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Too much carburator?????


MYRON

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I have been playing around with the

carmath website and I am finding some things interesting. It has a formula for calculating

the optimum cfm carb. Based on the information given on my car (355c.i.) running at near perfect conditions I should be using a little over 600cfm. May carb is actually 830cfm(750 body modified with 850 base plate)... My buddy that has a 383 with 602hp (very trick motor) is using the same carb... the carmath claims 820cfm @ 100% efficient with his 383c.i......

 

Am I way over carbed here?

 

Myron

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Myron, do you have access to a smaller carb?

You have the Gtech. Give a 650 a spin! If you haven't run a smaller cfm on it yet, you'll be amazed at the off idle response. Smaller bores have higher velocity. And you thought you had traction problems now! Still, knowing you have one healthy motor, I'd bet 600 won't be enough. Your 355 might have a big appetite and need way more. Can't remember, is your carb a vacuum or mechan. secondary? Vacuum secondaries work on a wider range of applications.

JS

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John, My carb is a mechanical unit.

The only time I ran a 620cfm carb was when I had the 700r4 and the original dry rotted

goodyear 70's. It spun like mad! but given the very low first gear and no traction,

I just figured I would be much better off with a big nasty carb. The other problem is the Holley 620 cfm carb that I have requires a base plate adapter that makes it stick up another inch.. I am trying to put a stock hood back on the car. If we miss out on the rain this weekend I will defanately try it though....

 

Myron

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Yeah, bigger isn't always better, at least in carbs. I found that a moderately built 400 ran twice as strong with a 650 than my 780 or 850. I really wanted to have that big honkin 850 in there though. My 400 probably had 2/3 the hp you're pushing, so its hard to say. I'd like to hear how the experiment goes.

JS

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Guest Anonymous

For those who havn't seen them, the carmath calculators can be found at:

http://www.prestage.com/

 

Pretty neat calculators.

 

Should be interesting to see if the real world test agree's with the calculator. As I recall some fairly large HP numbers are/were achieved on some form of dirt track cars using a 2 bbl carb of about 500 cfm on a SBC.

smile.gif

 

Lone

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I've been conversing with Earl Parker from Parker Carburetion, a guy who has lots of carburetor experience from saturday night special, to Nascar. Has been very willing to help me resolve my carb. problem (not yet). He claims its acceleration that wins races, meaning your carb must be producing power throughout the rpm, not just cabable of a top mph. His way of figuring an optimum carb is to determine the absolute lowest rpm that you need it to perform and engine displacement. Obviously a track racer and drags will have different applications. I've read the circle tracks guys like a carb thats right on the nut for cfm or slightly smaller, and drag racers just the opposite.

JS

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Voo Doo mathematic...

 

That what it all comes down to!!!

I know I'm sort of entering this discussion a little late, but I'm hoping I can still shed a little light (and no shadows wink.gif ) on this subject.

 

I've debated carb size over and over again with myself and others for years. One thing I've learned... it's all voo-doo witchcraft!!!

 

Seriously though, one thing that I recently taught, is to make sure the carb is "listening" to the engine demand. I know this sounds stupid, but it made sense once I read up on it. The basic concept with a carb "listening" to the engine relates to engine speed and engine efficiency.

 

As we all know, an equal amount of air passing through two different diameter tubes, will travel with a higher velocity through the smaller of the two tube... up to a certain amount. With a higher velocity venturi, a stronger "signal" is being pulled through the carb boosters. A strong signal equates to excellent drivability (assuming the jets are correct) at that specific RPM where the signal is strong.

 

Most people use the typical formula of displacement vs. RPM to calculate carb CFM (see Lone's post above). I've always stuck pretty close to these calculations, as they make great base lines for street motors. However, this equation neglects air speed and density.

 

...enter the voo-doo magic.

 

Basically, when the air flow starts to increase above a certain point, the linear correlation between RPM and CFM starts to go astray. I've still yet to find a good representation of where this "magical" point occurs. Most of it is due to the V.E. of the engine.

 

This next part is what blew me away when I first heard of it. When playing around with large CFM carbs on high revving motors, "fore sight" is needed when calculating CFM. For example, when the motor is at 6500, the carb should really be supplying it with 6800 - 7000 RPM worth of air. The RPM increase is guess-timated by the V.E. of the engine. The higher the V.E. the further forward the carb needs to be listening. Sounds messed up, doesn't it? Took me a while to believe it as well.

 

As for real word examples,

I ran three different carbs on my 355 small block. Estimated output is around 350hp. At the track, I recorded the following results:

 

Test 1:

Holley 600 CFM vacuum secondaries

Shifting at 6100 RPM

1.86 sixty foot

12.61 @ 107.7 mph

 

Test 2:

Holley 670 CFM mechanical secondaries (modified 650)

Shifting at 6300 RPM

1.87 sixty foot

12.53 @ 110.2 mph

 

Test 3:

Holley 780 CFM vacuum secondaries (modified 750)

Shifting at 6300 RPM

1.94 sixty foot

12.77 @ 111.1 mph

 

As the results show, I lost a lot of off-idle response with the larger 780 while the upper end seemed to fall off with the 600. All three tests were run back to back to back on an 80 degree summer afternoon (N20 solenoids seized, I had nothing else to do except play with carbs the rest of the day).

 

Well, I hope I haven't bored everyone to death. I've got a great article laying around somewhere that described the "listening" process in much better detail. I'll try to dig it up tonight.

 

-Andy

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Unfortunately I didnt get the time to swap carbs this weekend. I spent all morning Saturday on a tractor in the freezing gold rain trying to get the floats to the convention center. I was talking with a buddy of mine after we got soaked saturday

and we were talking about the carmath formula. One thing that was mentioned was that alot of hotrodders say that you can get buy with a larger carb on a lighter/more areodynamic vehicle.. I makes sense to me because your engine can burn more fuel/air at a faster rate.. I wonder if their is a varible that can be added into the formula

for less weight/frontal area.. to get a more

accurate idea of carb size..

One thing I do remember is the off idle throttle response of the smaller carb was better than it is now. I never got the chance to try running the car on the big end

with the 620 cfm carb... I will swap them this weekend just for grins though...

 

For those of you in the south central U.S.

look for me on the TV driving a tractor

that is pulling a float in the Thanksgiving

Parade in downtown Houston..

 

Myron

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Guest Rick Johnson

quote:

Originally posted by Andrew Bayley:

Voo Doo mathematic...

 

That what it all comes down to!!!

I know I'm sort of entering this discussion a little late, but I'm hoping I can still shed a little light (and no shadows
wink.gif
) on this subject.

 

I've debated carb size over and over again with myself and others for years. One thing I've learned... it's all voo-doo witchcraft!!!

 

Seriously though, one thing that I recently taught, is to make sure the carb is "listening" to the engine demand. I know this sounds stupid, but it made sense once I read up on it. The basic concept with a carb "listening" to the engine relates to engine speed and engine efficiency.

 

As we all know, an equal amount of air passing through two different diameter tubes, will travel with a higher velocity through the smaller of the two tube... up to a certain amount. With a higher velocity venturi, a stronger "signal" is being pulled through the carb boosters. A strong signal equates to excellent drivability (assuming the jets are correct) at that specific RPM where the signal is strong.

 

Most people use the typical formula of displacement vs. RPM to calculate carb CFM (see Lone's post above). I've always stuck pretty close to these calculations, as they make great base lines for street motors. However, this equation neglects air speed and density.

 

...enter the voo-doo magic.

 

Basically, when the air flow starts to increase above a certain point, the linear correlation between RPM and CFM starts to go astray. I've still yet to find a good representation of where this "magical" point occurs. Most of it is due to the V.E. of the engine.

 

This next part is what blew me away when I first heard of it. When playing around with large CFM carbs on high revving motors, "fore sight" is needed when calculating CFM. For example, when the motor is at 6500, the carb should really be supplying it with 6800 - 7000 RPM worth of air. The RPM increase is guess-timated by the V.E. of the engine. The higher the V.E. the further forward the carb needs to be listening. Sounds messed up, doesn't it? Took me a while to believe it as well.

 

As for real word examples,

I ran three different carbs on my 355 small block. Estimated output is around 350hp. At the track, I recorded the following results:

 

Test 1:

Holley 600 CFM vacuum secondaries

Shifting at 6100 RPM

1.86 sixty foot

12.61 @ 107.7 mph

 

Test 2:

Holley 670 CFM mechanical secondaries (modified 650)

Shifting at 6300 RPM

1.87 sixty foot

12.53 @ 110.2 mph

 

Test 3:

Holley 780 CFM vacuum secondaries (modified 750)

Shifting at 6300 RPM

1.94 sixty foot

12.77 @ 111.1 mph

 

As the results show, I lost a lot of off-idle response with the larger 780 while the upper end seemed to fall off with the 600. All three tests were run back to back to back on an 80 degree summer afternoon (N20 solenoids seized, I had nothing else to do except play with carbs the rest of the day).

 

Well, I hope I haven't bored everyone to death. I've got a great article laying around somewhere that described the "listening" process in much better detail. I'll try to dig it up tonight.

 

-Andy

Thanks for the excellent real world example of carb sizing. The math is great for establishing some ballpark numbers but I will take real data any day. I wish we had more real data for many of the things we discuss.

For your next topic how bout cylinder head port sizing for different size small blocks?? Just kidding.

 

Rick

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On head port size versus dyno performance, Chevy High Performance Mag. this month has an article on 5 different Dart heads with different port volumes to see what the results are. I just got mine yesterday and haven't read it yet.

 

------------------

Pete Paraska - 73 540Z - Marathon Z Project - pparaska@home.com">pparaska@home.com -

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I will post as much info on the test this weekend as I can.

 

I will be driving the "beauty & the beast"

float. Look for me!!

 

BTW: I have the dart heads with the 2.05/1.60 valves 64cc chambers and 215 cc

runners. I got that article in CHP and read just a small portion of it....

 

Myron

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Well, Now I am having ignition and fuel problems of another type.

My daily driver (97 TransAM Lt1) is giving me fits. Does anyone know anything about these motors?. Here is what it did.

2 weeks ago after sitting for a couple days

It misfired bad causing the starter not to be able to turn it over. Kinda like way advanced timing and the engine popping back agains the starter. I let it sit a minute

then tried again. It started and ran fine

all week to and from Dallas.

 

This weekend my wife went to drive it and she said when she tried to start it that

it made a loud "pop" and then sounded funny

when she cranked it and it woudlnt start.

I get in it and it cranked for along time before sputtering to life. It resumed

idling and would rev up plenty strong but

I could hear a rattle from the bottom side of the motor. I eased it up to my dads shop

and removed the tourqe converter cover to find that the backfire popped back so hard on the starter the the whole head/bearing

portion of the starter was broke off and rattling around in there. I replaced the starter and when I tried to drive it home

It is performing something like a sick 6 cylinder car. It shudderes and runs real slow.....

 

Any ideas?

Myron

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With all the computerized ignition and fuel on those things you'd think thats where to look. Backfiring at start has to be ignition? Are these engines crank fire or do they still have distributors? If it were an older vehicle, I 'd guess at the timing gear/chain. Starts running poorly then progresses into a barely running engine. Old test used to be to put a rag loosely over the intake, with the coil wire disconnected, and turn over the engine. If air blew past the rag from the engine you could bet the timing had jumped. Common with the old nylon gears, hard to buy with the newer engines. Good luck! Hope you figure it out sooner than my last fiasco. Its the $#%@# when its yur daily driver!

JS

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Hrm, can you say OptiSpark? I'm not real clear on how to troubleshoot those things but people bitch up a storm about the LT1 ignition systems. Apparently some parets ARE optical and can get screwed up with dirt - yick! frown.gif Head over to one of their discussion forums and see what's shaking - I'll bet someone can help you out. You did say it was raining like mad right? We had water get into the coils on the Impalla and it would refuse to start. Tinkering with it I finally found a bad coil wire that finally arced itself in two pieces - literally. Hope your problem is just a plug not firing man....

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