77 carb CFM?

[Ken A.]

What is the CFM of the L-48 & L-82 carbs on a 77? We will be rebuilding my son's tired 77 L-48 and we want to give it a little more guts and are trying to figure out what size carb to use.
Thanks
Ken

[Duke W.]

Re: 77 carb CFM?

It's about 750 CFM @ 1.5" Hg depression. The carb is not an issue with power. The restictive exhaust system is issue one. Issue two is head flow. Issue three is the restrictive inlet path to the air cleaner.

Duke

[Ken A.]

Re: 77 carb CFM?

Thanks. He's already got headers and improved cat w/o mufflers. Probably stick with the stock heads (money), but thinking of a better intake. Would you suggest a stock L-82 or aftermarket intake? Probably go with a K&N air filter.

Ken

[Duke W.]

Re: 77 carb CFM?

A better flowing inlet manifold won't do much, if anything, until head flow is iimproved. Headers are of little value until you get into high overlap cams and open exhaust. A full dual exhaust system (pre-'75) is the best exhaust system upgrade, but won't pass emission tests.

K&N filters don't filter and don't flow any better than a good OE replacement cellulose filter. They are overpriced junk!

There is really no such thing as "bolt on horsepower" other than lowering exhaust back pressure and removing restrictions to the air cleaner. Beyond that you have to go inside the engine - improve head flow and raise CR on low compression engines.

Duke

[Greg L.]

Re: 77 carb CFM?

K&N filters don't filter and don't flow any better than a good OE replacement cellulose filter. They are overpriced junk!

Duke[/quote]

That's an interesting comment Duke and I can't agree more!

I bought a car once with a K&N filter and held it up the light to see how dirty it was and it looked as though I could see actual light through it... Then I held up an orange screwdriver behind it and sure enough I could see specks of orange so I was in effect looking straight through it! K&N has a great marketing department.

[Terry M.]

[Duke W.]

Re: 77 carb CFM?

Hot Rod magazine of all sources did an air filter test a few years ago, which included both common OE replacement cellulose air filters and some non-cellulose types including K&N. All were the same size. They tested the filters on both a flow bench and an engine on a dyno looking for power difference.

The K&N did not flow the best nor make the best power, but all were pretty close.

Bottom line: There's no way a K&N will provide "more power" compared to the same size quality cellulose filter. Add the poor filtration and you know why I recomned avoiding them like the plague, but I'm just one guy against a huge advertising campaign.

If you don't believe you can easily find this test on the Hot Rod mag web site.

Duke

[Stuart F.]

Re: 77 carb CFM?

Interesting discussion on the K & N filters. I guess I too bought into the ad campaign and put one in my daily driver and my 63 340 hp just recently (hard to find p/no. except at Corvette Central). Before I correct my mistake, could someone give me a read on how the old 63 foam filter elements stack up. I have several new foams and only went away from them because of their mess. No matter how much oil I squeezed out of the after cleaning and reoiling, my air cleaner housing and manifold always showed some oil residue. Oh ya, one more thing; how do I best remove that pretty decal?

[Duke W.]

Re: 77 carb CFM?

The OE foam filters and repros of same are junk - poor filtration. At 115K miles the only measureable wear inside my 340 HP engine was at the top of the cylinder walls and I attribute this to the poor filtration of the OE foam filter. Save it for judging.

For normal use the NAPA 2070 cellulose element or equivalent in another brand should work.

The '63 air filter is rather short and is probably a little restrictive compared to the later taller elements, but the difference in power is not an issue compared to the potential cylinder wear that poor air filtration can cause.


Duke

[Stuart F.]

Re: 77 carb CFM?

Duke;

Thanks for the advice. I have a paper element in my stock I can throw in for the time being until I get to NAPA or a good parts house for a new one.

Guess I was thinkin I was cool to have a K & N in a 63, for as you say they are an odd size and people mentioned it at shows and all. I even put the sticker on the housing to show it off. My son about flipped, course he's a purist.

Roger on the foam elements too. Again, Thanks.

Stu Fox

[Clem Z.]

Re: 77 carb CFM?

if your corvette has a auto trans a higher stall speed converter will pep up the take off from a dead stop. one from a 6 cylinder with the same transmission is the cheapest wayto go

[Ken A.]

Re: 77 carb CFM?

OK, now that you have brought this up, exactly what is a 'higher stall' converter? I assume it has to do with the shift pt as it applies to the engine RPM. So, what would the stock one be and how much is too much. I've always assumed higher shift pt equals harder shifts.

Ken

[Duke W.]

Re: 77 carb CFM?

In a safe place to test put your left foot hard on the brake and floor the throttle in Drive. The engine will quickly accelerate to "converter stall speed", which you will find is about 2000. Now release the brake. Does it light the tires?

A higher stall speed converter will drop your fuel mileage 10-20 percent because of excess slippage.

I have a neighbor with a '77 L-48, TH, 3.08 axle. I've driven it. In normal around town driving it's quite peppy, but clearly power peaks at about 3500-4000 due to the restrictive exhaust, etc. At 4500 it just died.

These L-48s make 90 percent of peak torque at the OE converter stall speed of about 2000. High stall converters are for drag racing engines with high overlap cams than barely make 80 percent at 3000. They have no place on a base Corvette engine.

GM knows how to put together a good drivetrain combination, but emission control using the crude technology of that era took a high toll in top end power.

The only way to address this issue is the basics - get the engine to pump more air, which means minimizing inlet restriction starting with the most restrictive element, which is head flow, minimizing exhaust pumping loss, and raising CR to levels than today's high octane unleaded fuels will support.

"Bolt-on performance" is a myth promulgated by the aftermarket to separate you from your money!

A few months ago an owner with a '69 300 HP/TH/3.08 let me drive his car after I consulted with him on improving engine performance - massaged the heads and retarded the OE cam 4 degrees. My first attempt at a "hole shot" (flooring the throttle to converter stall speed and then releasing the brake) lite up the tires when I released the brake, so I tried again by just flooring the throttle from a dead stop. It dug in and exhibited a strong, linear pull to 5300 (the beginning of the red zone on the tach) with a little rolloff to 5500 at which point I popped the shifter into second gear. Revs dropped to about 3500 and it pulled strongly until I lifted off shortly due to traffic ahead. An OE '69 base engine is wheezing at 5000.

This shows the potential of what a well thought out performance improvement can do while retaining the normal smooth, pleasant driving characteristics, OE appearance, and reasonable fuel economy of the base engine - TH combination. I estimate that peak power was up at least 15 percent, with no loss of low end torque (note my first launch attempt, above). More importantly, the range of near peak power was extended more than 500 revs - actually slightly beyond the "redline", so average power through the gears increased more than peak power alone, and it's average power through the gears that detemines acceleration.

Your L-48 is basically the same engine with a single exhaust, low compression, and a more restrictive inlet into the air cleaner to accomodate the heat stove, etc.

I'm not an auto trans guy, but it was a very pleasant and easy combination to drive - very satisfying and certainly no slouch. I also judged the car (mechanical) at a Chapter meet, and I recall it got a Second Flight. A full deduction on paint had to be taken due to the wrong color. Without that it would have been Top Flight. The Mechanical Section deductions were minor.

Duke

[Clem Z.]

Re: 77 carb CFM?

the higher stall speed will only be affected when you "flash" the converter,floor the gas from a dead stop and the converter will "lock up" going down the road so the fuel mileage should not be effected

[Duke W.]

Re: 77 carb CFM?

I don't think so. These 3-speed TH transmissions don't have lock up converters. If you add throttle below stall speed the converter will allow revs to approach stall speed. The more converter slippage, the less efficient the transmission, so a high stall converter will consume a lot of fuel on a tall geared, low revving engine. The basic operating range of a L-48 is off-idle to about 4000 and the gearing keeps it in this range at normal road speeds. A high stall converter on a base engine will cause more problems than it will solve. I doesn't make any sense to have a 3000 stall conveter on a 4000 RPM engine.

Did you ever install one on a base engine?

Duke