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Author Topic: Long Pipe Tuning Question  (Read 796 times)

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Dan_Lockwood

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Long Pipe Tuning Question
« on: March 04, 2019, 05:28:45 PM »

As in the automotive genre, shorter pipes, relative to long pipes, do tend to provide the most power/torque overall.

In my next somewhat off wall project I'll be running a Harley motor with long pipes.  The pipes may end up being around 6' to 7' long.  I plan on 1.75" pipes with single per side and mufflers probably 75% down the pipes.

My question is this, if knowing the long pipes are going to be used, can one do things to the motor with cams, timing and fuel to make them run well with the long pipes?

I remember decades, like 5, ago a couple older friends of mine were running Sportsters and for Halloween they dressed up and got some exhaust pipes bent up to just clamp on the ends of their drag pipes.  I saw them a week or so later and they both complained about how poorly their bikes ran.  One said that he could hardly hit 60 mph.

Would you need a different duration or overlap on the cam to optimize the longer pipes?  I'll be running a S&S Super E new carb and the motor will probably end up in the 95" to 97" range.  It will be an '02 "B" 88" with a big bore kit.

Thanks in advance and have a great week.
« Last Edit: March 05, 2019, 02:15:30 PM by Dan_Lockwood »
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Dan

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Dan_Lockwood

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Re: Long Pipe Tuning Question
« Reply #1 on: March 06, 2019, 07:33:56 AM »

This is what I'm talking about for "long pipes".

For the Morgan Three Wheeler, they use the S&S Wedge 2L 56 deg V-Twin mated to a Miata 5 speed trans.  The pipes run from the motor to the rear on both sides of the trike.  The mufflers are under the shield and the CAT is up front near the flex joint.

Initially I was thinking true duals like this, but I would have the option to do a Y pipe and run through a single muffler system.

If I went with a Y pipe 2 into 1, would it be best to run 1.75" all the way or at the Y collector increase to 2" on back through the muffler and tailpipe?

Thanks and I hope to get some feedback if anyone has thoughts or experience with anything similar to this.

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Dan

1976 FLH
2009 SERG Orange / Black
Board Track Racer Project, Ultima 113"/6spd
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hrdtail78

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Re: Long Pipe Tuning Question
« Reply #2 on: March 07, 2019, 09:01:00 AM »

I haven’t tuned anything with as long if pipes as you are talking, but....IMO longer touring exhaust that comes out past the fender has always tuned in better and has been more forgiving of the cam choice over shorter exhaust.

Keeping in mind that the piston does not push out exhaust.  More that the exhaust is racing to get to the less pressure of atmosphere.  I would think keeping the runners non stepped and on the smaller side, as you mentioned, should help keep the velocity of the slug of exhaust moving toward exit and help with scavenging if you connect them.
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Para Bellum

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Re: Long Pipe Tuning Question
« Reply #3 on: March 09, 2019, 12:59:56 AM »

As in the automotive genre, shorter pipes, relative to long pipes, do tend to provide the most power/torque overall.
It's actually the amount of resistance to flow that determines the characteristics of the pipe/muffler combo.  Low resistance is good for HP (high RPM and speed), while higher (but correct) resistance is good for torque (low RPM and acceleration).  The factors affecting resistance to flow are (1) the cross-sectional area, CSA, of the pipe, (2) length of pipe, and (3) the number and type of turns/baffles in the path.  Keep in mind that larger engines and higher RPM mean more exhaust gas that has to get out.

CSA is highly dependent on the radius, since area of a circle = pi x r2 (r x r). A small change in radius has a large effect on CSA.  [Sidenote: remember the crossover exhaust pipe that's slightly flattened on many Harleys?  That is a higher resistance, which makes a 2-1-2 exhaust send more of the gas out the right-hand pipe.  Think of that flattening as being part way to completely pinched off, thus more resistance to flow.  That's why mandrel-bent pipes flow better, and why baffles and bends in the pipe or muffler decrease flow.]

Since the exhaust gas "rubs" against the wall of the pipe as it heads toward the exit, the length of rub (the pipe) also affects resistance, but in a linear way:  double the length = double the resistance.  Bends in the pipe add resistance b/c the gas runs into the wall and rubs more.  So does a rougher interior.

This shows us the simplest formula for amount of resistance (leaving out pi, since that's a constant):  R (total resistance) = radius2 x Length (for a constant diameter).  A muffler adds resistance by adding bends, baffles, and length.

Straight pipes, stepped headers and mufflers, and megaphones all reduce restriction by some combo of increasing radius and decreasing bends.

From this info, you can see why your friends adding pipe length made them run poorly.  The extra restriction prevented exhaust gas getting out as quickly and easily, keeping their HP and speed much lower.

As hrdtail78 mentioned, it's the higher pressure and temperature that are the driving force sending the gas down the pipe toward the lower pressure and temperature of the atmosphere.  As the gas flows along, it cools down and the pressure decreases, decreasing the speed of the exhaust gas.  Stepped headers and megaphone mufflers serve to decrease the restriction and keep the gas flowing better.

For your application, using a smaller displacement engine means you can use a smaller diameter pipe--but if you run the Y pipe 2-1, you'll want a larger diameter after the muffler, however, in the 2-1 the higher restriction of the single vs dual muffler will play a larger part.  So remember, too much restriction will reduce top speed, while too little will hurt your take-off.  It will probably take some trial and error to get the right balance.  Good luck with a fun project.
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Dan_Lockwood

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Re: Long Pipe Tuning Question
« Reply #4 on: June 10, 2021, 01:01:10 AM »

Para Bellum, thanks for the lesson!

I had originally thought that I would do a 2 into 1 as that would be simpler in the long run.  I will use the usual 1.75" header pipes, but will then do a "Y" into a 2".  from there I will go into a 2" in 2" out muffler and keep it simple with the single exhaust.

When I get to that point, I will contact you and run some things by you for your opinion.

Thanks again and have a great week!

Dan
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Dan

1976 FLH
2009 SERG Orange / Black
Board Track Racer Project, Ultima 113"/6spd
Dan's Car - Bike - Projects Photo Albums
 

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