rob75383

I've been chewing over the reasons why my car had a big drop in hp at around 4,300rpm on the chassis dyno.
The dyno tech told me "something" was wrong, but he couldn't elaborate. He wouldn't let me tune the ride either, just do three quick pulls, and get the next car on the rack.

4 yrs ago, I went to NAPA to get a replacement fuel pump, 'cause mine was leaking. The pump they had put the lines in the wrong spot, and the body was not the big and long pump, but the short, squat, pump. I checked other places, and they didn't have the same "big" fuel pump, either.
So I ordered one from Eklar's or MidAmerica.

Now, my question is, how much do you fellas ESTIMATE, in terms of hp, the "stock" pump can maintain?? Probably this pump was used in 165hp L-48's all the way to BB 400+hp ones.

I don't really think the fuel pump caused the dip in power, but I made a list of things that are suspect, and I am trying to eliminate the possibilities before my next dyno session (probably at Bloomington Gold again).

I was considering the holley replacement pump, which flows 7psi at __gph, and has that rotating base.
I'd rather not have to contend with running a regulator, especially with return lines, and until I re-do the wiring, an 'lectric fuel pump is out of the question.

There is currently nothing wrong with my current pump. It puts out about 5-6 psi. (I know, Holley recommends 7psi for their carbs) And doesn't leak.
I just figured for about $60, I might eliminate a high rpm power loss.

vettfixr

I wish I could help you on this because it sounds like an interesting problem. I don't know the specs for the stock pump but unless the engine really laid over because of lack of fuel I would look to other things. From your sig it looked like you've got a nice setup. It may just be a jetting or adjustment issue with the holley. Maybe if you posted something asking for carb help you might get a little more response. If you go to a non-stock pump you may have problems plumbing the return lines and getting the right pressure. Maybe someone else can be more help. Good luck in solving your problem.

rob75383

vettfixr- I took your advice, and asked the "carb gurus" over in engine mods about the power dip. http://forums.corvetteforum.com/zerothread?id=496257

SanDiegoPaul

See my earlier threads about the fuel pump in my new motor. When I put the motor in last month, I purposely used a new pump b/c I had no idea how old the old one really was :blueangel:

When I put her on my dyno at work, it croaked above 4,000 and only showed just under 275 HP at the wheels. Air fuel was really lean above 3500 so there was no doubt it was outta gas.

I put another pump on it (an OEM G.M. pump) and now it runs like a bat outta you know where! :yesnod:

When I get good dyno numbers I'll let y'all know what the fuel pump gained me.

MARKUS_P

I run exactly that pump.
Holley polished with rotating base.
I drilled the lower housing and tapped it so I could use a little regulator valve between the return line and the pump.
Normally the valve is closed, because the diaphragm in the pump allows a certain pressure until it releases so that the carb doesn´t get too much pressure. But because I´m running an Edelbrock carb and they state to use less pressure than the holley (750cfm) I machined the pump for my own "pressure relieve valve"
Because my car isn´t streetable at the time I can only tell you that the pump didn´t overfeed the carb. No leaks, everything worked out well.
Markus

UKPaul

Hi,
If you need 7psi but your pump is giving 5-6 then that could be the problem. From bitter experience I know that a low fuel pressure can cause a low fuel level in the float bowl at high rpms. If your pump is giving 5psi then you're down 2psi on the required, which may be enough pressure. I guess what happens is that not enough fuel is pumped into the float bowl while the valve/needle is open & that fuel frothing is causing the float to cut the supply while the level is slightly low? I'm not at all sure as to the physical reason for it, but it once took us a long time to realise that a hole in a piston was due to fuel starvation at high rpms. Increasing the fuel pressure solved the problems with that engine & I've always made sure to have the required fuel pressure since then (fuel starvation causes loss of power, but can also cause extreme loss of money!).
:cheers:

MotorHead

I was going to get a Holley pump but was told by a mechanic friend that the Carter was actaully better as he said he had some problems with sealing with the Holley.

The Carter is not shiny but who cares you can't really see it down there anyway and it has the rotating base and the Cartert flows 120GPH at 6-8psi. I can get you the part number if you are interested. :D

Fevre

I would look at your ign/timing first, many people loss high end power when there ign/dist crap out above 4000 rpm's. Do/did you have a pressure gauge hooked up to see if you loose pressure above 4000 rpm's on the dyno? Fuel pumps are a PITA to replace when the eng is in so I would do like you said and check everything else that is easier to deal with first.

IETEK

I could write two pages on this in short I learned the hard way I race a circle track car and have to run a 4412 Holley carb. (2 Barrel). Even the 2 barrel ran out of gas with the stock pump at the end or the straight away.
I went to a holley six valve pump and that solved the problem.
Another area to look at is your ignition/distributor a stock HEI starts playing out about 4000/4500 rpm because of lack of coil saturation time do yourself a favor and invest in a good ignition.
If you run anything less than 20w/50 oil in a motor without a cooler you can expect to see a HP loss above about 3500 rpm while this will not damage your motor it will decrease the HP gains you see on the Dyno.

IETEK :D

randy72

I have a stock 72 LT1 pump (from Paragon I think) and with my 383, I got 290 HP and 340 Tq. at the wheels..780 Holley was actually rich according to the 02 sensor. HP peak was 4800 and dropped off at 5200. Car will pull 6,000 under load, but I shift it at 5200 based on the HP curve.

rob75383

--MotorHead--
Yes I'm interested in the carter part #.
Please post it when possible.

P.S. the deciding factor for me was the fact that it comes dull, not chromed... makes it easier to paint my own color!

Thanks fellas for all your posts! :seeya

MotorHead

Carter Street Super Fuel Pump# is M6900 , $66.99 at Jegs.

rob75383

Okay, I just got the pump from UPS yesterday afternoon.
About an hour ago is when I finally opened it to check it out, and read the instructions.
<I KNEW I SHOULD'VE ASKED> This pump has threaded inlet and outlet ports, but no nipples included (so I guess I hafta goto the hardware store now), and I count 1 inlet and 1 outlet...
Where does my return line go???
Do I have to make a tee somewhere??
I have the M6900 Motorhead uses.
How would I go about using the return line?

Jughead

I tried the LT1($19) pump and it drove my fuel pressure gauge crazy. :crazy: I fluttered between 6-9psi. I never got a steady reading and was told that's typical of the LT1 pump. :confused: I switched to the chrome Holley($49) and get a steady 6psi.

I never had a return line so I can't help you there. I have seen the return port plugged on street rods and read that you simply block off the return line. I can't remember if the chrome Holley had provisions for the return line---I don't think it did. :confused:

Jughead

I tried the LT1($19) pump and it drove my fuel pressure gauge crazy. :crazy: It fluttered between 6-9psi. I never got a steady reading and was told that's typical of the LT1 pump. :confused: I switched to the chrome Holley($49) and get a steady 6psi.

I never had a return line so I can't help you there. I have seen the return port plugged on street rods and read that you simply block off the return line. I can't remember if the chrome Holley had provisions for the return line---I don't think it did. :confused:

rob75383

Yeah, I spoke to a buddy, who says that most likely, I'm s'posed to block off the return line. But he wasn't sure.

I'm starting to think that that is the solution... I've got time to wait, its still too cold and snowy.

MotorHead

I don't think any aftermarket pumps have a return line, just block it off. Same with the Holley Edelbrock etc. I don't think any come with fittings either. :D


[Modified by MotorHead, 11:49 PM 3/8/2003]