpaul]

Hmmmmm lots of different thoughts here, to me I would have thought back in the day if Mack could have got more power out of there motors by putting a bigger exhaust they would have done it cheap selling point extra hp from just an exhaust I wouldnt think it would be heaps if any at all

Paul

[bulldogmack58]

IT'S JUST A THOUGHT, IF YOU KNOW SOMEONE THAT'S INTO TRUCK PULLIN' I'D RECKON THEY'D GIVE A GOOD GUIDE! I CAN'T SEE HOW A BIGGER EXHAUST WOULDN'T HELP! JUST HOW MUCH BIGGER WOULD BE THE QUESTION AS I'M SURE THERE WOULD BE AN OPTIMUM SIZE VS OUTPUT OF EACH ENGINE DEPENDING ON WHAT'S BEEN DONE TOO IT! I'M JUST THINKING THAT TOO BIG MAY BE NOT GOOD JUST LIKE TOO SMALL CAN BE ALSO! MY TWO BOB'S WORTH, MICK

[farmer52]

Turbocharged engines are developed as a system (inlet, fuel, combustion, and turbo) using various components and settings. As I previously stated, increasing the exhaust diameter will yield little if any increase in horsepower. The best way to determine any change is to put it on a chassis dyno, measure the power with stock exhaust system then remeasure with new system. My bet is the change will be minimal.

As for the 4" exhaust...when I started working at Mack in 1985 (after spending years at the CAT house in engine development), I asked the question "why does Mack use a 4" exhaust system." The answer was "because EPA states a minimum of 4" to be used for the horsepower setting." My response was, "EPA sets a minimum, not maximum." During the development/evolution of the E6 I doubt anyone thought about putting a larger outlet turbo on the engine. The E7 was just being designed and if I remember correctly, it was initially going to use a 4" outlet turbo. I suggested changing to a 5" on the E7 for all ratings. The rest is history. To retrofit an E6 with a 5" outlet turbo would require "resizing" the turbo to get the best efficiency. That's my story.

BTW, it is interesting to note the engine label....1990 standards, built 05/1996 and set to 1979 EPA standards. Very interesting!

[b61mack]

At work we have a 1990 E6 350 4-valve and it has a 5" outlet on the turbo.

[farmer52]

1990 would be near the end of the E6s. Perhaps Mack decided to incorporate the 5" turbo on the later engines. I left Mack in 1986 for greener pastures (so I thought).

[Timmyb]

This is a bit different. We have an e7 that was reduced to 4in between the chassis and the engine.

Had the 4 inch section cut out and replaced with 5 inch pipe and it made a noticeable difference. No boost gauge in this truck so not sure what difference it made but according to the seat of my pants its was a bit!

[gearhead204]

the other thing I think would help is the reduction of angles and larger radius elbows ,any thing that accelerates air flow will improve power. by looking at your photo it looks as if your exh. goes under frame. Do some checking to see if you can run 4.5 or 5 in. pipe above frame (you might have to add a heat shield under the cab) a lot of e-9 powered trucks did this. if you do this then use 45 degree fittings or 22 1/2 off the turbo to improve air flow.

[rsb502]

You can't out flow a 5" intermediate pipe with that engine, stock is 4" from the turbo to the muffler on all older 6 cylinder Mack's up to the ASET engines. The etech and aset had a 5" outlet muffler but still the 4" intermediate pipe (turbo to muffler) I have a 5" downpipe, y pipe and everything under the cab then to 6" elbows and stacks on my turned up E9 and I never have exhaust temp issues. I have seen several high performance E7 or etech engines that have been swapped to 5" downpipes back to the stacks and none had heat issues but none made anymore power with it either. I'd say do whatever you want its your truck but trying to do 6" under the truck will be a pain and not give you any gains on that engine at all. If you swap to a Borg warner turbo from Antrim you can go up to the 5" downpipe that bolts right on and go from there.