How do I stop belt slip - Rotrex SC

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JalbKay

Full member
Points
33
From
Townsville, AU
Car
MX5 NC 2.5L SC
G'day All, I have Rotrex powered 2.5L MX-5 that just won't maintain boost. I know I'm getting some positive pressure as I've blocked off the BOV and before things get too hot I'll get back pressure noise through the SC coming off the throttle but once the temperature gets up there's nothing.

All the troubleshooting I've done is pointing to accessory belt slip on the water pump pulley. That pulley is supplied by Kraftwerks in their kit and it's a ribbed jobby to replace the OEM smooth and the main accessory belt has been changed to a 6DPK to suit (OEM is 6PK). The actual SC drive belt and pulleys are toothed and I just can't see them slipping so it has to be the water pump pulley... right? Any thoughts or tips appreciated. |B
 

obi_waynne

Administrator
Staff member
Moderator
Points
887
From
Deal, Kent UK
Car
A3 1.4 TFSI 150 COD
Slacken everything off and retighten. Are the belts all straight, being slightly off can cause them to slip.

Have you checked for boost leaks? Is the BOV activating correctly? How did you map the engine to run on a supercharger?
 

JalbKay

Full member
Points
33
From
Townsville, AU
Car
MX5 NC 2.5L SC
When you say "toothed "do you mean that it is the same as a cam belt that can't slip??
Yes mate, exactly what I mean; specifically a Gates GT3 640-8MGT-2O

Did you get anywhere with this issue?
Yes... and no. It's... weird. Everything has been checked, loosened and tightened. The intercooler/intake path is happily holding 25 psi. As I said driving with the BOV blocked off I get a 'wuffle' back through the SC coming of the pedal but my boost gauge never moves off vacuum. I've also used my compressor to test the gauge and it reads true up to 20psi.

Having said that, it's quick... Quick enough to beat several cars to 80kmh on different occasions, most notably a Focus XR5 Turbo, Gen4 Supra and a 6.0L SS Ute and I don't 'think' fuel and timing alone is enough to keep me in front of that sort of power. I'd probably get this sorted quickly but for two things, no where to go to legally test getting past 110kmh and having to pay $120 to get dyno time... I'm tempted to pull the Rotrex and send it to Bullet Cars for testing as they're the AU dealers.

Edit* Tune supplied by Mike at Moto-East, he knows the car and he knows the SC kit.
 
Last edited:

JalbKay

Full member
Points
33
From
Townsville, AU
Car
MX5 NC 2.5L SC
Quick update for any that may be interested:

I pulled the SC and sent it down to Tom at Bullet Cars (QLD AU). Tom was pretty happy the SC was OK and we went into some detail about belts, routing and pulleys (photos and measurements aplenty) as well as confirmation of intake/intercooler pressure etc, then the penny dropped. I'm running a kit that has been designed for a 2.0L while I'm running 25% greater capacity with the 2.5L. Short answer is I need a smaller pulley.

So currently trying to find a 28T70 25mm wide Rotrex pulley but I'm pretty sure I'm going to have to mill one from a blank, modify something similar or change all the SC circuit pulleys over to ribbed... Waiting on a response from Kraftwerks as to availability but at least I have a way forward.
 

JalbKay

Full member
Points
33
From
Townsville, AU
Car
MX5 NC 2.5L SC
Well, 4 months down the track... I've just fitted a custom made 48-8RPP 120mm main drive pulley because clearances on the nose wouldn't allow a smaller RPP pulley to fit. I thought it was quick before :woot: I'm glad I've got the 245s on the rear instead of the stock 205s ;) I've got to pull some logs and get it on the dyno but for the most part I think I've finally got it sorted :)
 

obi_waynne

Administrator
Staff member
Moderator
Points
887
From
Deal, Kent UK
Car
A3 1.4 TFSI 150 COD
Sounds Nice. Thanks for keeping us updated and I'll be interested to know the results you get back.
 
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