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Matthew E. Herd
Jedi Warrior
Offline
Like I said before, I believe a single 45 is better than dual 40's for most engines. The carb is cheaper than buying two, the manifold more available (and cheaper), and I've found even with my very well breathing engine that full throttle versus say 85% the response is indistinguishable because the carbs are slightly bigger than necessary. On a poorer breathing engine, its probably in the interests of driveability to go with the 45 because of the faster gas speed, inproving low end torque (and therefore making it easier to drive) at the sacrifice of your top end horsepower. You won't get to use whatever you might lose anyway, so why bother?
If you're thinking of cheap performance mods, don't go w/ a camshaft (because you need to pull and rebuild the engine). But if you're already pulling it, make sure you select one that isn't too radical because max rpm is limited on these long stroked engines (figure 6500 TOPS for a street motor). The spitfire 1500 and 1300 engines are identical, with the only difference being some versions had cam bearings and some didn't (midget 1500's don't have camshaft bearings) and the crankshaft is smaller and has a shorter stroke in the 1300, making it able to rev higher. This is why some really racy cams are offered, although they can't be effectively used in a 1500 without excessive risk of engine damage. Go with a fast road cam, or no more than the kent cams TH5(4)--4 indicating cylinders-- the VB "fast road" cam is actually a kent TH2 cam, which would also be a good choice and would be extremely driveable and safe with no modification to compression ratio, rocker ratio, valve size, or valve springs required (which the TH5 may in order to improve low end driveability and/or preserve the engine at higher RPM (standard valves would likely float at 6500 with such high lift of the TH5).
Basically, if you're looking to build a fast engine, I can give you practical advice and experience, but the theory is basically a summary of David Vizard's "Tuning the A Series Engine for Performance or Economy" which you should read as a priority before doing anything other than standard parts replacement to your engine (i.e. bearings and gaskets). Due to the relative scarcity of the Spitfire engine as compared with the A series, parts are MUCH more expensive. Expect to spend a LOT. However, everything in David Vizard's book is applicable, except that, obviously, the info on specific engines does not apply.
[ 04-16-2004: Message edited by: Matthew E. Herd ]</p>
If you're thinking of cheap performance mods, don't go w/ a camshaft (because you need to pull and rebuild the engine). But if you're already pulling it, make sure you select one that isn't too radical because max rpm is limited on these long stroked engines (figure 6500 TOPS for a street motor). The spitfire 1500 and 1300 engines are identical, with the only difference being some versions had cam bearings and some didn't (midget 1500's don't have camshaft bearings) and the crankshaft is smaller and has a shorter stroke in the 1300, making it able to rev higher. This is why some really racy cams are offered, although they can't be effectively used in a 1500 without excessive risk of engine damage. Go with a fast road cam, or no more than the kent cams TH5(4)--4 indicating cylinders-- the VB "fast road" cam is actually a kent TH2 cam, which would also be a good choice and would be extremely driveable and safe with no modification to compression ratio, rocker ratio, valve size, or valve springs required (which the TH5 may in order to improve low end driveability and/or preserve the engine at higher RPM (standard valves would likely float at 6500 with such high lift of the TH5).
Basically, if you're looking to build a fast engine, I can give you practical advice and experience, but the theory is basically a summary of David Vizard's "Tuning the A Series Engine for Performance or Economy" which you should read as a priority before doing anything other than standard parts replacement to your engine (i.e. bearings and gaskets). Due to the relative scarcity of the Spitfire engine as compared with the A series, parts are MUCH more expensive. Expect to spend a LOT. However, everything in David Vizard's book is applicable, except that, obviously, the info on specific engines does not apply.
[ 04-16-2004: Message edited by: Matthew E. Herd ]</p>
