Harley Port, flow and velocity Question

[afterburn 549]

First off, all Harley Haters move on, I do not care what you ride or like, please do the same for me.
Now-
Is this idea worth postulating-?
The ports and length of the intake system on a Harley is absolutely short.
Combine that with the long stroke, things do not get going very well for port velocity.
It has to be a bumpy ride to the combustion chamber.
Lots of stops and go signs untill about 4500 RPM.
I would postulate the manifold should be longer .
I will also say that because of "looks" the carb or throttle body sticking 6 inches further out into the rider's leg, is why this is not addressed.
Your thoughts?

[84CAB]

Yes, you are correct.
There is a local guy that is a factory trained HD tech that opened his own shop and specializes in performance engines.
He has different lengths of "extenders" he will use and tune the engine accordingly. It's been a while but I don't think any of them were over 2" or so... At least for street engines.

[asphaltgambler]

Yes, there have been carb spacers around since the late 60's originally pioneered by S&S. With long duration cams on hot street / drag engines the turbulance in the manifold is high. Some so severe at mid-RPM's it causes carb stand-off which is where the atomized fuel is pushed out backwards past the venturi.

Unfortunately, there is only so much length that can be added due to space / location limitations. One way is actually lengthen the area outside venturiusing a longer velocity stack ( hot street/ drags application ).

Testing has been done using a 90 degree curved intake placing the carb / throat inline with the travel of the bike. The reasoning is twofold: you lengthen the intake manifold considerably isolating the carb from all the turbulence and increasing the ram effect of forced air into the carb with the bike at speed.

BUT - in reality it never works, makes less power because the large volume of air has to make a sharp 90 degree turn slowing velocity down and heating it up. Plus the forced in coming air creates more turbulence at the outside of the venturi.

Modern aftermarket fuel injection systems (S&S again) have just about solved this as now the fuel is delivered directly behind the intake valve allowing the manifold and throttle body to be sized exactly for air flow only. The space / locations still exist though.

Also, big power today in modern; read aftermarket HD engines are now square ( bore / stroke ) to just slightly under square with very high flowing, efficient designs. This creates less turbulence in the intake tract

[84CAB]

This is how Erik Buell designed the early Buell models. Notice the long "runner" that is shown as #7.

Folks report loss of torque and overall performance when this tube is removed or shortened for a more traditional type of setup.

[asphaltgambler]

I believe his design was more aimed at reducing overall noise, which includes the carb / incoming air.

[afterburn 549]

I am thinking as an engineer with bad math skills will, why not build the manifold that comes out about as far as the carb does now , only loop it up and back between the cylinders so the carb ends up on the left side. This would have a gentle 180, but it would add a foot of length too!
I am unaware of any who have tried this or the pitfalls.

[Brando]

Wouldn't cam design have more of an impact than intake runner length? Most all motorcycle intakes I have seen tend to be incredibly short (including forced induction). Maybe it's a by-product of the limited space or design? Maybe optimal port design + cams would net better return than intake runner length.

[afterburn 549]

I will think forced then it will not matter.
Yes to cam design and length of runners.
I have DM cam 530 lift with 1.6 something R ratio.
Which of course is about 850 something lift.
I think if I smooooothed out the flow with a longer runner it would be a good thing.
I just do not need a carb hanging a foot off the the bike.
Plus I was curious if anyone had ever tried the design I described .

[Jeff Higgins]

Yeah, HD motors are a real dichotomy of design. An undersquare motor with a very long stroke, very low operating rpm's, they should have a vastly different intake tract. That said, the improvements made in the Evo, TC, and now Milwaukee Eight designs have been moving them in the right direction.

Take a look at any old Ironhead Sportster, Shovel, or Pan for examples of the very worst of the worse. Tremendously oversized ports and valves for the displacement and rpm's they have to feed. One has to wonder what on earth they were thinking.

The last Buells, with the intake up under where the gas tank normally is (the gas was carried the the big aluminum frame) were the best they ever did. That design carried through to the XR1200's as well. The Buells made the best power of any Harley motor ever made.

[84CAB]

Originally Posted by asphaltgambler

I believe his design was more aimed at reducing overall noise, which includes the carb / incoming air.

Yes, that was part of it too.
The whole air box setup was designed to take advantage of the Helmholtz design which had to do with performance and sound.
They were big and ugly hanging off the side of the Buell but quite functional.

[asphaltgambler]

Performance cam designs primarily optimize the bore / stroke/ connecting rod length of a given engine. The cylinder head / port size, configuration, length contributes to how well the cylinder is loaded ( volume). There is always some weak link or compromise in that HP/TQ chain.

My 96" drag iron Sportster. The stroke was increased a full 1" (4-13/16) retained the stock rod length as no one made a longer one at the time. It had a Red Shift 560 grind ( .560 lift at valve) over 300 degrees duration. I used the stock heads that were ported as no aftermarket was available. That cam was the largest you could install as the crank main roller bearing shell had to be fly cut so the lobe would swing past.

At 6000 RPM on paper, it required so much airflow the heads couldn't pass the volume so there was @9% loss in total output.