[skyshadow07]

I spoke about this on another thread, didn't want to highjack it so I started a dedicated thread. I milled my throttle bodies out from 35mm to just over 38mm in the search for more power. Why 38? Well the manilfold joint behind the TBs is just under 37mm with the gasket between them being 40mm. So I figured I can smooth the casting out and squeeze another mm in the joint without too much work. Bike pops a lot on decel now, probably running fairly lean. I need to do the Ivan reflash before next season. Here's some pics.

 

[Bobbyvstrom]

3 mm is a nice improvement. About 16% more flow per my calculations. That's great. That could be 10+HP on the dyno on top of the other typical mods. 90 HP???

 

[Bobbyvstrom]

Do you think that the TBs could stand another mm or two? It looks like there could be a little more meat available.

 

[skyshadow07]

Possibly, I have been considering 40mm. My concerns are the throttle plate shaft bushings and how thin the intake joint is at the cylinder. Going to 40mm without opening up the intake joint would just move the negative pressure wave to the cylinder. I could do a sacrificial run on a spare TB and see what happens.
There is a point of diminished returns on bigger throttle bodies. Too big and your throttle it can't flow any more air it just reaches max flow at partial throttle. This causes problems with throttle response, you may only have 2/3 blade opening to get full flow. Throttle becomes like an on/off switch with little smooth modulation. As well, the larger you open it up, the slower the air travels into the cylinder, possibly loosing the Venturi effect. This is the natural condensing and cooling effect of choked air. And lastly, After you reach optimum size (I have no idea what that is on this engine) then you are just moving the power curve higher, not increasing power. Only way I know to determine optimum size is to measure vacuum post TB. You want to see very little to no vacuum at full throttle. If you see vacuum, you can go bigger.

 

[Bobbyvstrom]

Thank you for this insight. I thought I'd need to open up the manifold to match the throttle bodies. My limiting factor is that I don't want to take the heads off and rework them. I haven't tinkered with the SCR yet, so I'm assuming that there is an intake manifold and that it can be worked on fairly easily.

 

[skyshadow07]

It does have a intake manifold. But since the two TBs do not share a common plenum, it is called an Intake joint. It is two, individual cast aluminum tubes that turn 90 degree into the engine. For the most part, the joint it well made with equal tubes that are smooth in their transition. Only change I would do is polishing to 38mm from 37.

 

[skyshadow07]

Looking at the intake joint today. Was going to attempt to bore it to 38mm to match the TBs but I'm noticing the cast walls are only 3.5mm. If you need a reference, that's two quarters stacked on each other. so it's fairly thin. I will attempt to polish it up a little but that's all.

 

[Bobbyvstrom]

Will you put it on a dyno pretty soon?

 

[skyshadow07]

No plans for dyno. Considering measuring vacuum in intake joint and possibly comparing acceleration with a draggy.