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Old 03-05-2005, 05:32 PM
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Carb size to small????

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

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Old 03-05-2005, 05:38 PM
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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.
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Old 03-05-2005, 05:40 PM
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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
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Old 03-05-2005, 05:50 PM
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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
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Old 03-05-2005, 06:18 PM
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I would get an 830 Holley. They are #9381. I see them used on Ebay for $175 to $225. (Take that Machine Shop Tom ).
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Old 03-05-2005, 10:09 PM
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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.
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Old 03-06-2005, 07:02 AM
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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.

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?"

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.
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Old 03-06-2005, 04:26 PM
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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.

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Old 03-06-2005, 04:34 PM
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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.

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Old 03-06-2005, 06:45 PM
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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
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Old 03-06-2005, 06:52 PM
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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?
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Old 03-06-2005, 09:48 PM
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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
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Old 03-06-2005, 10:25 PM
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Quote:
Originally Posted by brainsboy
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.
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Old 03-07-2005, 07:15 AM
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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 ...Mush.....if you ain't the lead dog the view sucks..
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Old 03-07-2005, 07:27 AM
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I have to agree with most said here. the 750 CFM will support 6000 RPMs without a problem, I to would personally ditch the Eddie & use a BG or Holley.
but also, like already said, verify timing, fuel volume/pressure etc.
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