Michael Ghia

Catflap, Thanks for the info. I'll be making my own exhaust. I was just wondering if anyone has every run a completely straight through pipe (running less than 3 foot in length) and a seperate pipe for the wastegate which vents purely into the air. Also what diameter pipe to run on the exhaust.

Dean, cheers mate. I'll be running the motor with a WRX Version 5 ECU, without a cat and with a K&N air induction kit. Hopefully it'll make close to 270hp. Should be enough for 2 wheel drive

MG

[Edited by Michael Ghia - 12/7/2001 8:18:39 PM]

Michael Ghia

Ok, as my MY98 motor is going in the back of this bug..
I have the choice of what to do with the exhaust system.

I've read a few posts on here about running straight through pipes... are these really straight pipes? or mufflers which are of a straight through design?

I've also read about the different downpipes which you guys run. Some of you go for a one piece down pipe... others run a double downpipe which then runs back into one section. Has anyone run a seperate pipe from the wastergate just straight out?

The exhaust from my engine is going to be a lot shorter than anything on an Impreza. The car is a weekend only car aswell as being used on track. So noise isn't really an issue. The car is stripped out and will be fittted with a weld-in rollcage.

catflap

Speak to longlife in bristol, they can make you anything yo uwant, and i meen ANYTHING

Just had mine done and cant say enought about them, they also had a verry nice viper in at the same time, they know their stuff.

They will ask you what you want it to sound and look like.

Verry cheep aswell, all stainless steel

@Dean

Can't offer any advice I'm afraid, but I just wanted to say 'Cool car, Cool idea'.
Make sure you post some pics when it's done...

Sith

If you run straight pipes IMO you will get big flames and you won't be very road legal. Not to mention the noise. Mufflers will be required I would have thought. Surely you need a certain length of pipe for back pressure, or am I barking up the wrong tree.

P.

Michael Ghia

Sith, you have a good point with back pressure. Do I need any?

I doubt that I'll get flames without using something like an antilag system but the noise is not a problem. As I said, the car is for weekends only and for track use. Power is the important thing.

Anyone?...backpressure?

MG

Michael Ghia

Any Exhaust guru's around?
MG

MAFFA

That's gonna go like s*it off a stick going to just two wheels

Good luck

MAFFA

Michael Ghia

I hope so. The car only weighs about 800kg Trans will be from a 1984-ish Porsche 911 3.2 Carerra with an LSD. Still wondering what rear tyres I should run.

Can't anyone tell me if I need to run backpressure with the motor or not?

MG

pat

Michael,

the determination of whether you want back pressure or not is largely dependant on what you want from the engine and whether you'll be running it normally aspirated or blown... what kind of power delivery are you after?

Whatever you want, Hayward & Scott in Basildon will make an exhaust system to suit

Cheers,

Pat.

Michael Ghia

Pat,
The engine is an MY98 with a WRX version 5 ECU, the exhaust system I plan to build will have either a single straight through muffler to take the edge off the sound or with no muffler at all.
I've never heard one of these engines unmuffled. I've heard other turbo motors and they aren't that noisey, enough to live with and ok for track use.
I have read the thread on "Twin dump, single dump downpipe???" and found it very interesting but no one really commented on the double downpipe after Pete Croney said "The wastegate valve opens to approx 30 degrees max where the gas leaves the valve and then hits the separating flap before finding its way back around the valve flap into the small, second exit pipe. As a result of this flow path there is a restriction on the wastegate gas flow, wastegate control is much less accurate and the ECU is less able to regulate surges in boost".
Would it make any difference having the wastegate releasing into the atmosphere without running back into the system?
Does anyone else care to comment on the Twin downpipe? I'd like to hear more than one persons view on this downpipe.

Also... do I need backpressure within the exhaust system for an MY98 engine?

Mike Ghia

BugEyed

Mike

You don't need the back pressure of the exhaust to get the power, but you will find that the car runs (very) lean on wide open throttle.

Here are a couple of pictures from an Impreza being raced in the USA that has the wastegate vented separately;




I don't have Pete's knowledge of the Impreza, but don't fully agree with him about the downside of the twin dump downpipe. As I see it, the flow in the main pipe (from the turbine, not the wastegate) will be substantially improved by keeping it separate from the flow from the wastegate. Whilst the flow from the wastegate may be restricted (possibly leading to boost control problems) I would expect this to cause less of a loss than the gain from the main pipe.

Oh, the other advantage of the separate pipe would be flames!

Finally, the flow through the wastegate pipe will only be in use when you are running full boost.

Just my 2p's worth, but you are going to have a seriously noisy car for road use. How are the Rozzers up your way? Round here they would prosecute us if we ran it unsilenced!

Duncan

[Edited by BugEyed - 12/12/2001 12:11:32 PM]

nom

Reading this thread with interest (and no useful knowledge ) but I was just wondering how many times it's going to need to be mentioned that the car's going to be for track use & so noise on-road doesn't matter - nor flames for that matter
Just so we don't look like a pack of ninnies

BugEyed

Nom

quote from mike's first post
I took this to mean that he will use the car at weekends on the road as well as the track. If I've misinterpreted it, then I'm sorry to make us look like ninnies!

Duncan



[Edited by BugEyed - 12/12/2001 11:15:43 PM]

Neil F

My interpretation of backpressure is that it will affect the torque band, or, the point in the rev range that you receive maximum torque.
There was a thread a while back that detailed how the Impreza WRC cars were fitted with 2.5 or 3 inch pipes depending on which events they were running at and where they wanted the power.
It is also widely mooted that twin dump pipes like the MRT unit, which terminates in a 3 inch section, (less restrictive?) will give you more top end advantage than a single dump like a Scoobysport which narrows to 2.5 inches (more restrictive?) where you will gain in low/mid torque.
A final point that springs to mind is that the BMW M3 engine when fitted to the Z3M produces around 20 BHP less power (don't quote me on the figure, but it is less) than in the regular M3. BMW's explanation for this is that they can achieve a longer exhaust length in the saloon which I can only assume means that there is some back pressure issue there?.
I'm not a guru by any means, but in the 17 odd years that I have tinkered with cars I have regularly heard that back pressure is a requirement for any engine except for very specialised "optimum use" setups (drag racers etc).
Hope this is of some help.

Neil.

PS, the setup you are considering will be facking loud, make no mistake, and you will have flames, especially on trailing throttle.

chuckster

It'll not be allowed on a UK track if it's too loud - bear that in mind!
Even some scoobies got black flagged at Bedford.
You may need to manufacture a baffle to keep noise levels down when on track.
Chuck

nom

Ah, oops, I see the thing in the first post.
OK, I'm the ninnie

Delta

Hello,

Just wanted to give you my 2p worth...

I have installed the same engine in a VW bus ('74)

The exhaust I had manufactured dropped straight down from the turbo - with the wastgate dump integrated with the main after approx 7" free run

I used 2+1/2" main pipe.

The pipe then did a gentle bend around the back of the engine (where the usuall vw muffler resides - with a single freeflow silencer - ie. straight through with perferations in the pipe and glass packed surround, much like a motorbike exhaust) - after the muffler the exhaust dumped down toward the ground.

On the dyno the engine produced 240 hp at the rear wheels with only some very minor mods (electronic wastegate controller)

and my Bus was capable of sub 15 second quarter mile passes - which is nothing compared to your bug - but i was hauling a full camper interior also

The system was capable of passing noise restriction tests at all the tracks i visited drag racing - and did spit some tasty flames on overrrun which was always fun on the road... grin.

If theres any questions you have - gimme a holler.

Michael Ghia

Great info.. thankyou to all.
I don't mind the powerband moving up the rev range so long as it doesn't move so much that I drop out of it when I change gear
I will go with the seperate wastegate dump pipe and use a 3inch pipe from the turbo.

Bugeyed... do you know what the diameter of the down pipe was on the racing scooby? I can see that it's fairly large in the pics but not sure if it's 2.5" or 3".

Is there enough room to get a 3" pipe on there plus a 1" pipe for the wastegate? I take it that 1" is big enough for the wastgate?

I'll use a straight through 3" muffler to take the edge off the sound. It'll probably be noisey enough with just the one. I just don't want to put too much of a muffler on it to keep the hp.

Thanks again

Mike Ghia