Quote:
Originally posted by niles
I would appreciate some serious dialog on this subject.
given that 10:1 is about the limit for pump gas. How does a blower allow a 9:1 motor to have 6-8 psi boost without detonation? Is the max compression psi the detonation limiting factor? If so, then I don't understand how blowers help?
I would like to get a technical blower thread going. Can I put a "little " blower on my 10:1 or do I need to build a new motor?
The ease of bolting on an additional 100 hp is interesting; but somehow I think there must be a catch-no free lunches normally apply
gn
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Niles,
Factors such as cylinder head combustion chamber design (quench), cylinder head material, camshaft lobe centre and duration, piston shape, supercharger design and efficiency and fuel octane rating all affect the wellbeing of your engine when running a supercharger.
Because you have a Cleveland, I'll jump to conclusions and guess that you're running open chamber 4V heads. this chamber desing is as blower friendly as you can get in an iron head. Alloy heads can dissipate heat more efficiently than iron - therefore making alloy heads the best option for combatting detonation.
A camshaft with a fair amount of duration will bleed cylinder pressure, making things safer. A bit of boosted air straight out the exhaust is no big deal!
I think over 240 degrees duration would be nice.
Good quality forged pistons would be a mandatory option for any blown engine of mine.
A roots blower (B&M, Holley, Weiand, BDS, Littlefield, Kuhl, Mooneyham etc) is by far the easiest installation, and can use your carburettor on top. Downside is the extra height of the blower on top of the manifold, and the significantly lower efficiency of the blower. They use more horsepower through frictional loss, and add more heat to the compressed air. More heat = less air density (less oxygen), and greater chance of detonation. Intercoolers are also more expensive and add even more to the height of the motor.
A centrifugal blower (Vortech, ATI, Powerdyne, STA etc) is more efficient and can be packaged below the hood - anywhere in the engine bay, as long as the blower pulley still lines up with the crank pulley
I've never had anything to do with carb'd centrifugal blown applications, but I'd say that there'd be a lot of sorting out to do. Otherwise, it's $$$ for the EFI conversion.
A Screw compressor (Kenne Bell, Sprintex, Whipple, Lysholm etc) is similar looking to the roots blower, but has a more advanced internal rotor design to compress the incoming air within the blower casing, rather than just displacing XXX cubic inches of air per revolution (as the roots blowers do). It is more efficient than the roots, but cannot match the centrifugal for peak horsepower.
I had a B&M 144 blower on a 302 with 10:1 KB hyper pistons, and running anywhere near the ignition timing that the motor needed to make power resulted in crank pistons - around the ring land. Forged 9:1 pistons alone stopped the carnage, but detonation was always a concern on a hotter day. Running Avgas 24/7 would have solved the problem entirely, but I wanted to use the car daily and I was too tight to spend the extra money.
My new motor is 8.5:1 compression, and I'll be running more than 20lb boost. The old theory was that you halve the boost value, and add it to your static compression ratio to give you an effective "running" compression ratio. So how does 18.5:1 compression work with 98 octane pump gas? Custom designed camshaft, 18 degrees total timing, alum heads, lots of fuel (96 lb/hr injectors), 24"x12" intercooler, JE custom pistons and careful mapping on an engine dyno.