600 CFM vs 750 CFM...

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7milesout

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Hey guys,

Although a lot of you guys have mentioned that a 750 CFM carburetor is not too big for a LA 360, I just can't seem to get my tuning where I want it on the 750. To back up a bit, the Scamp came with a 1407 on the original mains and rods. I now have a new exhaust and a wideband on it and have been tuning.

I've got the idle at a good spot. I've got WOT at a good spot. It cranks up fairly quickly compared to when I took ownership. But I cannot get the cruise mode or power mode lean enough. I'm at the limit of the calibration kit, and from this point it is a matter of guessing smaller mains and bigger rods, and paying for those piece-mill will get expensive.

I've been reading several threads on 750 CFM vs 600 CFM, and it appears that on an engine the size of a 360, drivability improves with a 600 CFM. As I can tell, the main difference between these 2 sizes of Edelbrock carbs is the primary venturis and primary bores. With smaller venturis and bores, the air velocity will be higher. That's going to better atomize fuel. And that seems to be where the drivability improves.

I've also read that I may give up 2 or 3 hp on the top end. That ... I don't care about. In order to achieve a 13.9 second E.T., I think maybe the 60 foot time would be better with a 600 CFM carb. And I have all the same ability to achieve a 12.5 AFR with a 600 CFM as I have with the 750 CFM.

As I see it, it would be a win-win to go to 600 CFM. Plus I have a $50 discount buying it through Advance Auto. That puts it at about $289 + tax.

Does anyone want a 750 CFM that has about 500 miles on it? I will put it back to the original mains and rods.
 
i can only tell you what i have used for different 340's.I had tuning problems with an Edelbrock 750.I used a 650 Thunder on one 340 and am currently using a manual choke 600 eddy on a 340 .It was easy to change metering rods to get my A/F where i want it and both 600 and 650 ran excellent.
 
Always remember the WB is a tuning tool. The numbers aren't set in stone. There are some really good threads over on speed talk about WB tuning and how the WB can produce different numbers.

Tune for best performance. Then check your numbers. I'd bet money your tune is better than 95% of the stuff out there.

Don't get buried in the numbers or the minutia.

As an example, if you run enough compression and enough cam, you'll never get your WB idle numbers as lean as some guys do, or claim to do.
 
yellow rose - I'm finding that very thing, at idle anyway. I've heard idle AFR's from 13.0 to 14.7. Ideally I would like it to idle at 14.7. But getting near that it gets VERY rough, sounds like it could stall at any second. I set it at 13.0~13.5.

Interestingly, my WB has been mounted on my left collector, and was where the readings came from. I think it was wise on my part to ask my exhaust guy for a bung on both collectors. Yesterday I got to thinking, "what if my left bank is at 13.0~13.5, and my right bank is at either 12 or 14.5?" During idle tuning, when I would turn 1 idle screw I would turn the other approximately the same, but that doesn't guarantee evenness.

So last night I switched the sensor over to the right bung. Lo and behold the right side is idling at the exact AFR's ... 13.0~13.5. I was surprised I had kept them so even.

I've heard that little spikes (at idle) to 13.5 or higher is a misfire, and the WB reads that as lean. Well, now that the AFR is painting the picture, it has frequent misfires at idle. I can see it and feel it. I thought a misfire would be a rare occurrence. But it is not. At least not for this engine.

I decided to go with a 1405. It's already set up for a manual choke, and I can feel exactly what the choke is doing through the cable ... and it was $31 less expensive anyway, and no additional wiring. I've just put the order in for the 1405. I'm hoping the higher air velocity will also lead to less misfires due to better atomization even at idle. We shall see.

In the other thread, I mentioned that I would like this car to be as close to an every day driver car as is possible. I won't daily drive it. But I want to be able to turn the key, have it fire up. And have good road manners. I don't want to be ready to put it on craigslist due to it being a PITA to deal with. This tuning stuff is FUN. But like I said, I'm as far as I can go with the 1407.

As for the question about "how much." I will sell the 1407 for $200. I think it's a $348 new carb, from what I remember. So I think that's a good deal for a like new carb. Includes the original mains and rods, intake gasket and carb gasket. Ships in the original box.


7milesout
 
What are the overall effects of undercarbing an engine?
For example if I have a car with serious HP but dont really want the kid or wife to have access to all that power will a smallish carb curb the power but still have driveability?
The wife tolerates my current endeavor because she wants to drive an old car with a 4 speed..... I have not disclosed to her the total of power and money spent on the engine quite yet.... Lol
So Im thinking I can choke it down with a 500 Eddy for her use inittially then swap the 850 DP when I go to the track.

Hey!! At least Im not sleeping around.... Lol
 
I'm currently testing a 600 holley double pumper in place of a 750 holley street hp. It idles better, had better throttle response, get better mpg, and has a choke.

Definitely lost a lot of seat of the pants power but all around drivability is way better.
I'm on the hunt for a cheap 700 (0-4778c) or 750 (0-4779c) holley to test out.
 
mbaird said:
What are the overall effects of undercarbing an engine?
...
So Im thinking I can choke it down with a 500 Eddy for her use inittially then swap the 850 DP when I go to the track.
Click to expand...

Just a guess here. If your wife were to drive it just cruising around on the street, and it were tuned properly, I think it would be more than fine. It would probably drive a whole lot better around town. But, if you were to go WOT, you'd have to have a big enough secondary jet to feed it enough fuel. The Edelbrock 500 CFM has the same size secondary venturis and secondary bore as a 750 CFM carb. So I'm guessing with the right jet in the secondaries, it may work out ok. It would be down on power due to the lack of air / fuel through the primaries (and thus through the whole carb), but I'm guessing you could tune it to safe AFR's and not be harming the engine.


7milesout
 
mbaird said:
What are the overall effects of undercarbing an engine?
For example if I have a car with serious HP but dont really want the kid or wife to have access to all that power will a smallish carb curb the power but still have driveability?
The wife tolerates my current endeavor because she wants to drive an old car with a 4 speed..... I have not disclosed to her the total of power and money spent on the engine quite yet.... Lol
So Im thinking I can choke it down with a 500 Eddy for her use inittially then swap the 850 DP when I go to the track.

Hey!! At least Im not sleeping around.... Lol
Click to expand...
Put a throttle stop on the 850 when you want to tone it down.
 
That was my 1st thought. But I was curious
about the overall effects. And throttle stopping would leave you with all the negatives of a large carb and none of the benefits.
 
7M-out Indont know your engine specs but a larger carb can be used IF the rest of the engine is up for it.

I am currently running a 600 AFB on my stock 5.9, minus the rpm intake and 1-3/4 super Comp headers into a 2-1/2 exhaust.
It is a little limiting on the top end but it is a true 100% drive the street car.
(727 & 3.55's round it out.)
 
rumblefish360 said:
7M-out Indont know your engine specs but a larger carb can be used IF the rest of the engine is up for it.
Click to expand...

rumblefish360 - The 750 CFM works, but having the WB opens a window into what is for real going on. In my opinion, a 750 CFM is good, but I think the 600 CFM will be better. At least for my engine. I can't tell you the specs on the engine. I took ownership not given the details. If I didn't have the WB and had it tuned it to where it is, I would have thought it was as good as it could be. The 750 probably gives me all the top end power I'm ever going to have with this engine as-is. But there's room for improvement on the street. And since that's going to be 99% of my driving, I'm looking for those improvements. Looking forward to receiving the 600.

I wound up buying the 1405 instead of the 1406. I'll save $31 and retain my manual choke. I can easily jet & rod to the settings of the 1406.

7milesout
 
Last edited:

1: Reply outside of the quote tags :thumbsup:
2: I thought you did indeed use the 600, like myself. I wasn't suggesting you move to a 750. Just stating that it could be used and if the engine perameters were up to it.

3: I understood that this is street driven. That's why I left my example.

Please read my reply again.
 
I think we're mixing it up. The car came to me with a 750 CFM, and I've tuned it as good as I'm going to get without buying jets and rods piece-mill. I'm downsizing to the 600 CFM. I have ordered the 600 CFM but haven't received it as yet. Should be middle of next week. I had modified your quote and got in a hurry (here at work) and left my stuff inside. But fixed now.
 
7milesout said:
I think we're mixing it up. The car came to me with a 750 CFM, and I've tuned it as good as I'm going to get without buying jets and rods piece-mill. I'm downsizing to the 600 CFM. I have ordered the 600 CFM but haven't received it as yet. Should be middle of next week. I had modified your quote and got in a hurry (here at work) and left my stuff inside. But fixed now.
Click to expand...
Looks like were on the same page! I think your going to like the 600 even though it is a little short on high rpm power. That is how I feel about my 60's on my 360's. Truly, no complaint. It's not a big deal/
 
I'm just thrilled you are putting in the effort and TIME to learn. Like I said, you can offer help to people now you couldn't have just a few months ago.

Keep us posted on how it goes.
 
Guys - I received the 1405 on Monday. Monday evening I had it converted to the 1406 set up and set it on the intake. There was a crack in the rubber fuel line, so I stopped there and went and got another foot of fuel line. Tuesday I finished the line and finished the car install. When I pulled the filter off, all manner of nasty schmutz oozed back out the backside of the filter. It concerned me.

The 1405 came with a clear plastic filter. I was glad, in that way I could see if the fuel is real nasty, meaning the tank needs attention. Luckily the fuel looks very very clean going into the filter. The filter may have been 30 years old.

Tuesday night I took it for a quick spin. It idles nice, comes off idle very well. It was very lean (too lean) in Power Mode (accelerating) so I did not attempt WOT. Cruising AFR's seemed close. I figure (but may be wrong) I need 8% more fuel to accelerate, and keep the same fuel level for cruising. I'll figure out what to do about that after I see what effect the below has.

I had an interesting conversation with an Edelbrock technical rep today. I told him about the off-idle lean spike. I read on the internet that a lot of Edelbrock owners experience this. I had it with the 1407, and now with the 1405. It seems that at about 1.5 seconds after beginning a slow acceleration, the pump shot is consumed and it goes lean. Sometimes bad enough to stumble. It's as if the metering rod needs to come up quicker to deliver more fuel. He said that if I switched from total vacuum to timed vacuum that it should resolve that or reduce it. And either in the videos or in the manual, I do remember being instructed to use timed vacuum. I will try that ... maybe tonight.

Yesterday I checked vacuum. At a warm idle, it's pulling 17" of vacuum. The Edelbrock manual says to take half the vacuum and use that size spring (at least as a start). 17 / 2 = 8.5. So I took the 4" spring out of it, and put the largest available 8" spring in it. This tends to make sense as the stronger spring would pull the rod out of the jet quicker. Which should also help with the lean spike. I didn't get to drive it as it was raining last night.

7milesout
 
Well, I got to drive it last night. The Edelbrock tech is full of bull. Timed advance, max pump shot and the biggest step up spring didn't do squat to what I'm calling "lean spike." I'm going to go back to manifold vacuum. I went WOT a few times and that AFR is perfect, and it feels strong.

But I feel stuck. I'm too lean on Power Mode, and too rich on Cruise mode.

On the current main jet (0.098") there's no metering rod that will make my cruise mode leaner than a 75/47 (which is what I'm running). 0.075" is as big a diameter as I can see in the manual.

If I go to a smaller main jet, I can lean the cruise, but the power mode can only get a tad richer (+3.5%). And I think I need more power mode fuel than that. That and the lean spike is annoying.

What would be great is if I could make my own custom metering rod. I would stay with my current main jet (0.098") and run something like a 78/37, and make the step higher up on the metering rod, so it starts getting fuel sooner. That change in the step location I think would help with the lean spike.

Does anyone know if there is a custom metering rod shop?


7milesout
 
what is your initial/total timing and what are your a/f readings for idle,cruise and wot?
 
ir3333 said:
what is your initial/total timing and what are your a/f readings for idle,cruise and wot?
Click to expand...

Main Jet: 0.098"
Metering Rod: 75/47
Step Up Spring: 8"
Pump Shot: Max
Initial Timing: 11°
Initial + Mechanical: 36° - 37°
Idle AFR: 13.0 - 13.5
Cruise AFR: 13.5 - 14.0
Power AFR: 15.0 - 16.0
WOT AFR: 12.5 --> 12.1

My current plan is to switch to a 75/37 metering rod.


7milesout
 
7milesout said:
Well, I got to drive it last night. The Edelbrock tech is full of bull. Timed advance, max pump shot and the biggest step up spring didn't do squat to what I'm calling "lean spike." I'm going to go back to manifold vacuum. I went WOT a few times and that AFR is perfect, and it feels strong.

But I feel stuck. I'm too lean on Power Mode, and too rich on Cruise mode.

On the current main jet (0.098") there's no metering rod that will make my cruise mode leaner than a 75/47 (which is what I'm running). 0.075" is as big a diameter as I can see in the manual.

If I go to a smaller main jet, I can lean the cruise, but the power mode can only get a tad richer (+3.5%). And I think I need more power mode fuel than that. That and the lean spike is annoying.

What would be great is if I could make my own custom metering rod. I would stay with my current main jet (0.098") and run something like a 78/37, and make the step higher up on the metering rod, so it starts getting fuel sooner. That change in the step location I think would help with the lean spike.

Does anyone know if there is a custom metering rod shop?


7milesout
Click to expand...
Exactly!
As far custom metering rod shop, it might be you. Sand paper will take the metering rods down, and I think you can get creative on where they need to be filed down. I've done this before more than once with great success
 
7milesout said:
I wonder if those metering rods are made from brass?
Click to expand...
I believe so, they sand easily. I've found Edelbrocks a lot more fussy than Holleys to try and tune. Not starting a debate, but just sayin,
 
7milesout said:
Main Jet: 0.098"
Metering Rod: 75/47
Step Up Spring: 8"
Pump Shot: Max
Initial Timing: 11°
Initial + Mechanical: 36° - 37°
Idle AFR: 13.0 - 13.5
Cruise AFR: 13.5 - 14.0
Power AFR: 15.0 - 16.0
WOT AFR: 12.5 --> 12.1

My current plan is to switch to a 75/37 metering rod.


7milesout
Click to expand...

.. your a/f readings look good and it's best to error a hair on the rich side.If your calling light to medium acceleration "power" 15 is normal ..your getting lots of air without much gas.
.. try adding a bit of initial timing, if it helps you may need to re curve your distributor.
 
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