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Fuel pressure modification.Is this for real?

rdgfx3

Well-known member
Joined
Jul 25, 2001
Messages
202
Location
Rowlett,TX.
Corvette
2008 Silver Coupe
I was "slumming" and found this.Is this safe and/or workable?


This modification was performed on an 87 Vette with approx. 100K on the odometer, a gutted MAF, a K & N air filter, Jacobs 8.5mm plugs wires and Delco sparks that are 2 heat ranges hotter than stock.

I tried this modification and my fuel pressure went from 30# to 40# with only this fifteen second mod. The modification is that I removed the vacuum line from the factory fuel pressure regulator. Also, I capped the fuel pressure regulator connection, so no dirt would get in, and plugged the vacuum line, to maintain vacuum on the engine. I tested the car and noticed a serious change in acceleration. I then took it on the highway, set the cruise for 70 with this mod I was getting an instantaneous reading of fuel mileage of 28-29 miles per gallon. I pulled off of the highway and switched back to having the vacuum line connected. I immediately noticed the power had reduced. I again took the car over the same stretch of highway and the mileage went down to 26-27. I thought the second mileage test would be higher than the first. I checked with my mechanic on this and he said the car is just running more efficiently with the mod than without. One other thing that got better for these test runs with the mod was the temperature of the engine went down. Normally, when the car is warmed up and going down the highway, it'll usually run about 208-211 degrees. With the mod it went to 184-187. My mechanic said that that may be caused due to the cooling effect of more fuel entering the cylinder. I don't know if the temperature differential is going to be a constant, because as you can tell, the tests I did were not scientific in any way. But, I feel there is an improvement, if only in my car.

I was told about this mod. via someone who works on, restores and sells parts for nothing but Corvettes. So I thought I'd give it a try.

Just make sure that the pressure you attain, if you do this mod, is not too high for your system. Of course you can always put a small needle valve in the vacuum line to reduce the press., if necessary.

If it does work,how would I know the pressure I attain would not be too high?
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Interesting.........

I have to look at this a bit......

Vig!
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
So fuel pressure facts

With the system up to pressure....

Static ( not running )
40.5 to 47psi ( stock setting )

Car running @ idle
This should drop by 3-10 psi


If you now pull vac (10") on the pressure reg
You should DROP another 3-10psi

So the more VAC you pull the less gas ( fuel pressure) you have available.

and your largest VAC is @ idle ( or decellerating down a hill)

I assume that they (GM) would decrease the fuel pressure @ idle since not as much fuel would be needed.

Still thinking......

Vig!
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
The "modification" is not "for real" in fact, it's not even a modification. It's a malfunction.

GM and other manufacturers design the fuel pressure controls in an EFI system to be partially adjustable according to engine load.

The higher the load on the engine, the more fuel it needs. Since the bandwidth of the electronic controls is not wide enough to cover all the fuel needs from very light throttle to wide-open-throttle (WOT), that bandwidth is extended by using engine vacuum, which is a indicator of load, ie: the lighter the load, the more vacuum is present and the heavier the load, the less vacuum is present.

The system is designed such that, at high vacuum, fuel pressure is reduced (and fuel flow decreases) and, at low vacuum, fuel pressure is raised (and fuel flow increases). This is acomplished by engine vacuum being applied directly to the fuel pressure regulator diaphragm.

When you pull the vacuum hose off the fuel regulator, you essentually force the regulator to operate all the time as if the engine was at WOT with little or no vacuum present.

All that does is make the engine run rich at part throttle which increases wear and reduces part throttle fuel economy. It does not make a practical increase in fuel flow at WOT.

The mechanics who sold you that idea as a good modification are "mechanics" only in name and, clearly, know little about how electronic fuel injection works.

What you, first, need to do is determine if your engine needs more fuel flow. If it does, then you can increase it by either altering the calibration, changing injectors or changing fuel pressure. If you need to change pressure, do it with an adjustable regulator or a different pump. Do not disable the application of engine vacuum to the regulator.

Keep in mind that many stock and near-stock L98s were calibrated rich from the factory and actually need to run leaner to generate best performance.

Lastly, if you are running spark plugs two heat ranges *hotter* than stock, take them out. You could damage your engine. Plugs two ranges hotter than stock may cause detonation or pre-ignition at WOT and high RPM. For street high-performance use, you're best to use a plug that is one heat range *colder* than stock.
 

rdgfx3

Well-known member
Joined
Jul 25, 2001
Messages
202
Location
Rowlett,TX.
Corvette
2008 Silver Coupe
Thank you Hib.
I did not actually modify my car in this way.I cut and pasted the post from another place. (read:lazy):)
I wanted a professional,knowledgeable opinion before I made any changes.:bang

Whaddya expect when you go to that "other"forum.:SLAP

Thanks again!:w
 

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