measuring fuel pressure

When measuring fuel pressure should you test the engine under load or can you just measure while in park and revving the engine. Two buddies telling me totally opposite methods.

When does it use the most fuel volume?
Not in neutral
I've drag raced for years and started dealing with fuel delivery problems on the first NHRA stocker I helped on, the original 1968/69 Plymouth GTX hemi automatic car that Steve and Linda Myers owned that she drove usually. I sold them the 426 Hemi motor and installed into their 440 car, I later rebuilt it and help them race it in NHRA during 1973 or 1974, CRS now
Fuel delivery has caused a lot of grief for a lot of drag racers, a LOT of them from stock to Top Fuel
To answer your question you should test it in high gear above 3500 RPM at WOT until the RPM you normally lift at
Let us know your results please, that can and will help others on here also

Well, you can technically "test" fuel pressure anywhere choose, but like Cab says, at the top end after the system's had to keep the bowls full under WOT is where you'll find the system's limit.
Basically, you need enough pressure under that kind of demand to keep the jets fed & the engine from leaning out.
Some carbs' needles & seats will only tolerate 4.5 PSI - lowest I've seen was 2.5 PSI on a stock flathead Ford - and the Holley ballpark is around 7 PSI.
Those #s are easiest to achieve under low loads, but not so much under real demand.
I like to take that measurement as close to the carb as possible.

check the psi with a gauge hooked up right at the carb inlet at the end of the run however/wherever that is. Fuel demand/consumption is the greatest at that point & that is where it needs to be checked.

Thanks for the replies. I've been chasing this problem for a while. Car starts fine, idles fine, runs great up to 70-80 mph. When I press the pedal to 3/4 throttle she starts sputtering, anything past that she just lays over.
I initially thought it was fuel. Swapped carbs with a buddy, swapped fuel pumps, checked for kinks in fuel line, blew compressed air back thru line into tank, same issue.
Then tried timing, have adjusted it every which way, swapped the coil and distributor with my other Charger (both using Firecore RTR system), same issue. Sent a video to a Charger buddy in Texas, he thinks it's fuel related. LOL talk about chasing my tail.
I just swapped out the fuel pump push rod, even thou the existing comp cam one looked fine, since I had it out I replaced it was one I got from board member Maccanix, no change.

Could be a bunch of things. Valve springs are shot, timing chain loose, partial blockage in your exhaust system, etc..........

New engine build. Hope it's not something the engine builder did. The last 440 he did for me runs great.

Does this in all gears or just high gear?

I need to make sure you understand that pressure is not the true indicator of enough fuel volume delivery, if it is running out of fuel at 3/4 to wot in high gear make sure the needles and seats are big enough to flow enough fuel into the motor to make sure it isn't being starved by them
I had that happen on a new Holley list # 9375-3 1050 CFM dominator carb, the factory sent it out with one .110 needle and seat in the primary bowl and the secondary bowl had a .120 size in it. It ended up needing .120 in both bowls to not run the front cylinders out of fuel in the 1/4 mile
Lots of things can influence are results

Doesn't this all depend on what the motor size and HP is? Are we talking 1000hp race car, or a 500hp street car? Service manuals have the test procedure for mechanical pumps, which I assume you are talking about. Pressure test at carb inlet, and volume test. There also is a static bench test and vacuum test.

What electronic ignition? What motor, carb. etc

Have seen Mopar electronic boxes that stop working effectively at higher rpm.

So this can be a lot of things including intake to fuel pump, filter etc...

Sorry, should have provided more info. 440 out of a 76 motor home. Motor didn't run when I bought car due to numerous electrical issues. Motor was rebuilt, .30 over, stock pistons, cast crank, Weiland Intake, 750 Eddy, Firecore RTR ignition system. My buddy who is a mechanic and diehard Mopar guy gave me a cam that he had but wasn't going to use on his stock 67 GTX. It's a Direct Connection P4286677, 272 duration, 48 overlap.He thinks the ignition system is faulty but I have the same set up in my other Charger and swapped distributors and coils and still had the same problem.

I have tried all different combinations of jets and metering rods without any change. As I mentioned I swapped carbs with a friend who have a 440 with no difference.

I just test drove it and tried it running in just second and first gear. Same issue.

I'm thinking not fuel (unless the fuel delivery system pieces was woefully inadequate/or has a malfunction from day 1). When you swapped ignitions has it had a non RTR firecore dizzy setup in there?

As stated in your other post, if the other fuel pump pushrod got ground off there's a good chance it ground off the lobe on the cam that it rubs against. That would be the first thing I'd look at. If that lobe is ground down it's not pumping much fuel.

Robert- The Firecore system is all this car has ever had in it. In order to use the RTR set up you need to modify the engine wiring harness by tying the blue and brown wires together and run a single wire to the coil. Then cut out the wires going to the ballast resistor and orange box. I contemplated swapping the engine wiring harnesses with my other Charger with the RTR system but that would be a PITA. I'm wondering now if the wire I ran from the splice with the blue and brown wires is heavy enough, not sure that would make a difference.I'll check the size of that versus the other harness and also check the splice tomorrow.

There was actually nothing wrong with the com cam fuel push rod I took out. But since it was out and I bought an NOS one I installed the NOS rod and will keep the other as a spare.

I'm wondering if you can "easily" swap back in an OE type ECU or even a points dist setup to confirm/elim the RTR. With some jumpers/temporary stuff so you dont have to do any (or very little) cutting/hacking.

What rocker arms are you using? Do you know how much lifter preload you have on the pushrod end between the retainer and the top of the retainer groove in the lifters?
I'm thinking like AndyF is going towards, maybe it is pumping up the lifters causing the problem

Did issue get resolved ?



1971 Superbee - My 383/432 Stroker is having the same issues since rebuild last year

Auto Trans Rebuilt - Turbo Action Auto Valve Body - Works Perfect

3:91 Sure Grip

Motor runs awesome , idle , cruise , off idle , full throttle perfect up until about 5,200 Rpms and then it starts falling flat , like running out of fuel - Get out of the throttle , and then back into the throttle and off it goes

Off idle this motor will destroy the tires - 20/30 mph and into the throttle car sideways

There are times say cruising in first gear , jam the throttle control the tires , Rpms come up until 5,500 Rpms shift into second and it starts sputtering going flat like mentioned after 5,000 Rpms in second

440 Source Stroker Kit

Edelbrock E Street Heads - 75cc

Comp Cams and lifters - XE 275HL

NGK BKR6E Plugs

16 initial - 34 Total Timing

10.5 - 1 Compression
All eight cold cranking compression , air cleaner off throttle plates closed 195psi dead nuts all eight , verified two gauges

1971 Carter AVS 4968 750cfm initially , converted over to Edelbrock jets and metering rods - Ran pretty damn good honestly up until mentioned

Now running Edelbrock AVS 2 800 - Plug color decent with factory jetting , went slightly leaner on cruise metering rods - Same issue at higher Rpms - Tried .107 Secondary Jets , same issue

This carb off idle , cruise is just plain awesome , so smooth , but again higher Rpms

Mechanical fuel pump - Pump Rod and cam is perfect
Factory 5/16” fuel line
1971 Factory vapor line to three nipple breather on valve cover

Tried three different coils

Now on second carb

Thanx Scott

Andy mentioned partial blockage in exhaust system in earlier post ?

My mufflers , Rhino Turbos many many many years ago , are pretty old but look great from the outside LOL

Sounds damn good

Rebuilt numbers 383 for the last 20 years , never had this issue at higher Rpms

Numbers 383 Was .030 over prior to this Stroker Kit , back then running the old Mopar 284/484 cam and built 906 heads

Flat tops where .025 in the hole until I kissed a piston one day after twenty years

Anyways

Hence this Stroker Kit with same numbers block

Running New Sealed Power stamped steel rockers and shafts with this Stroker Kit

Lifter preload I have written down somewhere

Comp Cams pushrods

Have not changed mechanical fuel pump or Chrome Mopar ECU with this higher Rpms issue
ECU has worked perfect prior for many years

If the motor goes flat at 5200 in each gear, it’s very unlikely a fuel issue.

This would mean you could roll on the throttle in first, experience the flat line of power at 5200, and then right after the shift into 2nd, and the corresponding drop in rpm....... the motor recovers and pulls good back to 5200.

My experience has been this situation has always been an ignition or valvetrain related issue.
I suppose it “possible” it’s a collapsed muffler condition, but I can’t imagine that having the on/off switch-like affect you get from ignition or valvetrain problems.

Fuel supply problems usually manifest themselves as; the motor just starts laying down, and no recovery occurs until the throttle is closed........ fuel bowls start refilling, then you step back down....... car accelerates for a bit, then lays back down as the bowls start to empty.

With the XE cam, you’re certainly a candidate for a valvetrain issue.

Another thing, if it’s somethiing other than fuel, you should be able to run it hard through the gears(wot), and not experience any issues if you keep the rpms below the point where the motor lays down.
So, if yours lays down at 5200, you should be able to go thru all he gears at WOT and shift at say 5000, and not experience any issues.

If it’s fuel supply, you wouldn’t be able to do that.
You’d probably get about 1/3 of the way through 2nd and it would lay down.

Thanx man for the quick response

Great explanation ! !



I will experiment with this subject matter / test , more over the upcoming weekend , shifting at a lower RPM

The quick and easy I guess would bee a ignition module change first

Second would bee pulling the valve covers and

Really appreciate your advice


“Another thing, if it’s somethiing other than fuel, you should be able to run it hard through the gears(wot), and not experience any issues if you keep the rpms below the point where the motor lays down.
So, if yours lays down at 5200, you should be able to go thru all he gears at WOT and shift at say 5000, and not experience any issues.

If it’s fuel supply, you wouldn’t be able to do that.
You’d probably get about 1/3 of the way through 2nd and it would lay down.”

With the Fuel, you want to measure the fuel delivery volume at pressure.
Easier to test with electric pumps, with a mechanical you may need a temporary fuel supply to be able to rev the engine with the mechanical pump disconnected.
You need to add some restriction at the fuel output to create pressure. If the pump is internally regulated, need to be careful to just bring pressure up to the regulated set point.
Setup fuel outlet into a empty gas can, and time how long it takes to fill to figure the fuel volume delivered.

This is just a static test with the car under zero G-Force.
You can also lift the cars front end to simulate fuel delivery under acceleration.

No. Have only dubbed around with it when I have time. A couple of things I recently tried.
Was wondering if I had some type of obstruction between tank and fuel pump. I took a two gallon container and strapped it to the 1" tube framing for the nosecone and ran a hose to the fuel pump, completely bypassing the tank and fuel line. Same issue.
I put a fuel pressure gauge on the car and taped the gauge to the windshield (that was interesting). The gauge fluttered a lot, not sure if it's supposed to do that. While under acceleration it fluttered between 5.5 to 7, smoothly out a bit at 50 MPH fluttering at 6-7. Once I stabbed the pedal it fluttered wildly between 2-7 with the engine just laying flat. (I have already installed two different fuel pumps).
My mechanic buddy wondered if I had an obstruction in the rubber fuel line between the pump and carb. Changed both hoses, same issue.

Had it out yesterday for a little bit

Took your advice and ran it through the auto gears shifting manually at say 4800 Rpms to 5000 Rpms

Definitely not fuel pressure related as it never missed a beat through all three gears

Then get into that 5200/5400 Rpm range on some more shifts , and it falls on its face

Didn’t have a spare Mopar Ignition Control Module on hand or in the garage to change quick - My bad

Been running the Mopar Chrome Module for a decade or longer , but recently had a few issues with MSD Blaster 2 Coils (Two to bee exact) going bad at much lower RPMs , intermittent missing and going flat under light acceleration since this new motor

.8 MSD Resistor - Now running a Mopar Canister Oil Filled Ignition Coil that I had brand new in the box from the 90 s - Made in da USA Lol

It works - Zero issues at any lower RPM s

Anyways that’s it for now until I swap in a new ignition box I guess , I would of thought they either work or they don’t , another words fail completely

I see the Rev nnator is sold out back ordered

Mopar doesn’t make boxes anymore

4 second s flat sells a box

There is a Jegs 7500 unit

I should probably just throw some China jobber junk on there for now to see what happens at higher RPM s

Have you checked the voltage at the coil while it’s running?
Checked voltage on the hot side of the resistor, and compared that vs the voltage at the battery?

Many years ago on my heap, I put in a higher rpm motor than I had run previously, which allowed some ign issuers to show up.

In my case I had lots of loss through the old factory wiring.
I peeled off some of the insulation in several spots and it was all green inside.

That’s when I went to a MSD 6A box...... since the power feed wires go direct to the battery, bypassing all the factory wiring........ and getting rid of the ballast.
I used the big yellow Accel Super Coil with it, which was already used when I got the car in 1981.

That being said, in my mind, it’s just as likely a valvetrain issue as ignition.

The pitfalls of fast rate hyd cams:
https://www.hotrod.com/articles/mopp-0510-440-engine-dyno-test/

Thanx for the link - Great read on the lifters

Hopefully I won’t have to go down that road , no pun intended

Anyways

Won’t get back to the car until the end of the week , hopefully will have more answers then questions

Thanx again

E Street 75 cc Heads - Angled Plugs

Came across this article and started with the spark plugs - They talked about toasted boots on #6 plus #8 later in the article

http://www.moparts.org/Tech/Archive/bb/Lobe.html

Never noticed anything on the plugs themselves or the spark plug boots for that matter , but once I looked inside the boots on #6 and #8
They where black - Went to pull the plug boots off and the metal terminals came with them

Serious heat issue and misfire with these two plug wires

Two new plug wires and those 1200 degree boot protectors , and I was off thinking for sure this was the issue - Fixed on the first day

Man what a difference overall , Smooth , Rpms come up fast , really fast , first gear all the way up to 5500 5600 Rpms shift into second

Engine feels Awesome , like I gained fifty horse overall

Second now coming up to 5200 5400 Rpms and it starts sputtering , PINGING like crazy



Cruise mode on the highway 3200 Rpms at 70 mph in Drive - Get in the gas to 5500 Rpms no problem

All day long in first to 5500 5600 Rpms then

Verified fuel pressure with gauge taped to my windshield - 4 to 5 psi at full throttle - Delphi mechanical stock 383 , verified pump rod at 3.22 “ factory original from 1971

Friends gave me a couple different ECUs and a new coil - Pertronix 40011 Coil - Jumped the ballast resistor

Idle 700 Rpms 13.5 volts to both sides of ballast , 13.5 volts positive side of coil - All wiring is fairly new and looks great
Volts come up to 14.5 with Rpms

Different ECUs no change



Obviously there is a huge difference between fuel pressure and fuel volume correct ?

I also pulled the valve covers - I wanted to check my rocker arm geometry and wear pattern with these Sealed Power rocker arms and shafts

Man I couldn’t bee happier - Rocker arms , comp cams push rods , spring retainers no rubbing , valve stems looked damn near perfect brand new after two summers

That’s where I am at - A much better running engine overall , all day long out of the hole in first to 5500 5600 , then falling off in second around 5200 5400 on a full run and PINGING like crazy

Thanx Fast 68 Plymouth for all your help

Pinging tells me it is running out of fuel at that time, check the needle and seats size
Try the next bigger to see if they are the restrictors or not
Some of these gremlins will run you through the hoops finding and correcting them You can lick them

These Edelbrock s come with .093 Needles and Seats

They make or sell a .110 Needle and Seat

Float level is at 7/16” verified on both - One float was out of whack from the factory new when I opened it up

Article

https://www.hotrod.com/articles/mopp-0312-solid-vs-hydraulic-lifters/

Fuel pressure is an indicated 4-5 psi........ while it’s experiencing a problem at high rpm?

If so, I’d probably rule out a fuel delivery issue.

The massive PINGING when it runs out of gas and falls flat at RPM


Thanx Scott

How does you fuel pressure change between normal cruise or driving verses WOT.

I would guess that this pressure drop causes the the vaporization temperature to drop low enough on a hot day to cause the vapor lock. Throttle closes, pressure goes back up.

I won’t know again until probably next summer , but it doesn’t fall off more then a few psi in this cooler weather at WOT

If it only bothers in hot weather, it’s likely just a vapor lock issue.

5400 RPMs is about all

Yes and no , maybe LOL


Motor just runs flat out Awesome with everything I have improved on , especially finding those two bad spark plug wires


However 5,000 RPMs comes on really really fast especially on the highway running 3:91 gears