exhaust pulse tuning modification

[powerabout]

The other thing you have to consider is an outboards bore centre lines are very close hence the loop charged porting is severely compromised compared to a sled engine and the induction has to go right across the crankcase then meet a slot due to the Vee config

[wax]

Yes we have the same issue on pwc engines with 118 bore centres
luckly our induction is a better setup than what you have to deal with

[Mark75H]

Not quite. A couple of the more advance engine builders have experiment with a different firing order crankshaft to use cylinders 1,2 and 3 as one set of three firing 120 degrees apart and 4,5 and 6 as the other set of three. This made the distance between the 120 degree phasing cylinders closer together in length trying to make the pulse and length to the next firing cylinder closer. Had to make a custom exhaust chest cover to group the banks of three correctly.
Great in theory for exhaust tuning but not so good for balancing and firing harmonics.

The Merc T2 motors used 2 sets of 3 close together rather than alternating even and odd ... this made the motor more closely resemble the 3 cylinder 650XS, x and 700x. When they did this, they did not retain the original close exhaust chamber depth ... an extension box was added to the side ... making the depth & distance somewhere between the longer Twister box and the exhaust on the triples.

The center exhaust still has to take a longer route to equal the distance between the top and bottom cylinders. Changing the firing order also will not change that. In any combination of 3, there will always be a top to bottom or bottom to top in sequence.

[FMP]

Ttt

[speedinstream]

There was a guy came to jasper one year with a v6 merc with 6 expansion pipes sticking straight back out of a custom divider it was very strong caused a rule change

[FORBESAUTO]

Wax is talking about exactly what I'm experimenting with the angles in chest to shorten distance between ports. I need some clarification. I understand two stroke exhaust tuning from messing with motorcycles. Which is why I'm having a problem with stuffing part happening in a v6 merc. I understand how it's supposedly working just not mathematically adding up to me on how fast the waves travel and the limited space in the chest. For instance on a single cylinder dirt bike motor with exhaust duration of 170 and a tuned length of 25 inches on an expansion chamber. The exhaust leaves the port is pulled by low pressure wave of the divergent cone,(which is the tuner in our application, especially a bell tuner) then travels to the convergent cone ( which we don't have) and goes back as a positive pressure to stuff the fresh charge back into the port. If the length from the port to the convergent cone is 25-30 inches that puts the pulse traveling 50-60 inches in 150-170 degrees crank rotataion at a tuned rpm of around 8k. The cylinders fire at 120 degrees apart on a v6 which means when the pulse leaves one cylinder the next port begins opening 120 degrees later and will need the stuffing effect around 150-170 degrees later which makes that 290 degrees crank rotation to stuff next cylinder in line, which by then exhaust could have traveled approx 85 inches . If the pulse of the next cylinder fired is used to stuff previously fired cylinder that's only 1 inch away that was fired 120 degrees earlier don't add up either. An exhaust chest is only 10-12 inches long from cylinder one port to exit at tuner depending on where you measure from. Once wave reaches tuner which is a megaphone which causes low pressure wave, its pulled out and has no way of returning. Which is why I feel v6 tuning is more about scavenging than stuffing. I'm not debating, would like to know what I'm missing here because it seems waves are traveling way to fast to stuff the next cylinder in line which is only an inch away.

[Whaaaaat]

1100 super jet? Can I ride it?

[Chaz]

Forbee ,
FM only pulled up this dead-thread to torture me because of the "V-1 pulse-jet I posted in the Chizzler thread ....

[powerabout]

Wax is talking about exactly what I'm experimenting with the angles in chest to shorten distance between ports. I need some clarification. I understand two stroke exhaust tuning from messing with motorcycles. Which is why I'm having a problem with stuffing part happening in a v6 merc. I understand how it's supposedly working just not mathematically adding up to me on how fast the waves travel and the limited space in the chest. For instance on a single cylinder dirt bike motor with exhaust duration of 170 and a tuned length of 25 inches on an expansion chamber. The exhaust leaves the port is pulled by low pressure wave of the divergent cone,(which is the tuner in our application, especially a bell tuner) then travels to the convergent cone ( which we don't have) and goes back as a positive pressure to stuff the fresh charge back into the port. If the length from the port to the convergent cone is 25-30 inches that puts the pulse traveling 50-60 inches in 150-170 degrees crank rotataion at a tuned rpm of around 8k. The cylinders fire at 120 degrees apart on a v6 which means when the pulse leaves one cylinder the next port begins opening 120 degrees later and will need the stuffing effect around 150-170 degrees later which makes that 290 degrees crank rotation to stuff next cylinder in line, which by then exhaust could have traveled approx 85 inches . If the pulse of the next cylinder fired is used to stuff previously fired cylinder that's only 1 inch away that was fired 120 degrees earlier don't add up either. An exhaust chest is only 10-12 inches long from cylinder one port to exit at tuner depending on where you measure from. Once wave reaches tuner which is a megaphone which causes low pressure wave, its pulled out and has no way of returning. Which is why I feel v6 tuning is more about scavenging than stuffing. I'm not debating, would like to know what I'm missing here because it seems waves are traveling way to fast to stuff the next cylinder in line which is only an inch away.

Whenever the wave hits a change of pressure it creates a reverse wave, so at the end of the megaphone it does exactly that.
The hotter and faster the gas is when it hits that big change the bigger the reverse pulse

[FORBESAUTO]

Which is the point if the convergent cone in an expansion chamber which causes an increase in pressure, which is where the pulse reverses. The mid has lower pressure. The tuner is like a divergent cone which causes a low pressure wave and true the hotter the faster it travels.

[powerabout]

Which is the point if the convergent cone in an expansion chamber which causes an increase in pressure, which is where the pulse reverses. The mid has lower pressure. The tuner is like a divergent cone which causes a low pressure wave and true the hotter the faster it travels.

The end of the cone is where the exhaust gas suddenly sees a different pressure, so creates a reverse wave.

[FMP]

One more time, 120* spacing. Assume 6000rpm, port opens a few or more degrees of rotation before pulse escapes up to 15* at high speed. Next cylinder is on compression stroke closed , next cyl is finishing scavenging on upstroke, pulse travels through chest using hot gas as medium. Next cyl to close is 240* behind originating cyl. Distance to travel on a triple will always have two short and one long to the next in sequence to close. No pulse down the pipe to tuner and back just port to port. Ideal time for arrival is near 30* before closing, two of the three will get stuffed earlier than than longer run. Regard the V6 as two triples, independent of each other. You want a formula?

[BUZZIN' DOZEN]

There was a guy came to jasper one year with a v6 merc with 6 expansion pipes sticking straight back out of a custom divider it was very strong caused a rule change

Lentzkow

[FORBESAUTO]

I understand the principle of how it works on paper. My problem is that a chest is only roughly 12 inches long which it too short for the rate of travel the pulses are moving in.

[FMP]

Assuming a travel distance of 8 " port to port and ex has temp near 1050 and port height near upper 90*s arrival will hit around 15* prior to close. Shorter run will get pulse around 20* prior to close. This is at 5000-6000 rpm. As crank speed increases degrees before closing decreases, giving way to higher ex ports for optimum arrival at higher rpm. An early arrival is far worse than late. High ports give early arrivals and crap low end but optimum at higher rpm. Too late and short circuit type spill with possible lean mix remaining.