mega_stream

Is torque lost with a catless exhaust system, on an otherwise standard car?
I've been reading some reports stating having too free a flowing exhaust system can actually decrease performance.

As I'm no engine expert, I've no idea if this is true..I'm wondering if I should fit my original downpipe back on my car.
I know a shed load of people have free flow systems that crank out high bhp/torque, but noticed many have also upgraded the fuelling/turbo/FMIC etc etc..

Should I put my original dp back on's the short question

John



[Edited by mega_stream - 2/2/2002 8:47:58 PM]

mole

mega_stream,

Seeing as none of the well informed people have answered you, I will start the ball rolling!

As far as I know, our engines need some back pressure, too much no good, too little no good.

When I fitted a de-cat downpipe (albeit ppp ecu and ppp exhaust), The first thing that became apparent was massive torque increase and quicker spool up.

I have not had, (nor will I), one of those huge back box things, on some cars they can reduce back pressure so much that the torque drops dramatically.

Some cars also suffer from "overboost" with a de-cat, more likely in colder weather, and with higher octane fuel (inc booster).

Hope this gives you a starter for 10, (sounds like university challenge)

Cheers, Moles Dad.

Steve Breen

A question to mega_stream.....did you notice a drop in torque when you put the system on? If you didn't and feel that the car is faster now why worry?

Now personally, I had a back box on my P1 which I found to boom once hot at the very same rpm as what I cruise at. Also I was sure there was a loss in low down torque. After three days I put the original STi silencer back on. What a difference! The low down torque was back, and my head stopped hurting. This was a plus point over the slight top end power I had gained.

Steve

mega_stream

Emm...trouble is I did my system in stages, had a SS backbox, the rest original at first, then had the dp and centre done.

What I was getting at is, is it just the mind being tricked into thinking the car is running better after these mods by the all of a sudden fantastic sound the car is making. I know there are some benefits of a better exhaust system even on a standard car, but are you really going to notice a 5-10bhp increase?

Trouble is I spose, unless you remap and up fuelling gains to be had with an exhaust change are always going to be fairly small.
God knows what my p0xy insurance company would think of doing things like that

NBW

John,

I suggest you put your original DP on and sell me the catless one for a tenner.

Cheers
Tim
PS Seriously, I've heard nothing but good things about catless downpipes, which is why it's my next mod. Look here some comments on this thread re: effect on torque etc. of different designs:

http://www.scoobynet.co.uk/bbs/thread.asp?threadid=69500

Cosie Convert

IMHO losing backpressure on a turbo car can only be a good thing.

On a normally aspirated car, which has been designed to operate with specific backpressure, loss of bottom end torque can be experienced due to the extra exhaust flow reducing the charge volume in the cylinder (just like the effect of longer duration/overlap cams)

On a turbo car this is not an issue. Less backpressure allows the turbo to start to spool earlier, this extra boost more than compensates for the charge gas lost down the exhaust.

The effect of a less restrictive exhaust is even more pronounced at higher RPM as this is where backpressure is at it's peak.

With a low BP exhaust system the torque starts to rise earlier and the power extends higher up the rpm range. As the boost is rising earlier, you get a smoother transition into the power band. This can lead to the illusion of lost midrange torque.

As a trial, I replaced my backbox with a 3" straight pipe, apart from the WICKED sounds My 'seat of pants' impression was that I had maybe gained a little power over 5K rpm ?

The stopwatch told me different, my 50 - 70 time (4th) had reduced by the same ammount as my 70 - 90 time My boost pressure had gone up 1 psi and my exhaust temp had dropped 40 deg C

Hope you find this helpful

cc

paulwadams_my99

I don't think that engines "need" backpressure. Its an obstuction however most normally aspirated engines have back pressure because of the legal requirements of exhausts and cats and the engine/ecu are therefore tuned that way. Reducing back pressure on a normally aspirated engine could therefore have consequences if you don't retune. I don't believe the same is to be said for turbos as the back pressure is already induced by the turbo...Might be talking crap but from what i have heard back the less back pressure in a turbo the better.

mega_stream

Ahh its all become clear to me now

Thanks

AndyMc

Just to add a bit more,the reason exhaust back pressure has an effect on engine torque is because the moving gas in the exhaust has inertia, and different back pressures creates different amounts of inertia.
As the engine fires exhaust gas is released into the exhaust one cylinder at a time so the flow isn't constant, at the end of each exhaust stroke the exhaust valve closes cutting off the flow of fast moving gas,because this gas has inertia it tries to keep moving and this causes a low pressure pulse in the manifold while the gas slows down.If this low pressure pulse can be made to coincide with the opening of the next exhaust valve then it can help to suck the burnt gases gas out of the combustion chamber.This is why the design of the manifold and the exhaust has an effect on the torque produced.

By the way the same pulse tuning effect can be used on the intake side of the engine too, which is why some cars have variable length inlet manifolds.

The thing is, if you compare how much extra power and torque an engine produces if it makes use of pulse tuning effects the difference is small compared to a conventional engine,and very small compared with adding a turbo.

If you took the back pressure arguement further and fitted smaller and smaller exhausts to increase back pressure its obvious the engine would lose power and eventually stop completely so it seems logical to me,to assume if you reduce back pressure you will increase power and torque .As was said above I think the often quoted theory that bigbore exhausts lose torque has come about because the extra power higher up the revs can make lower engine speeds seem flat.

Andy

Cosie Convert

It's interesting that on turbo cars very few (if any) standard exhaust manifolds take advantage of pulse tuning. Most just dump the exhaust into a non-flow friendly casting relying on the pressure and not the pulse to do the work 4x4 Cosworth being a particularly bad example !

As road going turbo cars don't tend to use much cam duration or overlap, this doesn't harm power as much as on a high reving N/A car.

mole

Well, I'm still learning!

Moles Dad.

BugEyed

Cossie Convert

I knew that I'd have to come clean about this eventually.

When I left college in 1987 I worked for Cosworth and was responsible for a number of things, some that I'll even admit to. One that I normally choose to forget is the Sierra Sapphire Exhaust manifold!

As you know, the original Sierra Cosworth (YBB) engine used an expensive cast mainfold that used long separate runners that bent round 180 degrees to provide a true equal length tuned exhaust prior to the turbo. Unfortunately, the manifold was also used as a support for the turbo, and with the unbalanced secondaries on the ford based block (no balancers etc.) and the thermal stress, the exhaust castings used to crack on a regular basis. Ford were unhappy with this, and when we started to add the extra mass of the front diff onto the sump the cracking on the exhaust (don't ask why!) got worse. So, the Newbie was told to design something "cheap" that wouldn't crack. So I did.

In my defence, I will also say that it dropped less than 3 BHP in comparison to the original design, cost less than 1/3 to manufacture, and ended our cracking problems. However, it was as ugly as sin!

I feel better having admited that, but now you see why I'll never dare criticise the Scooby exhaust manifolds (despite their alleged short-comings).

Duncan

Cosie Convert

Mmmm very interesting What do you design now ?

I'm about to build an exhaust manifold to put a T3 turbo on my 1100 Kawasaki engined Radical. I'm currently see-sawing back and forth between a simple, short, easy to fit set up and a proper stainless tubular equal length system.
Problem is, I can't decide on the length of the primarys, plenty design data around for atmo exhausts but not much for turbo's.
I'm retaining the high duration cams as it revs to 11,000 so I'll still need the pulse tuning........ I think

thoughts ?

cc

BugEyed

CC

For a living, slow websites .....
For fun, go faster parts for vintage / historic cars ....

For a serious race engine, go with a short but equal length manifold. This combination worked best with the F1 turbo engines (even at 13,000 rpm). The Indy Car race engines (larger capacity, but similar speeds) run longer manifolds, but this is more due to the packaging requirements and the formula rules that mean you are only allowed a single turbo for V8 and have to mount it low down. Pulse tuning in theory gives a better result, but as you stated you loose the gas inertia from the last part of the combustion process (it is still expanding as it leaves the cylinder), and that is very important in providing a responsive engine.

Engine response is very important for a road race car where less than 60% of the time the car is on full throttle. The figure may be higher for your chosen form of the sport (say hill climbing), but even so a wide power band is still important.

Duncan

Cosie Convert

Thanks Duncan

cc

Jacko

Bugeyed, I thought it generally known that short equal length runners are good for high speed torque and longer runners for low speed torque? Then the next step would be to have a combination of engine specific tuned lengths of both primaries and secondaries i.e. 4:2:1 manifold??

However I could be a muppet talking cack ...?

Jacko.

BugEyed

Jacko

The tuning of exhaust manifolds consisting of primary and secondaries is a black art, but broadly you are correct. The use of the correct length primaries for high speed power and then secondaries for mid range torque can be achieved, but not without compromising each other. Broadly the lengths used to tune modern engines is for the top end only, with other methods (variable throttles, tumble generators, variable valve timing, etc).

Duncan

Jacko

Fully agree with Cosie Convert, although the CSA (cross sectional area) and/or length will move your torque/power curve. It does vary to different vehicle applications, but if you take a standard scoob and compare a 2.5" with a 3" straight thru sys, as cosie convert says, you will probably find that at higher rpm you will have more power/torque than the 2.5". However, the 2.5" sys will probably indicate an improved bottom and mid range improvements over and above the 3".
IMHO (as a general rule of thumb), if you want improved midrange go for 2.5", if you want more top end go 3". A good analogy is to think of a garden hose flow and what happens when you squeeze it. As pressure rises, the smaller bore will increase the flow to a point, until the back pressure/restriction becomes to much. On a slighter bigger bore, it will intially require more flow than the smaller bore before the throttling effect is observed.
However, less backpressure/restriction of the bigger bore will allow higher peak flow than the smaller one.
On an ECU modified car, either would not be a concern as they can be mapped out to a certain degree.

Jacko.

[Edited by Jacko - 2/4/2002 3:16:08 PM]