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I've heard it mentioned before that some of the SP BOVs were installed the wrong way around. While looking on carsales I found that some of the recent listings do in fact have the BOV the opposite way around.



So my question is - does anyone know what the correct intention for the BOV direction? From my understanding you can run the BOV in either direction but they will change the way it works (one way will act as a progressive safety valve, the other as a switch). https://www.youtube.com/watch?v=lI9rJoNzNVc

This ties into the T-Piece mod.

The T-Piece mod as I understand it, basically ups the boost by 2-3PSI by moving the reference point for the waste gate actuator closer to the valves. Depending on which way the BOV is it will have two possible effects:

1) Lifting off the throttle will be extremely harsh as the vacuum actuates the BOV and you suddenly got from 8PSI to vacuum
2) You have more progressive boost pressure but ultimately lose boost as it is wasted by the BOV opening progressively

Neither is really 'wrong' but the original intended use would matter...

This comment http://mx5cartalk.com/forum/viewtopic.p ... 83#p553291 seems to imply that the ECU was tuned for a higher boost than what the cars were originally shipped with - I was wondering if anyone could provide any more information on that? I find that a bit suspect because this dyno run of the T-Piece mod http://mx5cartalk.com/forum/viewtopic.php?f=43&t=20505 seems to show that the engine is running quite lean.

I don't know a lot about this stuff, so if anyone can clarify anything or correct me I would really appreciate it!

Those were my dyno results- an AFR between 10 and 11:1 is actually quite rich, not lean. SPs are all generally that way, as evidenced by the dirty exhaust and the popping and farting on the overrun. An aftermarket ECU and good tune will extract more power and a smoother drive. I did the T-piece mod for that dyno day so I could see if it was too lean with the extra boost, which it wasn’t. The power increase was significant, so I left it that way 8psi is only .5psi more than the intended boost for the SP - mine would have been seeing 6psi max before the mod.

Thanks for that!

And which way is the BOV supposed to be fitted?

It's pretty strange that they didn't reference closer to the valves when the car was built but the fuel map still seems to keep up!

If the point of the T-Piece mod is to get the boost reference point as close to the cylinders as possible - wouldn't it be better to run it to point circled, which is directly above cylinder 1?

I dont have an SP, but my car does have an aftermarket ECU, which references manifold pressure to work out fuelling etc.

The reason that often MAP is not taken from there is that immediately behind the throttle there is usually turbulence which causes the reading to fluctuate a fair bit.


A steady reading is a predictable reading, and makes tuning a lot easier since you dont have unexpected spikes/drops that can stuff things up.

I can only assume when the SP was designed the same concept was applied.