Inline6Merc
Well-known member
Why do most of you guys put the 500cfm on your 6's? WHy not a 350cfm carb? Just wonderin.
Inline6Merc
Curtis
Inline6Merc
Curtis
Holley 2300 500cfm or 350cfm?
- Thread starter Inline6Merc
- Start date
A
Anonymous
Guest
because of the extra volume of air u suck in for more power...if the head is worked for improved flow and all that then 350 isnt enough...i found an equation that i forget now...but u take the size of the engine and multiply by max RPM and divide by some number and u get the amount of cfm that your carb should have...and the number i came up with for a 250 with a 6000RPM peak was about 430 CFM...so yea...pretty much a power thing. hope that made sense cause i'm realllly tired.
The formula is:
(CID*maxRPM)/3456 = CFM needed @ 100% VE
With a 250 at 6000 RPM you'll need 434 CFM
With a 200 at 6000 RPM you'll need 347 CFM
Now you have to bump down those numbers by about 20% because I do not think any amount of head work will get a Log head above 80% VE. So now that 250 at 6000RPM only needs 347 and the 200 only needs about 280 CFM.
I think anything more then 350 on a log head is too much and a waste unless you do multicarb and that's only because you end up with a higher VE with set ups like the Offy or MustangGeezers.
Now with the Aussie...another story all together. 500CFM seems to be working well for many people. While personally I think for a NA 200, 500 CFM may be a bit too much, I think 350 is a bit too little with the AUssie head. The 500 gives you some breathing room on the Aussie. I'm running a 390 and I use every bit of it on the 200, but I also have enlarged valves and higher lift cam.
So...to my original arguement, with a turd head...a 350 will be more then enough.
(CID*maxRPM)/3456 = CFM needed @ 100% VE
With a 250 at 6000 RPM you'll need 434 CFM
With a 200 at 6000 RPM you'll need 347 CFM
Now you have to bump down those numbers by about 20% because I do not think any amount of head work will get a Log head above 80% VE. So now that 250 at 6000RPM only needs 347 and the 200 only needs about 280 CFM.
I think anything more then 350 on a log head is too much and a waste unless you do multicarb and that's only because you end up with a higher VE with set ups like the Offy or MustangGeezers.
Now with the Aussie...another story all together. 500CFM seems to be working well for many people. While personally I think for a NA 200, 500 CFM may be a bit too much, I think 350 is a bit too little with the AUssie head. The 500 gives you some breathing room on the Aussie. I'm running a 390 and I use every bit of it on the 200, but I also have enlarged valves and higher lift cam.
So...to my original arguement, with a turd head...a 350 will be more then enough.
Howdy Curtis and all:
THe Dolly Parton factor is another issue. More is Better! It's the American way!!!
The new and rebuilt 350 & 500 are about identical in price. Holley 2300s are very tuneable. With other carbs it is sometimes very difficult to tune out a low speed bog or a high speed hesitation. Not so with the 2300s. Jets, Accelerator pump cams, squirter nozzles, power valves, all easily available, make for a very adaptable, forgiving carb.
Slade makes some good points. A well built, well tuned, correctly geared 200 with a modified log can use a 500 quite well. Put the same carb on a relatively stock 200, through a stock log head, with an adapter and it will be a miserable beast.
The formula detailed by Slade is a good generality for most engines. It is probably on the optimistic side for a stock, log headed 200, and probably conservative for a well built 200 with an Aussie 2V headed engine.
Also note that VE is achieved at a very specific place in the RPM curve, typically near peak torque. VE is considerably lower at an idle and at transitions. An engine with a relatively flat torque curve is more effective for a street engine because it stays nearer to the peak VE longer.
Adios, David
THe Dolly Parton factor is another issue. More is Better! It's the American way!!!
The new and rebuilt 350 & 500 are about identical in price. Holley 2300s are very tuneable. With other carbs it is sometimes very difficult to tune out a low speed bog or a high speed hesitation. Not so with the 2300s. Jets, Accelerator pump cams, squirter nozzles, power valves, all easily available, make for a very adaptable, forgiving carb.
Slade makes some good points. A well built, well tuned, correctly geared 200 with a modified log can use a 500 quite well. Put the same carb on a relatively stock 200, through a stock log head, with an adapter and it will be a miserable beast.
The formula detailed by Slade is a good generality for most engines. It is probably on the optimistic side for a stock, log headed 200, and probably conservative for a well built 200 with an Aussie 2V headed engine.
Also note that VE is achieved at a very specific place in the RPM curve, typically near peak torque. VE is considerably lower at an idle and at transitions. An engine with a relatively flat torque curve is more effective for a street engine because it stays nearer to the peak VE longer.
Adios, David
A
Anonymous
Guest
Just don't forget guys, 2bbl carbs are usually rated at twice the flow pressure than 4bbls are (3" of Hg vs 1.5" of Hg). Therefore, a 350 Holley is really about 250 cfms. A 500 Holley is about 370 cfms.
VE is the critical factor when you use that formula. 80% VE on a log head? Not in this world. You really need to drop it down to about 70% (or less...). A stock Aussie head would even be under 80% VE in my book. Like David said, VE will peak at peak torque (somewhere around 4,000 rpms). It falls off significantly by redline.
As for rpms... I'd be surprised if anyone here drives a log head over 5,000 rpms routinely. So....
A 200 @ 5,000 rpms with a 70% (still being very generous) VE only needs about 200cfms of carburetion. That's a lot more than the factory 1bbl, so you'll see improvement with a bigger carb BUT ONLY if you can utilize it by not using a funnel type adapter.
VE is the critical factor when you use that formula. 80% VE on a log head? Not in this world. You really need to drop it down to about 70% (or less...). A stock Aussie head would even be under 80% VE in my book. Like David said, VE will peak at peak torque (somewhere around 4,000 rpms). It falls off significantly by redline.
As for rpms... I'd be surprised if anyone here drives a log head over 5,000 rpms routinely. So....
A 200 @ 5,000 rpms with a 70% (still being very generous) VE only needs about 200cfms of carburetion. That's a lot more than the factory 1bbl, so you'll see improvement with a bigger carb BUT ONLY if you can utilize it by not using a funnel type adapter.
A
Anonymous
Guest
see a big improvement in what?
performance or gas milage?
performance or gas milage?
A
Anonymous
Guest
yeah but thats a 4v....shouldnt I get btter gas milage on the 2bbl? 32/36 setup?
and performance since it is higher cfm if I jump on it
I can do about 100mph now....but it feels like it is straining due to lack of air...and a vibration in my rear end
and performance since it is higher cfm if I jump on it
I can do about 100mph now....but it feels like it is straining due to lack of air...and a vibration in my rear end
A
Anonymous
Guest
Gas mileage...i modified my stock 1bbl...the venturi is bored out (cant say the size but i took about 3-4 mm around out) the jet is twice the size and the juice is set to be shot quite a bit earlier than normal...gas mileage is as slade said..pretty much equal if driven well...i can easily go through twice as much gas as normal with a lead foot...but the power difference is worth it...1 have hit 120 MPH, very smooth...it actually feels smoother than 80mph (must be the spoiler 
)..wouldnt a 4bbl get better mileage than a 2bbl as long as u didnt open up all 4 barrels, i heard that before, is it true? i mean if u got a 4 bbl u will obviously use all 4 now and then 
)..wouldnt a 4bbl get better mileage than a 2bbl as long as u didnt open up all 4 barrels, i heard that before, is it true? i mean if u got a 4 bbl u will obviously use all 4 now and then 
With a holley 2300 series carb, they are flow rated at 3" hg to convert that to 1.5" hg. the cfm flow rating at 3.0 in/hg divided by 1.414 = flow at 1.5 in/hg. the 350 goes down to 247.52 cfm, the 500 goes to 353.60 cfm at 1.5 in/hg.
I would love to see a dyno test or strip times with an engine using a modified log on a late model head, better valves, camshaft with at least 264 degrees @ .050, 1.6 roller rockers then to a dual outlet header.
: :idea: william
I would love to see a dyno test or strip times with an engine using a modified log on a late model head, better valves, camshaft with at least 264 degrees @ .050, 1.6 roller rockers then to a dual outlet header.
: :idea: william
