SoCar72
Well-known member
I'm in the earliest-most stages of supercharging my 170. Actually, I'm planning to swap for a 7-main 170, if I can find one, for the extra mains and (more importantly) getting away from the LOM distributor, among other design advantages.
So, the carb; I'm debating either :
Blow through the carb with a bonnet.
Blow through the carb in a box.
Pull through the carb and duct to the intake.
For any course, I'm planning on a small 4bbl, either a 390 vac sec or a 450 mech sec both of which I have from previous projects. I bought a used GM M62 which, according the the Eaton flowcharts, is the best overall size for the 170. It looks like driving for about 7psi puts the s/c in its most ideal island for my expected rpm band, topping around 4500rpm. Given this application, what may the the pros/cons of each set-up? I am intending to keep all of this under a stock hood.
Through a bonnet : seems like the charge could blow through seals and such. How big of a concern is this really? How may this affect a vacuum secondary system?
Carb in a box : both carbs have the plastic floats, not the older brass-style. The plastics should resist implosion considering the carb will be in a pressurized atmosphere. If the vacuum secondary circuit senses the pressure differentiation between the "atmosphere" and the primary venturis to determine operation, how may a pressurized "atmosphere" affect this? Let's ignore that fact that I'd have to build a box and engineer it's plumbing and linkages as I'm 95% certain that the typical V8 style boxes will be too big for the carb mounting position. Or, am I wrong about that?
Pull through : theoretically, the carb would not know the difference between being drawn by a s/c or a naturally aspirated intake. I suspect no carb concerns. However, there is the ducting concern between the s/c and intake manifold and how well the charge remains homogeneous en route. Pull through's typically dump directly into a manifold to the ports, and not through a ducting before the manifold. I have seen a few photos of this arrangement both with the s/c on the same side as the intake and opposite, but no documentary of performance. Could I be over-analyzing this?
Mopar back in the day ran a dual 4bbl set-up with the carbs practically on the fenders, driver's side forward and passenger side aft. Word had it that this set-up was horrible on the street and not worth dealing with on the track. The downfall was manifold vacuum and the engine trying to pull the charge that distance and the cold runners let a lot of the fuel drop out of the mixture. No matter how you sliced it, the engine was working hard for a hard-to-digest drink.
This pull through has the advantage of the s/c pushing the fuel/air charge, when the by-pass valve is closed, and the s/c rotors "blending" the fuel and air as well as heating the mixture up, allowing for a better retention of a homogeneous mixture. At least so I theorize...
I have no experience with s/c-ing yet, but I did read Pat Ganahl's SA Street Supercharging book. EXTREMELY informative. However, carb mods are only covered across a couple of short paragraphs and only basically mentioned the blow through concerns. And "remote" s/c'ing wasn't covered at all, since the book is based around carb'ed V8 hot rods and modern EFI installations. All I have is my gut to rely on right now. If it's not evident, I'm leaning toward a pull through arrangement.
Any input would graciously appreciated.
So, the carb; I'm debating either :
Blow through the carb with a bonnet.
Blow through the carb in a box.
Pull through the carb and duct to the intake.
For any course, I'm planning on a small 4bbl, either a 390 vac sec or a 450 mech sec both of which I have from previous projects. I bought a used GM M62 which, according the the Eaton flowcharts, is the best overall size for the 170. It looks like driving for about 7psi puts the s/c in its most ideal island for my expected rpm band, topping around 4500rpm. Given this application, what may the the pros/cons of each set-up? I am intending to keep all of this under a stock hood.
Through a bonnet : seems like the charge could blow through seals and such. How big of a concern is this really? How may this affect a vacuum secondary system?
Carb in a box : both carbs have the plastic floats, not the older brass-style. The plastics should resist implosion considering the carb will be in a pressurized atmosphere. If the vacuum secondary circuit senses the pressure differentiation between the "atmosphere" and the primary venturis to determine operation, how may a pressurized "atmosphere" affect this? Let's ignore that fact that I'd have to build a box and engineer it's plumbing and linkages as I'm 95% certain that the typical V8 style boxes will be too big for the carb mounting position. Or, am I wrong about that?
Pull through : theoretically, the carb would not know the difference between being drawn by a s/c or a naturally aspirated intake. I suspect no carb concerns. However, there is the ducting concern between the s/c and intake manifold and how well the charge remains homogeneous en route. Pull through's typically dump directly into a manifold to the ports, and not through a ducting before the manifold. I have seen a few photos of this arrangement both with the s/c on the same side as the intake and opposite, but no documentary of performance. Could I be over-analyzing this?
Mopar back in the day ran a dual 4bbl set-up with the carbs practically on the fenders, driver's side forward and passenger side aft. Word had it that this set-up was horrible on the street and not worth dealing with on the track. The downfall was manifold vacuum and the engine trying to pull the charge that distance and the cold runners let a lot of the fuel drop out of the mixture. No matter how you sliced it, the engine was working hard for a hard-to-digest drink.
This pull through has the advantage of the s/c pushing the fuel/air charge, when the by-pass valve is closed, and the s/c rotors "blending" the fuel and air as well as heating the mixture up, allowing for a better retention of a homogeneous mixture. At least so I theorize...
I have no experience with s/c-ing yet, but I did read Pat Ganahl's SA Street Supercharging book. EXTREMELY informative. However, carb mods are only covered across a couple of short paragraphs and only basically mentioned the blow through concerns. And "remote" s/c'ing wasn't covered at all, since the book is based around carb'ed V8 hot rods and modern EFI installations. All I have is my gut to rely on right now. If it's not evident, I'm leaning toward a pull through arrangement.
Any input would graciously appreciated.
