The Difference An Exhaust Makes

steve wright · Post by **steve wright** » Tue Nov 28, 2006 11:03 am

I’ve been dabbling with a few different exhaust combinations to try and optimise power, driveability and noise (both quality of sound and absolute decibels) and thought you guys might be interested in the results. As a reference point the engine in my 2.2S is not standard: 2.3 litre B&P, 10.5 compression, enlarged (40mm) throttle bodies, RSR cams, twin plug heads and MSD ignition.

Firstly the standard exhaust: a good compromise in terms of driveability and noise but restrictive in terms of power, which began tailing off after about 6,300rpm – certainly it’s not ideal in any way for a breathed on motor. My guess on noise is about 90 decibels at 5,000 rpm. It also takes any edge off the exhaust note.

Secondly, outright megaphones with no baffling. Purchased from EBS Racing, these are identical to those fitted to the ‘73 RSR. Driveability suffered, with a flat spot that couldn’t be driven through (you’ve got to change down or feather the throttle to get through 400rpm of popping and crackling that starts at 3,000rpm). But they definitely accentuate the cams and come into their own from 6,000 to 8,000, providing the most outright power of all combinations, but I’m not sure they were optimal in terms of torque. Noise is just unreal, with a fantastic “crack” to the exhaust that characterizes a race motor, but not really streetable at about 140 decibels! No back pressure whatsoever made fuelling a real problem with the 2.2S injection pump as it could not deliver the flow capacity the engine now required. Not even a change to a 2.7RS pump was enough. The suggested solution of an RSR pump was met with a polite decline….

Thirdly, megaphones but this time with baffling. I made up straight sections of pipe from perforated exhaust baffling that was inserted into the now switchable megaphone. Only improvement was a marginal increase in driveability, with the removal of the majority of the flat spot. However, constant throttle driveability was still very on/off with no low down torque and a slight wuffle and definite zinging from the exhaust.

Fourth and probably the most disappointing test as I had high hopes for this configuration. I added exhaust baffling for motorcycle canisters (looks like fibreglass matting but heat resistant) around the perforated baffle. Back-pressure dramatically improved with good driveability but the quality of the sound was completely horrible, with a real muted wuffle to the exhaust note and no crack or edge to it at all. Torque was still fairly poor with nothing below 5,500rpm and the edge definitely taken off it beyond that.

Finally, I fitted the Autofarm rally exhaust. This is the stainless steel switchable one they sell with three outlets, the normal outlet as well as twin straight pipes. Talking with Josh and Mike, who have been around forever, it was interesting to learn that they cut open a factory rally exhaust as fitted to the 911 ST back in the late ‘70’s. They copied the baffling and chamber design, which was based on a standard exhaust, but with most of the baffling removed and the expansion chambers moved around – they’re comment was that someone had obviously spent a LOT of time designing the internals. Their other comment was that the 50mm outlets allowed for fitting without risk of burning the paint on the rear valance, something the larger 70mm silencers might do, especially since there is no latitude for positioning the outlets any lower on the silencer box. The car now has huge amounts of torque for a watch-winder sized motor and at 5,500 it really comes on cam with a real power hike that revs freely around to 8,000. Driveability is as good as a standard car and the sound is great: just the right blend of edge, quality of sound and muffling. It’s something you can live with at 4,000rpm on the motorway or really enjoy up high. Decibels were 105 using a traditional stationary noise test at 1.5 metres, with the caps on the twin pipes so it’s okay for some track days. I don’t want to sound like an Autofarm disciple but having spent so much time and effort it’s obvious this is a good fast road/track setup.

The only other system I would’ve liked to have tried is an RSR silencer setup which has sectioned megaphones attached to an un-baffled silencer, which has a twin outlet set up that exits under the bumper over-riders. This apparently takes some outright noise out of the setup but I doubt it provides sufficient back-pressure for my little watch winder.

I know all of this could have been optimised with changes to fueling and timing at each stage and I could’ve been a lot more scientific by using a rolling road each time, but without the expertise or bottomless wallet, I decided to keep as many variables constant while playing around with just the exhaust.

Cheers, Steve

hot66 · Post by **hot66** » Tue Nov 28, 2006 11:07 am

great post Steve

Interesting because I'm considering a new rear muffler & have been tempted with the Autofarm one ( although I notice its gone from their web site )

steve wright · Post by **steve wright** » Tue Nov 28, 2006 11:39 am

thanks. i'm pretty sure they still make it. it's not cheap in stainless steel but it can be polished up nicely. the only thing i'm thinking about doing is getting barry to cut off the 3rd outlet and put ever so slightly larger twin outlets on, purely for cosmetic purposes - Josh & Nick don't think that will make any difference to back pressure or breathing as there's virtually nothing coming out the 3rd outlet with the caps off and my local MOT testing station is Paragon, who are sympathetic to old Porsche's

Mike Usiskin · Post by **Mike Usiskin** » Tue Nov 28, 2006 3:31 pm

I'm talking to M & K n the states about acting as a distributer for them. they're incredible units, thicker grade steels etc, dog's gonads etc

see the post re M & K or look at this website (http://www.mkexhaust.com./)

it's arrived and is georgous. I'll get it it fitted on thurday of this week all being well and will tell all after that.

Cost wise fully polished 2in 3 out $845 inc $100 shipping, so as you can see very competitive.

regards

MIke

1972_911t · Post by **1972_911t** » Tue Nov 28, 2006 11:25 pm

How important is back pressure? I have a M&K twin in twin centre out for 993 HE's that flow better than SSi's. The muffler is basically unbaffeled on the twin centre outs as the baffle tube runs through the center from side to side so the gases only flow through this if using side out. Im now concerned that a lack of back pressure may cause me problems.

Steve

steve wright · Post by **steve wright** » Wed Nov 29, 2006 8:52 am

back pressure is hugely important as it regulates how efficient the engine is. think of the engine as a pump, with an inlet (the carb/injection system) and an outlet (the exhaust). anything you do to improve on (or the efficiency of the pump itself) requires equal changes in the other. Improving the flow through the exhaust can provide a lot of improvement because the factory has to take account of noise, emissions, etc, which is usually restrictive. however, if you significantly improve it, it may need richening of the mixture to ensure enough fuel is getting in.

I've no experience with modern cars, I presume the electronics automatically take car of it, but on an older car, checking to ensure fueling is accurate by getting it checked on a rollnig road is never money wasted as an engine running lean has a tendency to go bang

Luke · Post by **Luke** » Wed Nov 29, 2006 9:05 am

james if you take in your old Stainless box I'm pretty sure Autofram can modify it and it saves a couple of hundred quid.

L

hot66 · Post by **hot66** » Wed Nov 29, 2006 3:14 pm

Luke wrote:james if you take in your old Stainless box I'm pretty sure Autofram can modify it and it saves a couple of hundred quid.

L

sounds like it could be a good option

I wonder what the purists would say about a loverly stock 2.4S with triple outlet system on it

Mick Cliff · Post by **Mick Cliff** » Wed Nov 29, 2006 8:49 pm

James - think long and hard about twin centre outlets - you know what mine sounds like! It's great for about an hour but then the noise gets tiresome, especially if you're just getting from A to B. Definitely need the 'Cyberman' get up (as you've witnesed!) OK for 'playing' in the lanes, though.

You're welcome to sample the noise first hand next time we meet up.

Aren't you concerned about noise levels at trackdays? Autofarm's measured 105dB at 1.5m! Aren't most trackdays limited to 100dB with a (very) few at 105dB? Would be a shame to expend all that dosh to have to have them capped?

hot66 · Post by **hot66** » Thu Nov 30, 2006 10:14 pm

exhaust replacement is way down the list of jobs for my car ... new seats is the priority job ... watch this space

sladey · Post by **sladey** » Fri Dec 01, 2006 5:11 pm

I know it's not the same as my experiences are with a 3.2 motor, but I've been living with my M&K single out now for about 6 weeks.

From outside the car it's fantastic-sounding - not excessively loud but plenty of menace, and a lovely harsness comes into it under acceleration.

From inside the car it is louder, but when I'm on my own I don't notice it - just turn the radio up a bit louder. When I'm with others though I'm conscious that I can hear it more than before.

It's definately changed the driving charactyeristics of the car, and not all for the better. Before the change the engine was really sweet. Since the change, and once it's settled in, I've got more torque in the mid-range, but have the feeling that there is less there higher up the rev-range. I don't know how much of this is perception (if you raise the mid-range and the higher revs hasn't changed then it feels as though you've lost something when you haven't). The engine feels less revvy, more torquey.

I'm thinking of a Steve Wong chip and wonder if that might help.

The system is nicely made, and I really really love the sound but part of me feels I've lost some driveability

Mark

Mike Usiskin · Post by **Mike Usiskin** » Tue Dec 05, 2006 10:22 am

I'm running a 2 in 3 out M & K stainless system and am very happy. With both 'straight through' pipes capped off the sound is rich and sonorous, with the real induction noise coming over 4000rpm. Over 4000 it develops a gorgeous crack and that F1 sound takes over. With the twin outlets uncapped, she breathes freer, and revs more cleanly. sound wise she's loud, inside and out, though i drove back form the garage the other day across London in rush hour with both outlets uncapped and across Hyde park she sounded like nothing else I've ever had the pleasure of driving

I'm going to take her to Bob Watson's next week (work allowing) and am interested to see the difference with caps on and off. It sounds freer but could well be an audio illusion as she's louder at 800rpm so might be my ears playing tricks again.

ttfn

Mike