Many of you know I have a boost referenced regulator. I previously had the vacuum port hooked up to my brake booster, which would make the regulator see vacuum AND boost. So at idle, my fuel pressure was 60psi, it rose to 70psi at 0 vacuum/boost, and rose to 80psi by the time I hit 10psi of boost (OK, it fell a little short because my pump was running out of steam, but it was TRYING to hit 80psi).

I decided to hook my vacuum port up to the nipple on the throttle body which was previously capped off, and now my regulator sees boost only, and not vacuum. I don't have a check valve hooked up to it at all, and the car idles at 70psi and doesn't start increasing pressure until I get into boost.

I'm a little baffled -- if the back of the intake can see vacuum, why is the throttle body not showing vacuum to my regulator?

Has anyone else observed this behavior? Am I just missing something stupid? I know a few guys on here pulled off some tricks to make their boost referenced regulators see boost only so that fuel pressure didn't rise too high on 10psi of boost. Is it really just as easy as hooking the regulator up to the TB instead of the brake booster line?

 

Yea, you are missing something simple. There is no vacuum in front of the throttle blade and that is where that port gets air. There will also be a spike there as the TB initially shuts and the blow off valve opens.

 

OK, thanks.

Does the pressure spike only apply to supercharged cars, or would my wastegates keep this kind of spike in check? Did you have yours setup this way at some point? Were there any drivability/tuning issues associated with doing it?

 

It would be the same on turbo and sc cars, wastegates only control the boost ceiling and don’t play much of a role when the TB closes.

I’ve never tried hooking up the FP regulator to the TB port, but common sense tells me it would be a bad idea. It might actually work to some extent, but I think it would be a pretty distorted signal.

I like the approach of just hooking up the regulator to the intake manifold vacuum and then flattening out the IFR table in the PCM.

 

As always, thanks for the advice.

 

One other thing, Reference regulators are to be setup with the port vented to atmosphere. So, with the Vac line disconnected, you dial in your base fuel pressure (60psi) and hook the lineback up. You will find at idle now the fuel pressure will be LESS than the the base of 60.

Phil