Exhausts

[av8rirl]

While reading in the Formula 1 forum about "Renault to abandon wide-angle engine" I started wondering about exhaust lengths (and numbers) on other racing cars.

For a four stroke, four cylinder engine with a V of 180 degrees (ie flat), what would the optimum exhaust length be? Should it be short (like current F1 exhausts)? Or should it be long? Now I'm sure that there are formula's for working this out, but what is the formula? does it depend on cc, compression ratio, exhaust diameter, exhaust bends, etc???

Also would it be better to have 4-2-1, 4-2, 4-1 configuration? What are the pro's and con's for each? Should the exhaust have its exit facing downwards? horizontal? or upwards (like the current F1 trend)??

I'm sure there will be much debate as to what is best and what is personal opinion but ALL answers are much appreciated.

[Jukebox]

You definitely have to change to a performance exhaust system for a four cylinder engine because you have to remember that from all strokes (intake, compression, exhaust and power)...only the power stroke does the job actually, while the other three absorbs power from it. That is why you need to maximize your power stroke's performance
especially getting rid of the back pressure produced by the exhaust stroke.

The actual length depends on what car you want to fit it in....different cc prefers different diameters and so on.

boyracer, if you happen to read this....would you mind forwarding the table for calculating the correct lenght of the extractors to av8irl....i seemed to have misplaced it.

[boyracer]

Sorry but I also appear to have misplaced that file.
As for advantages between different systems, A four-into-one system is best suited for high revving motors (not sure of science behind this all, has to do with many factors including scavenging and reversion), this system sacrifices some low end performance for better breathing at full throttle (F1 for instance) Whereas a 4-2-1 system provides a more useable power band in the low to mid range, more suited to road car.
Upwards or downwards the outlet of the exhaust is not a huge performance issue. On a road car it is nice to have it pointing either rearwards or slightly downwards, mostly so water doesn't enter the exhaust. However I guess if angled down in the wrong spot (say within the coachwork of the car) the sound waves may bounce off the road and cause reverberation. F1 exhaust's pointing upwards (as I understand) is to aid in the extraction of the exhaust gases with the various aerodynamic devices on the car. In other words the exhaust gases are dumped into a low pressure region (air moving from high to low remember) where they will have as little effect on the downforce of the car.
Older F1 cars used to vent exhausts into the venturis to push more air through the venturis and thus obtain more downforce, but it was RPM sensitive and obviously they don't do that anymore.
Hope this has helped.

[av8rirl]

The engine will have only a very narrow power band (approx 1800 revs). There will be two options: 1) 4500 - 6300 and 2) 5500 - 7300.

What are your thoughts on 2x 2-1 exhausts? Would this be worse, as good or better that 1x 4-1? Do bends in the exhaust matter (if they do not change the diameter)?

[Jukebox]

High revving engines usually for racing will always go for the 4-1 type of manifold and F1 engines have dual extractors for the V10 engine by having 5-1 exhaust manifold on each side.

Reasons being for Ferrari introducing the periscope shape in 1998 exiting at the rear top of the car has everything to do with FIA's regulation of not allowing extra air running through the diffuser as where the exhausts used to be.

The other would be since higher rev of rpm were able to be achieved year after year, the length of the whole process of the exhaust had to be shortened as to not disturb the frequency and the engine moved further forward hence making the center of gravity of the car to near perfect. Seemed like a perfect combination and others tend to follow.

One thing I know for sure is that the advantage with dual exhaust system is that the hot air exits more freely, thereby lowering the back pressure which is inherent in an exhaust system. With a dual exhaust system, a sizable increase in engine horsepower can be obtained because of the "breathing" capacity of the engine is better, leaving less exhaust gases in the engine at the end of each exhaust stroke. This, in turn, leaves more room for an extra intake of the air-fuel mixture. In which might be a good solution for the common back pressure problem for four cylinder engine.

I suspect that the engine would be use in a racing environment? Then using a smaller diameter pipe would increase the air velocity since volumes of the air that enters the cylinder would be the same…in other terms it will have to go faster. But at higher speed there would be restrictions.

Standard calculation of the diameter

Less than 250hp – 2 – 2.5 inches
250 – 350hp – 3 inches
400hp & above – 4 inches

You could only maintain the diameter through all those bends by using the mendral bending process and if you are thinking of pressed bent piping method, better not cause in each bend the diameter would be smaller. F1 engines bends their exhaust pipes cause they need to narrow the rear end of the car not because of better airflow. One other thing to remember is to make sure the inside of the exhaust pipe is smooth.