Originally Posted by
Yellowjacket
Hmmm
Quadruplng the power by FI is probably not the best idea... When I was in high school 40 years ago I read the book "The Sports Car, it's Design and Performance" by a Brit named Colin Campbell. One line in that book stuck in my head and it's been there ever since...
"As a means of finding the weakest point in an engine, supercharging has no equal..."
True then, even truer today with the advent of "turn the screw" boost and turbochargers that any fool can crank in more boost than is prudent...
Crank and rod forces are primarily speed driven and I think that you can live with that, but in order to get more power cylinder pressures are going way up and these engines are close to detonation as they sit. If you go to higher cylinder pressures you're going to have to reduce compression ratio and really, a deflector arrangement isn't the best thing since it is not only heavy, it results in a big chunk of hot metal sticking up into the combustion chamber that is a detonation anchor...
Gordon Gecko is quoted as saying that "Greed is Good", but if you get too greedy you're going to find lots of things that are going to lead to a trail of broken bits and that tends to take the fun out it....
If you get to maybe 150 hp total you've done as much as the basic architecture is good for. More than that and you're going to start breaking lots of things and sticking pistons and that takes the fun out of it.
I think you will probably have to bite the bullet and make a new crank. These cranks aren't that robust in the first place for reasons I'll discuss below, so it may be something that you will have to do in the long run anyway, and doing it up front may be the price of doing business... These cranks can bend and it would be an advantage to better support them with additional bearings in what was the reed blocks. Jeff was concerned about offset imbalance, but remember that the two cylinder pairs would actually not need counterweights if the crank was robust enough to transfer the forces between the two adjacent cylinders. Another way of saying it is that each crank pair is a mirror image of each other and the forces balance, but the forces are huge. Racers have for years trimmed the counterweights in the mod classes without issues, but as noted, the unbalanced forces in the crank goes up and that's something that you could readily calculate and see how much bending you have in the crank.
What I was thinking about was the fact that the reeds are now unnecessary and the area that was used for the reed blocks and for reed travel is now open for crank counterweights. This would also help address the issue that Jeff brought up in that the offset weight is increased. I don't know how you were going to get the charge into the case, but the reeds are unnecessary now so you can use that area for a new crank. Similarly for the middle bearing, while you don't want to reduce the bearing itself, there is area off of the centerline that could be trimmed back. Reed travel (per the APBA tech manual) is .185 (max) which when added to the thickness of the reed stop is close to a quarter of an inch. With the reed blocks gone you could replace them with a bearing support that is bigger in diameter than what is there now. The stock crank diameter was minimized in that area to minimize leakage and get intake area. You want the opposite, a bigger diameter and a bearing support there so that's another reason to make a new crank.
You'll have to look at the existing cases and see what you can fit inside and how it can all work with an additional bearing between the cylinders, but by using the space were the reeds were you may be able to get the area you need to fit a decent design of master/slave rod arrangement and that may get you over the biggest hurdle and let you break it in other areas sooner.