Date registered: Feb 2005
Vehicle: 1989 W201.029/M103 3.0
Mentioned: 0 Post(s)
Quoted: 4 Post(s)
RE: Supercharging a '67 250SE
Supercharging is definitely a good way to get loads of extra grunt without a complete engine rebuild but I'm thinking the '67 being so long in the tooth will probably want a good reconditioning anyway if it's not going to let go once you start planting the foot.
Three types of supercharging are available:
Roots-type superchargers are like the classic 6:71 GM style blowers and work like a big fan shoved onto the engine. Gives great low-mid range power and just keeps building boost as the revs rise.
Eaton makes some good low-blow units you can get in variously manufactured kits/bodies for stock and near stock small-medium capacity engines. People like B&M and Weiand make the serious stuff.
Roots-type superchargers can be piped to blow through or draw through your stock throttle body, or mounted to a custom intake manifold for a draw through setup with whatever aftermarket stuff you want to put on it.
The biggest consideration for one of these is making sure you get enough airflow cfm per cycle for your needs (manufacturer guidelines are very rough, better to do your own homework and don't trust the salesmen).
Turbochargers work like an air compressor run off an exhaust turbine. They compress a charge and then release it into the engine, giving a sudden boost in performance and sized right for your engine's needs, maintaining it as the revs climb. Biggest consideration with these is making sure you've got the right compressor diameter for your car, so you get boost at optimum revs.
Also something called an island has a cfm rating that determines the max boost a given unit is capable of on a given engine.
For yours, a 350 island-cfm rating might top out at say 6psi, a 500 at 10 and a nice 800 at 14.
You control the unit-available pressures with a wastegate and blow-off valve (better internal than the generally unroadworthy external ones).
Once again, a little bigger on cfm rating is better than too restrictive, but make certain you get the right diameter compressor piping to bring the boost in just right and then control it on the wastegate and blow-off accessories.
Centrifugal superchargers are like a turbo driven by a belt drive rather than an exhaust mounting, but in action work closest to turbo-supercharging of WWII era fighter design.
Once again they compress the charge before it goes into the manifold but they build up boost somewhere between turbo charging and supercharging and in race applications work best at mid-high revs.
Low-blow units available in these types are a great compromise, delivering a couple of psi at low revs and giving good power gains for stock engines, then hitting strong at mid-range with its designed boost range through to redline.
To drive you can feel them come on down low without any of the lag associated with turbos.
These you have to control a little more directly with their turbo-like island-cfm ratings and get the right one for the boost you want, then control where in the range it comes in with the belt gearing. Also a good idea to pop pressure-release valves on them (works like a wastegate/blow-off valve).
But like I said your engine will want a freshen up anyway, with new bearings and some regrinding and flushing. The bonus of this is you get to improve a few points here and there with a total project in mind, to get the most out of the application and setup you've got planned.
Get low compression, stock flat-top pistons fitted for around 8:1 static compression ratio. With good cam timing this will drop to around a 6.5:1 dynamic compression and is good for mucho boost and massive power gains on supercharging. Throw your manufacturer's guidelines out the window here and go for 15psi street boost for some real wheelspinning action and good quater mile times.
If you're going to go full house (ie. chasing 400hp out of the old 2.5E), inquire about Porsche pistons and rods resized to fit, most of 'em are pretty close to the Merc bores.
You can also flow bench the head for the right intake/exhaust ratios for your supercharger type, getting the most out of the setup (can be worth as much as 20hp just there). And all new double valve springs, etc.
Make sure you have a good flowing, tubular exhaust fitted.
Most of the stock internals are actually pretty good for mid-range performance applications, the Mercs are generally designed to spin to 6500rpm safely as is so can take low-medium boost designs and return high performance without trouble, so long as you get the compression and flow ratios sorted.
Stock Bosch manifolding is good for blow-through supercharging and on a basic setup there's no real need to change the throttle body. You can source larger injectors from other models and you'll need those.
Get some specialized supercharging cam grinds too.
A windage tray fitted to the sump will help. So will an electric fan, gapless piston rings and it is a must to have the whole engine expertly balanced during the process.
And don't forget a bigger radiator. When you're playing around with boost, think about intercoolers and water injection (especially for turbos and centrifugals).
driving a fast car should feel like falling off a building.