(2)450 DP's vs (2)750 DP's

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Newbomb Turk said:
It would be more interesting to fix the big carbs and put them back on.
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He made this statement earlier:

MomsDuster said:
My 750’s have annular boosters. Just to clarify, I wasn’t having any trouble with my 750’s. I’ve gotten them tuned pretty darn good IMO. In fact I’ve been holding off on putting the 450’s on because the 750’s do work very well. I’m playing with the 450’s because I have them and I enjoy the process.
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Obviously the 450's are doing something the 750' aren't to improve performance so you need to figure out what that is.
 
Hysteric said:
He made this statement earlier:


Obviously the 450's are doing something the 750' aren't to improve performance so you need to figure out what that is.
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I already know what it is. Read what I wrote and think it through.

Fix what he had. Choking an engine is a bad way to make it run.

I know at least 5 guys (not me) who can make the 750's do everything the wrong carbs do and male more power doing it.

No offense to the OP but we don't even know if the timing curve is correct. That right there is why small carbs feel better than big carbs that aren't set up correctly.

And that isn't considering the tune on the carbs.

I'm betting when done the way I know other people do it, that the OP would probably end up with only one emulsion bleed.

That should be a hint right there.
 
I believe he has a quicker throttle response on the hit with the 450s. The 750s he had no issues with. Mark is a tuner for fun so this was an experiment as he owned the carbs already
 
Newbomb Turk said:
Choking an engine is a bad way to make it run.
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You think 900 cfm is choking a mild 440?

There's nothing to fix as the engine isn't built around the carbs the fueling needs to suit the combination that's under it. If the fueling is better with the 450 because it needs some vacuum then so be it. That combination isn't really a miss match of parts nor is it an all out racer.
 
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Hysteric said:
You think 900 cfm is choking a mild 440?

There's nothing to fix as the engine isn't built around the carbs the fueling needs to suit the combination that's under it. If the fueling is better with the 450 because it needs some vacuum then so be it. That combination isn't really a miss match of parts nor is it an all out racer.
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Yes. It is choking it.

You should be asking yourself why the “big” carbs didn’t do what they should.

You know the answer. And the tunnel ram and pump gas is the only clues I’m giving.

That and he ain’t in Australia either.
 
71GSSDemon said:
I believe he has a quicker throttle response on the hit with the 450s. The 750s he had no issues with. Mark is a tuner for fun so this was an experiment as he owned the carbs already
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I know all that. I’ve already posted why the big carbs don’t do what he wants.

Use the search feature because if you run pump gas you can learn why most fail at it.

Tunnel rams make it far worse.
 
Newbomb Turk said:
I know all that. I’ve already posted why the big carbs don’t do what he wants.

Use the search feature because if you run pump gas you can learn why most fail at it.

Tunnel rams make it far worse.
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Thanks. I am happy with my pump gas tunnel ram street 440 with a pair of ancient Holley 660s and 1:1 linkage.
 
Newbomb Turk said:
You should be asking yourself why the “big” carbs didn’t do what they should.
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Bruce had to resize his carbs smaller on his race engine when he went to a larger camshaft to get back the performance he lost. Something to think about from some who knew more than anyone else about this stuff.

"If we look at the influence of intake valve closing point we find that at some point when the piston is coming up the compression stroke, the pressure in the cylinder will be the same as the intake runner. If we shut the valve after that point in time we have shut it too late. Once the pressure in the cylinder exceeds the intake runner the air will go back out of the cylinder. If that happens we can do something about it without tearing the engine apart, we can install a SMALLER carby. What that does is it restricts the fill of the cylinder so that the incorrect closing point of the valve is not incorrect any more. Essentially a smaller carby makes the cam appear smaller.THATS one of the factors why sometimes engines make more power with smaller carbs
Our own drag car has had various cams ranging from 269 to 289 intake duration in it, the longer the intake duration the smaller the carbys had to be to get max power out of it. And with all the different cams its always made exactly the same power because we never changed the head flows. BUT i had to make the carbys smaller to get the power back with each successively larger cam. Every time we went larger in the cam it lost power on the old carby size"
 
Hysteric said:
Our own drag car has had various cams ranging from 269 to 289 intake duration in it, the longer the intake duration the smaller the carbys had to be to get max power out of it. And with all the different cams its always made exactly the same power because we never changed the head flows. BUT i had to make the carbys smaller to get the power back with each successively larger cam. Every time we went larger in the cam it lost power on the old carby size"
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IF in the end of the day each combo made the same power, what's the value of over camming and have to run a smaller carb ? Which sounds like it was a Band-Aid solution, and smaller by how much of a difference 50/100/200 or more cfm's ?
 
We're talking 2.46 HP per cube. Why would you need to make the carbies smaller to gain the power back?
 
Hysteric said:
We're talking 2.46 HP per cube. Why would you need to make the carbies smaller to gain the power back?
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Why would you need to run larger cams just to make the same power ?
 
Its what the engine "needs" fuel wise to make the most power with the camshaft it has and it isn't always with a Bigger Carb.
 
Hysteric said:
Its what the engine "needs" fuel wise to make the most power with the camshaft it has and it isn't always with a Bigger Carb.
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Yes I've noticed your crusade of the small, I was just curious if the larger cam and smaller carb had any advantages over the smaller cam and larger carb since he stated power was the same for both. Guess we'll never know.
 
Pretty much, It's the only thing I ever see you post about.
 
Hysteric said:
Bruce had to resize his carbs smaller on his race engine when he went to a larger camshaft to get back the performance he lost. Something to think about from some who knew more than anyone else about this stuff.

"If we look at the influence of intake valve closing point we find that at some point when the piston is coming up the compression stroke, the pressure in the cylinder will be the same as the intake runner. If we shut the valve after that point in time we have shut it too late. Once the pressure in the cylinder exceeds the intake runner the air will go back out of the cylinder. If that happens we can do something about it without tearing the engine apart, we can install a SMALLER carby. What that does is it restricts the fill of the cylinder so that the incorrect closing point of the valve is not incorrect any more. Essentially a smaller carby makes the cam appear smaller.THATS one of the factors why sometimes engines make more power with smaller carbs
Our own drag car has had various cams ranging from 269 to 289 intake duration in it, the longer the intake duration the smaller the carbys had to be to get max power out of it. And with all the different cams its always made exactly the same power because we never changed the head flows. BUT i had to make the carbys smaller to get the power back with each successively larger cam. Every time we went larger in the cam it lost power on the old carby size"
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I’m not disagreeing.

I’m saying that’s not the only way to do it.
 
I want to thank everyone for all of the input. I appreciate the civil discussion and try to absorb all the pertinent information.

I do enjoy the tuning/learning process. I’m a firm believer in even negative changes in the tune are a positive in the learning process. Keep up the dialogue, and I’ll keep playing with the tunes and I’ll learn and get this thing running as best I can.

And by no means should anyone think that I’m done with the 750’s! The 450’s are on there for the time being and at some point I’ll put 750’s back on and use what I’ve learned and make the 750’s better. At the end of the day I’m enjoying the game win or loose.
 
Ok, for those interested. Took her out for a “data” session and here are the results. (See attached photo’s)

Would like to lean it out just a touch everywhere. May try to lower the floats 1 flat and see what happens. Also need to get smaller jets for the secondaries, it’s got 66’s in there now and the next smaller size I have is 60’s. Thinking that’s probably a bit too small. New PB’s on all Dragy data points. Not super dramatic but improvements are always good! As always, all comments are welcomed.

IMG_9573.jpeg


IMG_9572.jpeg
 
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MomsDuster said:
Ok, for those interested. Took her out for a “data” session and here are the results. (See attached photo’s)

Would like to lean it out just a touch everywhere. May try to lower the floats 1 flat and see what happens. Also need to get smaller jets for the secondaries, it’s got 66’s in there now and the next smaller size I have is 60’s. Thinking that’s probably a bit too small. New PB’s on all Dragy data points. Not super dramatic but improvements are always good! As always, all comments are welcomed.

View attachment 1716279249

View attachment 1716279250
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Awesome Mark. Now, with the motor sucking thru a smaller straw, figure it needs to be leaner for the amount of air it is moving?
 
Hey Eric, @71GSSDemon my thinking is that considering the AFR seems to be fairly consistent throughout. If I drop the floats it should lean out as a whole. With carbs mounted sideways and the stance of the car with the straight axle the carbs are at a 4.5* slant. It’s tough to find that happy medium with the level of the floats. One side of the carb is higher than the other. If that makes any sense.
 
One flat may do the trick. Middle of the sight glass has always been best for me. It looks like the PJ comes in at 3400 by your readings.
 
mderoy340 said:
One flat may do the trick. Middle of the sight glass has always been best for me. It looks like the PJ comes in at 3400 by your readings.
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Hmm….I was thinking that at 3400 the secondary was might be starting to open. The stance of the car give this thing the aerodynamics of a brick at those speeds. I could be wrong. I’ll have to look at that next time out.
 
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