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Carb size to small????

12K views 50 replies 19 participants last post by  xntrik 
#1 ·
I have been trying to tune up my motor, and I'm starting to believe my carb size is too small. After about 4,500rpm the motor will still rev up to 6500 but it gets there slowly, and the power maintains but doesnt have that neck breaking pull that it does under 4500. After playing with the timing alot with no effect, I was wondering what you guys thought about my carburator size. Im running an Edlebrock 750. Here are some of the specs on my motor.

408 stroker/windsor
Aluminum 215cc runners 2.08/1.60
10.68:1 compression
Roller Cam 224/.568 /112cl/ 3200-6700rpm range
Victor Jr intake
Edlebrock 750CFM carb
 
#2 ·
igniton and valve springs are the first to check.

I don't like edie carbs. You can't tune the secondary's easily.

750 should pull to 6500 easliy, try another carb or check the above items.

Your cam seems short for 6500 anyway. I'd look into something like 235@.050 and .600 lift.

try a 1" spacer on the victor Jr. Or swap to a super victor.
 
#3 ·
I think it may be the Manufacturer rather than the CFM
Demon recommends a 750 CFM Speed Demon either Vac
or Mech with that CID and Cam
http://www.demoncarbs.com/demon/default.aspx?page=5
 
#4 ·
Sorry I was wrong the cam card shows 2000-6000rpm (valve float at 7000)

based on 110@seat and 348@open..

The springs Im running are 135@seat and 420@open which crane said is the max spring pressure for that cam.

I also hate edlebrock carbs, the secondarys have a big lean spot that wont go away, but it was 1/2 the price of the hollys, so I figuered it would be good place to start while I tune things in.

So you guys think I should junk the edle 750? Try a demon? Is 750 going to be big enough or should I go with the 850?




Ben

I
 
#6 ·
Carb too small??????

Sounds more like a timing situation to me. Not getting enough total advance. That 750 should pull you quite easily up to 6000 RPM, Holley, Edelbrock, Carter, Rochester, Ford, Brand X, whatever, if its properly tuned. Of course I would also recommend going to a Holley or a Demon.
What are you getting for total advance and at what RPM? What is your intitial advance? With your setup you should be pulling about 38 degrees total advance at around 2500-3000 rpm, and about 10 degrees initial.
That 224 cam is a little on the short side for 6500 but should pull 6000 easy enough. If you have access to another carb of the same size that you know works well, try using it on your engine and see what happens.
 
#7 ·
So you spent ....what...$10K on the engine or more?

And you bolt on the cheapest carb money can buy, those 750's are the worst of the worst they all have chronic mid range lean out and you can't fix it.:nono:

I have a customer who did almost the same thing with a 408 in a 95 Stang. Went to the track and it wouldn't run past the 1000' mark just layed over, best pass was a 12.64.

I told him he needed to increase the fuel line to minimum 3/8", use a BG SB Ford Mechanical pump and dump the Edel-Broke and bolt on a 750 Mighty Demon. He insisted that his lines we're plenty big enough (5/16) and the Carb just needed tuning, "so why did you ask me?" :spank:

The next thing he asked me for was a ride home.....LOL

He messsed with it for about an hour and made his LAST pass....about 800' out it went up in a ball of smoke...melted 3 pistons out of it.

Back to the machine shop, $2800.00 later he brought it over to us. We ran 3/8 Braided fuel lines, fuel cell, BG Mechanical Pump, 750 Mighty Demon and curved the distributor.

200+ mid to low 11 second passes and he's still driving it to and from the track hasn't needed a tow truck since.....he did have to add a way bigger tires to make it hook. :)
 
#8 · (Edited)
While the Edelbrock/Carter thing is my least favorite, I doubt those pistons woulda lived any longer no matter what carb was sitting on it given the lack of fuel making it to the carb. He obviously just left his foot in it while it was running out of gas.

I think I can make an edelbrock/carter thing work as good as any Holley though.

EDIT: Oh yeah, max rpm x cu.in. divided by .3456 will give you the proper sized carb (CFM) and if you wind up say, halfway between two available sizes, go for the smaller one, you'll like it much better.
 
#9 · (Edited)
Uh oh. Not the formula again. Plug the dual 1250 CFM Dominators into that equation. You know the ones the racers use atop the mile high tunnel rams? Those should never work according that equation.

I agree with Cuda about the Edelcrocks. You can have em.
I was just on the phone telling a friend of mine that they are good for the average consumer. They kind of run half as--sed in most mild builds. Holleys can give you grief, but can also be tuned to run well. Edelbrocks just kind of stay at the half way point.
 
#10 ·
I have to agree here, that carb has no business on your engine. They are OK for street rods and mild street cars. Even Mr. Edelbrock doesn't run Edelbrocks on his fast cars. They are great carbs for hot rodders 101 because they are easy to tune well enough to run.

I also think the carb is on the small side. See if you can find an annular booster 830 CFM Holley double pumper. You will really like the throttle response and since you aren't turning huge RPMs it is a perfect fit (IMO).

Even if that carb isn't causing the "problem" (could be timing or springs, doubtful) it is definetly not letting you see maximum performance.

Royce
 
#11 ·
Cuda, Im running a fuel cell and number 8 line. You know interesting point tho... maybe I'm not running enough fuel pressure. Im running a holley fuel pump which was at 4psi. About two weeks ago I raised it to 6psi and it seemed to help out. Could I be accelerating faster then the pump can pump?
Today I noticed that the motor seems to run better when its cold around 140degrees. It even sounds better when its cold, once it warms up the motor sounds different.

The timing is set at 14 idle and 34 total. Im not sure at what RPM the total is coming in at, is that important?
 
#12 ·
Ben,
Do you have a fuel pressure gauge you can see while driving? 4 psi is low (IMO) 6psi is OK you might want to try 7psi. One other thing if you are setting your fuel pressure with the pump and gas cold, the pressure is going down after the pump and outside temp goes up. This will happen even on a larger scale if you are not running a return line type system.

I have seen as much as a 4 - 6 psi drop from cold to hot. Just something to think about.

Royce
 
#13 ·
brainsboy said:
Cuda, Im running a fuel cell and number 8 line. You know interesting point tho... maybe I'm not running enough fuel pressure. Im running a holley fuel pump which was at 4psi. About two weeks ago I raised it to 6psi and it seemed to help out. Could I be accelerating faster then the pump can pump?
Today I noticed that the motor seems to run better when its cold around 140degrees. It even sounds better when its cold, once it warms up the motor sounds different.

The timing is set at 14 idle and 34 total. Im not sure at what RPM the total is coming in at, is that important?
You want to verify fuel pressure under wide open throttle conditons. You may be 6 PSI at idle but under full flow it certainly could be dropping on you. You also want to watch that you don't run too much pressure as you'll overcome the seats on the float. You should baseline all your advance in by 3000 RPM. If you can get it in a little sooner without detoantion then great.
 
#14 ·
Fuel Pressure is only a measurement of restriction in the fuel system and has very little to do with fuel VOLUME.

Sounds like you running a Holley Blue, thay actually will flow at about 75GPH at pressure, combine that with the Blue Reg which flows maybe 50GPH and you'll see that a good mechanical will out flow it. Holley Blue Systems, IMO are good for about 300 HP max.

There lot's of thing's that determine fuel requirements, let's analyze a few.
1. You have a 1/2 line, that's very good, but what size are the inlets and outlets on your pump? The answer of course is 3/8" so your pump will only flow at the rate allowed by the smallest orafice in the system, remember electric pumps are gravity fed so although the outlet side will flow more volume due to the pressure the inlet which is gravity fed is the limiting factor.

2. Let's use a 12 second flat car for an example. I would think we can all agree that this car would burn about 1/2 gallon of fuel to make a 1/4 mile pass. So if we do the simple math to determine GPH (Gallons Per Hour) 1/2 gallon x 5 (60 secs. / 12 sec pass = 5)= 2.5 gallons per minute x 60 minutes = 150 Gallons per hour, far beyond the capacity of a Holley Blue or any mechanical pump.

3. So we've determined how much fuel we need to make a pass and the capacity of the pump required. But what about the gear ratio in 1st of about 2.50? Does this have any affect? Of course, we all know from experience that a car will use way more fuel at 30 MPH in 1st than it will in 2 or high gear so we need to make allowances for the huge consumption in low and 2nd gear. Some of this is built into a Demon with the larger float bowls creating a built in reserve, but if the pump can't keep up the floats will drop and the carb will not be able to maintain a nice flat fuel curve as demand increases. Do you think that maybe 20% more fuel would be a fair number to use?...so we're now up to 150 + 20%= 180 GPH

4. Let's not forget G forces, does your fuel line run from the cell straight up the carb taking the sortest route possible? By running the fuel line in this manner you further complicate the G forces working against the pump, all the fuel in the line is pushing back against that pump and will actually stall the pump (Remember the main feed line from the tank to the pump it's going in the opposite direction also starving the pump of adaquate supply)

The correct way to plumb a car for optimum fuel delivery would be to have the main line come all the way to the front of the car, we run them through the radiator core support, a big loop and back to the regulator.. Now when the car launchs the fuel is driven by G forces to the regulator giving it a supply while the pump recovers from the shock. So if your running your lines in what is probably not the best location then I think a 10% increase in pump capacity should be built into the equation....180 + 10%= 200 GPH

(How's your fuel system holding up so far?)

5. I'll bet you have your regulator mounted to the engine or chassis right? Another No-No....as the engine starts to wind up and the car builds speed that regulator is getting the ever loving ***** rattled out of it. In allot of cases so violently that the spring can't hold the ball on the seat so it releases fuel and can overflow the float bowls. You've all seen this at the track...a car gets about 300' out and it starts leaving a black haze behind it.

6. Feeding the carb from the regulator is another consideration, if you running a dual feed line that uses a T and splits off to feed the bowls your probably OK to about 350 HP aafter that we reccommend to use a 2 port regulator and supply each bowl with a -6 line from seperate ports on the regulator. Thjese lines need to be as short as possible we use a 7.25" line with a 45* on one end and a straight on the other with the BG 2 port it works perfect. Too large of lines or too long of lines will cause the regulator to respond slowly as there's more fuel volume to drop in pressure and more volume to fill to get back to full pressure.

7. Weight consideration and engine size are another factor to look at. If this 12 second car is a 4000# Hemi Super Bee or a 350 SBC in a 2800# 67 Camaro? Again I think we can all agree that even though both of these cars may run 12's their fuel consumption will vary drastically so the rule of 12 second car requires a fuel system that can deliver 1 gallon in 30 seconds becomes a rule of thumb and not an absolute number.

In this scenario the Hemi Bee would probably need a BG 280 whereas the Camaro would have plenty of fuel with a BG 220.

Your Fuel System is just as important as choosing the correct cam, the right Carb, tuning the Ignition, selecting the correct tork convertor, compression ratio etc. it's all part of building a good dependable and consistant car whether it's a Street Muscle Car, Sunday Bracket Racer or a Top Comp/Pro Stocker you must have an adaquate fuel supply to feed the power you've created, if you don't want to or can't afford to build an adaquate fuel system then don't build the power. Piston manufacturers love you guy's it keeps them in business.

I've been in this business for most of my life, been tuning carbs and ignition systems since a was a young kid, worked with my Dad who was probably one of the best tuners ever, he started teaching me this stuff when a was about 6-7 years old....that was about 45 years ago. But if I need a cam I call Jim at Racer Brown a real cam grinder, if I need a Convertor I call on Frank Lupo at Dynamic, when I need pistons I call Ronnie at Diamond, I use the experts available to me, I draw on their experience in their specific fields to obtain the best results. It works for us.....our All Iron, All Motor 318 Mopar in a 3100# car will crack off a 11.60 pass with 6200 RPM shift points anytime I hammer on it.

So in closing I can only strongly reccommend that if your not a expert on fuel systems then consult with someone who is and stop relying on a salesman at a mail order house or some rediculous magazine article to sway your decisions on your fuel system or ffor that matter any other component. Deal with that special individual who has risen to the top of his field and forget the mass producers of fancy catalogs and generic one size fits all components.

If you follow what everyone else does then you will follow everyone else, I prefer to lead, the view is much better up front :D...Mush.....if you ain't the lead dog the view sucks..
 
#16 ·
Based on 1/4 mile calculators I should be in the low 10's for my current setup, so It may be that my fuel system is not good enough. As one person mentioned I did actualy mount the regulator infront of the motor. The problem with that I cant see my pressure gauge while driving, so I have no idea where its at. I will relocate the regulator tonight and check it hot and cold to see what kind of pressure I have. I'll post my results here tonight.

thanks

Ben
 
#17 ·
This oughta be in a How To!

cuda66273 said:
Fuel Pressure is only a measurement of restriction in the fuel system and has very little to do with fuel VOLUME.[/b]
I ommited the rest of this Extremely Valuable information about getting your fuel system right that Cuda66273 wrote. But I just want to say thanks for the tips and for making it clear about fuel systems.

This thread definitely persuaded me to put my Edelbrock 750 on the shelf on go for the BG mighty demon and an appropriate pump & regulator for my new 383ci/505hp engine.

Thanks for the Math!

--Dan
 
#18 ·
Of course fuel pressure has everything to do with fuel volume. Just look at any pump manufacturer pump curve graphing pressure versus volume, as pressure increases volume decreases, no matter whose pump. When sizing pump you size at the designed fuel pressure for the application.

It is fairly easy to size a fuel pump without a lot of confusion. Basically flow = horsepower * brake specific fuel consumption. BSFC typically will average around .5 lbs/hour for an N/A application and .55 to .6 for a turbo. These are pretty safe numbers.

So if you make 300 hp * .5 lbs/hr = 150 lbs/hr of fuel. Fuel weighs aprox 6.25 lbs per gallon so 150/6.25=24 gallons per hour delivered into the carb at the designed inlet pressure. Of course we have to make allowances for pressure drops in the fuel line and such BUT, the thremodynamics are such that the motor only is going to consume a fixed max amount of fuel and the above calculation will get you very close absent the printout off the dyno. BSFC is a standard measurement on the dyno.

Cuda, we don't all need to be driving down the road with a Mighty Sumo hanging under our car feeding our 400 HP small block. The margins on those things aren't THAT good! :D
 
#19 ·
66Caprice

I'm glad you enjoyed the article and equally happy that you made the correct decision.

Have a look here:
http://www.4secondsflat.com/Barry_Grant_Fuel_System%20Specials.html

And here:
http://www.4secondsflat.com/specials.html

Real Parts for Real Gearheads
 
#20 ·
Brainsboy:
Quote:
"Based on 1/4 mile calculators I should be in the low 10's for my current setup, so It may be that my fuel system is not good enough. As one person mentioned I did actualy mount the regulator infront of the motor. The problem with that I cant see my pressure gauge while driving, so I have no idea where its at. I will relocate the regulator tonight and check it hot and cold to see what kind of pressure I have. I'll post my results here tonight."

Your fuel pressure gauge should be mounted on the cowl so you can see it as you make your pass, a $29.00 Carter Electric will give you 6-7 PSI at idle or free revving.

If you fill out a spec sheet from our wesite:
http://www.4secondsflat.com/specsheet.html

....and email it to us I'll analyse your fuel system and make the appropriate suggestions. tech@4secondsflat.com will work.

Don
 
#21 ·
With my gauge being by the grill, I tried to black tap my buddy to the grill and he didnt like that idea, so we relocated the pressure gauge, but due to having short lines my buddy still had to hang out the window to see it, so I didnt feel safe going with full throttle, but I did get a few good passes in and got it up to 5000rpm. The first few passes were at 3.5psi, the next few passes I went a little bit harder but not full throttle by any means and psi went down to 3psi. These were all 2nd gear accelerations from about 30mph.

As long as I have pressure then I should ok as far as fuel supply right?


Ben
 
#22 ·
lluciano77 said:
Uh oh. Not the formula again. Plug the dual 1250 CFM Dominators into that equation. You know the ones the racers use atop the mile high tunnel rams? Those should never work according that equation.

I agree with Cuda about the Edelcrocks. You can have em.
I was just on the phone telling a friend of mine that they are good for the average consumer. They kind of run half as--sed in most mild builds. Holleys can give you grief, but can also be tuned to run well. Edelbrocks just kind of stay at the half way point.
Yup its the formula again. Reread his combination, its is not 18-20 to 1 c/r that utilizes a chamber that enhances turbulence and maximizes flame travel. It is not reworked 1250s with smaller venturis tuned to specific port runners, cam, port, primary tube, collector and gear ratios. It is not running from 9000-10,000 + rpm only, nor is he giving any evidence of having an ignition system that could support any of that. Stray from the formula on his combination and it will slow down everytime. Your example of the dual 1250s will likely confuse this guy into buying two more carbs. One way to big and then the right one. Unfortunately he'll likely spend a few thousand chasing "problems" until someone comes along and puts the right carb on it for him.

Further, the fact that he is reporting good pull to 4500 indicates to me at least that it aint nothing to do with 750 cfm or Ed Edelbrock. He has other issues there and that carb should stay so he can at least maintain his baseline until he figures out what is really going on. I think I'd be looking at fuel delivery to the carb and I don't really like his springs either. Hell it could be air delivery to the carb at speed or, ignition, exhaust, or all of the above.

About the only thing I can tell from this far away is that (If clean) the 750 cfm carb is not his problem and he needs that carb to sort his problem out in the quickest, most affordable manner.

All any carb is ever gonna be is a mixing device. Some flow better then others. The tuner is responsible to make the proper Air/Fuel ratio throughout the entire rpm band. An edelbrock is just as capable of making that A/F ratio as anything else and in some cases the edelbrock is more adjustable. I personally dont use them but if I had to I could make it do anything a Holley can do.
 
#23 ·
brainsboy said:
With my gauge being by the grill, I tried to black tap my buddy to the grill and he didnt like that idea, so we relocated the pressure gauge, but due to having short lines my buddy still had to hang out the window to see it, so I didnt feel safe going with full throttle, but I did get a few good passes in and got it up to 5000rpm. The first few passes were at 3.5psi, the next few passes I went a little bit harder but not full throttle by any means and psi went down to 3psi. These were all 2nd gear accelerations from about 30mph.

As long as I have pressure then I should ok as far as fuel supply right?


Ben
I think what others are trying to point out here is that pressure and volume are two different and separate values when moving liquid. I dont think dropping to 3.0 during a second gear stab would be very desirable either. That would tend to indicate that the pressure gauge needle would be laying on 0 or very close to it during a launch. You need to maintain all of your pressure while running for reliable fuel delivery.
 
#24 ·
Quote:
" About the only thing I can tell from this far away is that (If clean) the 750 cfm carb is not his problem "

I would like to debate this statement but I have already wasted too much of my short life expectancy trying to tune the chronic mid range lean out of these carbs, they're a $289.00 piece, why spend 10 hours at $60.00 and hour trying to tune it and fix it when a real carb that is capable of handling the task is $400.00?

If you find the problem and a way to fix it please let me know, I have 4 or 5 near new ones sitting here that could be fixed and sold.
 
#25 ·
The pressure drop indicates a fuel flow restriction, if your base is 6 or 7. Could be something as simple as clogged filters, pickup problem in the fuel cell or venting in the fuel cell. Voltage at the pump also must be maintained at 12 volts nominal for rated pump capacity.
 
#26 ·
Hey guys no High jacking or taking over my thread please, besides at 60$ an hour this is getting expensive for me:) ....

So in simple terms just because I'm reading 3psi doesnt mean I'm getting enough volume? That doesnt make sence because if its under pressure (3psi) then the fuel is still waiting to get into the carb, But either way 3psi is not great, I need a new fuel pump?



Rick is it restriction or G-force pushing back on the fuel? Im using a standard 5 gal fuel cell with #10 going to the pump and after the pump is all #8, the vent on the top I would have to check I forgot about that..
 
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