6 replies to this topic

[Justin Casey]

Just had mine break at Thunderhill today. Luckily the two screws are both still in the shaft itself and not somewhere within the engine.

I am curious to see how many others have had this issue. Please comment if you had this issue and/or have any insight on how to further prevent this from happening again.

I have heard a few people putting epoxy on the shaft but I don't quite think that'd be legal...

TIA

- Justin

[chris haldeman]

Increase throttle cable slack and or install a pedal stop between the floor and pedal allowing only full throttle. What happens is the blades open all the way long before the pedal stops moving and you basically rip the shaft

[Justin Casey]

Increase throttle cable slack and or install a pedal stop between the floor and pedal allowing only full throttle. What happens is the blades open all the way long before the pedal stops moving and you basically rip the shaft

 

I'll definitely incorporate that when installing the new throttle body assembly.

 

Thanks.

[Bench Racer]

Here's a bit more info and not pissing on anything Chris stated. Chris is spot on with the floor pedal stop. 

 

Don't cheat and remove the aluminum arm that supports the intake manifold to the engine and don't remove the steel bracket that supports the backside of the intake manifold square surface the throttle body mounts to. Also remember the gasket between the throttle body and intake manifold is not specified within the rules, think thicker vibration absorbent gasket material.  

 

For a production car one may be more creative towards eliminating the vibration at the throttle body because the rules are less restrictive. When these same engines/throttle body/butterfly and shaft are used in the production class cars with engine rpm's of 8,000 plus a bit it is believed the engine harmonics breaks the shafts. They use two process to get around the issue. Weld the shaft/butterfly which is illegal by rule, weld or braze the screws to shaft is rule legal and there are units where the butterfly is secured to a special shaft (EDM'ed slot in shaft) with epoxy which is legal. I've viewed both shaft welded and shaft epoxy unites brake. 

[cam]

Adding another data point, I've had multiple throttle shafts snap over the years in my 1.6.  While a more relaxed throttle cable helps, that is just part of the problem.  Bench is on the right path, the harmonics really play havoc with the throttle shaft.  IMHO, keeping the manifold brace helps.  But I would be VERY wary of using the thick intake gasket.  It is my empirical evidence that the thick isolated style of gasket caused more problems than it solves.  In my case on my now STL 1.6, studs sheared off and caused a lean condition when the manifold created a leak.

 

When I get my car running again, I'll epoxy the shaft to retain the bolts.

[obxer]

A bit late to reply, but our experience was a bit different regarding this problem. I run a 1.6 in ITA with a megasquirt. So I redline a good bit higher than stock. We broke 3 or 4 butterfly shafts in quick succession with one breaking only about 30 minutes after changing it. After the first break we thought maybe we were over-torquing the shaft at WOT by not having the pedal stop adjusted correctly. So we adjusted the pedal stop so the butterfly was just barely to the WOT stop when we stood on the pedal. After this we quickly broke another. The looser we made the cable at WOT the faster they broke. We finally wised up and figured the less the cable was tensioned at WOT the more the butterfly could vibrate/flutter. So after the 3rd or fourth one broke really quickly we adjusted so the cable was tight at WOT - putting a little tension on the shaft at WOT. Since then, we haven't had one break in well over a year.

[Steve Scheifler]

Interesting, thanks!