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Is a blow-off valve really needed on a drag only bike?

Off Topic Discussion

If it's not really about tuning or wiring. Then it belongs in here.

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There are different ideas about this. I was thinking about not running a blow valve on my drag only bike. Only after a burn out I let the throttle close rapidly but this is without boost so I don’t think compressor will go deep into surge.

What do you guys think?

What's the harm in having one? If it was me, I'd install/keep it on although you are going to be much more knowledgable on your bike than I!

The BOV is there to protect the turbocharger when you lift the throttle from full boost. Whether or not it's an essential item will depend on your turbo and the amount of boost. For example on a ball bearing turbo, the core assembly is much stronger in thrust than a journal bearing turbo and hence a BOV could be considered optional. In a journal bearing application I'd recommend using a bOV, particularly if your boost level is high.

Ultimately though the BOV is cheap peace of mind so there is probably little downside to employing one regardless of your turbo.

I’ve heard stories about BOV leaking under high boost that’s why my question about this. Is there a way of trouble shooting a BOV on leakage? At the moment I have a BOV on my bike.

My turbo is a Garrett ball bearing GT30 and on the dyno I’ve pushed 30 psi of boost. I’ll keep my BOV and hope it will not leak under high load. Thanks guys for the replies on this matter.

I also run a turbo bike. I noticed leakage, so I added some washers on the BOV to help stiffen it up. Problem solved!

How did you find out it’s leaking? Is there a way to test it or where to look for leakage?

It wasn't building full boost pressure, so I went over all the fittings and lines, found no leaks. The wastegate is a brand new Tial 38, so I went under the assumption that it wasn't causing problems. So we stiffened up the BOV and sure enough, boost pressure came back up.

The easiest place to check for a leaking BOV is on the dyno. If you're not going to get in the way of anything, you can simply put your hand on the BOV outlet during a ramp run. If this is likely to be dangerous then the other option is to place some race tape across the outlet and watch it carefully during the run. If the tape blows off or ruptures while you're still at WOT then it's leaking.

Hi @Dolfo, to answer your question from yesterdays webinar about the decay rate on the G4/Vipec ECU:

The information I gave you was essentially correct. The decay rate defines how quickly the cut is removed when a function such as launch control is disabled. In this case the value defines the percentage of cut that is removed every 20 milliseconds. A low value will remove the cut slowly, softening the shock loading, while a large value will reintroduce engine power faster and more aggressively.

I believe from my own testing that leaving the decay at 0%/20ms disables the function and reintroduces full engine power instantly.

I will check it on the dyno for leakage.

Thanks for clearing that up on the decay!

See attached data log there you can see what happened in my situation. Decay setting at 100%/20 ms and you can see my RPM jumps up at the hit then falls back, flat lined then go back up. I think it comes off too aggressive. Look at the wheel speed it’s not moving at the hit. This means that the clutch starts engaging later on in the run. In this case I need to lower my decay to get it flat lined from the start right? Is there a better way off logging to fine tune this? Or will it be something I have to keep playing with to get the ideal number?

Attached Files

In my own experience, you will always see the sort of rpm spike on the gear change that you are seeing in your log. The DI speed trace looks to me like it shows some momentary wheel spin after the shift? Is the sensor on the driven wheel or front wheel?

I'd suggest simply making a change to the decay time and noting the effect. This is the easiest way to see what is going to work best for you. I would also recommend making quite a dramatic change - perhaps halve your current decay rate. This way the difference should be very obvious.

Actually I was referring to the launch. There is a spike when leaving the clutch out and exit the 2-step. There is also a big jump in the RPM and then the clutch starts to engage bringing the RPM back down. Here also I have to deal with the exit decay rate on the launch control settings. But about the shifting there I also have to deal with some type of exit decay rate after the shift.

Yes you are right about the wheel spin! When I shift I’ll get wheel spin and I think it’s my chassis that’s not working properly. I’m working on my rear shock to eliminate my wheel spin between shifts.

My wheel speed reads from the output shaft. Next time I’ll make some big changes on the decay rate and see what happens. Thanks for all that information!

Sorry, I see what you meant now. What you are seeing is likely an aspect of the clutch engaging slightly after the clutch switch deactivates. This is one of the aspects i mention in the launch control course - It's important to have the clutch switch activate at the same point that the clutch actually begins to engage.

Can this be achieve by adjusting the exit decay rate in the launch control settings? A low value to remove the cut slowly or it’s better to adjust the clutch switch to activate at the same point that the clutch begins to engage?

No, you need to correct this by adjusting the actual clutch switch position.