halfshafting the throttlebody on a miata

Apparently no one knows what im talking about on the miata forums. Anyone here done it?

Im a bit concerned since the shafts break or something like that, so I hear.

I don't know what you are talking about either.

This?

to

?

I have heard it makes a difference in CFM but I would love to see some hard data on power improvements.

Interesting.

Yeah, I've heard of it with Fox Bodies, but was always skeptical whether it really made any difference...

Where do Miata throttle shafts break? I doubt the weak part is the part you'd be cutting on.

Yes ditch. That. Good for 1-2 whp on a neon. Lots of info on it on neons.org. its like going 2mm bigger on the throttlebody.

Supposidly the screws break off or something? But mainly on race cars. Not entirely sure. Im about ready to just ignore the internet and build it like I would any other car.

Is the difference/result in the 'broken' third image?

In reply to Jaxmadine:

Hell, replace the screws while you're at it. I'm sure it'll be fine.

Is the Miata TB something of a bottleneck? I am vaguely remembering that the TB in the fox body 5.0s was considered a bit of a bottleneck and this was on the list of "free" ways to fix it.

Not sure. The intake is one for sure tho.

Anything that helps it breath cant be a bad thing.

One would think you could almost achieve the same thing by smoothing out the ridge at A or B. But that does not answer the OPs question, does it?

And me thinks the OP meant this, which is on a Honda BTW. Original: Modified:

I have pics for days. Just not sure if anyone has done it to a miata. Guess ill be the first again.

The thing to be wary of by thinning the center of the shaft is not so much full throttle, but closed throttle at high revs. The shaft provides a fair bit of bending resistance to all the suction form the motor side. With a thinner shaft, the bending is carried more by the throttle plate and may cause the throttle plate to deflect and dig into the side of the throttle body bore. This could cause sticky throttle response going back to the gas.

I have seen this on throttle blades that were machined with no shaft thickness across the plate and the shafts only being supported in the throttle body housing. It was not good for driver feedback when the throttle had to be pushed harder to get it open when they were trying to feed in throttle slowly on corner exit.

If someone wanted to halfshaft my throttlebody, I'd be rather concerned.

Crikey, they been doing this on SU carbs for years.

I believe the correct term for this is "throttle blade machining," as a "halfshafted throttle body" raises images of a CV halfshaft stuffed into a TB. I was confused as hell when I read the thread title.

spitfirebill wrote: Crikey, they been doing this on SU carbs for years.

I did this to a dual carbed Mini , about a hundred years ago. It's been a standard modification to SU carbs forever

If any bottlenecking in the intake is downstream of the TB, you won't see much improvement there. If the TB is well sized to the engine, i.e. it will flow way more air than the engine can possibly use, then ditto.

Have to check with a Miata engine specialist to know. MG people have been doing this for years even though it doesn't really get you much.

It's not an uncommon mod on DSMs as well but I'm not sure if it's going to make a big difference on a Miata. ISTR that especially on an NA, breathing mods that are external to the engine don't do that much for performance, if anything. I've also never heard that the TB is that much of a bottleneck at WOT on an NA, at least when we're talking mostly stock engine. Once we're talking an engine with either a later head or a gasflowed head things might change somewhat.

DeadSkunk wrote:

spitfirebill wrote: Crikey, they been doing this on SU carbs for years.

I did this to a dual carbed Mini , about a hundred years ago. It's been a standard modification to SU carbs forever

I did it, too.

Its a 99. And apparently the shafts tend to crack a bit. So ill just leave it be.

Main thing ive noticed when doing the mod tho is partial throttle gains. Theres no more hump when you cut the shaft off.

And yes. I do it to su carbs as well.

I would be carful doing that. You can do some significant porting to the "barn door" style AFM that will improve things. I did this to my RX7 and it made significant improvements.

If it's a 99, it shouldn't have a barn door AFM...