exhaust on track cars

evil_weevil · Postby **evil_weevil** » Wed Feb 02, 2011 6:11 pm

Hey guys n gals!
So, I have seen some regular mx5ers at the track with more a-racecar-than-streetcar MX5 with some decent exhaust systems, mainly the rear section. I have a full system but sounds nothing like them and infact, mine now sounds too quiet!

just wondering is there any benefit in changing the rear muffler to anything else? or for the gains vs cost it isnt effective to do? some ive seen have the straight cannon with no bends coming out the left rear to centre of the bumper

and for the ricer/young hoon in me - whats a straight through pipe with no muffler going to be like? LOL

cheers!

broady · Postby **broady** » Wed Feb 02, 2011 10:07 pm

Dyno tested my car.

Intake/headers/exhaust added ~13-14rwkw

Silencer vs no silencer 6rwkw

bensale · Postby **bensale** » Wed Feb 02, 2011 11:05 pm

evil_weevil wrote:
and for the ricer/young hoon in me - whats a straight through pipe with no muffler going to be like? LOL

cheers!

You'll probably loose all torque but if you're lucky pick up some high rpm power although my feeling would be that you wouldn't have enough back pressure to get a drivable amount of torque without some form of muffler. Also, I think an mx-5 with no muffler sounds terrible, something like a cross between a wounded sheep and a 1970s datsun. I haven't dyno'd my car like Broady but the seat of pants feel from my cat back (RB mid pipe, n*power muffler) was that I picked up high end power but lost an equal amount of low end torque, mid range is about the same and the noise is awesome!

Broady, what did your system do to the torque/low end power of your car?

broady · Postby **broady** » Thu Feb 03, 2011 12:03 am

Yeah my power is pretty peaky. But still plenty of torque as it is a 1.8. I need to fix up the intake to lengthen the pipe. That should bring back some low down torque.

Dyno graph here: viewtopic.php?f=57&t=39496&p=575245#p575245

hks_kansei · Postby **hks_kansei** » Thu Feb 03, 2011 8:44 am

I've got an NB that has factory exhaust, and an RSR muffler.

with the silencer out the RSR is basically a straight through (I can fit my arm inside), prob about 5inch to 6inch tip?
with the silencer in it blocks it down to a 2inch outlet.

anyway, in regard to the top end difference it is noticable, unsilenced mine revs and pulls cleanly to the top end, however with the silencer in it seems to be a bit rougher in higher revs, and not pull as hard.

low end torque is effected also, but nowhere near as much. say if the top end is increased 10%, the torque is only effected by 5%. (random figures to illustrate what I mean)
the only thing I can say is that the NB has a smaller muffler further up the exhaust, so I'm still getting the backpressure from the rest of the system, so it's still very drivable, the NA will probably be a bit different.

one more point,
the noise of open mufflers is great for once in a while, but it's not that great for people around you, especially at night when people want to sleep.

in Victoria the maximum legal noise for an MX5 is 90db.

most straight trough mufflers will probably fail this, the unsilenced RSR is somewhere above 100db, so I expect most similarly open mufflers to be the same.

project.r.racing · Postby **project.r.racing** » Thu Feb 03, 2011 9:08 am

i have a 2.25" system with high flow cat, 12" hotdog and a muffler that is nearly straight through, but the inlet and outlets are offset buy 10-15cm.

i ran it without the high flow cat for the 1st 18 months. and it was alot quieter. with the cat, i gained alot of noise and mid range pick up.

92.7db @ 6500rpm at Lakeside Park Raceway.

Hellmun · Postby **Hellmun** » Thu Feb 03, 2011 3:45 pm

Backpressure is a terrible thing and never good for performance. Air velocity however is the difference that effects exhaust performance at difference RPM. It's all about getting as much scavenging as possible to reduce pumping losses where the engine operates the most.

Ive got 2 mufflers and they're both straight through but one is and full of packing. The other is flogged out and incredibly louder (20db+). I found no performance difference. So a good muffler can still not be a big restriction but reduce noise. A cat deletion pipe gives more power for a lot more noise but then the car is only track legal. I noticed about 5kph difference on the back straight of Wakefield with the cat in. No cat also means a Lot of rasp on a stock motor.