Has anybody seen this ?
Not sure what to make of it ....Thoughts ?
http://www.seniorsix.org/faq/2_4bblconv.html
Not sure what to make of it ....Thoughts ?
http://www.seniorsix.org/faq/2_4bblconv.html
4bbl conversion
- Thread starter bossrobot
- Start date
Belgiumbarry
Well-Known Member
i was playing with the idea of testing that …. but no more. Problem is indeed lean outside cilinders, with the longest runners.
Don't know if BMW did it on the oem cars with 4bbl but US straight six ( Chevy ? ) had different cam lobes between cilinders to compensate that.
Don't know if BMW did it on the oem cars with 4bbl but US straight six ( Chevy ? ) had different cam lobes between cilinders to compensate that.
Mike Goble
Well-Known Member
Here's the big problem with using a square bore carburetor like the Holley in the picture. The flow into the manifold is going to be severely disrupted by the casting configuration. I'm making an adapter plate for a Q-jet for my 4-bbl setup, and the barrels match up very well.
jmackro
Well-Known Member
This is an interesting problem. I wouldn't say that the outer cylinders get a "lean" mixture, as "lean" implies a lower fuel/air ratio. The carburetor can only deliver one fuel/air ratio, and that will get to all six cylinders. The problem is simply that the outer cylinders get less fuel/air relative to the inner ones, due to friction in the longer chambers and delay = passage length / speed of sound. It would be like the inner cylinders "seeing" a half throttle, while the outer cylinders only "see" a third throttle.
And I suppose that Chevrolet (or whoever) could sort of compensate for this by putting more lift / more duration on the cam lobes for the outer cylinders. So those valves would open wider / longer, letting in more mixture. But both effects - restrictions in the long passages and more mixture with hotter cam profiles - are going to be nonlinear across the RPM range, so getting one to cancel the other would be tough.
Clearly having one choke per cylinder (and equal length exhaust headers) is the ultimate performance solution (like the Offenhauser engine below). But for inexpensive, simple engines intended for street use, a single carb on an inline engine is probably good enough.
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These setups were somewhat widely used on old Jag I-6 motors when the SU’s failed. They drove just fine.
Belgiumbarry
Well-Known Member
yes Jay , i thought the outside cam lobes have more duration.
On the other hand , we lost a lot of torque with triple Webers.... the "runners" are to short. That's why we fabricated long inlet tubes and a airbox. ( from 240 NM to 300 NM ! )
On the other hand , we lost a lot of torque with triple Webers.... the "runners" are to short. That's why we fabricated long inlet tubes and a airbox. ( from 240 NM to 300 NM ! )
I thought about doing the 4bbl conversion with an EFI throttle body from FiTech. Seemed like a really simple setup, but ultimately figured it would not suit my eye when opening the hood.
I was wondering about the 4bbl injection as well. I have more of a muscle car background and simplicity and speed tend to be the answer there. Not pleasing to the eye though, that makes sense.