The Stovebolt Site Forums The Shop Area Engine Shop McGurk 5 carb Manifold - why not 6 carb?

Hello All,

It has been a long time between my posts! I just finished watch one of Karl Fisher of Make It Kustom in Canada is getting a 302 straight six cylinder motor up and running .

302 straight six cylinder motor up - video link

The engine Karl is working on came with a McGurk manifold with 5 carburettors fitted to it. I have done some preliminary research on the McGurk manifolds. While I can understand why the manifold was used - I cannot understand why if they expanded the number of carbs from 1 to 5 did not they match cylinder to carb and have 6 cylinders for an inline six cylinder motor? There must have been a reason why McGurk stopped at 5 carbs?

I am aware that the McGurk's could be modified for a 2, 3 or a 5 carb arrangement. During the 1970-80s in Australia Holden six cylinder motors were souped up by fitting triple carb arrangements. Typically SU carbs. Also that the manifold does not make it critical to perfectly match the number of cylinders to the number of carbs.

If anyone could fill me in why McGurk's stopped at 5 carbs I would greatly appreciate it

Kind regards
Lionel

One would have to know the cfm of each carburetor to know how many you can add. They may not of had smaller units to choose from so 5 was the limit of what was available at the time. I would guess, out of the five, three may have been working and the other two came online at a higher rpm.

For example, my 250 ci had one single Monojet when new. I added a second one on a dual intake manifold. It would run, I drove it, seemed to have decent power, BUT it drank fuel faster then I could add it! I'm talking maybe 4 or 5 mpg.
I then switched to dual Carter W-1's off a 216 ci engine, a much smaller CFM rated. The smaller carbs kept the air velocity higher in each one so it still worked as it was designed to.

Carburetors are designed around a certain CFM rating, what the engine needed for everyday driving, and the occasional high speed run up. It doesn't matter what engine you put it on, the design and calibration does not change as long as the correct air velocity for the carburetor is met. Too much air speed or too little will change the metering, so if you can can keep the air speed right, you can pile on as many as you want.

The proper CFM single carb is ALWAYS better than multiples. The more carbs you add, the potential for problems increases by a massive amount. Intake manifold and cylinder head design has a lot to do with how efficient a multi carb setup will be, and the stovebolt Chevy design sucks like a vacuum cleaner- - - - -GMC heads and manifolds are only slightly better! Since both types of head have only three intake ports, the best choice for a multi-carb setup is a carb for each port, one that operates efficiently at the desired top RPM to supply sufficient volume for ONE cylinder. That's because each intake port is only fueling one cylinder at a time each turn of the engine, due to the 1-5-3-6-2-4 firing order. More carburetor CFM than that makes the engine only perform close to properly at wide open throttle, max RPM. The laws of physics are not "suggestions"!
Jerry

Quite the story on Karl’s engine ,he makes cool
tools as well

My guess is that there is only physically room for 5. I understand a stromberg 97 flows about 150 CFM so that's a total of 750 CFM. That otta do it and then some for a 270. A fellow I knew back in the day had a 53 chevy coupe with a 270 GMC sporting 5 Stromberg 97s, headers and a big cam. There was great sport betting when he'd go up against 55 Chevys with 283s both in street racing and at the drags. That was the hot setup then when performance 283s were current production. Nyle's 53 would blow the doors off all but a few of the 283 challengers. Now, 327s were another matter... They had just come out an a few guys had sprung for crate motors.