2 BBL carb to head without and adapter - why?

[aribert]

I have read several posts about not using an adapter to mount a 2 bbl (Weber or Holly) to the log head - cutting a larger hole in the log and welding a plate with the carb mounting pattern directly to the head.

What is the reasoning behind this? Is it only the desire to have a port opening the same size as the carb footprint or does the (lack of) height of the adapter come into play also?

I am asking these questions since I am about to fab a carb adapter to put a Weber DGV onto a D8 flat top log head. I have a Clifford adapter that I bought about a decade ago and plan to cut the base off of it and make a thicker base so that I can mount my '61 (California car) PCV valve into the base of the carb adapter and so that the float bowl will face forward. My intention was to keep the final height of the adapter similar in height to the Clifford part but I could shorten the height of the adapter by about 0.5 inch if that would improve things.

Your thoughts?

 

[aribert]

I don't know how the blasted smilely got into my post - was supposed to read '78 and I was trying to debug it when I hit submit instead of preview. I have never figured out how to use the smileys and get one when I do not want it!

 

[rbohm]

8) one issue is stuffing the added breathing capacity into the stock opening in the intake. if your engine is basically stock it wont matter much, but if you have done any mods, like a large cam and or headers then you want to open up the log to accept a direct mount carb for better airflow.

the second issue is the added height of the spacer pushes the carb up nearly 2" and that can cause hood clearance problems.

 

[66 Fastback]

As mentioned, direct mount probably has marginal benefit if the head is not modified. The limiting factor is going to be the heads. If you plan on future mods like camshaft, bigger valves and porting, then the direct mount would likely be the best option. But if you are pulling the head to make a direct mount, you may as well do the head work at that time.

Mike has some dyno runs and comparisons posted on his website.
Look at some of these Dyno runs on Gene's Maverick for some test results.
http://www.classicinlines.com/dynoroom.asp

Granted the direct mount picked up some horsepower, but it used a modified reworked head. Would the gains been as significant without the head work?? Probably not.
Doug

 

[JackFish]

If you want the two barrels of the carb line up parallel to the log, the plate would give better airflow.
If I understand you correctly, with the float bowl forward, that puts the venturis at a right angle to the log? If that is the case, the taller adapter would be the only option.

 

[shmoozo]

I don't know how the blasted smilely got into my post - was supposed to read '78 and I was trying to debug it when I hit submit instead of preview. I have never figured out how to use the smileys and get one when I do not want it!

The expression you typed was this:

(78 ) [but without the space between the "8" and the ")"]

An "8" followed immediately by a ")" is converted by the forum software into a sunglasses-wearing, smiling emoticon like the one that popped up in your message. [i.e. one of these --> 8)]

There are a few ways to prevent that from happening.

One is to add a space (or some other character) between the "8" and the ")" in the text like I did above.

Another is to use either the square brackets -- the "[" and "]" -- or the "curly brackets" -- the "{" and "}" -- instead of the rounded parentheses.

But you can also simply check the box for "Disable Smilies in this post" that is below the message when you are composing or editing it.

8)

 

[AzCoupe]

A few months ago we did a baseline dyno on a stock 250 with a 1V carb, then did several more pulls using various 2V carbs mounted with an adaptor. Depending on the carb, we gained 4-5HP using the adaptor. Next we pulled the head and had it modified. With a direct mount carb, slightly larger valves (175/150), and a little port work, we gained 22-25HP. Personally, I think the direct mount carb was responsible for atleast half of that, but that's just my opinion.

 

[rommaster2]

thats great news either way because the two barrel adaptor mod was a great boost in power for my falcon and I plan on taking a spare head and working it over myself (bigger valves, porting, direct mount) so it should add quite a kick .

 

[AZstang66]

According to theory, if we have a carburetor and add an adapter as described here in this thread to the same carburetor, the performance will increase. This is because the adapter is a diverging nozzle. Diverging nozzles increase fluid velocity, density, pressure, and temperature. Additionally, in the case of performance, the specific mass-flow is increased which means more air-fuel mass for the same conditions (as the carburetor with no adapter/nozzle). This theory is essentially identical as adding venturi stacks to the top of a carburetor. Another benefit of nozzle usage is the absence of increases in the required work to supercharge/compress the fluid.

However, using the adapter with a 2-barrel carburetor as opposed to a 2-barrel carburetor mounted directly onto the intake will be inferior performance-wise. This is because the inlet diameter and/or cross-sectional area of the 'hole' is larger. Therefore, a higher mass flow is automatically obtained, with the necessity of a diverging nozzle.

 

[aribert]

Thanks for the replys!

I have figured out what I need with respect to hood clearance to keep my stock '61 aircleaner cover in place. Since the Clifford adapter was intended to be used with the stock carb adapter/water port, I have a resonable ammount of carb adapter height to play with.