MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Exhaust back pressure question

I took my 73B roadster to a local muffler shop to have a new front muffler installed. It was one they had in stock. I did this because I wasn't happy with how loud the car was with the new Moss front and rear mufflers I had recently installed. The muffler shop felt that by replacing the Moss front muffler with one of their's the car would be quieter......and it is.......although still louder then I would like.

I took a closer look under the car today to see what they had done. Turns out the new front muffler has a 1 1/2" pipe as versus the standard 1 3/4" pipe on the rest of the system.

Does anyone that knows a lot about engine performance think that this downsizing of the pipe through the front muffler section of the system is a problem? I've always been told that pipe sizing was designed to place a specific back pressure on the engine. It seems the 1 1/2" diameter pipe would be putting more back pressure on the engine. The 1 1/2" pipe is about 3' long (inlet pipe + muffler + outlet pipe).

Thanks for your input.
BH
BH Davis

Mate
If I had laid out the cash for a 1 3/4 system and someone cut the middle out and made it 1 1/2 I would be very unhappy and let them know it.
As far as your backpressure question goes. If you are after performance which I guess is why you went to 1 3/4 in the first place-- The answer is the less back pressure the better it breathes and the better it goes with the mixture adjusted to suit.----- But in your situation where the exhaust has been reduced it will not harm anything-- The only things that will be affected will be a slightly richer mixture compared to the bigger system and not quite as good performance in the top end of the rev range.
Basically if you are not a high revving full throttle driver forget about it-- but it does seem to be a waste of a good exhaust system though
Cheers Willy
WilliamRevit

Willy,

I had the 1 3/4" system on the car simply because that is what the pipes are from Moss. As such I assume that is what they originally were when the car was built.

I'm not looking for better performance. This is just my enjoyable and fun to drive LBC.

What I am after is a quiet exhaust system.....which I had before the old Walker rear muffler rusted out at the seam.

The new front/rear muffler setup from Moss was way too loud for my liking. That is why I went to the muffler shop. He suggested to leave the Moss rear muffler (which I'm now told is a resonator as vs. a muffler) and replace the front one. The car was indeed quieter after doing this, but still louder then before.

I now have in hand a NOS Walker muffler that matches the old one that split. Unfortunately the old front muffler got tossed.....but it was on its way out anyway.

So, my debate is whether I should just put on the new Walker rear muffler and leave the 1 1/2" piped front muffler from the muffler shop on the car, or hold tight until I find a 1 3/4" pipe size front muffler that I think will be quieter then the one from Moss.

My concern with the 1 1/2" front muffler is that it might add enough back pressure to damage my nice running 35 year old engine that has never been rebuilt (I'm only the 3rd owner of the car so have a good idea of its' history).

Thanks,
BH
BH Davis

BH-
OE pipes are 1 3/4OD.
The front box is a resonator, the rear one is the muffler, and does most of the quieting. Many people remove the front box to increase ground clearance on early cars, with only a small noise penalty.
The muffler shop guy is an incompetent jackass.

While " not looking for better performance" may be true, presumably you are not looking for worse performance. Based on pipe ID, the relative flow rate is about 121/169 with the pipe reduction. If you just dawdle about, it will have no effect at all; but, at road speed of say 3000 rpm & up, that's enough to make a significant difference in power, fuel consumption, and engine temperature.

I suggest you call The Roadster Factory, who have all manner of OE type stuff, including all the correct hangers, and quite possibly the matching Walker front intermediate pipe/resonator/center pipe. A pet peeve of mine is muffler shop morons who screw up the perfectly designed MGB OE mountings, causing all manner of clunks, knocks, rattles and reduced ground clearance while installing a system that is half as good as the OE ones.

FRM
FR Millmore

FRM,

Thanks for the detailed answer. I actually did try to buy the front muffler from the Roadster Factory but they were out of stock with no clear delivery date.

The one thing the muffler shop guy did right was to use my new OEM hanger kits from Moss. I had already installed them myself with the Moss front and rear mufflers and made a point of telling him I wanted these used on his revised system.

I really want a quiet car.....I'm not interested in hearing that "nice throaty sound" that MG's are known for. Do you think that the front muffler/resonator from the Roadster Factory (when they are available) will push me in that direction?


Thanks,
BH


BH Davis

BH-
Mufflers are a can of worms. Everyone from OE through aftermarket at all levels change designs at random, often continuing the "original" part numbers, all whilst lying to you about what the part actually is. I doubt that many of them actually know themselves, certainly at the retail level. Vizard has pictures of the "Cooper S" system, as originally and latterly supplied by BL - criminal misrepresentation!
I have replaced OE Mazda systems on my car with
"OEM spec" parts, to find that performance and fuel mileage dropped noticeably; turns out the replacement muffler had the 1 7/8 ID pipe reduced to under 1 1/4 ID inside the muffler - about half the basic flow capacity, and worse due to the configuration.
You almost have to find someone who has experience with the exact pieces under consideration. Walker in general seem to be reasonably good and definitely toward the
OE end of things. OE MGB didn't actually have much of a "throaty" sound, though it does have a characteristic exhaust tone due to the siamese port/LCB basic design.
TRF has a bunch of odd ends and knowledge that you don't have access to in the usual ordering method. Call them on the tech help line and ask to talk to the resident tech expert graybeard, probably John or Dave.

FRM
FR Millmore

Fletcher is right - the front box has little to do with the noise level - I run a 2" system on my car with only a big back box and it is very quiet - I think the size of the rear box is the determining factor - also heavy gauge or double skinned boxes are better at killing that harsh rasp in the exhaust note.
Chris at Octarine Services

Thanks again guys. I will call the Roadster Factory again tomorrow.

Do you think I might get the quiet sound level I want with just the NOS Walker on the rear and a straight pipe to it with no front muffler?

BH
BH Davis

BH,

Originally MGB were equiped with 2 resonator and the rear one outlet was a bit restrictive. As said, it is hard to find original stuff now.
You can find rear muffler from Bosal
# 115-070 but middle is N/A.

I tried many exhaust systems, Free-flow originals and custom made.

FF are a bit on noisy side
Custom made:

#1= Headers+ 2" tubing+ single Dynomax turbo muffler #17732= silent system, but skightly noisy if you accelerate promptly.

#2: 2" tubing + 2 glaspack cherry bomb muffler. Accepable but less silent than #1 set-up.

Cheers,

Jean G.

Jean Guy Catford

Addendum:

I made a mistake

2" is #17762
and I found a 13/4" possibility which is:# 17771.

JG
Jean Guy Catford

BH=
Walker only at the rear is worth trying.
Re JG's comments - It was a Bosal "OE spec" muffler that tried to strangle my Mazda, and I am less than impressed with build/fit of Bosal stuff I've seen lately.
Chris brings up a good point re wall thickness/construction., especially applicable to common SS systems. These usually have very thin wall tubing, making them noisy - a good bit of sound comes through the tube walls.
If you still want more quiet than the Walker provides, it's worth looking around for an OEM "organ donor" muffler from something modern. Since they are actually paying some attention to performance/efficiency today, modern designs tend to be much better acoustically, rather than simply being restrictive as was formerly the case. The combination of small displacement, relatively high performance, good mileage, and Federally mandated lifetime/sound levels means that a >>factory<< muffler off a wreck could be worth fooling with. Something in the 1600-2000cc range - Mazda, Honda, Mitsubishi ETC. Mitsubishi/Chrysler make some beautiful SS mufflers. Main problem is finding something that fits space available; with luck you can also find enough matching pipe to hook it up.

FRM
FR Millmore

This thread was discussed between 03/10/2009 and 04/10/2009

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now