Tuning with a blower
First let us say that we were tempted to call this section tuning with a blower/turbo. Both force feed the engine, and as far as we know and have read, tuning for them is similar. But since we haven’t yet had first hand experience with a turbo engine, we wont claim to know enough about it to include it here.
Tuning with a blower is not for the faint of heart. When tuning non-blower engines the margin for error is pretty wide. How many engines have we destroyed because they were out of tune? One. Did that engine have a blower? Yes. All the warnings about not going too lean, not having too early ignition, they apply here even more. When you force feed the cylinders, the pressure and temperature in there is much higher, and tolerances lower.
But it can be done. We once had a boat with a 502” engine and a blower, it produced 550-600 HP, and it could be sailed at 5000 rpm for long stints without complaining. Tuning with a blower should be done as described for a normal engine except as noted in the following paragraphs.
Ignition
It’s important to have the absolutely best ignition system money can buy. For two reasons. First of all, for reasons we really can’t explain, it’s more difficult to ignite the air/fuel mixture when under higher pressure. The second reason is related to carburetion. To avoid being lean it’s recommended to have a richer than normal setting of the carb. And to have colder spark plugs to avoid detonation. Having colder plugs can lead to fouled plugs, to avoid that you need the best ignition system you can, one that fires more than once, like MSD. Besides that the tuning will be the same steps as for a normally aspirated engine.
carburetor
You need a carburetor that’s built for a blower engine. The thing that is different is the power valve system. Normally the vacuum for the power valve is taken at the bottom of the carburetor, therefore from the intake manifold. However when the carb is placed on top of a blower, the vacuum gets too high, resulting in the the power valve not delivering fuel in the high rpms where it’s the most needed. You can compensate this by having a power valve with a higher vacuum stamped on it, but it may give you an overrich air/fuel ratio when not needed, resulting in excess fuel consumption, fouled spark plugs and ignition misses. It won’t work satisfactorily. You should buy a blower carb, on which the vacuum to the power valve is taken from a port which you then route to the manifold under the blower, the vacuum there is more suited for controlling a power valve.
When tuning the carburetor you go about it the same way as for normally aspirated engines. But given that the margin of error is smaller, be even more obsessed with reading spark plugs and being too rich is a lot better than being even slightly too lean (= new engine).
Blowers create pressure and heat. Pressure aids power. Heat removes power and causes detonation. You have to have a good cooling system on the car. Secondly you add a lot of power and produce a safer engine by adding an intercooler. That’s a good bargain, power and safety, usually it’s a trade-off between the two, here you get both.