Carbs:Holley Vs. AFB
In the NCRS Lingo section of the website in the section on RPO numbers and in particular the "L" series of engine options the carburetor used for the high horsepower small blocks is listed (for the L-79 anyway; no indication on carb for the L-76) as an AFB. I'm no expert on the C2s, but I thought that in 1965 both the L-76 and the newly introduced L-79 used the same carb and thus the only difference between the two was the camshaft. I also believed, but obviously I'm wrong, that the 1965 L-76 carb was a big Holley four barrel and, again, the L-79 in 1965 had the same carb. Maybe I dreamt this! Can someone set me straight on this? Seems as though the cfm on the L-76 carb was something like 780. So the first year of Holley carbs on small blocks was 1966, not 1965? Thanks.
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Re: Carbs:Holley Vs. AFB
The AFB was used on '62 to '63 L-75 (300 HP) and L-76 (340 HP).
Beginning in 1964 a Holley Model 4150 was used on L-76 (365 HP), while the L-75 continued with the AFB, and the new for 1965 L-79 (350 HP) also used the same Holley 4150 as L-76.
Beginning in 1966 the base engine became what was previously L-75 with a Holley Model 4160 and the same carb was used on L-79. The 4150 has replaceable secondary jets, but the 4160 has a secondary "metering plate" with fixed jets.
The AFB flows about 550 CFM at 1.5" Hg, depression and all the above Holleys about 585 at the same 1.5" Hg depression, which is the defacto standard for measuring four-barrel carburetor air flow. (Two-barrel air flow is rated at 3" Hg depression.)
Holley model numbers are available in various air flow ratings based on throttle bore and venturi sizes; 327 4150s and 4160s have 1 9/16" throttle bores. The Model 4150 used on SHP big blocks in 1965 and 1966 have 1 11/16" throttle bores and are rated at or near 780 CFM @ 1.5" Hg, and this same basic 780 CFM 4150 was also used on the 1970-72 LT-1.
The 1964 increase in L-76 horsepower was primarily due to the 30-30 camshaft that replaced the Duntov camshaft - more power, but it killed the low end torque big time! The larger valves were a minor factor, but the Holley had no real impact because it flowed about the same as the AFB.
Some of us still wonder why Chevrolet replaced the nearly bulletproof AFB with the leak-prone Holley.
Most of the above is from the NCRS 1953-1967 Corvette Specification Guide, which is a very good document to have.
DukeLast edited by Duke W.; July 11, 2010, 09:58 AM. -
Re: Carbs:Holley Vs. AFB
the AFB carbs did not cause fuel flooding/starving problems in hard cornering when auto crossing or racing but the side hung float holleys that came on corvettes did. ford found this out with their GT 40 so holley holley came out with the le manns float bowls which turned into the cathedral type now sold as center hung float type. i had to go to holley back in 64 to get these special ford bowls so i could win at autocross. i still have the letter from holley telling me that they would sell me these special bowls. i kept the bowls and later sold them on e bay because the cobra kit car owner wanted them because they were standard on the 427 cobras used for road racing.
the 65 396 corvette was the first year for these type float bowls and i bet if holley had come out with these bowls in the 50s there would have not been any need for corvette FI.
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Re: Carbs:Holley Vs. AFB
Duke......
Thanks for the info on Carter AFBs and 4150/4160 Holleys. I have the 1953-1972 NCRS Specification Guide which came out late in 1989 and don't remember seeing in it as much detail as you have supplied.........perhaps I need to read it closer. Nice to know I wasn't totally wrong on the introduction of the Holley on small blocks. My big gripe back "in the day" wasn't so much the Holley (4160) on my '66 L-79 leaking as it was the propensity for blowing power valves. And they weren't driveable for long with a bad power valve--that is if you valued lubrication on your cylinder walls. So the 340 horse with the AFB might stay ahead of the 4150 model L-76 up until 60 mph or so but from then on the horsepower of the 365 horse would begin to dominate the 340 horse? This scenario would be with 3.70 gearing. Thanks, again.Comment
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Re: Carbs:Holley Vs. AFB
the blown PV in holleys is over stated as i never had that problem. it was cause by poor tuning that cause a back fire thru the carb blowing out the PVComment
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Re: Carbs:Holley Vs. AFB
Well..............a hot start on my '66 L-79 would easily cause a back fire in the intake which would tear the diaphragm in the power valve. This might be related to tuning, but hot starts were difficult to make without giving it a little gas (and that's all it took) in any event.....but then 37 years ago is a long time.
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Re: Carbs:Holley Vs. AFB
i always blocked the heat riser crossover in the intake manifold for more HP so hot starts were not the problem but COLD starts were.Well..............a hot start on my '66 L-79 would easily cause a back fire in the intake which would tear the diaphragm in the power valve. This might be related to tuning, but hot starts were difficult to make without giving it a little gas (and that's all it took) in any event.....but then 37 years ago is a long time.
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Re: Carbs:Holley Vs. AFB
I think it comes down to the individual engine, gearing, and driver. I recall one late-sixties face-off against a '64 375 HP FI from highway speed up to about 130 on what was then US 10 near North Bend, WA, east of Seattle. Given my CR trans/3.08 axle I downshifted to second and ended up at the top of third. We were dead even all the way until he lifted. Even if he hadn't backed off I still had another gear left!
I never lost a race like that if there was enough open road ahead of me because few cars could achieve over 150 MPH due to either not enough power or too short gearing.
I did worry once about an XK-E, but I pulled away so fast from the get-go that it was over before we hit 80 MPH. E-types could do about 150, but they couldn't match a SHP/FI small block's acceleration below 130.
Everyone always thought I had an exceptionally strong running 340 HP engine. Other than changing the spark advance map and minor jetting changes is was all original Flint-built back then. My preferred MO was a rolling start from no less than about 40 MPH, and I would use first gear.
DukeLast edited by Duke W.; July 11, 2010, 12:55 PM.Comment
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Re: Carbs:Holley Vs. AFB
I'm with Duke with regards to my L-76 63 340hp vs. higher HP small blocks of the era, only I have the 3.36 final. My runs were generally from 30 mph on up. I never lost to any other high HP small block that had a 3.70 or higher numerical gear (4.11 or 4.56). Granted, much of the reason was the gearing which they simply ran out of. You'd be surprised how many fell for that sucker ploy - even for money. However, with a C-2 road racing above 100 mph, it takes a lot of b***s to keep it on the road -specially side by side on a two lane country road. Some of the victories just might have also been due to they ran out of guts as well as gearing.
One day my son and I may give it a go some place. He has a 96 Grand Sport with the LT-4 and six speed, plus he's flown fighter jets. Sooooo, I think I'll cede that one on all scores.
Stu FoxComment
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Re: Carbs:Holley Vs. AFB
Mine is the the second edition, first printing, 1997, but it's getting so dog-eared and wine-stained I may have to replace it soon.
Stuart - The only way you can beat a LT4 is to rebuild your L-76 to my "327 LT-1" configuration. Then it will definitely be a horserace as long as you run from a high enough speed that your gearing doesn't hurt you.
DukeLast edited by Duke W.; July 11, 2010, 10:57 PM.Comment
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Re: Carbs:Holley Vs. AFB
Duke;
At my tender age of 70, I think I'm running shorter on the "guts" not the "Gearing". Then again, when we convoy to shows, he (my son) always complains that I drive too fast for him to keep up.
Stu FoxComment
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Re: Carbs:Holley Vs. AFB
The 327 LT-1 makes about the same net horsepower as the LT4, but at higher revs and the LT4 has a fatter torque curve.
From a dead stop it wouldn't even be a race due to the C4s much greater initial tractive effort.
The LT4 has the advantage of more gears, but a C2 is lighter.
From a 30-40 MPH rolling start with Stuart in first gear, it would be too close to call - if his engine was build to 327 LT-1 spec, but in OE 340 HP configuration, the LT4 is a sure winner.
DukeComment
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Re: Carbs:Holley Vs. AFB
This thing (thread) continues to run and maybe it should just be allowed to die, but I have to respond to an earlier post by Duke......in reference to the "supposed" 150 mph top end of the E-type (or XKE, if you insist on the American term). The factory seriously massaged the engine(s) which were used to make the 150 mph run. I seriously doubt any production model of the early E-type would actually get even close to 150. And this is just for comment, not questioning Duke's statement. The magazines (R and T at least) stated that the engine used by the factory had to be run up to 6100 rpm to get the 150 mph. The early 3.8 liter engines were readlined (the production engines) at 5500 rpm (and the later 4.2 liters at 5000 revs). That is ten percent above the redline.....and a ten percent reduction of 150 is 135 mph--which is about what the production cars would do without asking for serious engine issues (going DEEP into the red). And the 4.2 engines would not rev with the 3.8s I would not ask for an argument (I'm getting too old for that) with an E-type owner, but would not be too abashed to state to his/her face that I doubt a production car (their car) getting anywhere near 150 mph. I know times change and maybe with today's level of engine building exceeding 6000 revs in an engine with that long a stroke would not be so unusual, but we're talking stock/production here. And back in the day when the D-type raced and won at Le Mans those engines were redlined at 5750 rpm (but with Le Mans gearing and triple Webers they would exceed 170 mph--not exceeding engine redline for very long, though). Or........am I just too chicken to put the tach needle that far in the red? And others do it almost routinely? You might think so, because I have told an E-type owner to his face that I doubted the 150 figure in a production car and he kindly corrected me.....but I know what red means to a fighting bull and believe it to mean the same to engine internals. On the issue of top end to a solid lifter cam C-2, the Sting Rays had terrible issues with wind resistance (a Cx of 0.5 or higher) and I've always felt that to get those (327s anyway) above 135-140 mph would indeed be a feat.
Remember, opinions are like navels and I'm not looking for an argument.
BillComment
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