'progressive' throttle body mod

[wayneschofield]

Hi All.

Just thought I'd share a little something I am up to.

I found the throttle a little 'snatchy' off the bottom for some of the types of riding we do. I played with the throttle cals etc but no amount of 'ramping' the ignition is going to make the airflow for a given throttle angle any less, so I set about changing just that.

I bought a spare body with some other bits a while ago so I stuck it on a flow bench and graphed airflow vs throttle angle and it confirmed what I felt - that the initial throttle angle change creates too much 'open area' for it to be 'nice' on initial opening. This also gives the ECU a harder job, expecting interpolation to blend this area is asking a bit much of it really.

I mixed some favourite JB and started filling just by the throttle plate, eventually ending up with a small portion 'shaped' so that there is a radiused 'ramp' on one side so that only one half of the throttle opens initially. Thus the area opened for a given initial throttle angle change is less.

Bizarrely, there is almost no change in full-throttle airflow, providing the throttle is opened slightly PAST what used to be full throttle, so the throttle plate angle at max airflow is actually about 93 deg.



Check the pics .....

[Canniboomer]

Note you may also have an even-radiused throttle bellcrank, which has the same overstroke affect down low in the throttle movement, for a snappy and uncontrolled feel. I am curious which one you have. The later bellcranks were intended to alleviate that issue. Some added filing to the bellcrank cable groove, nearer to the end pin may give the same result, and would not obstruct the full-open intake path. Pictures of the 2 shapes are available somewhere around here. A billet bellcrank is also available, and may be better to experiment with.

[wayneschofield]

Thanks for your input.

I did look at the bellcrank route, it would have been a much easier 'fix', but that tends to make the cable chaffe on the outer as it moves from side to side as the bellcrank radius changes, made worse by the fact that the cable outer is quite close to the quadrant. This would have meant no re-mapping was needed though, but that's no big deal for me as that's all I do all week.

It doesn't, however, address the TPS volts change being very small for a given throttle open area change. Doing it this way allows the maps to be blended much nicer at low openings, more logarithmic so to speak.

I go on trails with the kids all the time, and sometimes it gets real rocky so I have to go very slowly with them so they don't have a problem trying to go too fast. I ride at the back so I can see what they're doing all the time. With a snatchy throttle I tend to have to ride on the clutch a lot of the time. I'm on a 13T front sprocket so can't really go any lower on gearing, unless someone has a lower first gear ratio going spare...

Thanks for reading anyway, input and advice always welcome....

[cannondale27]

Thats very interesting.When you flowtested wouldnt it need to be on a head with valves opened and closed though for flow claims at wide open to be accurate?

[wayneschofield]

It was just flow tested mounted on a parallel tube with a radiused ram pipe on the top side. To be honest, I doubt whether the stock heads I have at the mo would out-flow the t-body anyway, at least with the stock valve lifts employed.

I figured, if it's posing no more restriction than a stock body, it's not going to reduce the power anyway up. I was using the throttle angle as the variable vs flow anyway, adding valve openings into the equation might have confused my simple mind!

I am happy with how this turned out for now so, since I can now use this on my motor (if I ever get the thing done), I'll get some testing done with the (now spare) stock one on a head and see where the restrictions are. If it sees a real improvement from opening up the body outlet, along with the rubber joiner and the head, then I'll look at this again perhaps later on.

I figure my motor should be around 55 bhp when done, and to be honest, it was fast enough before for what I do with it, so increasing the power much more would be moot. I only tore it down to do the (ever increasing number of) updates.

Thanks for the input.

[cannondale27]

55bhp is about a 13hp increase over stock.Thats not easy to come by.Would be cool to see those numbers accomplished in a whole different way.Drivability is also a worthy goal for sure.Keep the ideas coming.

[kdeal]

Here's the newer one... It has two holes for the cable. They combined the ATV & motorcycle style throttle hook-ups (twist vs thumb)