You can type here any text you want

Correction to a fact

Welcome!

By registering with us, you'll be able to discuss, share and private message with other members of our community.

SignUp Now!

bruce

Rest In Peace
Joined
May 25, 2001
Messages
10,367
For those really into it.
The Fuel Pressure Regulator is not manifold vac referenced. It is referenced thru a ported orfice.

If you've watched the F/P and never understood why it didn't track right this explains it.

Also, explains some oddity in the DE, DFCO stuff.

Also, some overtones to BOV operation, in some applications.
 
Bruce, please explain.

My fuel pressure seems to track just fine, pound for pound.

What did I miss?
 
I'd agree with you Bruce if there was actually flow through the line, but there shouldn't be. I guess there is a miniscule amount as the diaphragm in the reg moves, and maybe any rubber parts in the line swell a bit, but as long as there isn't an air leak from the regulator then flow should be just about zero and the pressure should track manifold pressure very closely. It's supposed to act very similarly to a mechanical oil pressure gauge, with the difference of the fluid being compressible. I think that effect is pretty minor. Just MHO. Talk me out of it :)

John
 
Interesting topic.I've been waiting for this one.:)
To a large extent,I agree with Bruce.
Reason?You've got a large mass of dense air passing a small orifice.That orifice is passing a sample of what is supposed to be representative of the actual pressure going into the manifold,but the air going into the manifold is moving,or dynamic,not static.Therefor you get an approximation,not a true representation of the pressure.
To me it's *kind of* like standing in an inlet at the side of a raging river.Step out into the river and you get swept away.Stay in the inlet and you can see and hear the water but you are largely unaffected.
I think maybe syphon effect might come into play here.
This reminds me of the guys who say that the pressure in a cylinder runner is at constant boost pressure.Wrong.
The air is dynamic,therefor the pressure changes with the opening and closing of the valves.
 
Maybe, possibly, but to me it's no different than the signal that a MAP sensor gets on a speed density car. If that static pressure measurement didn't track the "real" (whatever that is :) ) manifold pressure pretty well, I would think that system would work a lot less well than it does.

John
 
"Static" means just that. Under non-flowing conditions the pressure at all points in the system is EQUAL...

:)
 
It seems that John is saying that because the air in the throttle body is flowing, the pressure "seen" by the hole which leads to the pressure regulator is not the "real" pressure in the throttle body. While it is missing the velocity component, that is not a big number. If you run throught the numbers, you will see that the 231 inch six is at most going to take in about 350 cfm. That 350 cfm may be at 25psia, or at 35 psia, but it is still only 350 cfm. The velocity in the throttle body area is about 200 ft/sec. If you use that to find the (rho)(v-squared) term in the pressure equation, you don't get a very big number, although it does get bigger as the boost goes up (rho gets bigger) I agree that the line from the top of the throttle body to the regulator is essentially "static" so there should be no significant effect from any restrictions in that line. I suppose you could use a tap at the back of the dog-house, if you wanted to get a more accurate value for the pressure. But the pressure that the injectors see is from even faster moving air, moving past the tip of the injector. so what's the point?
 
I still think this is an interesting discussion to pursue.
That 200 cfm or so works out ~135 mph,linear.That's no small speed.
Factor in the air density,and you've got a real interesting situation.
As to the port situation,that would explain why guys with bigger cams find it adventagious to adjust their sensors away from spec for better top end fueling.
That might also explain why some guys tend to run out of fuel when everything is in line with their fuel systems.
Maybe they're not catching the highest velocity/lowest pressure points in their intake events?
I think Bruce is right,however not over the total boost/intake event,in reference to the intake "doghouse"/throttle body airflow situation.
The only accurate way I've seen to sense the boost/airflow[hope I'm stating this right],is to use a 90 degree tube facing into the incoming airflow that feeds a signal to the regulator,etc.
The factory setup is at best, a *reasonable approximation* of what is going on.
I'm betting that there will be transition points along the airflow curves where the signal at the ports will be "off",so to speak.
While we've got the ears of a couple of engineers,any idea what the velocity is in the intake ports,of say a pair of Champion irons?
I believe they flow 210 cfm @ 28 inches of vacuum,.500" lift.
 
Bruce, I really like your scientific approach to things, you really know your stuf!! What I would like to know is how much of a difference you measured between what the real boost in the intake is ,as compared to the loss of boost to the regulator. And do you advise the use of a rising rate fuel pressure regulator to compensate for this? Keep up the great work!

Nick
 
Originally posted by TurboDave
Bruce, please explain.
My fuel pressure seems to track just fine, pound for pound.
What did I miss?

Ported.

You have manifold source vacuum, and ported.
And within ported you have just straight ported and EGR ported.

The reference line to the Fuel Pressure regulator is reading a ported signal. Meaning at idle there is little to no vac applied to the port, and as you open the throttle it sees some vac., and as you progress from there it sees boost. But, depending on you exact intake tract, you may see some F/P drop due to the systems restriction.

So it will track with boost just fine. But if your engine is idleing at at say 50K/Pa, then you should see you intial fuel pressure reading drop 7 PSI from vac on to vac off at idle.

We'll skip hysterisis of the sensing elements for reading F/P and vac, but in a nutshell you want really small lines so as to not dampen the readings. But, that generally induces alot of chatter.
takes good guages, and or code filtering to really see all the details.
 
Dude, what's going on lately?? Small sensing lines DO have a damping effect on the signal. And no vac on the fp reg line at idle?? Haven't you noticed that when you reconnect that line at idle the fuel pressure drops like 5-6 psi? What car are you looking at??

TurboTR
 
Originally posted by TurboTR
Dude, what's going on lately?? Small sensing lines DO have a damping effect on the signal. And no vac on the fp reg line at idle?? Haven't you noticed that when you reconnect that line at idle the fuel pressure drops like 5-6 psi? What car are you looking at??

Your not going to beleive this.
Somewhere in it's life the vac hard lines had be rebent. And looked right. I had to compare it to another one off car to see the difference.
Matter of fact good enough to look factory.
The old one went for a trip.
Arrgh, sorry.
 
Originally posted by bruce
Ported.

You have manifold source vacuum, and ported.
And within ported you have just straight ported and EGR ported.

The reference line to the Fuel Pressure regulator is reading a ported signal. Meaning at idle there is little to no vac applied to the port, and as you open the throttle it sees some vac., and as you progress from there it sees boost. But, depending on you exact intake tract, you may see some F/P drop due to the systems restriction.

So it will track with boost just fine. But if your engine is idleing at at say 50K/Pa, then you should see you intial fuel pressure reading drop 7 PSI from vac on to vac off at idle.

We'll skip hysterisis of the sensing elements for reading F/P and vac, but in a nutshell you want really small lines so as to not dampen the readings. But, that generally induces alot of chatter.
takes good guages, and or code filtering to really see all the details.

Here's what I see. I'm pulling 16" of vacuum at idle. And when I reconnect the vacuum line to the pressure regulator, the fuel pressure drops by almost exactly that amount (something very close to 8psi) Isn't that the way it's supposed to be.

In looking at the underside of the vacuum block it appears that the three center ports are getting their vacuum from just behind the throttle plate, which makes sense to me.
 
Bruce, with that new vac line, you're gonna have to redo the MAF tables and scalars. Now if it was closed loop, it would've compensated for it!:p :p ;)

Just funnin' ya..

Eric
 
Back
Top