shop math, ideal header length

[mtiberio]

I'm busy setting up the "Worlds Fastest Guzzi Automatic" (TM) for Land Speed trials at El Mirage Dry Lake next weekend. Mufflers are  optional on the lake bed, and so I'm looking to cut my headers to an ideal length. Currently I'm running a stock guzzi (ditch pump) cam. I estimate (no tach) I was pulling about 6450 RPM when I hit 114 last fall. Given that I have bigger carbs this year (30 vs 28), I expect to pull a bit more (plus improved aero). So assuming 38mm OD (35 ID) head pipes (these are Eldo pipes on the Convert motor), what header length should I use to tune for about 6600 RPM?

[s1120]

I'm busy setting up the "Worlds Fastest Guzzi Automatic" (TM) for Land Speed trials at El Mirage Dry Lake next weekend. Mufflers are  optional on the lake bed, and so I'm looking to cut my headers to an ideal length. Currently I'm running a stock guzzi (ditch pump) cam. I estimate (no tach) I was pulling about 6450 RPM when I hit 114 last fall. Given that I have bigger carbs this year (30 vs 28), I expect to pull a bit more (plus improved aero). So assuming 38mm OD (35 ID) head pipes (these are Eldo pipes on the Convert motor), what header length should I use to tune for about 6600 RPM?

Back in my drag racing days we used to adjust collector length buy painting a line on it and seeing where it stopped being burned off..  Granted this is a different case, being that the pipes are for just one cyl...  That being said... here is a link with some background theory on the concept..  https://www.onallcylinders.com/2015/10/30/header-theory-part-1-looking-at-the-science-behind-exhaust-header-tuning/


All in all, I think just getting them to exit in the place you want would be the best deal.

[cliffrod]

The two 1969 Ambassador World Record bikes were specifically built to pursue top speed records in the 750cc and 1000cc classes and, in doing so, promote the potential of guzzi's new VTwin platform.  Multiple records were achieved during 2 days at Monza. 

Both appeared to have very similar if not identical exhaust.  The headpipes each turn in towards center soon after the exhaust port, are bent to pass above & parallel to the lower frame rail while hugging the engine block and have another bend away from the center before reaching the approx plane of the flywheel/face of bellhousing.  At that point, the pipes begin to flare slightly into the reverse cone megaphones.  Some other pics show the various aspects a little better, but this one is pretty good. 



I have no idea if the headpipe length used was on these bikes were tuned to length or just convenience, but would expect it was more a decision of function than vanity.  You can extrapolate the approx lengths from the pics and mock-ups against your bike.  I'm doing the same here, just haven't gotten quite that far yet.

Hope this helps.  Good luck.  Ride safe.  Keep us posted.

Edit- at least one of these engines used 38mm Dellortos with remote bowls, with redline around 6200 iirc. 

[bmc5733946]

Mike, I hope you can see this. Scanned from " THE DESIGN and TUNING of COMPETITION ENGINES " Philip H. Smith C.Eng., F.I.Mech.E., M.S.A.E..
Smith also wrote an entire book on exhaust design.





Brian

[Kiwi_Roy]

Note the Velocity stacks in Cliffrod's picture.

[mtiberio]

 Clearly a Dyno tuned system

[John A]

From Phil Smiths book, The Scientific Design of Exhaust and Intake Systems, there was not an easy answer. I didn't find a formula I could relate, there are too many variables. I did find in the chapter Pressure variations, on some testing of their 500cc one cylinder engine for max torque at 4krpm the pipe length is 4'3". at 6krpm it is 2'10" and at 8krpm it is 2' 1.5".  It further said that it is best to achieve the final result from experimentation. not much help but its all I have until I become smarter, but don't be waiting on that outcome!

[s1120]

The two 1969 Ambassador World Record bikes were specifically built to pursue top speed records in the 750cc and 1000cc classes and, in doing so, promote the potential of guzzi's new VTwin platform.  Multiple records were achieved during 2 days at Monza. 

Both appeared to have very similar if not identical exhaust.  The headpipes each turn in towards center soon after the exhaust port, are bent to pass above & parallel to the lower frame rail while hugging the engine block and have another bend away from the center before reaching the approx plane of the flywheel/face of bellhousing.  At that point, the pipes begin to flare slightly into the reverse cone megaphones.  Some other pics show the various aspects a little better, but this one is pretty good. 



I have no idea if the headpipe length used was on these bikes were tuned to length or just convenience, but would expect it was more a decision of function than vanity.  You can extrapolate the approx lengths from the pics and mock-ups against your bike.  I'm doing the same here, just haven't gotten quite that far yet.

Hope this helps.  Good luck.  Ride safe.  Keep us posted.

Edit- at least one of these engines used 38mm Dellortos with remote bowls, with redline around 6200 iirc.

Im thinking the head pipes are like bent like that to get them out of the air. Improving aero will have more effect on top speed then whatever improvement correct length pipes it has IMHO. Im also thinking its the flair of the meg, or reverse meg is where the science is..

[cliffrod]

Im thinking the head pipes are like bent like that to get them out of the air. Improving aero will have more effect on top speed then whatever improvement correct length pipes it has IMHO. Im also thinking its the flair of the meg, or reverse meg is where the science is..

Agreed on the bends.  The pipes bend to tuck better under the full fairings used in certain classes.  The reverse cone  megaphones are also flared wide at the back of the bike.  So from the side, the exhaust looks somewhat shorter if you overlook the bends & angles than when viewed from different angles.   I'm confident the overall package was tuned for the record attempts, but there's not lots of specific information available so didn't want to specify such things.

Given a week until the OP's planned attempt, this is all general info for consideration per his request.

Full side view-




Rear view-

[Kiwi Dave]

I have zero experience, but I thought the science is to expel the exhaust gases as quickly as possible to clear the combustion chamber of the greatest amount of burnt fuel in the little time available.  Thus the pipe should be as short as possible but curved to the rear to discourage air from the outside entering the combustion chamber when the exhaust valve is open.

Have a look at the Rolls Royce engine exhausts used on the Spitfire and Hurricane fighter planes of WW II in these short clips of Guy Martin's one.  Designed for maximum power and zero noise suppression.

https://www.youtube.com/watch?v=s_A9m21drwY

https://www.youtube.com/watch?v=wJ9aO5yvT3k

[wirespokes]

There's more to it than safely directing scorching exhaust away from bodywork. Pressure waves generated every exhaust stroke travel down the header, reach the end, then reflect back to the exhaust valve. The length of the header determines the timing of those waves returning.

That's important because those reflected pressure waves will prevent the intake charge from being sucked out the exhaust pipe - the intake and exhaust valves are both open for a period of time. This is RPM dependent and is one of the things an engine designer considers for its intended use - what RPM range it will be used at. It will be most efficient and strongest there.

[garbln]

The Rolls engines were low RPM supercharged engines, and so exhaust tuning wasen't really of much benefit.   Notice most aircraft engines of that era had pretty stubby exhaust headers for that reason.  Atmospheric induction at a higher RPM is a whole different ballgame, the intake and exhaust length must be tuned to work together for best results.  It's really an art.   

[Rough Edge racing]

 Exhaust tuning can be exhausting..From my actual experience.....The intake, cam and exhaust work as a team ,change one and you may have to makes other changes...On the dyno my 650 Triumph land speeed racer running in modified production made the most power and was the fastest in it's class for nearly five years with two indivudual 1-1/2  x 34 inch straight pipes.Only a strong tailwind helped another Triumph to best me by 1.5 mph..Different exhaust systems ,velocity stacks and so call tuned intakes just made it slower....I ran my 105,000 mile stripped down but stock engine Cali 2 at the Ohio mile LSR track with just the down pipes..Ran consitant 117 MPH....
   The Subtle Crowbar Guzzi hold the modified Production record of 157 mph with headers looking this...

         

      The key to all of this is testing on a dyno or the track....I was riding the bike for hours on the dyno trying different combination of parts..

        https://www.youtube.com/watch?v=-y94Yje2lzc

[Roy gardner]

My mate, Fred, who designed all of the pipes for John Britten, calculated 47" from the exhaust valve seat for his, and all other round barrel Guzzii.

[chuck peterson]

I wonder how close 47" is to stock..

[LowRyter]

I am the least knowledgeable mechanical person on the board.  So if you want to ignore my stupid question, no problem, but I am curious to know.  I read that headers should always be equal length for max performance.  Does the Guzzi (90 degree) firing interval affect the notion that headers/exhaust should be equal length for each of the cylinders?  Or should one of the two pipes be longer than the other?

First, when the twin pipes go into a common collector, should one header pipe have more volume (or length) than the other?

Second, since most Guzzis have dual exhaust, does the firing interval affect placement of the crossover between the dual pipes? 

I've not read anything that mentions this, so I would guess it's not an issue.  I think some more extreme engines (like a Harley 45 degree) actually do have some unequal length headers (but perhaps that's just packaging or cooling?).

[rocker59]

1- I read that headers should always be equal length for max performance. 
2- Does the Guzzi (90 degree) firing interval affect the notion that headers/exhaust should be equal length for each of the cylinders? 
3- Or should one of the two pipes be longer than the other?
4- when the twin pipes go into a common collector, should one header pipe have more volume (or length) than the other?
5- since most Guzzis have dual exhaust, does the firing interval affect placement of the crossover between the dual pipes? 
 

1- Yes.  they should be equal length.
2- No, the firing interval does not change the fact that they should be equal length.
3- No, one pipe shouldn't be longer than the other.
4- No, header pipes going into a collector should not be different diameters or lengths.
5- Yes.  Guzzi experimented with the "front crossover" in the late '90s and early '00s to help tune the exhaust pulses in the dual exhausts.  For the CARC bikes, they went to at 2:1 system which did away with the "need".

[rocker59]

 
   The Subtle Crowbar Guzzi hold the modified Production record of 157 mph with headers looking this...

         

     

And, Dr. John used a similar 2:1 exhaust on his bikes in the 1980s.

If setting up a racing Guzzi, I would look at using some version of the 2:1 systems used by other race teams.

[rocker59]

I found this calculator online:

http://www.wallaceracing.com/header_length.php

[mtiberio]

I found this calculator online:

http://www.wallaceracing.com/header_length.php

Looks great, too bad they don't tell you what the units are for all the parameters...

[Rough Edge racing]

 Land speed racing has more in common with drag racing and and the focus is on maxium power with not much concern for mid range like on a road course...
  While at the various land speed racing tracks I look at and talk to other bike race racers with push rod two valve engines. One thing I see in common is a great variation of exhaust designs on the fast bikes ...Like others, I found the formulas in books interesting but often not effective on the track..Of course you need a starting place..If the engine is not to radical, simplist is two staight pipes about 42 inches long..then make runs on the track or the dyno lenghting or cutting the pipes shorter to find the best HP and torque or best speed....This may require jetting changes, it does get time consuming..
 Most important ,especially for a novice, is a reliable consistant machine with a state of tune that is tolerant of air density changes...It's hard to run well if the engine is super fussy or blows up.....
  And learing to read spark plugs properly ..And using an otoscope to look for early sign of detonation...

[LowRyter]

1- Yes.  they should be equal length.
2- No, the firing interval does not change the fact that they should be equal length.
3- No, one pipe shouldn't be longer than the other.
4- No, header pipes going into a collector should not be different diameters or lengths.
5- Yes.  Guzzi experimented with the "front crossover" in the late '90s and early '00s to help tune the exhaust pulses in the dual exhausts.  For the CARC bikes, they went to at 2:1 system which did away with the "need".

OK then.   

[chuck peterson]

I wonder how close 47" is to stock..

Measuring the outside bend in get 31” on a stock head pipe, now add the crossover length...if you don’t cut them they’re is another 10-15” of straight pipe shoved down inside a muffler...that might make the 43”...if moving the forward crossover up the head pipe adds mid range, what effect would moving the crossover all the way to the back?

[Howard R]

I once read somewhere that "BUB" himself (Denis Manning, of BUB exhaust, they used to make a system for Guzzis with a crossover pipe just a few inches away from the heads) said that the closer he put the crossover to the exhaust ports, the more power he got, but that was for street use.  I remember Barbara Nowell from North Carolina had a Centauro with a BUB system on it, and it was NASCAR-loud!  If someone has access to a BUB system, maybe they could give you some measurements as a place to start.  Some Guzzis came from the factory with the crossover just below the alternator, maybe inspired by the BUB.  Overall, it sounds like this will truly be a "cut it and try" undertaking.

Howard

[Luke]

I think that the pipes should be as long as possible so that the fishtails are not cluttered. You are using fishtails aren’t you?