Wilsons Sandbox

Dear Kimberites: Flipping through my file on this subject I reviewed a message to the list from Tom Poole on 2-25-2001 where he said he swapped out his Judson J 26 blower for the new Moss unit. He then modified the manifold by connecting a boost gauge to the intake manifold and recorded boost levels of 6 to 9 lb. Wow! Doesn't this seem awfully high? I reviewed the Moss literature and find no reference to the amount of boost they anticipate will be produced. I know that in other street applications 4 lb. is considered high and most turbos have a waste gate the bleed exhaust out before hitting the turbo when boost gets over 4 pounds. Also, most turbo/supercharged engines are detuned, i.e., have reduced compression ratios. At these boost levels is anyone concerned about wrecking engines? Kindest regards, Alan & Victoria Campbell

There are no rules concerning boost, but there are some generalizations. Turbos and belt driven blowers are both superchargers, which allow more than atmosphereic pressure to shove air and fuel into an engine. Most of the time, a supercharged engine behaves like a normally aspirated engine running under vaccuum. But when you step on it, and allow more air into the unit, the superchager acts as a pump and goes beyond atmoshpheric pressure (14.7 psi or thereabouts) and pressurizes the intake charge, shoving the mixture into the engine. This allows a smaller engine to burn more mixture and act like a bigger engine. Turbos are exhust-driven centrifugal pumps. Belt-driven pumps, commonly called blowers, can be of the centrifugal (Paxton), the rootes (Magnussen, GMC, Marshall, etc.) or one of several others, like the vane-type (Schorrock)or screw-type. Turbos will often release pressure, via a wastegate, sooner than a similar blower setup, due to the incresed temperature of the boost charge. Since most blower setups are "wet" meaning that fuel flows through the blower, you can get away with higher boost, since the fuel cools the charge and gets atomized very finely to boot. Adding an intercooler to a turbo setup to shed the extra heat will allow higher turbo pressures. Turbos also have what is called "turbo-lag". After stomping on the pedal, the impeller takes a moment to spool up before it generates pressure. This is usually offset in modern engines by very high-pressure impeller designs coupled with a wastegate. This gives better driveability at the cost of wasted efficiency whenever the gate bleeds pressure. Blowers have no lag. Since they are belt-driven, thay always have the ability to make boost, but normally are kept from doing so by a mostly-closed throttle. When you stomp on a blown car, air is allowed right into the already over-driven pump, the misture is pushed into the engine, and off it goes. Boost is controlled by pulley sizing so that the blower's cfm is at some percentage over the engine's normally aspirated requirements, say from 20% to 70%, with around 40% over-driven being average. 6-9 is what I would consider a normal range for an engine in the 8:1 or 8.5:1 range. Lower boosts are required for higher compression stock engines. (Say, a Chevy running 9.2:1 or so, you want to go for around 4-5 pounds.) I built a special blower engine for my Toyota Celica with 7.8:1 compression and regularly ran 13-14 pounds of boost. (Of course I had fuel injection and an ignition retarding setup that kept things right on the bleeding edge of detonation...) And there is the rub. You can get away with quite a lot assuming that the lower end is built to be very strong, especially our long stroke tractors, and detonation is controlled. No hot spots, keep it cool, a careful timing curve, and a camshaft ground for a blower which will be more sedate than a stock grind. The idea is to run the engine at its safe limits AND NO MORE! Just on the edge of detonation is great, but start pinging a blower engine and you will do lots of damage in a hurry. Blower Engines, like nitrous oxide injection, are great fun and very reliable (your average fuel consupmtion will improve due to increased efficency) but if you get greedy, you will pay for it. My old Toyota college project is still running 14 years later and still screams. Wish I never sold it. But to get right down to it, there are no rules except, "Don't melt it!!" Depending upon lots of different variables, one can use a little or a lot of boost, as long as the bottom end can take it and you don't detenate. To get to specifics, I personally wouldn't bolt a blower right on a stock XPAG engine. It would probably work fine, but not at its peak efficiency. Blueprint the bottom end, since they are a bit flimsy to start with, compression at 8:1 or so, a good camshaft designed for blower use, and some porting and polishing to the head. Carefully fiddle with the timing curve to get the most advance without pinging. Now you are cooking! depending upon variables, you might get up to 10 PSI safely, but 6-8 would still be a kick in the pants! So, to quote Bill Nye the Science Guy, "Now you know!!!", probably more than the question needed, but I do like blowers. Cheers....Andy email me with any questions

Alan Back when supercharging was the common mod for T-types and there were several different kits on the market, 6 lb was considered normal for road use and people used more for racing. With standard compression there's no problem at six pounds, but with Stage 1 or more raised compression and a standard exhaust manifold, people (including me) blew engines 'cos the heat couldn't get away fast enough. I remember seeing the exhaust manifold and the first two feet of the exhaust pipe glowing red after a few miles test run on a summer night! Fitting an extractor exhaust allowed Stage 2 ie 9:1, plus supercharger. Of course the engine has to be in good condition to start with. I've read that prewar you could blow a P-type at 6 pounds and still retain the MG new car warranty. Regards Dave Dwyer J2, TA, TC

My experience-for those interested -is positive. I am currently running 3 supercharged MG's. TC 7563 with a Moss (Rootes type) at 6 # +, a P A 1510 at similar pressure using a vintage Centrics(vane Type) driven by the factory installed pulley and L 0414 at 10# + with a modern Volumex (Rootes type) driiven of the crank.All three blowers are on engines in excellent condition and really improve the performance. I believe they make our cars better, particularly the MMM cars. For what its worth, Tom Poole

Tom, Could you enlighten me regarding the Volumex blower. Regards Dick Shaler F 0285 F1319 TC5905

Sorry guys. In my post about the Bishop box rebuilder, I forgot to mention that I need the guy to be in the States.... Thanks, Ray McCrary "Speed is Life; of course Luck and Altitude are helpful, too."