Exhaust pipe diameter

Beste Hans,

Has been discussed a lot, see

http://www.ferrarichat.com/forum/showthread.php?t=109585&highlight=pipe


met vriendelijke groeten,
Ron

 

Maybe in theory but not in practice. The 2" diameter aids in the exhaust silencing w/o actually being restrictive. It also helps balance air intake flow with overall engine performance.

More importantly, it's the style of the muffler that dictates the restrictive nature of the exhaust gas flow.

Very few cars have exhaust diameters beyond 2". So, don't be fooled by the numbers. I had designed mufflers for V-12 Jaguars, and the ID was no greater than 2" (50 mm).

Exhaust designers in the 60's followed a rule that exhaust pipe ID should no greater than the daimeter of the largest valve in the block! You can imagine how small those pipes were.

Stick with the 2" and work forward on this.

 

Like the man says, power OR torque, but not both.
A free(r) exhaust will give you more power at higher RPM, but it may actually reduce your torque at lower RPM, as some fresh mixture may escape from the cylinders before the exhaust valves close.
This is the "Achilles Heel" of engines with 4 valves per cylinder, as the port area is quite large when the valves are open, while 2 valvers are known to have higher torque (but less power at high RPM).
To get what you want or a good compromise, you need to experiment quite a bit and the use of a dyno would be a great help.
Good luck.

 

Power and torque are not the same: you may have a motorcycle engine that produces a lot of power at very high rpm but that would not have sufficient torque to move a car (with a less powerful engine) from standing still.
Torque is generated by pressure on top of the pistons, by the surface of the pistons and by the length of your stroke.
For any given engine, you reach maximum torque when your cylinders receive maximum filling of fresh mixture.
Tipically the coefficient of filling of a cylinder of an aspirated engine is less than 1 (where 1 corresponds to the cylinder completely full of mixture without port losses) however, with clever intake and exhaust design and at certain engine speeds, you may have an overfilling situation through a certain fascia of rpm.
This is the surge of power you feel in your car above 4,000 rpm and up to 6,000.
By opening too much the exhaust, some of the fresh mixture may escape through the still open exhaust valves, thus robbing you of some of the energy that would go otherwise to produce the push on top of the pistons.
This is a common problem in 2 stroke engines, where the design of the exhaust is an integral part of their timing.
It is less critical in 4 stroke engines, although engines with 4 valves per cylinders are more sensitive than 2 valvers.
This is why I said initially that you may need quite a bit of experimenting with the pipes and that a dyno would help expedite the process.
Obviously variable valve timing makes almost all of these compromises redundant and the size of the exhaust less critical.

 

Let me assure you he is not the only one!

You can operate on all the theory you want, and you can wear out all the pencils in the world trying.

It's the actual practice and experience that will teach you.

If you have had neither, good luck trying to reinvent the wheel.

 

The beauty of a balanced V engine is that the banks are isolated making tuning easier and sportier.

If you add an X pipe you are defeating this aspect, adding to exhaust gas flow resistance, and affecting tonal quality. manufacturers do this, because it's a cheap way to reduce exhaust sound.

I need to see a drawing of the OEM 456 system before I can tell you what would be the best way to go. I'll try to resource it.

If you replace the "mid-muffler" with pipes you'll make the system louder but not necessarily sportier.

 

In short for max performance go with 2 seperate straight pipes, no X-pipe. For a street car 2.5" OD would be your best bet.

 

That's a good idea.

I would drive the car a while to make sure it's warmed up fully so you are able to hear all the nuances of the exhaust in this configuration.

 

Is the sound much louder? And what about the performance?
Better mid-range?
Regards,
Ron

 

Dear Hans,

Dont forget the other potential bottlenecks downstream of the center pipes:
the bent tube from the central pipes to the rear muffler and the rear muffler itself

I wouldnt like to see your gains held up!
Regards,
Ron

 

LOL! Quite amusing to see some of the attitudes on here. Practice and experience is all very nice...what you don't seem to share is the factual results of this experience which make your statements on here nothing more than opinion.

-To quote David Vizard: "I don't do opinion, I have my flowbench and dyno!"-

You are correct in stating that the design of a silencer mandates its noise suppression characteristics, what you can be sure of however is that your basic 2" in 2" out silencer won't outflow an equal length 2" piece of pipe!!!

If noise suppression is your primary design goal stick with small bore pipes and silencers...easy as that. If maximum performance with street legal noise levels is to be the name of the game, things get a bit more challenging!

As stated in this thread, as a start you need about 2.2CFM exhaust flow to get within 1% of open exhaust power levels, this is FACT....it has been proven during 1000's of hours of dyno testing.

This makes it quite easy to compute the correct exhaust diameter, since pipe flows around 115cfm per sq.in. Silencers should be flow tested. Since a really good straight through silencer flows about 93-95% in comparison to an equally sized piece of pipe, silencer size should typically be at least one step up from exhaust size.

Then there's length.....a resonator box in front of the silencers takes care of wave tuning, you'd need a dyno to test the best position. Then there's primary size....about Mach 0.3 exhaust speed is what you aim for to get maximum scavenging with minimum backpressure.

Anyway, feel free proving me wrong with dyno sheets of cars using exhaust equipment of your designs compared to open pipe, or any other factual material.

Ciao,

Harald

 

I have read all of this technical information on exhaust pipe diameters and have found nothing in regard to the effect of the exhaust gas temperature on back pressure. As the exhaust reaches the back of the car it has cooled to the point it cannot burn the skin. Try putting your hand at the back of the cat! What this means is the rear muffler is not nearly the restriction for the same area flow path as it is is closer to the engine as hotter gas needs a much larger pipe cross section for the same flow resistance as does cooler gas.

The easiest performance upgrade you can therefore do is install cat back pipes as large as can be physically fitted in the space provided (3in probably) and then reduced down to the stock over the half shaft pipes to the mufflers. A further step is to replace the cats with pipes to match the cat back pipes. Obviously this isn't 'green' and with the bulk of the noise suppressed in the stock cats the sound would be 'full race' without mufflers. Hi flow 3in cats would be another option.

I have a set of early centre muffler rear 575 cans and the sides which have the valve are straight through when open. One then needs to ask why do I need these heavy rear cans (nearly 40kg) if I fix the valve so it is open all the time? While I am still waiting on my 2004 manual 575 held up in compliancing here in Oz (headlights), it came with a 'Tubi exhaust' which turned out to be no mufflers at all. The dealer (Furlonger) kindly shipped me the aforementioned mufflers gratis to enable compliancing.

 

Further to this and after looking at the drawings on Ricambi I can see that the flow through the 'over the half shaft' 2 piece pipe would also benefit by perhaps going down to 2 1/2 inch to whatever muffler/tailpipe you are using, the exhaust being significantly cooler by that point.

My car with just the cats doing the silencing is nowhere near as loud as 'full race' but it surely does scream. I would imagine that without cats, 3 inch front to rear and 2 1/2 inch over the half shafts to a proper 2 x 2 inch performance muffler at each side, I would have a very big power jump within reasonable sound limits. One then just has to take the cans off for the the 'full race' noise for a track day.

I will be doing before and after runs down the local drag strip and I will bet the big pipes will add a few mph through the traps. Remember, it is the speed through the traps not the ET which you should compare. You don't have to punish the car off the line as within reason, no matter how poor your start the mph will always be close to that of a perfect start. I have seen lows of 111mph for a 550 and as high as 117mph for a 575 quoted. Does anyone have accurate numbers? This site has all you need.

www.ajdesigner.com/phphorsepower/horsepower_equation_trap_speed_method_horsepower.php

This also brings up the question of the ECU. Is anyone here FULLY (before and after dyno figures) knowledgeable as to whether this has to be played with?

 

Great explanation, Greg. Thanks!

You're no doubt familiar with all this too: http://www.jagweb.com/aj6eng/technics.php

Barry

 

In the mid-eighties I was playing with a Holden Turbo Commodore with a single turbo 3 litre Nissan straight six. I had up-sized the turbo, added an intercooler and an auxiliary fuel injector with a manual controller using an aircraft exhaust gas temperature guage to keep things from melting. I was at the point where I was maxed at 14psi boost with the stock, I think it was a 2inch, single exhaust. I replaced the pipe between the exhaust manifold and the stock rear muffler with a 3inch pipe. After roughly adjusting for more fuel I gave it the boot, the boost jumped to 22psi before blowing the ends off the intercooler! Needless to say once that was re-welded I had easily double the power still using the stock muffler. The BIG hp gains are to be found where the gasses are hot, not at the rear muffler where they are much cooler and therefore do not need big diameters to flow the same. I did eventually replace the stock muffler and found a very much smaller hp improvement albeit with a nicer sound.

The obvious 'choke point' for hp in Maranellos is the cats and secondly, right after the cats. It would be so easy for the center muffler delete pipes that are available for sale to be made bigger than stock. Same with the cat delete pipes. Why aren't they?