Back pressure in exhaust system

[Pauljay]

Hi Guys! There's a lot of talk going on about exhaust systems on this and other forums. Which got me thinking. Do our cars really need back pressure for performance! What does back pressure do for your engine? I thought the faster an engine gets rid of exhaust gasses the better. I know your not supposed to fully de-cat for emissions but what if cats aren't needed. Recently I dynoed my car and the tech said I'd lost 20hp from removing both primary and secondary cats due to loss of back pressure. I'm going to re-do my exhaust from the headers back. I have AMS headers and I was going to put in 100/200 cell cats and then connect to a Y pipe before running a straight pipe to my box. (2.5in pipes from headers to cats to Y pipe, 2.5in inlet to 3in outlet on Y pipe, then 3in straight pipe to box with twin pipe exit). My car was a 430 so I don't have the AMG exhaust. I just wanted your opinions before I went ahead. I got the idea from PTE. He also removed his resonator like I did. I like the sound. All comments are welcome.

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[ProjectC55]

Originally Posted by Pauljay

Hi Guys!

There's a lot of talk going on about exhaust systems on this and other forums.
Which got me thinking. Do our cars really need back pressure for performance!

What does back pressure do for your engine?
I thought the faster an engine gets rid of exhaust gasses the better.

I know your not supposed to fully de-cat for emissions but what if cats aren't needed.

Recently I dynoed my car and the tech said I'd lost 20hp from removing both primary and secondary cats due to loss of back pressure.

I'm going to re-do my exhaust from the headers back.
I have AMS headers and I was going to put in 100/200 cell cats and then connect to a Y pipe before running a straight pipe to my box.
(2.5in pipes from headers to cats to Y pipe, 2.5in inlet to 3in outlet on Y pipe, then 3in straight pipe to box with twin pipe exit).

My car was a 430 so I don't have the AMG exhaust.

I just wanted your opinions before I went ahead. I got the idea from PTE.
He also removed his resonator like I did. I like the sound.

All comments are welcome.

You would lose some tq in the lower rpm's but I doubt if you would have lost hp. You could get rid of the primaries and put on high flow secondaries. I would perhaps try to get the car tuned as well.

That's what I need to do to my car since I have headers bigger diam exh pipes and high flow secondary cats.

[Pauljay]

Hi Carl.

Thanks for the info. I needed to know if I could get away with not putting my cats back. I like the sound without them (if a little loud). But after what the Tech said I wondered whether to add some.

If I don't have to use the cats then I won't.
Also with the Y pipe do you know where the best place along the exhaust to install it is?

After the exhaust mod then I'll dyno again.

[MarcusF]

If you lost 20 HP, something is wrong. What did you replace the cats with?

[speedybenz]

Backpressure only robs your engine of power, unless during the exhaust phase of the engine operation the exhaust port works too well and allows unburned air and fuel to get past the exhaust valve. That condition is called "Overscavaging" which may occur at low revs with big cams, but should not be an issue with the short duration cams our engines come with.

On some current motorcycle engines they put a valve in the exhaust system which allows for some slight back pressure at low revs to limit the amount of overscavaging at low revs and then this valve progressively opens as the revs climb to eliminate the backpressure and allow for full power.

Jeff

[Pauljay]

Hi Guys,

Marcus

When I took my cats out I replaced them with a straight pipe 2.5in.
Also where the primaries were I put an X pipe (I know they don't work on our cars) to quieten the exhaust a bit. Which worked.

Speedybenz

Thanks for the info.
I'm running a set of Schrik cams so maybe your onto something with the scavaging thing.
When I dynoed the first time the Tech said the car was over fueling and my AFR was wrong. He wrote something into the ECU and said my AFR is now correct and is 13%. I know nothing about air fuel ratio but have noticed other members are lower.


Surprisingly he said my torque lbs/ft went up from where it used to tail off and remained high even though at peak I lost 20hp.

That doesn't seem right which is why I'm asking about my backpressure.

Guys it seems the Tech screwed something up and backpressure has nothing to do with it.

Is that correct?

[MarcusF]

Jeff hit the nail on the head. The backpressure misinformation has been spread everywhere, and it’s wrong. Engines don’t *need* backpressure. To make horsepower, engines need high exhaust velocity. Velocity is the speed the exhaust gases are traveling. Velocity results in inertia, which results in scavenging (Newton’s law of motion). Scavenging clears additional exhaust residue from the chamber (a very good thing), and can throw off air/fuel readings at low revs (it takes long duration cams though, – got the specs for yours?). High exhaust velocity is dependant upon the exhaust system, which controls pressure waves.

That brings us to your problem - it’s just a guess, but your x-pipe may be jacking up, rather than benefiting, your exhaust pressure waves. Also, and I mean this very kindly, an x-pipe is for improved scavenging. That’s why the placement of an x-pipe is so important. It’s noise reduction is a side benefit. For sound quality, mufflers are a better tool for the job. Advice? Your cam manufacturer may have preferred exhaust specs.

[Fabio D]

Who said you our cars need backpressure to performance???? :S some crazy peolple round the world! you need to deleat aaalll backpressure in the exhaust system to talk about prformance buddy

Fabio Daniel

[ProjectC55]

Originally Posted by Pauljay

Hi Carl.


Also with the Y pipe do you know where the best place along the exhaust to install it is?

After the exhaust mod then I'll dyno again.

I plcd my Y pipe just behind my secondary high flow cats. Pretty much where the stock Y-pipe was located.

[Pauljay]

Thanks Guys!

I knew something was up.
Glad I can get away with not installing new cats. That'll keep the cost down.


Marcus

You're a wealth of information .

I'll find out tomorrow what spec cam I have. All I know is they're the middle spec. The 1st was a modest tune, good low-mid torque and Hp. The second was mid-high (fast road and track) slightly lumpy idle and the last was a race cam, very lumpy idle and short power band. That's the way it was explained to me.

Carl

Thanks for the info on position.

[blackbenzz]

I have dual 3" pipes, then again it is FI too

[Pauljay]

Originally Posted by blackbenzz

I have dual 3" pipes, then again it is FI too

Yeah I know, I read about your car!

Research has said the best route is 2.5 into 3in.
Does anyone disagree.

I'm after power as well as sound.

[MarcusF]

Originally Posted by Pauljay

Yeah I know, I read about your car!

Research has said the best route is 2.5 into 3in.
Does anyone disagree.

I'm after power as well as sound.

Think about this for a moment – a supercharged engine ingests more air/fuel than a NA engine. If that's true, wouldn't that same supercharged engine have exhaust requirements that exceed those of an NA engine?

I too have read where posters say the recommended exhaust size for an M113 is 2.5" to 3.0". I look and smile. So . . . . it's the same size for the 430 as it is for the 55? They’re both M113's, but one engine is over 27% larger. The 55K is 27% larger AND runs at 0.8 bar of boost. How can they possibly all use the same sized exhaust?

[SoCalCLK]

Originally Posted by MarcusF

....I too have read where posters say the recommended exhaust size for an M113 is 2.5" to 3.0". I look and smile. So . . . . it's the same size for the 430 as it is for the 55? They’re both M113's, but one engine is over 27% larger. The 55K is 27% larger AND runs at 0.8 bar of boost. How can they possibly all use the same sized exhaust?

Precisely. My thinking was the same in terms of the OEM Air Filter being same as in a CLK320 and in a CLK55!

[Pauljay]

Marcus and SoCal

I hear what your saying. Maybe the 3in is for sound. I admit I don't run cats or resonator which makes the exhaust louder, but maybe the bigger exit gives a deeper tone.

As I don't understand the finer workings of an exhaust could you help clarify that as I was going to re-do my exhaust tommorrow.

Can I get what I'm after from a 2.5in diameter?

[MarcusF]

You're asking about exhaust tubing, and what size to use. When exhaust leaves the head, it's HOT. For performance, it’s best to keep it as hot as possible until it leaves the exhaust system (tail pipe). That’s because cold air is dense. Dense air is heavy and slow, which impedes scavenging. So here are your issues - oversized exhaust tubing impedes velocity by allowing the exhaust to bounce around and cool before it exits. While keeping the exhaust hotter, undersized exhaust impedes velocity because it’s too small to move the volume needed. You've dumped your cats, and some cat manufacturers tout the fact that their cats HELP flow by keeping exhaust temperatures up while not impeding flow. Seeing as how some cats have flow numbers that surpass mufflers, so it's hard to argue with their point. But then I reread your posts and it sounds like you're mainly concerned with tone. For tone, forget the pipe size. Mufflers have a greater impact on sound.

[Pauljay]

Thanks Marcus!

You cleared that up perfectly.
The lost performance was what I wanted to regain but without losing the sound/tone of my exhaust. I guess my muffler gives the right tone for me. I'm going to stick with a 2.5in tube from headers to muffler.

I would post a clip when done but it never sounds the same on camera!