Pulley size/boost ratio?

speedingminivan

Registered User
Anyone know of approximate boost levels from different sized pulleys?

I'm new to the M90 area and don't know what its specific output is per pully size. What is the stock size pulley for an M90 off a T-bird SC? And what level boost is it running? Thanks.
 
boost is not always more power, boost is back pressure. However the 89-93 had a slightly smaller diameter pulley than the 94-95. The 94/95 blowers were redesigned to make more efficient boost so they made the pulley larger. Stock sc's will make 10-13 psi. Its really hard to guage what psi of boost you will have under certain condition. If you leave stock pulley on and remove cats then you will lose boost but gain slight power due to better flowing exhaust. Also do not reccomend any smaller pulley until you do something with the exhaust or you can hop on the head gasket bandwagon. Also you should consider better intercooler or at least a fan for it. Off the top of my head i dont know exact sizes but a 89-93 pulley is often considered a 3% overdrive when used on a newer blower. A 5% is usually ok with no work done to your car but you were cautioned about the hg's. And a 10% should not be installed unless exhaust work is done and intercooler is new or at least has one fan on it. Im sure others will chime in and add to the list. I also believe in the faq there is something about this same question ill look and post back!

When is it safe to run od

FAQ Forum, with many helpful threads
 
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Pretty sure a stock pulley is right around 3 inches, because a 10% overdrive pulley is 2.8 I believe. Someone may correct me on that.
 
What is the diameter of the crank pulley turning the supercharger? I'm matching this to a different car with a 135mm (about 5-5/16") crank pulley.
 
What is the diameter of the crank pulley turning the supercharger? I'm matching this to a different car with a 135mm (about 5-5/16") crank pulley.

The supercoupe employs three pullies in driving the supercharger...the crank, a jackshaft pulley and then the supercharger pulley itself. For your application, there is not really a way to tell what pulley will produce what boost. Everything about your project is different from a super coupe. Your engine is smaller than a super coupe engine and the entire intake is different, not to mention the exhaust system from the exhaust valve back.

For informational purposes, the stock crank measures 6.9910", the stock jackshaft measures 3.9710 and 4.930" and a stock '94-'95 pulley measures 3.2750" giving you a final drive ratio of 2.65:1, (unless the pulley measurements I have are wrong, which I doubt) supercharger to crank. That is the largest pulley combination that we have producing the lowest airflow (assuming an early style supercharger). You should shoot for no greater of a ratio than that. Not sure what your 3.0 runs, but our 3.8 runs 8.2:1 compression ratio and we make between 10-12 lbs of boost on a completely stock super coupe with this combination. That may be too much air to start with on your engine. You might try going with a 2:1 ratio just to get the supercharger on and working. Then once that happens, you can start dialing up the boost via smaller pullies. But until then, for the sake of your motor, you want just a little boost...like around 6 psi, get it working, then go up from there. Just my .02.
 
my motor stock has a 9.5:1 compression ratio. im thinking about running maybe 10 psi, which gives it an ECR of about 16. however the highest octane gas i can get locally is 91. the coolant when hot is usually around 180 degrees, and ambient air temperature is about 75 most of the time. i can richen the fuel mixture by 6% and safely retard the ignition just over 4 degrees (cam timing cant be changed due to this being an interference engine).


think the gas will put up with that ECR without pinging? or should i drop to 8psi which is about 14.7 ECR?


thanks
 
well u can start with wut pullies u have and wut boost ur makin right now.

along with whatever other upgrades ya got. ;)
 
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