The Difference An Exhaust Makes
Posted: 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
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
