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Exhaust backpressure?

rogerthechorister

New member
Joined
Sep 11, 2006
My 760T had a lovely Jetex aftermarket exhaust, nice throaty burble, and seemed to be quicker for having it on. Alas I chose the mild steel not the stainless because the stainless came only in gigantic single tailpipe - totally ghetto looking - and I wanted twin outlets that looked like the Bentley Mulsanne turbo.

Well, that was 4 years back and the back box just fell off days before my MoT test was due,

I rang up Jetex and the nearest back box was in Sweden, would cost ?104 + VAT (at 20%) and would not arrive until the middle of next week.

But the overaxle pipe was still good and there was a bit left pointing straight back - so off to Maidstone exhausts for a bit of pipe that would sleeve nicely over the Jetex pipe that was left. Fitted lovely, looked fine (just a little bit bigger than the Volvo 2.5 inch outlet).

Nice sound, a bit less burbly than the Jetex silencer and sounding actually quieter than with the Jetex rear silencer box!

Now here comes teh shock. Because I still have a standard CAT and the Jetex centre box (huge) I really didn't expect deleting the back box and substituting a bit of large-bore pipe to make any difference (and if anything the reduced backpressure to give better grunt) - but she feels quite a bit slower, boost is slower coming, and all the change points on the autobox (ZF22) seem lower.

Who'd have thought it? It looks as if I need to order the Jetex box after all.
 
Is your buttt dyno calibrated? I have people yell and scream to slow down when I roll by in the loud carat 15 mph, and nothing at 30 in the quiet one.
 
Does box= muffler in this thread? Or do you actually have a box on the exhaust?
 
English

I speak English in England, where a muffler is something a Yorkshireman wears round his neck in cold weather, very like a scarf, and an exhaust pipe has one or more silencer boxes in it.

Yes, calibrating the butt dyno) or, as we would say "arse dyno" or "bum dyno" had occurred to me as an explanation but I am showing lower speeds on my favourite test pieces of road and seeing changeup in the ZF22 at lower revs on the rev counter (whatever that is called in the USA), or tachometer for pedants.

Meanwhile, has anyone any views on the question posed?
 
My 760T had a lovely Jetex aftermarket exhaust, nice throaty burble, and seemed to be quicker for having it on. Alas I chose the mild steel not the stainless because the stainless came only in gigantic single tailpipe - totally ghetto looking - and I wanted twin outlets that looked like the Bentley Mulsanne turbo.

Well, that was 4 years back and the back box just fell off days before my MoT test was due,

I rang up Jetex and the nearest back box was in Sweden, would cost ?104 + VAT (at 20%) and would not arrive until the middle of next week.

But the overaxle pipe was still good and there was a bit left pointing straight back - so off to Maidstone exhausts for a bit of pipe that would sleeve nicely over the Jetex pipe that was left. Fitted lovely, looked fine (just a little bit bigger than the Volvo 2.5 inch outlet).

Nice sound, a bit less burbly than the Jetex silencer and sounding actually quieter than with the Jetex rear silencer box!

Now here comes teh shock. Because I still have a standard CAT and the Jetex centre box (huge) I really didn't expect deleting the back box and substituting a bit of large-bore pipe to make any difference (and if anything the reduced backpressure to give better grunt) - but she feels quite a bit slower, boost is slower coming, and all the change points on the autobox (ZF22) seem lower.

Who'd have thought it?
It looks as if I need to order the Jetex box after all.

I speak English in England, where a muffler is something a Yorkshireman wears round his neck in cold weather, very like a scarf, and an exhaust pipe has one or more silencer boxes in it.

Yes, calibrating the butt dyno) or, as we would say "arse dyno" or "bum dyno" had occurred to me as an explanation but I am showing lower speeds on my favourite test pieces of road and seeing changeup in the ZF22 at lower revs on the rev counter (whatever that is called in the USA), or tachometer for pedants.

Meanwhile, has anyone any views on the question posed
?
The only question I see is bold, and it's hard to answer. But my view of the question is on a small screen on a phone, so others will have a much larger view.
 
Simplify

SIMPLE version of question follows.

Would one expect replacing a 2.5 inch (internal) straight-through silencer box, the rear box in the system, with a straight pipe of the same internal diameter, to cause a loss of power?
 
I speak English in England, where a muffler is something a Yorkshireman wears round his neck in cold weather, very like a scarf, and an exhaust pipe has one or more silencer boxes in it.

Yes, calibrating the butt dyno) or, as we would say "arse dyno" or "bum dyno" had occurred to me as an explanation but I am showing lower speeds on my favourite test pieces of road and seeing changeup in the ZF22 at lower revs on the rev counter (whatever that is called in the USA), or tachometer for pedants.

Meanwhile, has anyone any views on the question posed?

OK, I asked because there are "empty" *boxes* that are custom made & used on big bore ultra perf V8 apps.(Article was in Hot Rod some time ago. I saved it)

And there is a formula to figure the volume of the "box". Was unsure if you were referring to a Silencer. Or the box I just mentioned
 
Pipe

The new pipe sleeves outside the end of the overaxle pipe. And I'm sure she's slower from 0-20 mph although on the slippery roads in the rain today I am having some trouble with wheelspin if I manually hold 1st and if I am brutal on changeup into second.
 
The short of the answer is YES. You've changed back pressure in the exhaust stream and have effectively changed what the 02 sensor is seeing imho. You have also changed the characteristics of your engine as far as fuel and power demand. Just like jetting a bike for a aftermarket exhaust. And that's a small motor. Bigger motor , bigger changes.
Rest your ecu, let it relearn. It will take approximately 300 miles i believe for you to get a good idea of how it will react. I had the same problem when my muffler fell off. Car got slower and louder. I reset the ecu, and she was a bit different after she learned! You didnt say what year it is , so i dont know what efi you have on it. But that's what I would start with.
 
SIMPLE version of question follows.

Would one expect replacing a 2.5 inch (internal) straight-through silencer box, the rear box in the system, with a straight pipe of the same internal diameter, to cause a loss of power?

No. If you let the ECU relearn what a/f ratio to give now. FWIW I modded my header pipe to have a 2.5" collector,installed 2.5" in/out MagnaFlow cat, and 2.5" mandrel bent sytem using 2.5" straightthrough MF mufflers, on my otherwise stock n/a B230. I saw a little bit of an increase in acceleration performance, especially at 60+ roll ons on the hiway.
 
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Generally speaking, turbocharged engines make the most power with zero backpressure. You want your post-turbine restriction to be as low as possible, so replacing a silencer box (muffler) with a section of straight tubing should improve turbine efficiency and power, if anything. Similar to removing a catalytic converter, removing silencers and restriction from the exhaust system is beneficial to performance. I've seen easy gains of +20 hp at the wheels on a chassis dyno from removing the cat on a turbo 4-cylinder.
 
Er -

May I point out we have diametrically opposed answers above - on the one hand "yes" and on the other hand "no"? I await enlightenment.
 
Run a catylic converter and a nice muffler with 2 inch pipe to keep it nice and quiet and drone free. Install a 3inch DP and install, say, a 44mm wastegate on the DP. Now you have nice and quiet exhaust but retain the wide open throttle free flow your after. I run twin 44mm gates on my setup.
 
Meow!

I have a stock Volvo cat, and the Jetex system like the Volvo system is 2.5 inch anyways and noise is not intrusive. Problem is my butt dyno tells me it is now slower.
 
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