Off Roading Forums banner
1 - 8 of 8 Posts
G

·
Discussion Starter · #1 ·
I just found out that the irrigation motor that I put in my Jeep is a Chevy 285/300 hp crate motor, with four bolt mains. I put a Holley 600 cfm carb on it, is this too small. It sure don't feel like it has that much, but then, I'm running 3.07 gears with 33 inch tires. It will do about 87 mph in the quarter mile.

Just a farmer
 

·
Registered
Joined
·
6,870 Posts
I'm sure TEX and some of the others will argue the point, but...

With the 3.07:1 gears, and the 33" tall tires, the 1/4 mile probably isn't a fare test. With that gearing/tire combination, you are not set up for drag racing...

I am assuming a stock 350 CID engine, straight from the crate...
According to HOLLEY, (and to simple math) the carb you have will be just fine to about 6,250 to 6,500 RPM.
--It wouldn't be the best idea to operate that crate engine past 6,000 RPM!--
Assuming about 85% (VE) Volumetric Efficiency. 85% is about normal for average engines. If you had 100% VE (not possible with out something like a super charger or nitrous oxide), you should run out of carb at about 6,000 RPM, so the lower VE lets you grab a few more RPM's.

If you operate above 6,500 RPM for very long, I suggest you get a slightly larger carb.
If you daily drive your jeep, then it sounds like you are doing really well right now.
What kind of gas milage do you get?

You didn't say if you had;
1. Vacuum secondaries (4160) or a double pumper (4150)?
2. Single fuel line, or dual fuel inlets (fuel line enters one on both float bowls)?
3. Changed the cam? (if you did, what's the lift and duration?)
4. What Intake Manifold?
5. What header size?
6. What ignition system?

On the front of the carb's choke air horn, there is a number or two stamped in the metal, please give these numbers.
I'll also need at least three vacuum readings, at rested idle (not in gear, just setting there), Curb idle (in gear at idle), and at normal cruise going down the road. Just tape the vacuum gauge to the windshild, and note the vacuum readings for about a half a day. Let me know what the lowest vacuum reading was while you were driving normal around town, not including when you were at wide open throttle, or trying to pass somebody.

If you email me with some answers to these questions, I will try to tell you how to adjust your carb. If we do it through email, I don't have to argue with the unknowing or misinformed, and it won't plug up the BBS.

Hope this helped, Aaron.

So many cats, so few recipes...
 
G

·
Discussion Starter · #3 ·
Actually, I wouldn't argue. I think a 600 is just about perfect for the motor you described. I've actually got one on my 408 & it really helps off-line jump, but it definitely runs out of gas (literally) on the top end.

TEX

/wwwthreads_images/icons/wink.gif Got Mud?
G.U.M.B.O. Mud Racing
 
G

·
Discussion Starter · #4 ·
I'm still having fuel starvation problems, from a cheap fuel pump. I'm going to check the fuel pump pressure at WOT, and see what it reads.

TeamRush, did you get my email? Not rushing you, just checking.

Just a farmer
 

·
Registered
Joined
·
6,870 Posts
I did not get your message.
Try [email protected]

Several others have responded though, so I know Off-Road.com's private message service is working.
If you send me the information, I will put you on the test & tune mailing list when we start.

So many cats, so few recipes...
 
G

·
Discussion Starter · #6 ·
How much fuel pressure should a Holley have at WOT? If I really get into it, it drops down to about 1 to 1.5 psi. It takes awhile for the gauge to go back to normal after I let go of the trottle. Down the road, it has about 4 psi. I know I need a bigger fuel pump, but how much pressure sould I have at WOT?

Just a farmer, who drinks to much coffee.
 

·
Registered
Joined
·
6,870 Posts
At 1-1/2 to 4 lbs. fuel pressure, You are literally running out of fuel!
You are about to start learning fuel systems first hand.
Welcome to the Wide, Wide World of Auto Racing... Gentlemen, Start Your Check Books!

Fuel pressure for a Holley should be taken from a line as close to the float bowl as possible. No Holley fuel feed line should be under 3/8" for any reason.
Fuel pressure for a Holley should be in the area of 6 to 7 PSI on a Gauge (PSIG).

If you are still using a mechanical fuel pump, it's going to be difficult to feed any high performance carb, mostly because of the lines that feed the pump.
(Usually around 5/16" ID for stock lines)
You can try one of the high volume mechanical pumps, but I haven't had much luck feeding the beast with them... The stock lines are usually to restrictive.
Generally, even if you go to Holley 'blue' pump and regulator to increase volume, you are still limited by the crappy little 5/16" or so lines that are factory stock.

I'm sure if you have a vacuum secondary carb, the stock Holley transfer tube to the back float bowl isn't feeding the back Venturi during hard passes.

One thing I would look into is the needle and seat. Some holley needle and seat arrangements used in the smaller carbs are too small to feed the beast. That problem shouldn't turn up as low fuel pressure though, it should turn up as lean plugs after a hard pass.

You need to start by finding out what line you have coming from the gas tank. If you have a 3/8" or larger INSIDE DIAMETER (3/8"ID) fuel line, and you inspect it from tank to fuel pump for damage, (bends that kink, bends tighter than 3" radius, mashed flat spots (anything that reduces the diameter of the line by 1/4 or more must be replaced),
3/8" fuel line should do the job you want to do with a stock engine.

--Or you can get new 1/2" ALUMINUM line for your fuel line. Aluminum doesn't really cost that much more, sheds heat a lot better, and is much easier to work with.
Remove the original mechanical fuel pump, fuel pump rod, and get a block off plate for the hole in the block. Run the new line right up the front of the block to somewhere you can mount a regulator.
Don't use any 90 degree fittings if you can help it. Every 90 deg. fitting reduces fuel flow by approximately 5 to 10%, depending on line size.
(changing the line gives you the opportunity to shield it from damage while 4 wheeling)

Install a Holley 'Blue' electric fuel pump as close to the gas tank as you can.
Install a Holley fuel pressure regulator as close to the carb as you practically can.
Use as little rubber line hooking things up as is possible. Rubber line on the suction side of the pump WILL get sucked flat, and become a restriction if allowed to.
Don't forget to get a fitting that will allow you to install a fuel pressure gauge, and install it AFTER the regulator.
Install as large of fuel filters as you can get away with.
Use a metal case one before the pump if you can, and use just before the carb also.

(a bunch of people will argue, but as the gas tank degrades with age, and as the gas quality gets worse here, a filter in front of the pump will keep that crud out of your expensive pump & regulator. A second filter, after the regulator, will keep any debris from a bad fuel system out of your carb, and insure a clean fuel supply.)

Mount a gauge OUTSIDE of the vehicle passenger compartment (in case the fuel line to the gauge breaks, you don't have gas inside the vehicle), where it can be seen by the driver.

If you still loose fuel pressure now when you launch, you may need to put a 'sump' in the back of the tank. The fuel may be climbing the back wall of the fuel tank, and uncovering the pick up tube for the pump.

See if you still loose fuel pressure now.

So many cats, so few recipes...
 
G

·
Discussion Starter · #8 ·
Thanks for the reply. I am using the cheapest electric fuel pump I could find. I was told it would be good for a 750 cfm carb, that's what I get for listening to anyone at Auto Zone.

I was about to order a Holley red pump, I guess I'm trying to save as much money as I can. I'll get the blue. I like the idea of a regulator. Could I use a big spin on type filter, the more paper the more flow right?

Just a farmer, who drinks to much coffee.
 
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top