Sound
It’s a cat-back exhaust, the stock item and as you’d expect, the stock exhaust is quiet, almost silent.
Honestly, I quite like it. You can go quickly while not drawing any intention. Much better.
Before it was 70mm, straight piped effectively and when the exhaust valve opened it was loud, obnoxiously loud. Granted with the valve closed it was quiet but the valved exhaust was so restrictive when it was closed I didn’t really enjoy having it on the car.
In other words, I prefer the sound from stock exhaust 99.9% of the time.
Power
But it’s not all puppies and rainbows. Power, as you can probably imagine has taken a big hit. Not a little bit, but a lot. My butt dyno says it’s down by 70bhp+
The car is fast with the stock exhaust sure, but it doesn’t rip like it did. The craziness has gone.
The owner is happy with it as is for a few weeks but no question, something needs to be done to get the power back.
Problem-Reaction-Solution
The problem is the power loss.
The reaction is “the power needs to come back with volume close to stock”.
The solution. This is a bit more difficult.
Realistically I think the only way to have fully stock noise levels is to have the stock exhaust on the car in some form. Which leaves us with what?
Options
At the time of writing, there are two possible solutions.
Some sort of bypass valve at the front of the exhaust for race use. Or adding extra pipework which is fully 70mm(?) front to back and if the stock exhaust is staying then that means two sets of pipes.
The first option is not ideal. Sure it get’s the power back but it will be noisy when the valve is open.
The second option would entail retaining the stock silencers but with a valve that opens and sends some of gases to the opposite side of the car. At the opposite side of the car we have a ton of space to put a silencer. So full bore exhaust back to the rear axle and then split the exhaust into two smaller pipes, one going into the stock rear silencer and one going into a baffled silencer on the other side.
Split
At the moment I’m thinking 76mm would be nice until the axle and then splitting into at least one 55mm pipe to the stock silencer and the other side diameter? Who knows.
Calculations
This exhaust calculator says 70mm is at the limit and possibly 76mm is the limit for 300/360 brake.
I’d like to be on the safe side and make it 76mm to the axle.
Using this pipe area calculator, it says that two 55mm pipes is about the same as one 76mm. And considering the stock silencer is baffled (which could be as bad as a 30% restriction) means the other pipe needs to be bigger than 55mm.
If we are using two baffled mufflers on either side, and both, worse case scenario, cause 30% restriction, we need the combined area of the pipework after the axle to be 30% larger than the area of a single 76mm pipe.
That means 5900mm squared total pipe area.
Minus 2375 sq mm of the 55mm pipe going to the stock silencer, that leaves us with a difference of 3525 square mm. And a pipe with that area works out as a 63.5mm pipe or 70mm.
63.5 could probably do the job assuming the silencers aren’t as bad as a 30% restriction which I don’t think they will be.
63.5mm diameter will be okay if they are a 20% restriction. And 63.5mm will be better for noise reduction.
One more issue
The only issue with the second solution, apart from cost, is the 76mm pipe back to the rear axle line. For sure we’d put the biggest possible centre silencer but the pipe is 76mm which will be louder.
Having said that, if we can run the exhaust gas through the two sides all the time, that should in theory make the noise less, because the exhaust gas is running through a greater volume of silencer (left and right).
And one more…
And one more problem with the 76mm plan is the catalyst which is 70mm at the moment. So that will need to be changed to 76mm. And its hard finding a cat that isn’t restrictive for 300+ brake. I suppose we could run two 70mm cats……
All-in-all
There is a cheap solution to getting the power back.
The solution for getting the power back and keeping it silent is expensive. Effectively a new exhaust system plus a bit extra and a new cat, be it an extra 70mm or one 76mm. My concern with running two cats is efficiency and heat. Is there a chance they won’t get up to temp?
To be continued…..