Does a Fuel Pressure Regulator need the vacuum pipe?

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Ferg
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Does a Fuel Pressure Regulator need the vacuum pipe?

Post by Ferg » Sun Mar 12, 2017 1:18 pm

I'm trawling threads on various forums about removing the IACV on my VHPD. It's a widely recognised mod but a lot of the threads I'm finding are old and missing pictures.

This mod sees the removal of the vacuum lines, or rather they get looped between each pair of open vacuum ports on the titan throttle bodies. The one thing that I haven't worked it is how a reference vacuum is provided to the Fuel Pressure Regulator. From all the text I've read it's never discussed.

Hence the question, does a fuel pressure regulator actually 'need' a vacuum feed? If not this mod starts to makes more sense.

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by 2F45T4U » Sun Mar 12, 2017 6:32 pm

Ideally yes. That way the pressure across the injector is directly proportional to the manifold pressure. That way you will be able to map much more stable idle conditions.
For example if you had a bar of vacuum and no vac reference on the fpr, you would be running the equivelant of 1bar extra fuel pressure at your injector and because of the extra flow, you would struggle to get a small enough injector time at low rpm.

There is no logical reason for not using the reference signal. You are technically running your injectors outwith their designed pressure without it.

So yes you can run either with or without, but because of the reasons above, you have to run the fpr in whatever condition the engine was mapped in.

When using ITBs i normally 'Tee' each of the vac signals together and run the commoned signal to the FPR.

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by steve_weegie » Sun Mar 12, 2017 7:16 pm

Ferg wrote:I'm trawling threads on various forums about removing the IACV on my VHPD.
What's your thinking there Ferg?

FWIW, both my bikes run their FPR's without a VAC line, and the only benefit I can conceive from this is that there's no lag in the rail pressure responding to the manifold pressure changes. As stated, any change to the VAC setup to the FPR will mandate a tweak of the ECU's fuel tables....

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by Scuffers » Sun Mar 12, 2017 9:08 pm

Answer - depends if it was mapped with it or not.

most OEM stuff will use MAP as load along with a vac link to the pressure reg.

Most aftermarket (NA) stuff use TPS and no vac link.

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by Ferg » Sun Mar 12, 2017 9:31 pm

Yes. I'm running an Emerald using TPS. In many VHPD cars I've noticed that they have the IACV removed including the vacuum lines. It's this I want to replicate. There's a clear guide on how to set up idle from Dave Edwards. In fact when working on it today I found the TBs were quite a way out of sync so already made an improvement. Will be doing an oil catch tank as part of the excercise too.

I understand the reason for the vacuum reference on the FPR for both idle and acceleration. But given that it reduces the pressure slightly there should be no overall harm as it will remain completely within the capability of the injectors. I'll be looking to get the engine mapped again later in the year to smooth out any kinks in the map.

The more I read it suggests that I will simply loose the capability to soften the fuel pressure at high vacuum. This is obviously a much bigger issue on forced induction cars, but for NA it should be ok.

Interestingly these were all recommendations from the service manual for the 190 upgrade.

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by Scuffers » Sun Mar 12, 2017 9:33 pm

that's not the only downside, the other is what you do about altitude compensation etc..

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by 2F45T4U » Sun Mar 12, 2017 9:46 pm

Scuffers wrote:
most OEM stuff will use MAP as load along with a vac link to the pressure reg.

Most aftermarket (NA) stuff use TPS and no vac link.
The problems still exist regardless of the load sensing method and doesnt explain why no one ever connects it.

Fwiw, i have a std M3 sat here with OE ITBs, MAF and a FPR with a vac reference, and a set of CBR ITBs with the vac connected to the FPR

Ive seen the arguments before about 'response times' but it doesnt make sense as the regulator has to respond whether its referenced to MAP or not.

Its all about flow and DP across the injector and you cant argue with the physical principles. If you keep the FP constant against a manifold under vacuum, you will increase the flow for the same injector duration...if you have zero fuel pressure and -1barg of vac you have the same as 1barg fuel pressure with a manifold at 0barg.

Conversly, if you run 1barg of boost against fuel pressure or 1barg you have 0 flow.

If you have the FP relative to the manifold, the flow is more linear across the map. You can normally tell whether it was mapped with it connected or not by looking at the map on a 3d graph. If it was mapped with it fitted the map is flatter as the fuel is already fairly proportional to the engine load.

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by Ferg » Sun Mar 12, 2017 10:38 pm

Agree, makes total sense. The pressures experienced in a forced induction are obviously large enough to need the DP capability. Out of interest how much vacuum pressure can a 4 cylinder NA car generate, or rather at Sea Level ( to remove the altitude question) what is the likely maximum differential pressure? My only reference is that at idle there a good bit of 'suck' on the vacuum tube. Don't have a gauge to hand.

There's nothing to stop me hooking up a vacuum feed just for the FPV. The reason for the original question was to work out why several examples I'd seen had potentially chosen not to. It does look a lot neater. ;-)

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by Scuffers » Tue Mar 14, 2017 8:30 pm

Ferg wrote:Agree, makes total sense. The pressures experienced in a forced induction are obviously large enough to need the DP capability. Out of interest how much vacuum pressure can a 4 cylinder NA car generate, or rather at Sea Level ( to remove the altitude question) what is the likely maximum differential pressure? My only reference is that at idle there a good bit of 'suck' on the vacuum tube. Don't have a gauge to hand.

There's nothing to stop me hooking up a vacuum feed just for the FPV. The reason for the original question was to work out why several examples I'd seen had potentially chosen not to. It does look a lot neater. ;-)
that's not really the reason..

it's all about maintaining a constant pressure across the injectors.

All that said, lot of stuff not to meet fuel tank emissions run dead head fuel rails with regs in tank and no vac link.

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by Ferg » Wed Mar 15, 2017 11:04 am

I think I'm going to hook it up, but drop out the MAP sensor. I'll find a way to plug the secondary vacuum points. It does look neater when loops between the pair of vacuum lines on each inlet, but obviously you loose the vacuum feed.

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by 2F45T4U » Wed Mar 15, 2017 3:52 pm

If its for asthetics then just make it look however you want.
If its optimum performance it maybe needs some more thought. The MAP sensor is obviously performing a function, either a fallback for alpha-N or primary control. It could probably be used with the Emerald to do some barametric correction with the speed density setup and then a remap to make it all come together.

Just do what you think is right.

I find its easier to plug the vacuum lines between each TB completely, balance them, then reconnect the lines. Otherwise you induce a flow between ITBs and it can be tricky to get a clean MAP signal or idle.

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Re: Does a Fuel Pressure Regulator need the vacuum pipe?

Post by Ferg » Wed Mar 15, 2017 10:50 pm

That's a good shout regarding isolation for balancing. Thank you. :thumbsup

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