Is a 650 cfm carb too big for a relatively stock 302? My neighbor offered his holley quick fuel carburetor that he isn't using. I was told to go with a 600.
Thanks
Thanks
wscott00
A 600 cfm carb is a big as I would go on a stock(ish) 289/302. It may not be optimum, but it will give room to grow if you have plans to add performance parts.
Well, Ford stuck a 600cfm carb from the factory on the '83-'85 GT which was the pinnacle of SMOG crackdown. So I'd say they felt it wasn't too big.
GM stuck 750 cfm QuadraBogs on 262 (4.3 liter) six cylinders....
Many people think that a carburetor's CFM rating is indicative of how much air and fuel its going to FORCE the engine to use. It's not. It's solely a rating of how much air, AT FULL THROTTLE, the carburetor is CAPABLE of passing through.
A lot depends on factors such as venturi design (downleg, straightleg, annular), shape (ability to generate a proper vacuum signal), how well the fuel is emulsified (air bleeds), etc.
Yes, "out of the box" (depending on your other engine specifics) one might find that a carburetor rated at a lower CFM might provide a better throttle response and fuel mileage AND a lot of people DO simply buy a "one size fits all" carburetor, bolt it on, adjust their idle mixture and that's it. However, even FORD didn't do that as noted by the 5 different choices of carburetor calibration for the 289, each with slightly different venturis to deliver the right fuel/air mix depending on engine spec. The same was true with distributors and, yet, many folks will buy a "one size fits all" aftermarket distributor and stab THAT in, as well.
From a technical/engineering standpoint, I never got that rationale. Same goes with headers, camshafts, etc. IMHO too much is left on the table because there is so much variation from one engine build to another. If you ask me, aftermarket vendors feed on this mentality to shill their products...
I run a pretty much "stock long block" '85 5.0HO. It has a 289HP dual-point (no vacuum advance) distributor, Tri-Y's (been on it since I got it) the stock cast iron intake and a O-4777 Holley (650 cfm dual-feed, double-pumper) that has no problem roasting the tires with a TopLoader and 3.00 rear, and returns 20-21 mpg on long highway runs at 70-75 mph. Throttle response is excellent... no bogs, no burps. Yeah, if I use the typical calculation to determine CFM requirements for my engine it come out to around 525 cfm. The O-4777 may be RATED at 650 but in the "real world" will flow a lot closer to 600, IF NEEDED.
So, do I think you'll be "okay" with using that QFT 650 on your 302? Probably. If you can find an annular booster kit for it all the better. You also have the ability to not only change jets but air bleeds, power valve and pv channel restriction, secondary opening rate, etc. to properly calibrate it to your car. If it has vacuum-operated secondaries, all the better for a near-stock application as secondary opening will depend on engine demand.
Many people think that a carburetor's CFM rating is indicative of how much air and fuel its going to FORCE the engine to use. It's not. It's solely a rating of how much air, AT FULL THROTTLE, the carburetor is CAPABLE of passing through.
A lot depends on factors such as venturi design (downleg, straightleg, annular), shape (ability to generate a proper vacuum signal), how well the fuel is emulsified (air bleeds), etc.
Yes, "out of the box" (depending on your other engine specifics) one might find that a carburetor rated at a lower CFM might provide a better throttle response and fuel mileage AND a lot of people DO simply buy a "one size fits all" carburetor, bolt it on, adjust their idle mixture and that's it. However, even FORD didn't do that as noted by the 5 different choices of carburetor calibration for the 289, each with slightly different venturis to deliver the right fuel/air mix depending on engine spec. The same was true with distributors and, yet, many folks will buy a "one size fits all" aftermarket distributor and stab THAT in, as well.
From a technical/engineering standpoint, I never got that rationale. Same goes with headers, camshafts, etc. IMHO too much is left on the table because there is so much variation from one engine build to another. If you ask me, aftermarket vendors feed on this mentality to shill their products...
I run a pretty much "stock long block" '85 5.0HO. It has a 289HP dual-point (no vacuum advance) distributor, Tri-Y's (been on it since I got it) the stock cast iron intake and a O-4777 Holley (650 cfm dual-feed, double-pumper) that has no problem roasting the tires with a TopLoader and 3.00 rear, and returns 20-21 mpg on long highway runs at 70-75 mph. Throttle response is excellent... no bogs, no burps. Yeah, if I use the typical calculation to determine CFM requirements for my engine it come out to around 525 cfm. The O-4777 may be RATED at 650 but in the "real world" will flow a lot closer to 600, IF NEEDED.
So, do I think you'll be "okay" with using that QFT 650 on your 302? Probably. If you can find an annular booster kit for it all the better. You also have the ability to not only change jets but air bleeds, power valve and pv channel restriction, secondary opening rate, etc. to properly calibrate it to your car. If it has vacuum-operated secondaries, all the better for a near-stock application as secondary opening will depend on engine demand.
Spot on. That VE calculation everyone uses to determine a 400cfm carb is the right size for a 302 leaves a lot of variables on the table. Richard Holdener did a series for Motortrend to replicate some of those classic engines from the 60's to see how much HP they really made. They ended up sticking a 750 on the Boss 302 engine (because they couldn't find a correct Holley 780) and it made 375hp in stock trim. Imagine if Ford stuck a 1.08 Autolite on a Boss 302 instead of the Holley 780.
Which specific QF carb is it? There's at least three different ones in every CFM rating, as I recall.
Cubic inches by themselves do not begin to tell the story when it comes to carburetor choice. Ford has made 302s as pitiful as 135hp in Mustang IIs in the '70s, and over 300hp on the other end of the spectrum. Two very different engines with two very different carburetion requirements. You need to make the right choice for your particular engine that very likely falls somewhere in between. Ford made many, many variants of each of its few basic carb designs they used over the years, and carbs would usually vary even between automatic and manual transmissions.. just to give you some idea of how specific carb setup is.
I recently put together a fairly hot aluminum headed 289 that runs quite well with a 670cfm Holley. But that same carb would be way too much on a stock '70s smog 302.
It also comes down to tuning. No carburetor will run perfectly out of the box. All aftermarket carbs (or OE carburetors that aren't stock to your application) require tuning to run their best. Tuning is not terribly complicated, but it is not guesswork, and is best done by somebody with experience, or someone who at least has mechanical aptitude and understands how to follow directions and pay attention. Your "mostly stock" 302 is almost certainly not the average base engine that they designed that carb for, so it will absolutely need tuning, or it will probably run on the rich side.
Most of the time, choosing a carb based on what someone has laying around or what their buddy has is not likely to get you the ideal carb for your application. Also keep in mind the linkage and other potential changes you will have to make to fit that carburetor on your engine. I typically recommend figuring out what the ideal carb is for your application first, then try to find that carb. That will always be the best long term solution.
Cubic inches by themselves do not begin to tell the story when it comes to carburetor choice. Ford has made 302s as pitiful as 135hp in Mustang IIs in the '70s, and over 300hp on the other end of the spectrum. Two very different engines with two very different carburetion requirements. You need to make the right choice for your particular engine that very likely falls somewhere in between. Ford made many, many variants of each of its few basic carb designs they used over the years, and carbs would usually vary even between automatic and manual transmissions.. just to give you some idea of how specific carb setup is.
I recently put together a fairly hot aluminum headed 289 that runs quite well with a 670cfm Holley. But that same carb would be way too much on a stock '70s smog 302.
It also comes down to tuning. No carburetor will run perfectly out of the box. All aftermarket carbs (or OE carburetors that aren't stock to your application) require tuning to run their best. Tuning is not terribly complicated, but it is not guesswork, and is best done by somebody with experience, or someone who at least has mechanical aptitude and understands how to follow directions and pay attention. Your "mostly stock" 302 is almost certainly not the average base engine that they designed that carb for, so it will absolutely need tuning, or it will probably run on the rich side.
Most of the time, choosing a carb based on what someone has laying around or what their buddy has is not likely to get you the ideal carb for your application. Also keep in mind the linkage and other potential changes you will have to make to fit that carburetor on your engine. I typically recommend figuring out what the ideal carb is for your application first, then try to find that carb. That will always be the best long term solution.
The CFM ratings on most carbs are generous. They are also not wet flow tested. Quick Fuels cfm ratings are probably a bit more accurate than in general though. It depends which Quick Fuel carb it is and it also depends if somebody has been monkeying around with the itsy bitsy brass gidgets. If it is a double pumper you probably will not want that and if it doesn't have a choke you probably do not want it twice. I ran my Q650 on my 289 but there weren't nothing stock about that engine.
I put a 500 cfm summit on my 67 289 last year. Only engine change is dual exhaust (and 2v to 4V). I'm very happy with mine.
Ford and chebby put 780 holleys on their boss 302 engine and z28 302 camaro engine. If that 650 has straight boosters you can have holley annular boosters put on a 4 holes and it will knock it down to about a 600 cfm flow. annular boosters are great for street and imo roadrace.
An engine will pull fuel from (with increasing load/speed):
The larger the carb relative to the engine, the more time is spent in situation 2, which is unfortunately usually not tunable, but can be on a more advanced carb (like a Quick Fuel) if it has exchangeable Idle Feed Restrictions and/or Idle Air Bleeds. Some main circuit designs also start to "kick in" earlier than others, e.g. due to the booster design. So, if you tune it well, a "too big" carb can work well and maybe even better than a "one size fits all" smaller one.
I would ask your neighbor if you could try if first before you decide to buy it .
- Idle circuit (fuel flow is a function of manifold vacuum and adjustable restriction sizes)
- Idle + transfer circuit (fuel flow is a function of manifold vacuum and (normally fixed) restriction sizes)
- Idle + transfer + main circuit (fuel flow is a function of air flow through the venturi and exchangeable jet sizes)
- Idle + transfer + main + enrichment circuit (fuel flow is a function of air flow through the venturi and flows through a fixed restriction, opened/closed by the power valve)
The larger the carb relative to the engine, the more time is spent in situation 2, which is unfortunately usually not tunable, but can be on a more advanced carb (like a Quick Fuel) if it has exchangeable Idle Feed Restrictions and/or Idle Air Bleeds. Some main circuit designs also start to "kick in" earlier than others, e.g. due to the booster design. So, if you tune it well, a "too big" carb can work well and maybe even better than a "one size fits all" smaller one.
I would ask your neighbor if you could try if first before you decide to buy it .
Well, the configuration is important. You can use a 600 or 650 on a stock engine, and by that I mean it still has the C3AZ-V camshaft, as long as it has vacuum secondaries. You simply won't use all of the potential cfm.
That said, you would get crisper throttle response and better mileage with something smaller.
Even with mechanical secondaries, you can get away with a larger carb, but you will need to develop a bit of skill with the throttle to get the best performance from the engine. I drove my 289HP every day for many years with the Ford Cobra 3-2V intake setup, which totals 785 cfm. Worked great, unless I pressed the pedal too fast.
If it has vacuum 2ndaries and is a NON- DOUBLE PUMPER, it's probably going to work OK.
STAY AWAY FROM DOUBLE PUMPERS FOR STOCK STREET APPLICATIONS. Typically they are just too much carburetor. They are great for racing though!
But yes, for a stock 289/302 of around 225 -230 horsepower, a 600 is an excellent choice.
I had a Holley 600 cfm on my stock 225 horse 289 and it worked fantastic. It was a Holley LIST #1850, which is a universal replacement carb with vacuum 2ndaries, electric choke, single pumper, single feed, model 4160.
The above mentioned carb would be the carb to go with, if U pass on the 650 U are talking about.
U could always try the 650 to see if it works 4 U, with the understanding U can return it if it doesen't work to expectations.
Something to consider ! 🤔
STAY AWAY FROM DOUBLE PUMPERS FOR STOCK STREET APPLICATIONS. Typically they are just too much carburetor. They are great for racing though!
But yes, for a stock 289/302 of around 225 -230 horsepower, a 600 is an excellent choice.
I had a Holley 600 cfm on my stock 225 horse 289 and it worked fantastic. It was a Holley LIST #1850, which is a universal replacement carb with vacuum 2ndaries, electric choke, single pumper, single feed, model 4160.
The above mentioned carb would be the carb to go with, if U pass on the 650 U are talking about.
U could always try the 650 to see if it works 4 U, with the understanding U can return it if it doesen't work to expectations.
Something to consider ! 🤔
I may still have the LIST 1850, 600 cfm Holley I had on my stock 289.
If U R interested, contact me at rreardon90@gmail.com
How close R U to Fort Wayne, INdiana?
If U R interested, contact me at rreardon90@gmail.com
How close R U to Fort Wayne, INdiana?
A Holley 650 would be too big for a completely stock 302, particularly the way Holley jets/calibrates.
A 600 is still a lot of carb on, once again, a stock 302.
A 600 is still a lot of carb on, once again, a stock 302.
It's the fuel that goes with the CFM that is too much
Get it jetted down correctly and you are ok with the 650
Get it jetted down correctly and you are ok with the 650
Some folks need to study up on bernoulli's principle and understand how carburetors work. Proper fuel metering depends on proper flow through the venturis, and proper sizing is important if you want a responsive engine with no flat spots under any circumstances. No different than cylinder heads. You don't run a huge tunnel port cylinder head on a stock 352 bottom end with a round smog cam from a truck and expect to have a good running combo. Sure, the heads have great flow numbers, but in the real world on a low displacement smog engine you end up with insufficient port velocity to maintain any kind of good idle quality and tip in throttle response. The same concept applies to carburetors. Going too large results in poor idle quality and tip in throttle response, and you generally have to run excessively rich mixtures as a result, and what you end up with is an all around substandard result.
The examples given of large CFM carbs used on lazy smog engines make me scratch my head, as these are NOT engines to look to as examples of good tuning and throttle response generally. Yes, GM used Quadrajets on a huge variety of engines, but they came in hundreds of different variations to accommodate the variety of engines they were installed on. It is bad advice to suggest that you can use any size carb you want on any engine and simple tuning/jetting will achieve the best overall result. This is false. The best results are always achieved by choosing the most appropriate carb for your specific engine, then tune as required to dial it in.
The examples given of large CFM carbs used on lazy smog engines make me scratch my head, as these are NOT engines to look to as examples of good tuning and throttle response generally. Yes, GM used Quadrajets on a huge variety of engines, but they came in hundreds of different variations to accommodate the variety of engines they were installed on. It is bad advice to suggest that you can use any size carb you want on any engine and simple tuning/jetting will achieve the best overall result. This is false. The best results are always achieved by choosing the most appropriate carb for your specific engine, then tune as required to dial it in.
That post from @Woodchuck is worth reviving. In fact someone should respond to this post annually just so people can read it again.
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