TR5/6 PI: measuring fuel pressure at the pump-outlet.

[Waldi]

Hi all,

I need to measure (and likely adjust) the fuel pressure on a friend’s TR6 PI.

I’m aware the recommended location is at the inlet of the MU.
Advantage over other locations:
A pressure drop across the line from pump to PRV to inlet MU is accounted for.

A partially plugged inlet is accounted for (or can be detected).

My friend’s car however has a rather stiff flexible hose at the MU-inlet, so I want to measure at the PRV-inlet. I have all the hard-ware to do this.
We will clean the line from PRV to MU with compressed air, so it will be “open”. 

Would this be an acceptable method/location?
I do not see any disadvantage, if the fuel line is clean and “open”. The velocity and hence the pressuredrop over the line from PRV to MU will be low, and will vary anyhow with engine speed/load.

Thanks,
Waldi

 

[michaelfinnis]

If you measure at the PRV inlet you will be reading the pump delivery pressure, not the pressure at the MU which is what you need to adjust. The flow is divided from the PRV, one line going to the MU (regulated pressure) and one line back to the tank (pressure relief). You could take a reading at the PRV outlet to the MU. It wouldn't take into account the pressure drop over the PRV to MU line but I doubt that would be significant. I can't see that being easier than the usual place though.

Mike.

[Mike C]

9 hours ago, Waldi said:

Hi all,

I need to measure (and likely adjust) the fuel pressure on a friend’s TR6 PI.

I’m aware the recommended location is at the inlet of the MU.
Advantage over other locations:
A pressure drop across the line from pump to PRV to inlet MU is accounted for.

A partially plugged inlet is accounted for (or can be detected).

My friend’s car however has a rather stiff flexible hose at the MU-inlet, so I want to measure at the PRV-inlet. I have all the hard-ware to do this.
We will clean the line from PRV to MU with compressed air, so it will be “open”. 

Would this be an acceptable method/location?
I do not see any disadvantage, if the fuel line is clean and “open”. The velocity and hence the pressuredrop over the line from PRV to MU will be low, and will vary anyhow with engine speed/load.

Thanks,
Waldi

 

From memory  the fuel flow to the MU is about 20% or less of the total flow through the PRV when I last  calculated it  using a fuel consumption of 10 mpg. Given that the pressure losses are a quadratic function of the line velocity I'd say the pressure at the PRV supply to the MU is virtually the same as the inlet to the MU.

The PRV inlet is essentially  connected to the MU supply with the PRV bleeding pressure back to  the tank through a branch as a primitive surplussing valve . I'd say the pressure at the PRV inlet would be pretty close to the pressure at the MU inlet.

Edited Wednesday at 02:16 AM by Mike C

[Waldi]

Thank you Mike&Mike.
I will fit the T-piece at the PRV inlet or outlet, I guestimate the dp should not differ much from the MU-inlet.

Given the recommended pressure (105-110 psi), and I can correct this if road-testing shows that is benificial.
 

Waldi

[Mike C]

5 minutes ago, Waldi said:

Thank you Mike&Mike.
I will fit the T-piece at the PRV inlet or outlet, I guestimate the dp should not differ much from the MU-inlet.

Given the recommended pressure (105-110 psi), and I can correct this if road-testing shows that is benificial.
 

Waldi

Yep, you have to road test and experiment  if you have problems with fuel pressure.

Years ago I found that my engine ran best when I set the PRV to open at 680 KPag with compressed air (I like to set valves using  a safe media)  , since that time I just set the valve to open at that pressure . But every PI system is different these days after nearly 60  years of being messed about with.