looong intake runners for HP

Just got this photo from Jesse showing his long intake runners. 13-1/2" from the valve to the end of the velo stack. Thats about the length Axtell used when developing Ron Woods 84HP 750 short stroke dirt tracker. Its also about the same as Leo Goff used in his world beating 750 drag bike. Needs a special low slung oil tank. - room for creativity here.

I would prefer to see solid intake runners. Since they work through standing waves I would worry that rubber intake runners would flex to a degree and dampen the standing waves.

I had good success with my BMW racer: The longer of these two bellmouths made 4 horsepower with no other differences, none! Pop them on, 4 horsepwoer, take them off, 4 less.

jseng1 said:

Needs a special low slung oil tank. - room for creativity here.

Click to expand...

A Commando oil tank can be mounted transverse down low, with just a couple of "P" strap clamps. It's a bit of a fiddle to drain, but works a treat.

pommie john said:

I would prefer to see solid intake runners. Since they work through standing waves I would worry that rubber intake runners would flex to a degree and dampen the standing waves.

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Weellll, they are not really standing waves - or they would be no problem.
They are PULSED waves - if you could see in slow-mo, for about 75% of the time nothing is happening,
and there may even be a bit of reverse flow - or dissipation of flow.
Its the pulsed motion that could cause rubber manifolds to flex.
But the proof is in the pudding ?, this bike has a proven record ?

BTW, ~14 inches is proven Manx G50 7R AJay KTT territory, so nothing really new under the sun ?

Long intakes fine for top end power, but getting it sharp coming off the bottom to mid range, will be a challenge. personally I would have concerns about the carbs icing, and the rubbers de laminating ran longer rubber inlets on Tridents back in the 70's and experienced both problems. Though I am sure you can get better quality hose these days.

Best Regards

Esme

pommie john said:

I would prefer to see solid intake runners. Since they work through standing waves I would worry that rubber intake runners would flex to a degree and dampen the standing waves.

I had good success with my BMW racer: The longer of these two bellmouths made 4 horsepower with no other differences, none! Pop them on, 4 horsepwoer, take them off, 4 less.

Click to expand...

That is perhaps why Steve Maney sells solid turned extension tubes...

http://www.stevemaney.com/products/bill ... 0tract.jpg

Many years ago a friend of mine had a tricked up Triumph 5TA with long intake rubbers, nearly as long as the ones shown. when he closed the throttle the rubber tubes just flattened under the suction created. They needed sleeving with steel or alloy.

Grandpaul - I'd like to see a photo of the "transverse oil tank"

Yeah sleeving would make sense - thats what I would do and its easy enough to add aluminum tube which will probably happen later on.

Below is a chart for a 500cc short stroke. Note that the length is longer for low end. It depends if you want primary, 2nd or 3rd primary wave boost etc.



13-1/2" may not be new but you don't see many Commando race bikes get it right because its inconvenient.

Ron Woods 84 HP short stroke with long runners


Leo Goffs dragster with long runners


My 1980s racer with long intake runners was very fast and got me on the podium at an AMA national.

gripper said:

Many years ago a friend of mine had a tricked up Triumph 5TA with long intake rubbers, nearly as long as the ones shown. when he closed the throttle the rubber tubes just flattened under the suction created. They needed sleeving with steel or alloy.

Click to expand...

And he wouldn't be the only one to have that happen. It is exactly why Steve Maney produces extension tubes as well as manifolds.

I use an inlet of similar length (10.5" head face to bellmouth end, you need to add the head face to valve distance, around 3"? ) on my 750 short stroke, but the manifold is made from straight stainless tubing all the way out to the carb and uses standard short Mikuni rubbers for carb mounting.

But the motor is vertical like an Atlas, in a Rickman frame, no oil tank, just a heat shield above the exhausts, which run above the gearbox.

The long tracts must make sense for racing, at least on a fast track.
Broken record I know, but for the road why give away midrange for some peak hp you will rarely use?
The stock Norton inlet tract is short and I believe that is why stock Commando in good tune will sometimes sit a new-to-Commandos rider on his ass. The throttle response is superb.
I've experience with one bike which utilized very long custom inlet tracts and that one also made some extra peak power. For racing on fast tracks it was quite good. On the road it was/is a dog. By the time the power comes on the stock bikes have already left the county.

Glen

worntorn said:

The long tracts must make sense for racing, at least on a fast track.
Broken record I know, but for the road why give away midrange for some peak hp you will rarely use?
The stock Norton inlet tract is short and I believe that is why stock Commando in good tune will sometimes sit a new-to-Commandos rider on his ass. The throttle response is superb.
I've experience with one bike which utilized very long custom inlet tracts and that one also made some extra peak power. For racing on fast tracks it was quite good. On the road it was/is a dog. By the time the power comes on the stock bikes have already left the county.

Glen

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Glen, I wouldn't advocate the effort of a long inlet for a road bike, but I don't recognise the power characteristics you are talking about. Are you sure the jetting was set correctly for the manifold? My short stroke pulls all the way from 4000 to 8000, direct to throttle input. Of course I am not talking about sub 3000rpm, because I never use that.

Steve, the bike I'm thinking of is a Vincent with a similar Rev range to a standard road going Commando. It topped out at 6500 and was a real dog up to 4500 or so. The last 1500 revs were good, it pulled quite well.
The power loss below 4500 made it a miserable road bike as most of the road riding happens 2500-4500 revs, same for Commando.

Glen

jseng1 said:

Grandpaul - I'd like to see a photo of the "transverse oil tank"

Click to expand...

I believe he means something like this



Sorry I don't have a better close up. That's a stock oil tank that's been rotated 90 degrees about a vertical axis, with one of the hanger arms bent 90 degrees as a mount and the other side clamped to the frame. It'll work in a pinch, although might not be a desirable long term solution.

Maybe there is some confusion here. Most of the long runner set ups are a reaction, a fix for the loss of low speed power that comes with a new bigger carb. Then, the new longer runner, which actually helps the bottom of the power band, still gets blamed for the loss of low end that was caused by the big carb in the first place.

For illustration, put a longer runner (within reason) on a stock carb'd engine and the low speed power curve likely will improve but that long pipe will choke the engine at higher speeds.

Another way to say it - if you retain a short runner and change to a big carb, then between strokes the velocity/volume/mass in the port falls off too quickly, between strokes/valve events, as compared to a smaller carb. If, however, you add a longer runner to that big carb, then more volume/mass of air is transported over a longer time and that pushes the average velocity/volume/mass higher between strokes/valve events.

So . . . . within reason, with larger carbs, longer ports broaden the power band, help retain the low end.
Tell me if I'm wrong.

Another question though - where is it best, within that 14" port to place the carb? Close to or far from the head? According to inside diameter of the carb, is there a ratio that predicts the ideal length for a velocity stack?

A olde Daytona I rode , had like truck / earthmover / digger - radiator hose for the runners ,
rather thick hard laminated reinforced straight stuff , to get the dual 30s behind the seat tube .
Mustve worked , it'd blow of a 650 , both two up .
And all the whizz bangs on the dirt / metal roads , on a ralley . While Two Up .
The only one that got past , as I was map reading ( the pillion was blonde ) then had trouble with
a farm gate then, as the road went left , then right , then straight across the highway , around a house , past a boat ( on a cradle )
( got a wave there ) , and back on the highway , or was that the bike . a few got past there , but tarmac dosnt count ! :lol:
The + 40 650 pistons mayve helped too , the 59 frame was good on gravel / clay . Didnt flex much untill 90/100 mph .

ANYWAY !

Moir Commando had the long intakes , Mk II Amals about that posn . ( Against the rear of the cross brace ) .
The smooth Curve of the pipe bends they were fabricated from , blended marelously into the port curve .

Cant help but wonder if / at the KINK in the Std. manifolds could get stalled - Tumbling turbulance & choke
there in some circumstances. Like when the engines running .

Thinking the one cylinder below 2.000 rpm was the lack of balance tube .
And think only materialised AFTER Id fitted the BSA advance unit . 30 Deg or whatever it was .

The pick up on the thing was outstanding on the higway on light throttle , and if you got it past 3.200 elswhere . Combat Cam .
And yes , as hobot states , past 5000 was eyeballs back in the sockets . :shock:

SO ,
Thinking , for the street , a balance tube ( maybe at the mounting flanges ) about 1/4 bore , may well be worth F'ing around with .

P. S. I think the figure was 14 in , valve face to jet , C/L , as they say .





and a nazi one or two .

P.S.S.S.

Now where were we . . . .

early-square-oil-tank-t22122.html

non-standard-oil-tank-commando-t8756.html



4 pics HERE : http://s1162.photobucket.com/user/scott ... 4.jpg.html

THOUGH youd be better to do a similar , sans tray ( if youve a 2MC capacitor , etc ) 1 Gal. Volume = , for 3 quarts oil ( IMPERIAL )
If youre using the throttle . Alloy set up . Theres a picture of a Norvil Team one somewhere . alloy , vented , 'd cool it some , too.
Thats why you want airflow through there .
Maybe a airbox too , if your being a real kook , Er wanna go faster .

Maybe I have it all bassackward then. That can happen with a sample size of one.
Here's another view on inlet tract length from Paul Dunstall.

" Just as important as correct settings (carb) are the induction tract lengths. For road use, spacers between the cylinder head flange and the carburetor should be 1 1/2" long. The race cam needs a longer induction manifold which gives us the chance to use flexible induction pipes 3 inches long to isolate each carburetor from the cylinder head.
Although long bell mouths may add to the racy look of a machine, they do not, in my experience , add any performance where concentric carburetors are used. Any theoretical ram effect is more than cancelled out by the fact that long bell mouths obscure the atmosphere compensation hole and, therefore, reduce the pressure difference between the induction tract and the float chamber."

Thats why they invented THESE Ones . ( See the F750 Tri / BSA threes )



uberwise the moving air depresses the float chamber. Or RAISES it . :shock:

You getta extra 1/4 odd , fitting the carbs up behind the cross brace. But that dosnt spell ' the end of the world ' .

Another way to do it is to just use long metal intake adapters, like on this Commonwealth Norton back in the '80s. Sorry about the picture quality. Didn't have a digital camera back then.



Better picture here



Ken