Hi all,

I was killing some time in the shed this week and after reading a few posts on various sites about exhaust tuning decided to measure mine

Anyway, my A1 1 3/4" sidewinder is 43" long from flange to collector flange with the last 7" being collector. I measured up a standard 4-1 and got 37". Just wondering if i am leaving anything on the table with it being that long?

Have tried a few on-line calculators and keep coming up with something around 32-34". I`m thinking i may go back to common style header if it would help.

Cheers,
Keith

Send dangerous a PM ... he has a crazy program that goes deep into this.

The shorter the pipe creates more peak power but a bit less torque wise. I'm running 41" on my bus as I wanted more torque than peak power to push a big brick through the air. Also can make a difference with back pressure and scavenging as well from what I've been told.

Its more about the primary volume, as Brennden said, Longer Smaller for lower rpm, Shorter Fatter for high rpm. Also smaller for small engine, larger for large engine.
Another myth is the megaphone, in testing its conclusive that a straight tube only a few sizes larger than the primaries after the merged spired collector will produce better results.

Perhaps those that did the testing on the megaphones should tell all those blokes running Pro Stock bike in the NHRA, they might all drop their megs in the bin.........

Exhaust testing is as much art as science, the tuned length can be roughly figured out using revs alone, but to be close involves much trickery and a truckload of testing. You can have two identical systems on identical high comp motors for example, but if one motor uses C16 and the other methanol, the exhaust temp is dramatically different, and so the best tuned length will be also. Might I recommend a program called Pipemax if you want to get right into it?

Is there a 37" length set that you could borrow to try it out, and see it's effect on top end and ET?

Thanks guys. I am going to try a different set of headers and see how it performs.

Modnrod - Will look into pipemax.

Cheers.

Flick me an email keith and I can run your specs through my pipemax program.
My gut feeling is the shorter system will make for a faster car, but
some times lower peak HP can mean a better average, and thus a faster car.

Thanks for the offer Dave.

Will do that, be interesting to see.

Regards,
Keith

My uncle builds formula vees and has spent countless hours on the dyno experimenting with pipe lengths and also megaphones as well. The thing that would surprise most is how the length of a megaphone can make a huge difference to the mixtures and power delivery as in where peak power is delivered in the rev range.

I find the stuff fascinating, how seemingly minor differences in collector length or type (straight or megs), can alter the tuning requirements so much!

HAHA! I really should get a life!

Another little trick that also alters the tune is the collector internal spire

A little bit off-topic, but Benito/Brennden/Dave, have any of you guys ever tried a simple 4-2 (back pair/front pair), or a tuned length set of 4 open pipes with no collector, instead of the 4-1 normally used? I'm thinking of trying a set of 4-4s on my bike, no balance or joiners, theory suggests I should lose mid-range a bit, but if rev-tuned will probably pick up over a narrow rev-range (tuned where I want it). I've always thought that a 4-2 on an ACVW for a drag car would give more tuning effect but over a shorter rev range than the traditional 4-1.
Perhaps a 4-2 with a short meg each side (to help broaden and strengthen the tuning range)?

To the OP, if you can't borrow a set of 37" pipes, maybe a cheap 2nd hand set of 4-1, and try to shorten the pipes to whatever Dave comes up with length wise out of Pipemax. It will probably be ugly, but a good way to see if they will work as you want.

Yup watched my uncle play with internal spire as well as a Venturi of sorts at the exhaust port to help stop scavenging. Top fuel Harley's use them as well as some bigger capacity race bikes. 4-2-1 header configuration would work but obviously designing it and making it fit is the challenge especially if you follow the traditional path of running all the pipes out the back. Running pipes forward as I have would make it easier as most the system on my bus is beside the gear box and between the chassis rail. On a beetle you don't have that kind of room to play with.

OK, found something for the OP that will get him to within an inch or two of the ideal length. I've plugged in various pipes I have tested different lengths on different bikes, and it also concurs with Pipemax.




I have some old notes here given me by a Harley tuner. These are for single, isolated header pipe per cylinder with no collector. This is old fashioned pocket calculator stuff. A good computer program such as Larry Meaux's PipeMax is likely more accurate, but here goes......

Step 1 is to determine header primary pipe (inside) diameter based upon port velocity of exhaust gas. The notes indicate between 280 - 300 cfm to be optimum. And recommend tuning for 290 cfm at the shift point for a drag bike (or middle of power band for a road bike).

First, determine piston speed at desired RPMs......

(stroke X 2, divided by 12) X RPM = piston speed in ft. per minute

In the final calculation below, we will divide fpm by 60 to convert to ft. per sec.

Then we take......

(piston speed in fpm, divided by 60) X (bore squared, divided by primary pipe ID squared) = port velocity of exhaust gas

Note: As exhaust tubing is measured by OD (outside diameter), you can take the tubing wall thickness X 2 and add it to ID (inside diameter) to give OD. So if the calcs indicate 1-3/4" ID and wall thickness were 1/16", OD would be 1-7/8", for instance.

Step 2 is to determine header primary pipe tuned length based upon pulse wave. The calculation below will give you total exhaust tract length, measured from exhaust valve to end of primary pipe...... So you will need to subtract the distance (measured flow path) from exhaust valve to header flange from overall tract length in order to determine primary head pipe length.

In the final calculation below, we will add 180 degrees to actual exhaust valve opening event BBDC (Before Bottom Dead Center) to convert from BBDC to ATDC (After Top Dead Center). By actual valve timing event, I mean that if the cam is not installed straight up, but is advanced or retarded, we will adjust degrees BBDC as stated on the cam card to reflect actual valve opening event.

For instance, if the cam card lists exh. valve opening at 64 deg. BBDC, but cam is installed 4 degrees advanced......The valve opening event will actually take place 4 degrees earlier, so we will subtract 4 degrees and use 60 degrees BBDC to correct for actual valve opening event. Conversely, if the cam is installed 4 degrees retarded, we will add 4 degrees and use 68 degrees BBDC to correct for actual valve opening event.

With that in mind, take......

(180 degrees + actual exhaust valve opening event BBDC) X 950, then divide by RPM = total (calculated) exhaust tract length

To determine primary header pipe length, subtract the distance (measured flow path) from exhaust valve to header flange from overall tract length.

This will give you the calculated length. To determine actual length, make primary pipes a couple inches longer than calcs indicate and shorten them an inch at a time until optimum power is achieved.


Best regards,

Harryenigma57Guru
Posts: 1279Joined: Mon Oct 08, 2007 11:59 amLocation: Galt's Gulch

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I give all due regard to Harry from Speedtalk forum, this is his reply to a similar question there years ago, and so won't mind having his excellent answer repeated again.
Still have stuff to do to play with megs/spires/collectors to screw up the tuning, so it's not all bad!

Thanks guys,

Dave has run the numbers and given me sizes etc i am to aim for. I am keen to see what difference it makes and how it changes the cars performance. Here`s hoping it goes in the right direction.

Regards,
Keith