JohnKoaWood

Lets also not lose sight of the fact that you are trying to compare a 2.5L 8V oversquare motor built in the 80s to a 2L 16V undersquare (85*88) motor built (likely custom built) within the last few years...

Hell, the Evo IX came with up to 300+Hp from the factory!

IMHO there are just too many differences between our cars and an Evo to draw a conclusion or parallel between them!

NOW

I think everyone would LOVE to see back to back comparisons from one exhaust to another... IF my red car ever gets finished, perhaps I will drag it out to a local fabricator budy, throw it on his dyno, and start playing with exhaust components... IF...

333pg333

Oh, clearly it's very hard to compare. This was really just a starting point in a conversation that took us to other subjects....but a motor is still just a pump. If it's pushing out 700whp down a 3" pipe with an even smaller downpipe then it's probably within the bounds of acceptability to believe that it could work on a Porsche i4 3L race motor, no?

Eric_Oz_S2

Patrick,

From what I have read in various books, a 2.5" system is good for 400hp and 3" for 600-700hp. A 3" system for 400hp is probably OK, but for less than 400hp, 3" would probably result in a loss of torque at lower rpm. It is of course dependent on each individual engine.

A larger system would supposedly make more peak hp at the expense of midrange torque/power.

thingo

I've dynoed my car without the exhaust, with about a foot extra off a three inch downpipe(my downpipe is a bit longer than stock), the car made maybe one horsepower extra, but it spooled about 500 rpm worse and was completely sluggish. Garrett gt3071 3" exhaust. I left my 4" oval pipes on the shelf.

Willard Bridgham 3

Hydraulic radius is what controls the pressure drop on oval/rectangular exhaust shapes. The ratio of surface area to cross-sectional area defines it. A circle has the least hydraulic radius and it's downhill from there for pressure drop.

Horsepower is a function of volumetric efficiency and anything you can do to increase volumetric efficiency (including less pressure drop in the turbine exhaust) will increase horsepower. Best operating point for a turbine is atmospheric pressure, ie, no exhaust at all with an evase to clean up the discharge in the casting.

refresh951

Do you have specific references for the 2.5" vs 3" info? I looked at this a while back and am interested in how this effects low end torque.

Eric_Oz_S2

I have one reference here, it is in "Supercharged - Design Testing....." by Corky Bell (now that's an American name!!).

You've probably read all the reasons before - including maintaining gas velocity to increase volumetric efficiency and promote filling of cylinders, etc.

Also slowing gas down too much (by opening up the exhaust dia as you move from the collector) cools it, increases density and generally reduces the upstream velocity as well.

DDP

The exhaust should be looked at as a system, as anything else. It must be sized to fit everything as part of a sum. There is a general range of size it should be depending on it's surroundings, bigger is not always better. Aside from the basic fluid theory, other considerations such as size and packaging should be considered. It seems the vendors are all about 4 and 5" exhausts and I wonder if they've ever installed them. That is excessively large. They're a absolute pain to work with. They're going to be close to hitting everything and there is no point for that in my opinion. I don't think there would ever be any reason to have larger than 3" exhaust.

The WG tie in does change things, however. That is best to be plumbed out to the back/side/hood of the car (if you're a hard ***) rather than being put back into the stock exhaust. Again, that comes back to packaging and what is easiest.

And I disagree with not being able to compare to the Evo. I understand the engines are different, but the physics doesn't change. I've learned plenty from what the DSM/Evo guys are doing because they're just pushing that much harder than us. I would trust their experienced advice before our speculated advice.

ausgeflippt951

Patrick, I don't care how much power that Evo is making. That has got to be one of the ugliest cars I've ever seen. In order for me to own that, it'd better be handing out BJ's laced with gold, diamonds, and crack.

I'm sure driving it is one helluva lot of fun though.

Thom

Thanks for that - this example just goes on to show that freeing up the exhaust should be balanced with freeing up the intake, otherwise there will be too little back pressure past headers and the turbine will necessarily spool later in the rev range, well at least when using a turbine more efficient than a KKK.

If we had more decent aftermarket intake manifolds and free-er breathing heads the 951 scene would probably look less different than the Evo scene IMO...

968 GUY

Nascar uses flattened exhaust systems that I believe are called boom tubes.
Have seen them online somewhere.
They come in various configurations and I think they have oval piping in different shapes.
Got it on the home computer.

Duke

A turbo does not want backpressure AFTER the turbine. That is just not how they work. And the empirical tests I've done without exhausts have always increased throttle response and boost.
However, most of the gains from a larger exhaust comes with the higher boost pressure as a result of the lower post turbine back pressure. If you adjust the boost pressure to match the level with the more restrictive exhaust the gains will be much, much lower.
But if you tune for it you can increase the overall power a bit more.

There is a reason the drag cars run the exhaust straight out of the side in front of the front wheels...
The reason why many Evo cars run 3" is packaging reasons. They have to run the exhaust under the engine. In this case it would be better to look at cars where space is no issue - like in the Supra tuning world...

Looking at engine X and saying something like "it makes 500 hp with 3" so it must be enough" is the wrong approach when discussing flow.
It is like saying "I make 450 hp on the stock head so no need to port the head at that level", or like saying "the WRC cars have 350 hp with a 36 mm restrictor before the turbo so a 3" turbo intake is excessive"....

Instead you should be thinking, "wonder what power it COULD be making"

333pg333

Collin, I think we are more used to the Time Attack style cars down here. Many of them aren’t pretty but they’re designed to go around tracks stupidly fast and it seems to work. These cars are actually faster than the Aussie V8s….and they’re on R spec rubber! Having said that I’m comparing to the V8s at the same track some years ago. Nevertheless, to be in the same ballpark is amazing. They are as fast as the fastest slick shod V8 Radicals on the same track which is very fast. They all say the final few seconds are down to Aero. Something that very few 944 guys have explored. My intention is to go splitter, canards, flat bottom, diffuser, wing at some stage in the future but who knows what will happen. Just driving will be a novel change!

I agree with you Thom. I wish there was more quantifiable info re intakes for our cars. I wonder just how imbalanced the stock one is. I mean all we ever know is what someone else has said. I’m sure they’ve been flowed but can’t recall seeing any actual figures. Of course LR say theirs works wonders and maybe it does but again, anecdotal evidence suggests there are question marks over that. Seems to be built well enough. I know there is one on the only 8v 3L racecar down here but they said they had to modify it. They didn’t say in what way these mods were carried out though.

Be interested to see what Nascar does. I'm 99% going with a 3" round system as I think this will give me back some clearance and perhaps spool.

333pg333

While I agree with you Derek, there is little knowledge of what is the best combination for the upper end of our modified cars. I’m sure there is base evidence which we can draw on but without doing lots of back to back expensive and time consuming dyno runs with different parts we just have to use guesswork. I’ve always been of the opinion that once you start changing/improving part of the VE system beyond a certain level, you need to change it all. Pretty sure I have this covered on this motor and ancillaries. Basically everything is larger…but that in itself doesn’t necessarily mean this will work. The intake is the last piece in the puzzle for me and I don’t have a solid alternative to what’s on there now.

333pg333

Oh, and the latest concept that some of us are thinking re my crash is that to gain clearance for the front wheels (18” x 10.5” et 50 w 285/650) the builder had to raise the front ride height a little. This in turn meant raising the rears and this is why I believe we have run out of rear droop or travel. While the Motons have a shorter rear shock body compared to stock the wheels / tyres hardly seem to drop down much even while on the hoist. I think the outside of the rear tyres are higher than the inside when viewed from behind so I guess this combined with the raised ride height contributed to running out of rear traction. We haven’t done any quantifiable measuring yet, this is all just theory. However the engineer is convinced that for whatever reason, we certainly ran out of travel as the logs show it ‘flat-lining’ through certain corners. The corner that I came off being one of them. So as ridiculous as it sounds I might have come off because we were being dictated to by the fender / flares &/or the wheels/offsets. To compound the irony, I bought the Fikses sort of via Broadfoot and between them they came up with the offsets. Broadfoot say that you can run up to 11” wheels et 47 on the front with their GT2 kits. Something is not adding up. Until we can measure ride height and the shock body / travel we won’t know for sure. Now I’m looking at possibilities of gaining some extra room in the front.