http://www.gt40s.com/ubbthreads/showflat.p...o=&fpart=1&vc=1



sorta kinda applicable

moi!



this one is applicable



http://www.gt40s.com/ubbthreads/showflat.p...e//fpart/1/vc/1

What were you doing in a GT40 forum?



Mark

um it was a link from the miata forum

I suppose that's ok then.



Mark

Oooh man. Talk about irony.



I don't know about Formula None, but the NASCAR guys realized long ago that for headers that have to fit in an actual chassis, it's kind of pointless, since any bend in the tube will affect how pressure pulses travel and are reflected. They don't seem to care about things many people think are important, like equal length runners. As long as the tubes clear the chassis, and have as few bends as possible, that's all that seems to matter.



And then I see things like equal length shorty V8 headers that look like a knot of earthworms. The NASCAR guys seem to be right, since they rarely make any significant difference over a similar unequal length header.

Yeah, F1 just has very short headers with no resistance/backpressure at all so there's very little point in trying to tune them, but knowing F1 I'd imagine they still spend wads of money on it.



Mark

he drives an mx6!

i read somewhere that one of the teams (i wanna say ferrari) has one guy who builds all the headers, out of iconel, they take 50man hours EACH and are replaced every race (season before last they had a couple failures during the race).



they are a little "weird" BIG diameter primary tubes, and smallish collectors.



on one of his posts that adam fellow mentions tuning the exhaust doesnt matter until you're turning over 10,000 rpms

I think the reason the collectors are small is to cut down on the air disruption caused by bigger holes in the bodywork.



I'm not sure about exhaust tuning being worthless below 10k rpm, I'd like to see his reasoning on that one.



Mark

Well he has totally missed the point with the weber inlet in his analysis, so for me that makes all the others suspect. You are looking at a totally different pulse tuning regime with IRTBs, which he hasn't spotted. There is a saying 'A fool with a tool, is still a fool'.



As for F1, they generally have 2 guys building headers. One always builds the left header, the other the right header. At Williams they are brothers !



So why don't nascar care, and F1 doesn't. Well for a start, unless you are running a flat plane crank then exhaust tuning is hard work. I suspect nascar don't go flat plane, as you loose a lot of the v8 sound.



Secondly lookat power/litre. F1 cars are approx 850HP from 3 litres. ISTR Nascar are about 650 from twice that (but my memory often fails me). The 2 engines are optimised for totally different requirements.

I must have missed that.



Intake and exhaust tuning are totally different, of course. The intake is working against a valve or port that is "closed" much of the time, while the exhaust isn't. The end of the port doesn't close, the beginning does! Top head porters have application specific flowbenches that actually pulse the flow, as in a running engine, and bow howdy does it make a difference. Funny how sometimes a port that has less CFM flow and less velocity will crap all over a port that "flows better", that must mean flow isn't it, and even velocity isn't be-all end-all.





Power/litre is merely a function of how fast they turn the poor engines. NASCAR guys are "only" turning their engines in the 9000-10,000 range, and remember that they also are running fairly low compression, a restrictor in some cases, and a fairly small carburetor. 390cfm carbs if I recall right, and 9.5:1. I am unaware if F1 has any similar type restrictions on power output other than mandating a given displacement and number of cylinders.



I don't know if the NASCAR guys run flat plane or 90 degree cranks. It probably doesn't matter, given that they use X-pipes in the exhaust now, which do a fair good job of negating the odd exhaust pulses. If not, then they could always run crossover pipes in the headers, similar to the systems commonly seen on Panteras, but as mentioned before, they have found that exhaust tuning doesn't matter all that much, and as restricted as they are you would think that they would take any angle they could to make an extra half a percent horsepower.



On further reflection, I doubt they would run a flat plane crank. A pair of 179 cubic inch four cylinder engines joined at the crank would make one hellacious vibration at 9 grand!

Dunno. Think Ferrari. They use flat plane cranks in (nearly) all their V8s. Georgeous sound. But good 'ol American V8 sound it isn't.



Lotus used a flat plane in the esprit V8 and managed to get it to sound like a cement mixer.

Doesn't a flat-plane crank lead to a better balanced engine? I mean that's the reason they use them for high revving engines. Funnily enough the other big advantage is better exhust tuning!



Mark

When you think of flat plane, think of Harley Davidson and how that engine runs.

F1 restricts the size of the air intake hole in the bodywork

I thought both pistons on a harley come off the same pin?



Mark

The problem with an even fire inline four cylinder engine is that there's a whole lotta shaking going on. The pistons go up and down in pairs. At 90 degrees from any given top dead center, the pistons are all even with each other. So far so good, until you think about it geometrically and see that due to the rod's being at an angle, the pistons are more than halfway down in the bores. This is just additional proof that piston acceleration is asymmetrical with respect to where it is in the stroke, and this asymmetrical acceleration causes a shake.



On smallish engines, with small pistons and small strokes, it's not really a big deal. Larger engines have problems. Counterrotating balance shafts were created to counteract the vertical shake, and even then they are not fully successful. There's a good reason why 2.4-2.5l seems to be the practical upper limit for a four, as Porsche seemed to discover with the 968.



What does this have to do with V8s? A 90 degree crank V8 doesn't have these problems because the pistons aren't paired in any meaningful way. One up, one down, two halfway in between per bank. Relatively low vibration, at the reasonable expense of one cylinder out of phase per bank. A flat plane crank has the problem because it can be described on several levels as being two four cylinder engines joined at the crank, with all of the shaking forces that it entails.



Ferrari and Lotus make production flat plane cranks, yes. They also make fairly small V8s, in the 2.5 to 3.5 liter range, where the shaking forces are not so bad. When Ferrari wants more displacement they make a V12 just like the olden days. Lotus doesn't even believe in displacement, just adding lightness.



A 5.8 liter flat plane V8 would be a nightmare of vibration forces tearing the engine apart, and I would estimate that it would actually become a performance detriment compared to the minor hassle of uneven exhaust pulses. If you look back at the old Busch series when they used V6s, they didn't even use even fire V6s, even though that is what the engines allegedly started out as! Shows you how much credence they lend to such things when other factors are taken into account.



I note that this is increasingly moving away from rotary engine building. But, it's always good to see that the grass is not only not greener on the other side of the fence, but it's patchy and brown and full of weeds and nobody's been picking up after the dog.

I dont know if anyone has bought it up but there is more than one (theoretical) way to tune exhaust lengths. I have a paper from a british reasearch group that was doing work for an F1 team which I will try and dig up. What they found was that basically as the engine speed increase the best tuning method changes from acoustic which I will assume the links are talking about, to inertial. Using CFD software the results from these theoretical approaches are very accurate. The only problem is the cost of the software. In regard to NASCAR there are a few SAE papers about the exhaust tuning so I guess they must do it to an extent.