What should fuel pressure be?

blykins · #1 (**permalink**) 03-20-2005, 09:47 AM

I stuck a fuel pressure gauge on my fuel system yesterday....showing 8 psi with a Holley Red electric fuel pump and a Mighty Demon 750. Is that a little high? Should I neck it down a little?

Also, another question......In what increments should you change jets to see a change? When I had my engine on the dyno, we kept jetting it up because a couple cylinders were a little hot....They thought it was running a little lean. We ended up with 88's and 98's on it......Now it turns out it's running a little rich. It was definitely idling rich, so I opened up the idle air bleeds and helped it considerably. It's still a rich running above idle though. How far should I take the jets down before I read the plugs the first time?

trularin · #2 (**permalink**) 03-20-2005, 09:55 AM

Actually that is not bad and it will drop when you use fuel. It should read between 3 and 7 pounds.

Initial Jet moves to me are a two size move and then tweek from there.

blykins · #3 (**permalink**) 03-20-2005, 10:19 AM

I thought maybe it might be a little high...and could have been part of the cause of the rich idle....

You think if I went down to an 84/94, it would be a considerable enough decrease to see if it's really made a difference or not?

Bill Wells · #4 (**permalink**) 03-20-2005, 11:09 AM

hmmm. the Holley BLUE comes with an external / separate fuel pressure regulator. however, the the Holley RED electric fuel pump is supposed to be self regulated by Holley to 7 psi with no separate fuel pressure regulator. and yet you got a 8 psi reading ? maybe it is a variance in the gauge ? you should be alright at 5-7 psi for those cubes and carb at high rpm's under heavy acceleration. bill.

Zoom This · #5 (**permalink**) 03-20-2005, 11:10 AM

Put the motor on a chassis dyno and an Air/Fuel Ratio sniffer. Your WOT A/F ratio should be around 12.5:1 - 13:1. At cruise rpm, 2500 rpm, under light throttle the A/F should be in the neighborhood of 13.5:1 - 14:1. My 418 stroker went through all the jetting and dyno results last year and this seemed to be a good point for a balance between performance, economy and emissions. I have an 850 Holley massaged by Jet Performance on a 418 stroker with 75 jets on the secondaries and 73's on the primaries with #31 shooters.

Bruce Edwards · #6 (**permalink**) 03-20-2005, 12:04 PM

5 to 7 is normally max for a carb. Much more and it will push the needle off the seats and cause flooding.
As for the jets what size are you running now? A good start would be around 72 on the front side.

blykins · #7 (**permalink**) 03-20-2005, 12:07 PM

Should I stick a regulator on it then?

As for the jets, I'm running 88's in the primaries and 98's in the secondaries.

If I remember correctly, it came with 69's and 72's from the factory. Wasn't enough.

Bruce Edwards · #8 (**permalink**) 03-20-2005, 12:44 PM

Holy Cow!! 88 on the front??? That was a huge step up. Split it. If you are running 88 now and it is rich and it came with 69 try around 78 or 79. The front side is the one that controls fuel mix at all speeds before the secondary kick in. Get the front side sorted out before you mess with the rear.
I would install a reg to smooth out the fuel flow and control the pressure.

blykins · #9 (**permalink**) 03-20-2005, 01:13 PM

Bruce, we didn't go straight from 69 to 88....

We stepped it up gradually. Apparently the dyno guys were watching the cylinder temps and were trying to even out the temperatures when on the dyno. I think they went a little too far.

I have a Vic Jr. intake.....and my engine builder buddies at a certain oil manufacturer engine lab told me that they were notorious for having hotter cylinders than others due to the runner configurations.

I think the dyno guys kinda picked out a cylinder or two that were hotter than the others and were trying to richen it up a little bit.

The thing runs like a top....takes gas extremely quick and well....starts easily.....and didn't hesitate when the secondaries opened on the dyno.

The plugs look good....they're a touch on the sooty side....but I've been doing a lot of fuel plumbing lately and have started it often and let it idle.

What leads me to believe that it's a little rich is of course the little carbon on the plugs.....and when it's up to operating temp, you can bump the throttle up to about 3 grand or so....and when you let off, it makes a nice pop out the sidepipes...along with a nice little light to go with it.

Bruce Edwards · #10 (**permalink**) 03-20-2005, 01:32 PM

If it is popping out the pipes it is to rich. If you had it on a dyno they are only set up, in most cases, for temp at the time of the run. Not real settings for everyday driving. Start with small changes of 2 steps at a time. I bought a full set of jets some time ago with every size from 65-120 if I remember the spread correctly.
Remember the primary does 90% of all the fuel mixing you use short of running full out on the track that is.
The unleaded fuel does not burn the same way as the old leaded stuff did and a light dusting of black on the outside edge of the plug is pretty normal now a days.
With that said it should not POP out the side pipes. That is a sign of raw fuel in the pipes lighting up by the stray hot carbon thrown out during combustion. If it only does it when you let your foot off hard, as when you goose it, you may be seeing a powervalve set a bit low and dumping raw gas in as the intake pressure changes.

relaxinrob · #11 (**permalink**) 03-20-2005, 01:40 PM

I have mine at 78 and 80 and it's good for 572 HP.

blykins · #12 (**permalink**) 03-20-2005, 01:50 PM

I have the same box full of jets that you were speaking of....

Yes, it only does it when I goose the throttle....and doesn't do it everytime even at that....

Can you change the power valve on a Demon?

Bruce Edwards · #13 (**permalink**) 03-20-2005, 01:55 PM

Yes. It is after all a new version of the Holley and is on a whole set up the same way.

blykins · #14 (**permalink**) 03-20-2005, 02:10 PM

I'll see if I can lean it up with the jets first.....Will try going down to like a 78/80 first and then see what happens.

The car is back home.....and there aren't many shops around. The place I had the engine dyno'd also has a chassis dyno....I'm making sure they have the stuff I need to tune it up there with an A/F meter.

As for changing the power valve, that will be a last resort for me. I'm only 27 and grew up in the fuel injected days.

I can change jets, change air bleeds....but that's about it. Won't learn unless I try though, right?

Bruce Edwards · #15 (**permalink**) 03-20-2005, 02:29 PM

The power valve just screws out it is an easy swap. Just need to get a set of different ones to "play" with.
A good start would be to check what the factory installed to get an idea of what way it was tuned.
Then check your idle vacuum and divide it by half. So if you have 14" at idle you would install a 7. Check that to what is in the carb now and go from there. Easy.

blykins · #16 (**permalink**) 03-20-2005, 03:17 PM

I had a vac gauge on it at some point....can't remember what it was to be honest though....I'm thinking 15-16 inches....Does that sound right?

The cam is a hydraulic roller....230 duration.... .544 lift on a 110 separation.

Anyone know what size PV a 750 Mighty Demon has in it?

blykins · #17 (**permalink**) 03-20-2005, 03:29 PM

OK....I found it on Barry Grant's website. Looks like it comes with a 6.5.

And it looks like I was smoking crack about the factory jet sizes. I know for a fact what it has in it now and what progression we used, but I was off on what it came with. The website says it comes with a 75/83 setup. I may go back to a jet setup maybe 2 sizes up from that and see if that helps.

The website says an 850 Mighty Demon comes with 85's and 93's. Everyone's told me that I should have went with an 850 over a 750 for 425 ci. Maybe I'm not that far off with the jets sizes that are in there......But then again, an 850 has bigger venturis and bigger buttefly diameters too...

So I really don't know what order to progress in now.....I'm gonna try to get a little dyno time......but what should I do first? Regulate the fuel pressure? Switch jets back down to factory spec?

The fuel should be just visible in the fuel bowl sights, right?

Bruce Edwards · #18 (**permalink**) 03-20-2005, 03:43 PM

When you pull the site plugs the fuel should be right at the edge of the hole. If you bounce the car some should leak out.
6.5 on the valve would be Ok for a setting of 13 to 14 on the engine at idle. I would check that first.
If you are in that area then go to smaller jets. The factory settings sound a lot closer to what I would see for a 351 to 400 ci engine and would have been a good starting point.
Remember that the total jet size will only come into play at full throttle. The idea is to get the best balance between full throttle and cruise mix.
This is why all the new cars are fuel injected. Much easier for the computer to control the mix over different loads and temps that a carb. With a carb you have to do your best to get a happy middle ground.
Have fun and do not let it scare you. Do one thing at a time and check it after every change. The big thing is to give the engine time to adjust to the new settings. So drive it for effect don't go by the way it sounds.
If you have a plug cleaner I would clean them before every change to see how you are doing or buy stock in the plug maker you like best and buy a case!
Tuning can be a real pain but if it is done right it can be a real kick when you find that sweet spot.

blykins · #19 (**permalink**) 03-20-2005, 03:47 PM

You guys think a fuel pressure regulator would be a good investment at this point? If it is 8 psi, it kinda worries me.....like some of you have said, the catalogs say the Holley Red is preset to "approximately" 7 psi. Should I set it a little lower? Maybe 6.5 - 7 psi?

blykins · #20 (**permalink**) 03-20-2005, 03:49 PM

Oh, just for curiosity......what's a good temp range for a cylinder at WOT? 1300 F? 1500 F?

And while I'm asking questions....I got another one.....Since I adjusted the idle air bleeds to lean up the idle...if I lean it up by lowering the jet sizes, that will affect the idle air bleeds too, right? I'll have to redo those now, right?