Exhaust Backpressure

[Nath 1,975]

What's the go with it? Do all engines need it or just some?

 

I took the baffle out of the exhaust on my SV, it sounded spectacular but seemed to have lost some low end responsiveness, so I googled it and the general agreement was that it was because of the lack of backpressure, as the size of the hole pretty much doubled. Something about it causing the engine to run lean on it's current tune.

 

Is it the same with crossflows/clevelands etc?

 

 

 

[slydog 7,873]

Its got very very little to do with back pressure and more about flow and sonic waves in the exhaust.

 

To quantify that statement I run twin 2.5" exhaust on a NAT ASP 250 4.1 ltr engine. Even modern cars run big twins on small engines these days.

[Stu5766 394]

Any form of restriction in an exhaust creates " back pressure "

You want as little back pressure as possible, how ever a small amount can assist with scavenge.

On a standard pressed metal system, I doubt that knocking out the baffle would have a negative effect on performance.

[Nath 1,975]

So how come removing said restriction made it noticeably less responsive then?

[PRO250 1,506]

The flatter feel is state of tune the engine has lost tq somewhere in the power range but you will find if it sorted as a package it will make up for it else where

by SV you mean commondore?

[deankxf 20,266]

doesn't it have a lot to do with the tune, air speed through the exhaust.
My crossflow lost low end grunt when i went to a 2.5 inch exhaust, but picked it up with efi manifold and tuning with GRA throttle(was impco 300a on std webber)

[Nath 1,975]

Nah bike.

[gerg 10,871]

Gotta jet up if you increase flow in the system, simple as that. More gas out means more air in, therefore more fuel needed to keep mixtures right.

 

Another way of looking at it... You've got more horsepower potential now so you need more fuel to feed it

 

Haven't touched bike carbies before but if you can take shims out of the needle or it has a circlip with multiple grooves, you'll make it richer that way.

[Nath 1,975]

It's fuel injected, would have to get it tuned.

[wagoon 2,429]

+1 on flow increase through the engine.

When I had my quad I tried a simple airbox mod by removing the snorkel like feed into the air box(snorkel helps keep water out) This was the only change I made to it and the bike developed a massive flat spot so bad I nearly fell off a couple of times as the engine would just die. Since I didn't have the snorkel with me I taped up half the hole and the flat spot was just about gone. Jets would have fixed the problem as Greg pointed out.

It's the exact same principal with cars but generally a car is a little more forgiving and you aren't making such massive changes, as you said Nath the exhaust pipe is now double the size. Imagine taking the standard 2 inch exhaust and putting in a 4 inch exhaust on your car, it would make a really big difference.

[ando76 4,354]

Yep - as highlighted by my learned friends above - you have created a situation where your engine wants more fuel. 

 

No engine needs backpressure - why else would be spend time improving exhaust (and intake) flow on our cylinder heads.  All engines need exhausts built to suit the application (noise limits etc.) and then tune to suit.

 

If your car is computer controlled it may even fix the issue itself.

[Nath 1,975]

Cheers guys, very edumacational.

 

I would have thought the computer would have adjusted to it but after riding around for a day with it out, it was exactly the same.

[wagoon 2,429]

ECU can make small changes and adjust for small changes. removing the butt plug from the exhaust is not a small change. If you put an insert in that had a larger internal diameter you may find the bike improves on what it was before, but increasing the flow to much will require more fuel than they ecu can adjust for.

[Nath 1,975]

The only reason I did it was for noise factor. On the off chance I take it in to get tuned for some reason I'll do it without the baffle, otherwise I'll just leave it in.

 

One thing I was wondering - can I saw the 4 inch tube off the baffle and just run the faceplate (so the hole is the same size) so I can have my noise cake and not lose any throttle response eating it?

 

 

That really seems like something that should work but I obviously don't want to cut first and find out it won't.

[Outback Jack 6,352]

Ok nath.

What ya do on bikes is remove the baffle and cut off 1/4 of the length on the baffle..... bolt it back up , take for a ride and see how you like it.

 

Remember these guys that talk NO back pressure run hi performance motors with hi compression.

If you remove all back pressure on a standard motorbike you will burn the exhaust valves out over time.

I have done this mod a few times on standard bikes with good results.

I used to find 1/2 to 3/4 of the baffle removed worked well but start small... can always take off more but cant put it back on.

 

Jack.

[Nath 1,975]

Ok nath.

What ya do on bikes is remove the baffle and cut off 1/4 of the length on the baffle..... bolt it back up , take for a ride and see how you like it.

 

Remember these guys that talk NO back pressure run hi performance motors with hi compression.

If you remove all back pressure on a standard motorbike you will burn the exhaust valves out over time.

I have done this mod a few times on standard bikes with good results.

I used to find 1/2 to 3/4 of the baffle removed worked well but start small... can always take off more but cant put it back on.

 

Jack.

Cheers mate, I'll give that a try. It's a little too quiet at the moment with the baffle as is.

 

 

Bringing the topic back to cars/Fords - So you could technically run an exhaust with no restriction at all as long as you richen it up to compensate?

[Outback Jack 6,352]

Yeah I agree with the guys on hi po motors. .. less restriction the better.

 

Only cars I see that run no exhausts are full on drag cars.

 

Jack

[Nath 1,975]

Maybe I should try it on my 3.3 XD... Because that's basically a drag car anyway right?

 

...

 

Guys?

[gerg 10,871]

Maybe I should try it on my 3.3 XD... Because that's basically a drag car anyway right?

 

...

 

Guys?

 

Well you're not far from the Cross so "drag" might well be apt after all

[Nath 1,975]

Maybe I should take it down to eastern creek for a couple of passes, that way I can say it's a drag car.

 

No rule that says it has to be fast

[gerg 10,871]

So you could technically run an exhaust with no restriction at all as long as you richen it up to compensate?

 

Not exactly. As stated already a set of "zoomies" like on a fueler or tough race boat are for all-out racing, but anything else will benefit from some kind of collector and a length of pipe to take advantage of the momentum of the gases coming out of the cylinder. On atmo or supercharged engines that is. Turbos are a whole different universe.

 

But no restriction as in muffler, that's a win-win.

[ando76 4,354]

I somehow missed this was on a bike - doh.  Here is something from my experience.  I made a new intake and exhaust for my single cylinder 250cc water cooled buggy engine.  Why - because the standard unit was basically what they run on the scooters that these engines are derived from and it sounded and performed puss.

 

So I set to work and made the exhaust and read all the stuff on the buggy forum I am on and they all said it needed jetting up.  Yep cause I am a freak I fitted an O2 bung in the exhaust and ran the old AFR gauge from the race car in the buggy.

 

Took it for a run with unchanged jetting - just the intake and exhaust mods.  Well it wasn't much of a run because by the time I got to the end of the shop driveway (about 200metres) I realised how bad it was.  I'm talking high 17 AFR's the whole way through the band.  It sounded awesome but was on the way to 'burn a piston town'.

 

Back to the shed and out with the jewellers drill bits and I hooked into the pilot and main.  The pilot was a 38 (.38mm) and the main was a 1.2.  Now they are like 55 and 1.6. and I've got awesome AFR's.  I couldn't  move the needle height as there were no clip positions (chinese copy carb) so I just gave the tip a little love with some 1500 grit. 

 

The performance difference was amazing and AFR's are now in the safe zone.  I fitted an adjustable CDI and I gave her a bit more timing and that meant another run through with the jewellers drill bits to get the AFR's back on track.

 

So what does all this dribble show you - small changes on single cylinder engines (or small capacity) make a big difference.  I would of thought the efi had some head room in it but clearly it doesn't. 

[gerg 10,871]

A bit off-topic, but not too much... Toyota turbodiesels like Cruisers and Hiluxes have a way from factory of limiting boost besides the wastegate... They put a really pissy size exhaust on. Best upgrade you can do on these is go 3" straight through with just a hot dog. That and maybe a chip.

[Grimmy 312]

And then they crack Pistons... lol

[wagoon 2,429]

Thats a perfect example Grimmy of modifiying well beyond the capability of an engine without improving other area.

Ando the ecu would learn from small changes but removing the baffle would change the muffler from 1 1/2 inch diameter outlet to 3 1/2 inch outlet which no ecu would be able to cope with. The other thing that has been missed and may help is removing inlet restriction by the same amount although it still wont get the ecu to put more fuel in.