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Re: Mikuni TM's and velocity stacks [Re: Pete R - R.I.P.] #529402 02/20/14 11:14 pm
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I guess it depends on what AMAL you mean.. I have seen both methods used, bleeds inside and outside. It will be difficult to get a stack that fits the venturi correctly, and it may yield better results, I do not really know for sure. But it would make for a smaller diameter thus possibly fit the bike better. What is really important is to make sure the rider does not foul the air intake.
Ron

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Re: Mikuni TM's and velocity stacks [Re: Ron - in California R.I.P.] #529436 02/21/14 8:02 am
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Using available formulas and plugging in my engine specification comes up with different results.

Engine Pro - Intake Port Length is 14.02 inches
Gordon Blair Formula - Intake Length is 10.69 inches
Simplified Chrysler Formula - Intake Length is 11.67 inches
Simplified Chrysler Formula (with bigger cam) - Intake Length is 11.15 inches
Simplified Formula - Intake Length is 11.00 inches
Simplified Formula 2 - Intake Length is 13.60 inches
Simplified Formula 3 - Intake Length is 12.83 inches

Obviously a formula is only a quess and time spent on a dyno and at the track is the only way to know for sure.
There appears to be two things happening at once from intake and or velocity stack length.
Longer length appears to move peak HP to a higher rpm and moves the torque lower and flatter at the same time.
Shorter length does just the opposite.
For my racing I need both torque and HP at higher rpm...
This is the reason for the vast differences in intakes and exhaust on very similar engines on LSR bikes. There is no absolutes here, you run what works best for your rider and machine.
And the new exhaust will need tuning along with carb and intake....
A lot of trial and error.....My recent ignition fiasco really sucks because it's still winter here and dyno operators are begging for business....
but I'll get it done...


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #529484 02/21/14 12:50 pm
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Good job H.B..!! You did some homework and found out all those books and computer programs only get you so close.! To fine tune, you have to do the work yourself. We found that even 1/4" on intake length made a noticable difference.

Same with exhaust. When my guy made the new exhaust header pipe (to take a bunch of bends out of it). All the lengths were to be the same. Well, the dyno knew better.. The dyno instantly found it and told us all our settings (carb) were now off and the power was down, even after changing the jetting. A quick tape measurement showed it to be almost 2" short. Added the 2" and voile, it went back to where it was.

And you are correct, tuning the intake tunes more than one thing.. That length also accounts for cam specs, and exhaust lenght, and of course desired result.

I should make a separate WEB site with all the pictures I have of a bunch of race bikes, no two are the same on either intake or exhaust. If you see an up pipe that ends near the seat, that is darn short..! So a good example is look at a BSA Gold Star with a Clubman's pipe, then look at a Gold Star with a Scrambles pipe. No tape measure is needed to see the difference..! Tuning can be frustrating or it can be fun, but it does take a lot of time.
Ron

Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #529608 02/22/14 4:51 am
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Using the Gordon Blair formula,and plugging in an ambient air temperature of 25 C,and 10.69",I come up with 8411 rpm and 6562 rpm.
You could be better off going for 5850 rpm and 7500 rpm,with a length of 11.988"

Any formula that doesn't account for air temperature is worthless,especially if they've been using it on car engines with under-bonnet temperatures of 100C or more.Your average intake temperature,between the carburettor and the valve,will be slightly less than ambient air temperature.

Re: Mikuni TM's and velocity stacks [Re: Pete R - R.I.P.] #529624 02/22/14 7:03 am
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Last year the intake length was 12.5 inches using 3 inch non tapered velocity stack and 9.5 inches of carburetor and intake . The bike ran 120 mph with 32 mm Amals, a stock Bonneville head, 37 inch long straight pipes and 4.76 overall gearing with a 27 inch tall tire. The bike was still accelerating at the end of the mile according to the rider.
Reworked head this year along with 34 mm flatslides several exhaust systems to try out and 4.66 overall gearing, same tire...
Probably start with a 12 inch total induction length


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #529625 02/22/14 7:07 am
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This is what I've done, could be neater but works. The air bleeds are blended, maybe that has an effect on how they work as air bleeds but I cannot tell. These are TM38s
[Linked Image]
Not sure how long the total inlet tract is but the carb is visible in this photo and gives an idea, good power from 2,500 - 7000 but I doubt the length of that is the main thing. A small change to the exhaust can have a much bigger impact, as can head work. I think the shape of the bellmouth is important to what it can flow and may be more benefit than such a long run as you have in the photos trying for a little gain with optimum length tuning which may be a bit elusive anyway. I'd be seeing what the power difference is with them at their longest practical length compared to the long set up in your photo, shorter may actually work better.
[Linked Image]


mark
Re: Mikuni TM's and velocity stacks [Re: Mark Parker] #529638 02/22/14 8:06 am
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Mark...As I said before,the fastest in class LSR OHV bikes have what appears to be nothing much done on intake tuning . And every exhaust system is different...
Maybe I should do no tuning...exactly what I did last year grin


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #529644 02/22/14 8:57 am
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Hope it all fits together for you HB.You think your doing the right thing, and you get to Ohio and it goes slower. Sometimes there's no ryhm or reason.


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Re: Mikuni TM's and velocity stacks [Re: konon] #530054 02/24/14 11:26 am
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After many discussions, looking at what many other top of the heap lSR bikers have done....There appears to be no parallel twins ( or Harleys) running intake and carb combined length longer than about 9-10 inches.
So I believe the tuning is done with exhaust and jetting to work with whatever cam profile is being used in their engine...


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #530076 02/24/14 1:11 pm
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Originally Posted by Hillbilly bike
Mark...As I said before,the fastest in class LSR OHV bikes have what appears to be nothing much done on intake tuning . And every exhaust system is different...
Maybe I should do no tuning...exactly what I did last year grin


I wouldn't alter anything until you have tested the mods you have done so far, you could be gaining with one thing an loosing on the other.


beerchug
Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #530081 02/24/14 1:27 pm
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Originally Posted by Hillbilly bike
There appears to be no parallel twins ( or Harleys) running intake and carb combined length longer than about 9-10 inches.
So I believe the tuning is done with exhaust and jetting to work with whatever cam profile is being used in their engine...
Have a look at what johnm is using on his Norton.You'll never be the fastest by copying what others are using.

Re: Mikuni TM's and velocity stacks [Re: Pete R - R.I.P.] #530087 02/24/14 2:09 pm
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Peter is exactly correct. I just had an ahem "debate" with some supposed engineer.. He told me.. "this is how it has been done for over 50 years, why do you want to change it..?" I then replied, well if I do only what others have done, then my bike can not be any faster than theirs.. He never got it.

Most if not all bikes I look at, show no signs of doing this kind of testing.. they all seem to take the easy route and live with it. And some go darn fast that way..! But, the very best will spend the time and money to find a bit more....

Ron

Re: Mikuni TM's and velocity stacks [Re: Pete R - R.I.P.] #530090 02/24/14 2:23 pm
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Originally Posted by Pete R
Originally Posted by Hillbilly bike
There appears to be no parallel twins ( or Harleys) running intake and carb combined length longer than about 9-10 inches.
So I believe the tuning is done with exhaust and jetting to work with whatever cam profile is being used in their engine...
Have a look at what johnm is using on his Norton.You'll never be the fastest by copying what others are using.


At the E.C.T.A track in Ohio we hold the class record. grin .But guys better than me are gonna be chasing.. I asked all the questions here during the original engine build and pretty much did what you and a few other suggested.
I know how to use tools and fabricate parts, but I am no more than an amateur tuner...And Johnm's expertise is far ahead of me...So I would be just coping his build?



I have some dyno time coming up... that and the limited track time is all I got...
I look at racing like making love to a woman. Have fun and smile and try to please. Take yourself too seriously and it becomes anxiety..........


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Mikuni TM's and velocity stacks [Re: David Dunfey] #530772 02/28/14 5:20 am
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Originally Posted by David Dunfey
I also thought the intake tract length began at the valve and ended at the carb intake, but Stuart Hooper disabused me of that idea. He told me it ends at the bell mouth and it does not matter where the carb is. The intake is tuned just like the exhaust. The negative pulse in the intake reverses as it moves the end of the bell and turns positive and heads back down the intake tract. The intake tract length needs to be tuned so the positive pulse hits the intake valve just before it closes. The pulse weakens each time it reverses, so the second pulse is the strongest. Unfortunately the second pulse needs the longest intake tract.

To hit the middle of your range using 6500 RPM the second pulse would indicate a 20.3" tract, third pulse 14.9" and the fifth pulse is 11.4". These are just starting points for the dyno work.

David


Absolutely right David, I agree with Stuart,
The only real factor of carb positioning is that close to the head you don't get the same fuel "dropout" at low revs that you would if the carb was mounted a long way out and needing high gas speed to keep the fuel particles suspended.
total length is what is important.
However Trial and error will be the key as with any theory. Sometimes on some applications of theory you need a different theory :o)

Julian

Cool looking bike by the way!

Last edited by Julian; 02/28/14 5:21 am.

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Re: Mikuni TM's and velocity stacks [Re: Julian] #534525 03/24/14 12:52 am
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Total length from the velocity stack to the exhaust outlet is what should be kept in mind. During testing any intake length change should also be tested with exhaust length and style changes.


Dennis M
Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #534541 03/24/14 5:07 am
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I posted this graph several years ago.

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=268898&page=1

It compares the calculated lenght for Blair and Vizard.


There are two things I would add from my experience.

Firstly as several others have said the inlet tuning needs to match the exhaust. For road racing I think you should tune the inlet to fill in the low points on the hp curve from the exhaust lenght. For example - tune the exhaust for a 6800 rpm and the inlet for 6300. This produces a smooth curve.

Secondly possibly the most important thing of all. Vibration of the carbs. With the long inlet tracts you will probably find the carbs vibrate badly. They are out on the end of a long tube and can really buzz at certain rpm. You must mount them on good quality Mikuni type rubber connectors and support the carbs so they can move but not buzz! If you find you are getting a misfire at the end of a long straight with the throttle wide open you are probably getiing frothing in the carbs and fuel starvation from vibration. (I assume you have already tested to ensure you have sufficient fuel flow through the lines fuel taps etc.)

This is really important to check this or all your tuning tests may be misleading!

Anyone wanting do this stuff must read the back to basics paper here. It is excellent!

http://www.profblairandassociates.com/RET_Articles.html

Last edited by johnm; 03/24/14 5:15 am.
Re: Mikuni TM's and velocity stacks [Re: johnm] #534552 03/24/14 6:47 am
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This bike was clocked at 131 mph on run at the Bonneville BUB meet last year. The fastest LSR speed by a naked frame modified Triumph 650 on gas.It's in altered class allowing very low seating which should help top speed . It has a Triumph 2 valve head and pre unit cases. I have discussed engines with this guy (Astek racing)....The engine has short intakes, about three inches. When I asked him about the long exhaust he said it was chosen because it looks "cool". He's a bit secretive about the engine build but claims the engine has never seen a dyno, the bike has no tachometer. My guess it about 7500 rpm to make the power needed?

[Linked Image]

This was Tim Joyce's race bike,or one of them. Very highly modified of course and a 750 ...Notice the intake length and exhaust length. 8500 rpm?
Circuit racing is far different than LSR racing but I'm thinking Tim's bike will run quite fast on top end at an LSR track.
I think these examples show relationship between exhaust and intake lengths

[Linked Image]



79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #536145 04/02/14 7:39 am
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OK, an update here. I finally got some dyno testing yesterday.. Learned a lot..! Unfortunately I had to make several changes at one time, thus it makes it more difficult to get to where I want quickly. New carb, with a totally different velocity stack design, and different jet designs, and some cylinder head changes. This is far less than ideal, where I like to make a single change at a time.

The first two dyno runs were aborted due to an extreme lean conditions. Made a 15 point main jet increase that made minimum difference. Quickly discovered an error in the main air correction jet. (new carb and old carb used different types of air jets and did not cross reference properly). Once the air jet was changed to a smaller size the jetting went rich, like it should. The air correction jet will be tested later to match that air jet to the needed fuel characteristics. Once we got our first full dyno run we noticed an increase in power at 6K, but slightly less at 7.5K. Aha..! This showed that we are either too long of too short on intake length. One of the changes with the new carb/stack set up is I went long (about 1.5") to start on purpose. Much easier to remove metal than add it on. Shortened the stack by 1/4" and the power peak started to move up, as expected. Ran out of time.. today will continue to shorten the stack until the power peaks, then starts to fall off. Also note that the 1/4" change in length required a main jet change. First change the length, then change the main jet. Each jet size (in my case) produces almost 1 H.P. So, it is impossible to make any meaningful judgement on intake length unless the jetting is also optimised for each change.

What is also interesting is that the more power we make, the cleaner the engine runs, emissions wise. And even more interesting is that the engine heat does not always increase with power increase. The change in engine heat may later allow for ignition timing changes as well. Currently I dyno with the timing slightly retarded until we get a good state of carb tune, then test for max power with ignition timing, while monitoring the engine heat. I finalize the ignition curve last. However I will likely do an initial ignition map later today, as the current readings tell me the timing is way off at some ranges. Being able to program the ignition curve in 500 RPM increments adds a lot of power at some steps.

Later..

Ron

Re: Mikuni TM's and velocity stacks [Re: Ron - in California R.I.P.] #536266 04/02/14 9:16 pm
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Not too sure about all the math involved but the Art and Design Department says to change out the pink overflow tubes to clear!:
[Linked Image]
Maybe some engine turning?

[Linked Image]

Last edited by JBMorris; 04/02/14 9:22 pm.

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Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #538590 04/18/14 1:44 am
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Quick update.. finally got back to the dyno... we got the best ever set up..! More power than ever, everywhere... But this was with my old carb. The new one is not as good.. yet. Tomorrow we start tuning the intake length.. we know it is off with the new set up. It is either too long or too short.. we will see. Worst case use the old carb.. we know it is awesome overall. What is amazing is that the power only tapers off a few tenths from peak to 8K... unlike before where it would drop a bunch at above peak RPM. All good so far... Oh yes and the engine is not running hot anywhere..!

All the hard work is paying off...

Ron

Re: Mikuni TM's and velocity stacks [Re: Ron - in California R.I.P.] #538711 04/18/14 4:21 pm
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Refresher.............your old and new carbs are ?


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #539114 04/21/14 3:01 am
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Sorry, both are Del Orto's... PFM. The main difference is how the velocity stack is attached. The other difference is the new carb has an enrichener circuit the old one has a non-functioning tickler, so it can be a chore to fire up cold..

Ron

Re: Mikuni TM's and velocity stacks [Re: Ron - in California R.I.P.] #539131 04/21/14 6:52 am
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I'm familiar with PHF's on my 1000 Guzzi rider. Dellorto carbs seem to be quality devices . Models with accelerator pumps can help lower speed throttle response on engines very large carburetors. I was considering them on the lSR bike but the lower priced Mikuni TM's got my attention.


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Mikuni TM's and velocity stacks [Re: Hillbilly bike] #539158 04/21/14 11:46 am
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Mikuni TM's are very good... just can't use them for racing... Even the Del Orto's have to be a cerain version to be legal.. no pumps allowed, can't block them off, must have factory non-pump version. Luckily they are readily available.. Just got a new needle to test.. the current needle was too lean at 3/4 throttle and slightly lean at 1/2 throttle and of course rich at lower openings... the new needle should fix this.

Ron

Re: Mikuni TM's and velocity stacks [Re: Ron - in California R.I.P.] #539336 04/22/14 11:22 am
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....LSR class rules follow the SCTA and there are no restrictions in modified classes about anything on or inside the engine other than displacement,2 or 4 stroke, fuel type, supercharged/nitrous and pushrod or non pushrod. Production classes must look like a stock bike but anything goes for engine modifications so long as the stock airbox, if equipped, and stock muffler outlet size.


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
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