----------------------------------------- Yep, they're motorcycle carbs, off of some Japanese inline-4 bike engine. Using the carbs like this - ie - one carb per cylinder in 'isolated runner' form on a Chevy V8, it'd definitely be way under-carbureted. That could be easily fixed without changing the carbs by mounting them on a common plenum chamber. Mart =======================
I think the how is simple enough, the big valve small valve thing is a pretty mute point as you can put bigger valves in the whole and in a genuine raceing application the extra money spent to rework the heads wouldn't be out of the ordinary. There have actually been a lot of engines built with a purpose (other than to get your attention) over the years that had the induction on the outside and the exhaust in the middle. I good piortion of the time it was to facilitate a logistical problem or to make them breath better or both. A lot of the formula cars for instance breath from the outside and exhaust down the middle. It makes it easier to run the exhaust and if your building a 180 degree header it cures a logistical problem at the same time. We could go on forever, but on a front engine car its probably not your best idea but in a mid to rear engine application it is a very good idea.
The above is not true. Suspose those are 1.6-2.02 heads. How much bigger are you going to get the old exhaust, now intake valve? 1.660? Maybe and then it runs into the old intake valve and the combustion chamber wall. You would have to weld up the intake valve bowl and move the exhaust valve guide and the intake in the head along with the ports so that they would now line up, to get the exhaust and intake to be even the same size. SBC engines run really good because they have very good ports and valve train. Swaping them may look neet to some. Looks dumb to others
--------------------------------- The Ford DOHC Indy engine could be had in two configurations too - the one shown in your pics, with the exhausts exiting between the Vee and another less commonly seen version, with the heads spun around and swapped side to side, to put the exhausts outboard like a conventional V8 p***enger car engine. Mickey Thompson ran one in this configuration in a Mustang Mach 1 at Bonneville in 1969 or '70 and set a bunch of F.I.A. records with it. That FED you mentioned that used a pair DOHC Indy engines was Jim Busby's "Double Trouble" junior fueler. The car was an instant sensation in junior fuel and got a huge amount ink in the car magazines of the day and then suddenly disappeared. The story is that Busby bought five 'ready to race' Ford Indy engines for $3500 when they were suddenly obsoleted by a USAC rule change and for kicks, decided to stick a pair of them in a junior fueler. After only making a couple of p***es with the car, but winning an NHRA "best engineered car" award at the '71 Summernationals, for his efforts, USAC suddenly changed the rules back again, making these expensive, exotic engines compe***ive and very valuable once more. I'll bet the guys running the "conventional" Chrysler and Desoto hemis and small block Chevies in junior fuel at the time must have heaved a huge sigh of relief at this! Seeing a chance to make a large quick profit and figuring that NHRA would probably eventually outlaw his combination anyway, Busby quickly converted the car back to conventional hemi power and sold the five Indy engines back to the original seller for $30,000......almost 10 times what he originally paid for them! There's a quote in a post on the net where Busby tells about this. Click here: http://www.performanceforums.com/forums/showthread.php?p=83 9921454#poststop Mart ==========================a
Can you explain why mounting them on a commen plenum chamber makes a difference? If each carb is putting out the same amount of fuel per cylinder (or intake stroke) how does adding a common plenum change the equation?
------------------------------ Yeah. With an 'isolated runner' manifold setup, each cylinder only sees one venturi and that one venturi has to p*** the all of the air required by that cylinder. With a common plenum joining the carbs together, instead of just one venturi, the cylinder can draw mixture from all of the venturis connected to that plenum, effectively increasing the amount of venturi area. Also, with an isolated runner set-up, air can only flow through the carb when that cylinder is on the intake stroke and the intake valve that carb is connected to is open. For the other three cycles the carb is not flowing any air and is effectively doing nothing. With a common plenum, particularly at high rpm, the airflow through all the carbs connected to that plenum is nearly constant. because all the cylinders on that plenum are drawing air through them. Even on a conventional, ''non-isolated-runner" single-plane type manifold,a general rule of thumb is that the bigger the plenum, the smaller the carb needs to be to make power at a given rpm and vice versa. Without any plenum at all, the carb needs to be a whole lot bigger just to flow the same amount of air in a given amount of time (or degrees of intake duration) Mart ============================
Thanks for that clear description of the physics of it all. It makes perfect sense and I can picture how a common plenum would make a difference. I just learned something new and very applicable to a project I'm planning. Thanks!!
The Jalopy Journal
