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88v8
Ivor Duarte
Gloucestershire UK
(471 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
TR6 74 2.5 injection six

Fuel pressure now and then.
Posted by: 88v8
Date: December 20, 2016 10:43AM

A Holley carburettor reconditioner ... we had a brief exchange about fuel pressures.

My shop manual of 1963 specifies a min pressure of 4 psi. I saw video of him running an engine at 3 psi.

He said ' It has to do with the leaded fuel weight and properties vs Ethanol or alcohol based fuel of today'.

First I heard of this. Have been unable to find other references to lowering the window of acceptable fuel pressure.

I am setting up a Filter King to ensure I have the correct pressure.

Could ask Holley, but our here in the real world what do we think?

Ivor


MGBV8
Carl Floyd
Kingsport, TN
(3139 posts)

Registered:
10/23/2007 11:32PM

Main British Car:
79 MGB, Buick 215

authors avatar
Re: Fuel pressure now and then.
Posted by: MGBV8
Date: December 20, 2016 12:32PM

Are you running 100% methanol or E85? If running E10 pump gas, IMO it is not an issue. I run 7psi in my Holley double pumper (Camaro).

This article mentions a vacuum-referenced pressure regulator for use in racing with alcohol.
Quote:
If you are going to use E85 or alcohol as a fuel, you need a fuelcompatible pressure regulator. Holley’s selection is a good place to start, but if you want to expand your range, check out Summit’s large selection. When alcohol is used, you need a large needle valve and adequate fuel pressure to flow enough fuel for maximum power. Fuel pressures need to be adequate at WOT and maximum RPM, but these higher pressures are often too much for the float to accurately control the fuel flow at idle. If the pressures are set for idle, the engine starves of fuel at the top end. This situation is by no means universal, so it depends on the design of needle and seat used in the fuel bowl as well as its effective diameter. I have a friend who has just wrapped up a championship with his dragster and has not run a single pound over 3 psi all season. Therefore, if the needle and seat pressures are good enough, high pressures are not needed. However, it may take a while to sort through your fuel system to get to this happy state.
The best way to run strong while still in the process of making adjustments is to have a vacuum-referenced pressure regulator, such as one made by Mallory. A few other companies, such as Aeromotive, also make vacuum/boost-referenced, fuel-pressure regulators worthy of consideration. All these regulators compensate linearly, and that means for every 1 psi the intake pressure changes, the regulated fuel pressure changes the same amount. Here is how this works for an alcohol-fueled engine. If, at idle, there are 6 inches of vacuum (3 psi), the pressure regulator reduces the pressure delivered by 3 psi. So if the base pressure is set at, say, 6 psi, then, at idle, it is 3 psi.

[www.musclecardiy.com]



88v8
Ivor Duarte
Gloucestershire UK
(471 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
TR6 74 2.5 injection six

Re: Fuel pressure now and then.
Posted by: 88v8
Date: December 20, 2016 01:00PM

Just running pump gas. The ethanol content is not disclosed to us poor customers, but between zero in some Super brands, and 5%.in cooking gas.
We do have E10 in some filling stations but I don't use it.

Ivor


Moderator
Curtis Jacobson
Portland Oregon
(4298 posts)

Registered:
10/12/2007 02:16AM

Main British Car:
71 MGBGT, Buick 215

authors avatar
Re: Fuel pressure now and then.
Posted by: Moderator
Date: December 20, 2016 11:00PM

5 percent? 10 percent? Either way, there isn't enough alcohol in your fuel to worry about.


joe_padavano
Joseph Padavano
Northern Virginia
(110 posts)

Registered:
02/15/2010 03:49PM

Main British Car:
1962 F-85 Deluxe wagon 215 Olds

Re: Fuel pressure now and then.
Posted by: joe_padavano
Date: December 21, 2016 10:45AM

The only issue would be the specific gravity of the fuel as it related to buoyancy of the float in the carb. Of course, I'd reset the float level instead of changing the fuel pressure, but the result would be the same. You need to be sure the needle closes with the correct level in the bowl. Of course, with a Holley, you can just adjust the float level with the engine running, so why even mess with the fuel pressure?


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5239 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Fuel pressure now and then.
Posted by: BlownMGB-V8
Date: December 21, 2016 02:30PM

Rate of delivery can be a factor. At 6 psi fuel can flow into the carb twice as fast as it can at 3 psi.

Jim


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