Is this too much Carb?
- Thread starter Billr57
- Start date
It will be perfect. Copy the Mustang Geezer's arrangement.
In my opinion, carb jetting is related to peak power, and some guys on this forum with engines rated around the 150 net hp mark have gotten away with main jets as small as 68 on there sixes.
With a header and just the carb, you'll gain a 25% improvement in peak hp, with a small loss in fuel economy.
Just watch the mounting with these tall 250 engines. There is little space between the hood and the top of a Holley. In addition, cornering round sharp left handers can uncover the power valve, causing starvation if you really hammer it.
More info for egg heads:-
If you look at some published charts, the peak total cc per minute needed for any given power with this 500 cfm #2300 carb is 5.5 times the target hp.
So a 150 hp engine will need 825 cc/min from two jets
125 hp engine will need 687 cc/min from two jets.
From page 125 of Barry Grants 1997 carby catalogue, each jet from 60 to 100 was given a cc/minute rating. Hence the 825 cc/minute , 150 hp car with a 500 cfm 2-bbl needs two jets of 412 cc/min, or a good old 68 jet.
The 687 cc/minute , 125 hp car with a 500 cfm 2-bbl needs two jets of 344 cc/min, or a good old 64 jet.
Each Holley carb has slightly different flow numbers. On a 350 cfm Holley, the jetting is usally a factor of 1.41 less at about 3.9 cc/min per target horsepower.
Most 4150 Holley4-bbls run at the 5.5 rate, and if it is seeing lots of race duty, you'll want to richen it up to 6.6 cc/min per target hp.
In my opinion, carb jetting is related to peak power, and some guys on this forum with engines rated around the 150 net hp mark have gotten away with main jets as small as 68 on there sixes.
With a header and just the carb, you'll gain a 25% improvement in peak hp, with a small loss in fuel economy.
Just watch the mounting with these tall 250 engines. There is little space between the hood and the top of a Holley. In addition, cornering round sharp left handers can uncover the power valve, causing starvation if you really hammer it.
More info for egg heads:-
If you look at some published charts, the peak total cc per minute needed for any given power with this 500 cfm #2300 carb is 5.5 times the target hp.
So a 150 hp engine will need 825 cc/min from two jets
125 hp engine will need 687 cc/min from two jets.
From page 125 of Barry Grants 1997 carby catalogue, each jet from 60 to 100 was given a cc/minute rating. Hence the 825 cc/minute , 150 hp car with a 500 cfm 2-bbl needs two jets of 412 cc/min, or a good old 68 jet.
The 687 cc/minute , 125 hp car with a 500 cfm 2-bbl needs two jets of 344 cc/min, or a good old 64 jet.
Each Holley carb has slightly different flow numbers. On a 350 cfm Holley, the jetting is usally a factor of 1.41 less at about 3.9 cc/min per target horsepower.
Most 4150 Holley4-bbls run at the 5.5 rate, and if it is seeing lots of race duty, you'll want to richen it up to 6.6 cc/min per target hp.
)
Even though I'm in Nu Zealand, and half a world away, I'll state my reputation on the fact that with two sixty jets, and a totally stock engine, you'll get by with a 500 cfm Holley.
The little 2 liter engines run up to 6500 rpm with a 500 cfm 2-bbl carb at 3.0 " Hg. It may produce 138 hp.The actual airflow demand on one of these little four bangers is 122*6500 all divided by 3456, or 229.5 cfm. At a 85% volumetric effciency, it would shift 195 cfm of air. The peak air speed through a couple of venturis totalling about 0.020624 square feet is 157.5 feet per second, fairly low. At a highway cruise, the car runs at about 3250 rpm, or needing only 115 cfm, or 79 ft/sec of air flow. Peak manifold vaccum at wide open throttle is about 0.46" Hg
A big 250 six, stock as a rock, can pull up to 4500 rpm with a good carb. It may only kick out 110 or so pnies, even with a 500 cfm carb. The 250 has an airflow demand of about ((250*4500)/3456)*0.75, or 244 cfm, since it has only about 75 % volumetric efficiency. This is a gas speed of 197 feet per second at these revs. At its normal 2250 rpm, it flows 122 cfm, or 99 feet per second of air, if volumetric efficiency was the same throughout the rev range. Peak manifold vaccum at wide open throttle is about 0.73" Hg
I know you can say anything you like with numbers, but a 500 cfm carb, jetted properly, will work just fine on a 250. The big 250 can suck a rat up an 8 ft down pipe backwards!
The little 2 liter engines run up to 6500 rpm with a 500 cfm 2-bbl carb at 3.0 " Hg. It may produce 138 hp.The actual airflow demand on one of these little four bangers is 122*6500 all divided by 3456, or 229.5 cfm. At a 85% volumetric effciency, it would shift 195 cfm of air. The peak air speed through a couple of venturis totalling about 0.020624 square feet is 157.5 feet per second, fairly low. At a highway cruise, the car runs at about 3250 rpm, or needing only 115 cfm, or 79 ft/sec of air flow. Peak manifold vaccum at wide open throttle is about 0.46" Hg
A big 250 six, stock as a rock, can pull up to 4500 rpm with a good carb. It may only kick out 110 or so pnies, even with a 500 cfm carb. The 250 has an airflow demand of about ((250*4500)/3456)*0.75, or 244 cfm, since it has only about 75 % volumetric efficiency. This is a gas speed of 197 feet per second at these revs. At its normal 2250 rpm, it flows 122 cfm, or 99 feet per second of air, if volumetric efficiency was the same throughout the rev range. Peak manifold vaccum at wide open throttle is about 0.73" Hg
I know you can say anything you like with numbers, but a 500 cfm carb, jetted properly, will work just fine on a 250. The big 250 can suck a rat up an 8 ft down pipe backwards!
Ignore the CFM numbers. They're all apples and oranges. If a 600cfm 4bbl is too big, you'd think that surely a 500 2bbl must be WAY too big. But a 500 2bbl is really 1/2 of a 750 and at the same pressure drop flows about half as much. If you applied the same pressure drop and adjusted for flow by using Holley's conversion factor of 1.1414, the 500 Holley is rated at about 370cfm.
With some tuning to get the right power valve, jets, and accelerator pump there's no reason to think a 500 won't work well on a 3.3 liter engine. It will not, however, work right out of the box. Holley tends to jet them too rich, with power valves that open too soon.
BTW, we beat this to death about two years ago:
http://pub41.ezboard.com/ffordsixcylind ... D=93.topic
With some tuning to get the right power valve, jets, and accelerator pump there's no reason to think a 500 won't work well on a 3.3 liter engine. It will not, however, work right out of the box. Holley tends to jet them too rich, with power valves that open too soon.
BTW, we beat this to death about two years ago:
http://pub41.ezboard.com/ffordsixcylind ... D=93.topic
A
Anonymous
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I have run both the 350 and 500 CFM Holleys on the 250 engines either will work but the 350 would be the better choice for a stock to medium set up engine. The 500 is best on hoter setups. Probably the best rating would be a 250 CFM for a stock to mild engine. I once blocked the one side of a 500 cfm Holley to get 250 cfm I needed.
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