Retune my entire carb after installing new heads?

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Do you still have anything constructive to add except to tell me it won't work?

I'm not running Pro Stock where I have to have every last hp on race fuel... It was working well at 10.3:1 on the old iron heads. Now it's around 10.6 (will have to look to make sure) with the new heads.

"Too low" is relative...street manners are decent, and I still haven't wrung it all the way out since there's no safe place around here to do it!
 
Yeah run a smaller carb that will improve vaporization to crutch the low compression time you have with that big cam. Should also improve the transition problems you were chasing with different restrictors you were trying.
 
I didn't notice any difference on the butt-dyno in low- and mid-range performance when I went from an 800 to a 950 (once I dialed it in). But a smaller carb may cost enough power that it kills some of the increase from going to good heads!

This is the first I've ever heard of "compression time"... not to say it isn't a thing, but isn't that why lower compression and less dense fuel mixtures benefit from more advanced ignition timing?
 
DrCharles said:
I didn't notice any difference on the butt-dyno in low- and mid-range performance when I went from an 800 to a 950 (once I dialed it in). But a smaller carb may cost enough power that it kills some of the increase from going to good heads!

This is the first I've ever heard of "compression time"... not to say it isn't a thing, but isn't that why lower compression and less dense fuel mixtures benefit from more advanced ignition timing?
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How much you compress the mixture depends on when you close the intake valve for cylinder pressure to build and compressing the mixture helps convert it to a burnable state. The problem is you think of an engine as an air pump so fuel condition doesn't enter into your frame of reference. Yes fuel needs to be conditioned for it to be able to burn. Liquids don't burn in cylinders gasses do so what comes out of you carb is a liquid droplet that needs to be converted to a gas to actually use it. Most likely you have every little vacuum in the intake tract to do this (gasify the liquid) you need more compression time to do just that on the compression stroke. When you have none of that its one big mess and a tuning headache....

How to Pick the Best Carburetor for a Street/Strip Car

Look at the ET from the 650 DP on 425 cubes and look at the vacuum it generated @ WOT. It ran the quickest ET and highest MPH. Surely it was choking that 425 to death......
 
I don't know WTF you actually want. I gave as good advice as I knew how and you threw it back at me so I told you to leave it as it is. What else do you expect? I'm telling you from seat of the pants experience that your combination is idling too high. I actually don't really give a CRAP if you believe me, follow my advice or not. I'm not gettin paid here. You seem dead set on doing it your way so frikkin do it! My stuff runs good. That's all I really care about. Have fun with it.
 
SpeedTalk thread on Carb sizes
I ran a 750 on a 700hp smallblock, changed to a 1000 type, and didnt pick up anything, just lost some throttle response, the 1000 runs as well, just not as snappy lower down.
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From the guy who built that 750:
As far as the throttle response goes, etween a small venturi and a large venturi carb, the larger venturi set up on an engine with the correct power range for that size carb and calibrated properly, will deliver good throttle response. So if the carb is too big for the power range of the engine, then generally a smaller venturi carb that is suited for the power range will deliver better throttle response, and quite often allow the car to be faster.
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Now go think about what that 5.5Hg might be doing for that 425 Ford earlier.
 
I said at the start of this thread, I was only asking if all circuits will need to be changed. I have that answer.

I don't give a CRAP what you think about my idling "too high". It works for me and everyone else who has run this cam. Your stuff is not my stuff and every combo is different.
 
DrCharles said:
I said at the start of this thread, I was only asking if all circuits will need to be changed. I have that answer.

I don't give a CRAP what you think about my idling "too high". It works for me and everyone else who has run this cam. Your stuff is not my stuff and every combo is different.
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If you don't get the idle and transfer right what makes you think WOT will be right? Idle and Transfer set up the engines ability to take WOT.
 
Hysteric said:
SpeedTalk thread on Carb sizes


From the guy who built that 750:
Click to expand...
Thanks for the interesting article. My engine and cam are a little larger than that 425 so I think a 650 would be too small no matter what...

I am also surprised that the 650 gave the best 330' performance, but not surprised that it was too small at the eighth and, as they estimate, would still be too small at the quarter giving up 14 hp on this particular combo.

Anyway if I only wanted to cruise around town, I agree a smaller carb would be easier to tune and drive ;) But I had it working with the iron heads, I'm sure I can make it work with the new heads too :)
 
Hysteric said:
If you don't get the idle and transfer right what makes you think WOT will be right? Idle and Transfer set up the engines ability to take WOT.
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I'm not sure what you are trying to say here. Idle and transfer contribution is a small part of the total fuel flow when all the main jets, especially the big secondary ones, are flowing...
 
DrCharles said:
Idle and transfer contribution is a small part of the total fuel flow when all the main jets, especially the big secondary ones, are flowing...
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You still don't get it because its all about combustion and nothing to do with airflow. It can take literally seconds for a cylinder to recover its ability to fire correctly when you have misfires and partial fires. The idle and transfer are the MOST important part BECAUSE they set the condition in the cylinder to take WOT correctly.

DrCharles said:
but not surprised that it was too small at the eighth and, as they estimate, would still be too small at the quarter giving up 14 hp on this particular combo.
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Yeah and that 1050 Dominator made 20 more HP on the dyno but ETed slower than the 750......
 
Hysteric said:
How to Pick the Best Carburetor for a Street/Strip Car

Look at the ET from the 650 DP on 425 cubes and look at the vacuum it generated @ WOT. It ran the quickest ET and highest MPH. Surely it was choking that 425 to death......
Click to expand...
You really like this article, you found one instance that backs your theory up, how many go the other way, unfortunately the only real way to know test a few different carbs, I'm sure once the OP gets his carb setup he'll be satisfied.
 
Hysteric said:
Yeah and that 1050 Dominator made 20 more HP on the dyno but ETed slower than the 750......
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For that particular car, if that was the norm most drag cars would be running smaller carb sizes.
 
273 said:
You really like this article, you found one instance that backs your theory up, how many go the other way, unfortunately the only real way to know test a few different carbs, I'm sure once the OP gets his carb setup he'll be satisfied.
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Here's another one I like:

Holley 950HP 1000HP????

Yeah that's 700HP on a 750DP......Ran quicker ET's than a 1050 that made 20 more HP on the Dyno.

What's my theory? Improve combustion and make more power?
 
Hysteric said:
Here's another one I like:

Holley 950HP 1000HP????

Yeah that's 700HP on a 750DP......Ran quicker ET's than a 1050 that made 20 more HP on the Dyno.

What's my theory? Improve combustion and make more power?
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No the over emphasis on that one aspect, a lot of time it's easier to make more HP/E.T. by displacing a lot more fuel and air over time at a lesser efficiency than try to squeeze every last hp out of lesser amount of fuel and air. Especially since there narrow window of efficiently available to most of us. And little need outside highly competitive racing, if getting between 1.2-1.3 lbs-ft per cid is doing really good for most.
 
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273 said:
No the over emphasis on that one aspect
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And you wonder why some engines make more torque than others........So put a bigger carb on and then complain it doesn't make any torque down low so now you need a big stall to get it to launch........

273 said:
a lot of time it's easier
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Its easier to put a smaller carb on to fix a problem than spend hours posting on forums searching for a solution to a problem you created. How much time has this guy spent changing TSR's to get the transfer working right.......The whole point of running a smaller carb is a smaller throttle bore increases vacuum to aid the idle and transfer circuit to do its job effectively and properly ie condition the cylinder to take on WOT mixture correctly. Remember my statement from a previous thread:

Engines with lack bottom end power?​


Less firing events equates to less force transferred to the crank does it not? So the quality of those firing events becomes critical in the torque generated does it not?

Its the efficiency of each firing event all the way through the entire rev range that accelerates the car not just the top end. The most critical is the low speed running circuits BECAUSE they set the stage for HOW the cylinder fires. So Mark Campbell fixes the low speed running on a Prostock car and the car goes FASTER. Did the car now ingest more air to make more power or did just do it more efficiently? If you're loading up plugs and creating a less than ideal cylinder condition then when you introduce the mains fuel its gonna take a long time to get back up to peak efficiency before it can make peak power.
 
Hysteric said:
And you wonder why some engines make more torque than others........So put a bigger carb on and then complain it doesn't make any torque down low so now you need a big stall to get it to launch........
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What you say comes across that smaller carb, head etc.. always equals better. I'm Not saying opposite is always true either.

Generally the engines putting out higher lbs-ft per cid run big cams, carbs, heads etc.. very built race car engines. Not stock very restrictive engines even at very low rpms.
Hysteric said:
Its easier to put a smaller carb on to fix a problem than spend hours posting on forums searching for a solution to a problem you created. How much time has this guy spent changing TSR's to get the transfer working right.......The whole point of running a smaller carb is a smaller throttle bore increases vacuum to aid the idle and transfer circuit to do its job effectively and properly ie condition the cylinder to take on WOT mixture correctly. Remember my statement from a previous thread:
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Yes it's possible a smaller carb maybe better for the OP, but it's no guaranteed fix and there's no reason why the one he's running won't work even if it ain't optimal which only though testing a bunch of carbs we'd find out, so we will probably never know and the gain probably not worth buying another carb anyways.
Hysteric said:

Engines with lack bottom end power?​


Less firing events equates to less force transferred to the crank does it not? So the quality of those firing events becomes critical in the torque generated does it not?

Its the efficiency of each firing event all the way through the entire rev range that accelerates the car not just the top end. The most critical is the low speed running circuits BECAUSE they set the stage for HOW the cylinder fires. So Mark Campbell fixes the low speed running on a Prostock car and the car goes FASTER. Did the car now ingest more air to make more power or did just do it more efficiently? If you're loading up plugs and creating a less than ideal cylinder condition then when you introduce the mains fuel its gonna take a long time to get back up to peak efficiency before it can make peak power.
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Yes in theory were only talking hundreds of power strokes per race. But not everyone has an Prostock budget to R&D the best possible combination.

It's only a guess, your educated guess that a smaller carb will at least fix and or be an all around improvement and would that improvement be worth spending hundreds of $$ to the OP? Maybe we should start with what he's got and go from there.
 
After watching a 1hr video with Campbell & some clown, I wouldn't believe a word Campbell says. Full of mistakes & errors.
 
DrC.
I saw on the other site you were using a 7.5 p/valve. Bit close for comfort if you have ~8" of vac at idle.
 
Bewy said:
DrC.
I saw on the other site you were using a 7.5 p/valve. Bit close for comfort if you have ~8" of vac at idle.
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The PV does nothing at idle since the mains aren't flowing! I've run as high as a 10.5 and as expected, it has no effect on my idle AFR at all. But it was too rich at light throttle because it opened too soon.

What makes it even harder to tune is the 8" at idle, but 15" at cruise. Most street engines with small cams show high vacuum at idle which goes down at cruise. It'll never be perfect, but Pro Stock-level concerns to the contrary, I think I've made it work reasonably well on the street.
 
DrCharles said:
I am not sure a fixed orifice is the best thing, because it may be too big at idle but still too small at wide-open...

I found my carb log with the most recent settings: (pri/sec)

IFR .028/.028
IAB .078/.079
TSR .074/.074
MAB (HSB) .026/.026
MJ #71/#86
PVCR .066
PV 7.5"

Tomorrow is supposed to be a nice day, hoping to get the Dart out and get a few AFR readings at low-speed, high-speed cruise, and a very brief WOT on all four barrels :eek:

Since the head swap, it's a little too lean at the start of the transfer slot (that small flat spot JUST off idle is now worse and leaner), but highway cruise at 60-65 mph is good and lean (15-15.5:1) without surging. I may need to open up my TSRs a little bit.

Depending on what WOT looks like, maybe go up two sizes on the secondary jets?

Hey @Mattax are you around? I've also posted on racingfuelsystems Holley forum :)
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Not really around.
One change at time.
Since there is a flat spot, I'd start by either reducing the PIABs or opening the primary IFRs.

I don't see any reason to change the secondary jetting at this point. Hopefully the new heads are not pulling less air under full load, high rpm.
If you are worried they are, then jet up all four barrels equally. Take it to the dyno or the track as brief bursts are effected by dynamic responses. Plus the AFR numbers aren't meaningful unless there is a way to correlate them to changes in performance.
 
DrC,
Fully aware that the PV has no affect on idle. You clarified my point in post #47 that it could be too rich on cruise &/or comes in too early.
 
Do you have square T-slot on both the pri & sec? If so, I would think the engine needed a lot of bypass air with that cam. If so, how is that supplied?
 
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