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measuring fuel pressure #2658896
05/24/19 09:27 PM
05/24/19 09:27 PM
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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.


My Dad always told me, when setting goals " avoid disappointment, aim low".
Re: measuring fuel pressure [Re: NHCharger] #2658901
05/24/19 09:42 PM
05/24/19 09:42 PM
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Originally Posted by NHCharger
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? work
Not in neutral tsk grin
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 whiney
Fuel delivery has caused a lot of grief for a lot of drag racers, a LOT of them from stock to Top Fuel shock
To answer your question you should test it in high gear above 3500 RPM at WOT until the RPM you normally lift at up scope
Let us know your results please, that can and will help others on here also thumbs


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: measuring fuel pressure [Re: Cab_Burge] #2658915
05/24/19 10:22 PM
05/24/19 10:22 PM
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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.

Re: measuring fuel pressure [Re: topside] #2658917
05/24/19 10:37 PM
05/24/19 10:37 PM
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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.


live every 24 hour block of time like it's your last day on earth
Re: measuring fuel pressure [Re: RapidRobert] #2658920
05/24/19 10:56 PM
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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.

Last edited by NHCharger; 05/24/19 10:59 PM.

My Dad always told me, when setting goals " avoid disappointment, aim low".
Re: measuring fuel pressure [Re: NHCharger] #2658926
05/24/19 11:13 PM
05/24/19 11:13 PM
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Could be a bunch of things. Valve springs are shot, timing chain loose, partial blockage in your exhaust system, etc..........

Re: measuring fuel pressure [Re: AndyF] #2658931
05/24/19 11:21 PM
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Originally Posted by AndyF
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.


My Dad always told me, when setting goals " avoid disappointment, aim low".
Re: measuring fuel pressure [Re: NHCharger] #2658941
05/25/19 12:05 AM
05/25/19 12:05 AM
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Quote
When I press the pedal to 3/4 throttle she starts sputtering, anything past that she just lays over.
Does this in all gears or just high gear?


live every 24 hour block of time like it's your last day on earth
Re: measuring fuel pressure [Re: NHCharger] #2658942
05/25/19 12:40 AM
05/25/19 12:40 AM
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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 scope work
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 puke shruggy
Lots of things can influence are results work


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: measuring fuel pressure [Re: Cab_Burge] #2658978
05/25/19 07:35 AM
05/25/19 07:35 AM
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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...

Re: measuring fuel pressure [Re: dragon slayer] #2659053
05/25/19 12:06 PM
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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.


My Dad always told me, when setting goals " avoid disappointment, aim low".
Re: measuring fuel pressure [Re: NHCharger] #2659063
05/25/19 12:40 PM
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Quote
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?


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Re: measuring fuel pressure [Re: RapidRobert] #2659065
05/25/19 12:54 PM
05/25/19 12:54 PM
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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.


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Re: measuring fuel pressure [Re: GODSCOUNTRY340] #2659176
05/25/19 09:03 PM
05/25/19 09:03 PM
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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.


My Dad always told me, when setting goals " avoid disappointment, aim low".
Re: measuring fuel pressure [Re: NHCharger] #2659187
05/25/19 09:30 PM
05/25/19 09:30 PM
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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.


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Re: measuring fuel pressure [Re: NHCharger] #2659204
05/25/19 10:45 PM
05/25/19 10:45 PM
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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 work scope


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)

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