DIY_EFI Digest V2 #330

[J.W. Harris]

From: Paul Tholey <pft101 at psu.edu>
Date: Sun, 21 Sep 1997 22:41:42 GMT
Subject: Re: I understand Q-jets

>If the mechanical fuel pump can't keep one bowl filled, how can it
>keep 2 bowls filled (assuming same rate of consumption & still only
>one fuel pump)?

Correct, It can't over the long run!

>        The problem is with your acceptance of one mechanical fuel pump
>being enough.  The pump runs off the cam.   Therefore the pump is working to
>feed 8 cylinders at only 25% of the time, The other 75% of the time it is
>idle.  

The rating of the pump takes this into account.


>The Holley allows you to run more pressure and at the same time more
>volume while not blowing past the needle valve, as would the problem be with
>the Q-jet.


Get a high volume pump and put a pressure regulator on it.  I had to put one
on to keep pressure from blowing past the crummy O-ring around the needle &
seat.  Holleys are definitely not a 10th as reliable as a Q-jet.



 
>>         To be more correct the reason the Holley carb is used over a Q-jet
>> is because the Q-jet has a single fuel bowl in the center to feed both the
>> primaries and secondaries.  Mechanical fuel pumps cannot keep this bowl
>> filled, and electric fuel pumps that can keep an average carb filled run at
>> a pressure that will blow past the Q-jet needle and seat, wich are only good
>> to 7 lbs. 
>>         The Holley has two fuel bowls, one on each end, which can be
>> extended to hold even more fuel.  

Unless you put the whole 20 gallon tank in there, sooner or later your'e
going to run out if the fuel pump can't keep up.  Any engine worth worrying
about this on would suck the bowls dry in a hurry!

Their needle and seats also hold 9 lbs.
>> and can be replaced with ones that hold up to 12 lbs.
>>         I am not trying to start an argument, but there are people who
>> subscribe to this list that run Holleys, and it is not because we can't
>> figure out a  Q-jet.


Q-jets are excellent carbs that will provice superior transient performance
if detailed correctly.  Unforutnately selections of tuning parts and hi-po
parts haven't been avialable (until Edelborck).  THe truth is that most
people don't use them because the Holleys have fewer variables are easier to
set up (although less accurate at metering fuel across the whole dynamic
range), are an industry standard,  and units that aren't worn out are easier
to get (until Edelborck).  It really doesn't much of a make a difference as
most of us are most concerned with WOT power anyway.  

JW 

jhlh at netusa1.net
>