L36 carburetion comparison to the L68

[Michael G.]

Good Sunday afternoon all,

Myself and a fellow enthusiast were comparing carburetor stats on the L36 vs the L68. I believe the '66 and '67 L36 carburetors were rated at 585 CFM. What we weren't sure about was the rating of each 2 bbl carb on the L68. He came up with a figure of around 1200 CFM total for all 3 carbs. If that were the case......how does that play out as all other aspects: cam, valving, compression etc. of the L36 and L68 are the same.?

[Joe L.]

Re: L36 carburetion comparison to the L68

Good Sunday afternoon all,

Myself and a fellow enthusiast were comparing carburetor stats on the L36 vs the L68. I believe the '66 and '67 L36 carburetors were rated at 585 CFM. What we weren't sure about was the rating of each 2 bbl carb on the L68. He came up with a figure of around 1200 CFM total for all 3 carbs. If that were the case......how does that play out as all other aspects: cam, valving, compression etc. of the L36 and L68 are the same.?

Michael-------


Vaguely, I recall that the 2 barrel carbs used for 67-69 3X2 were about 385 CFM.

The L-36 and L-68 core engines were identical with the possible exception that some L-68 may have been equipped with 4 bolt mains. The only significant difference was the induction system.

[Duke W.]

Re: L36 carburetion comparison to the L68

Two-barrel carburetors are flow rated at 3" Hg depression versus 1.5" for four-barrels, so the published numbers are apples and oranges. However since flow is a near linear function of the square root of depression up to near the speed of sound (beyond which point the duct is "choked" and further depression will not increase flow), the two-barrel 3" flow rating can be converted to flow at 1.5" depression merely by dividing the 3" number by the square root of two.

You do the math.

Further, a single four-barrel manifold usually has more equal runner geometry and better average flow efficiency. "Tri-power" is primarily a bling factor.

For a high performance road engine, a carburetor that delivers maximum air demand at no more than 1.5" depression based on 85 percent VE at maximum RPM is sufficiently generous. For a 427 this would be 741 CFM, so the early Holley is a little short, but being a bit "undersized" it is very responsive to throttle changes. The 750 CFM Quadrajet from later years is right on the money.

So the higher flow is offset to some degree by a less efficient manifold, which calls into question the validity of the 10 HP different in engine ratings. That's only about 2.5 percent difference, which is not far outside the 1-2 percent range of back to back dyno run repeatability.

I recall a similar discussion back in the early days of the TDB comparing the L-72 800 CFM four-barrel to the L-71 trips. The latter's 3" rating combined to 906 CFM at 1.5" depression, and given the less efficient manifold, I'd make an even odds bet the four-barrel would make at least one to two percent more average power in a handful of back to back tests on the same engine.

Duke

[Michael G.]

Re: L36 carburetion comparison to the L68

So Duke, could the '66 or '67 390 HP engine benefit with a slightly larger CFM carburetor....if so could anything be done with the original carb that would benefit output?

[Duke W.]

Re: L36 carburetion comparison to the L68

If you installed a 750 in place of the 585, you'd probably see a bit more power starting at 4500 to the redline, but no more than two percent.

There are better ways to get more useable power out of a L-36 by first optimizing the spark advance map to get more low end power and then massaging he heads for more top end power.

Duke

[Mark E.]

Re: L36 carburetion comparison to the L68

... I'd make an even odds bet the four-barrel would make at least one to two percent more average power in a handful of back to back tests on the same engine.
Duke

Even still, as you mention, the bling factor of opening the hood and seeing three Holleys is very cool. If I was choosing a set up for a race car, definitely the efficiency and simplicity of a single four-barrel is the way to go. But for a street or show car, I'd take the bling over 2% more power. My LS5 has a Quadrajet, arguably one of the most efficient and responsive street carbs available. It performs great, but doesn't quite have the wow factor of a 3x2.

[William F.]

Re: L36 carburetion comparison to the L68

Would 3X Holley carb headaches be worth the "bling" factor? One Holley is enough or 1 too many for me.

[Mark E.]

Re: L36 carburetion comparison to the L68

Would 3X Holley carb headaches be worth the "bling" factor? One Holley is enough or 1 too many for me.

I've never owned a tri-power Corvette. Is this set-up troublesome? Or does it just require knowledge how to set up and tune?

[Michael G.]

Re: L36 carburetion comparison to the L68

I've never owned a tri-power Corvette. Is this set-up troublesome? Or does it just require knowledge how to set up and tune?

Believe you got it Mark, they are only a issue if you don't have the skill set to 'tune them in'. Bling factor for me is off the charts!