How much boost?

Kit Sullivan

Registered User
How much boost is the stock set-up supposed to make under full throttle?
My 1990 (according to the boost gauge) makes about 9-10 at full throttle, and never goes beyond that. Is that normal?
 
It varies some with climate. Around here I pushed about 12-13psi stock in my 89.

I would check for vacumn leaks either way If I were you just to make sure

chris
 
About normal assuming a stock car. The thing most don't seem to understand is that boost is a measurement of the mismatch between blower output and what the engine can use at that instant. The engine is being fed with a positive displacement pump that is pumping a compressible gas (air). If the blower is putting out more air than the engine can use at that instant you see a pressure at the inlet of the engine (ie Boost pressure). But given the same blower output if the engine is flowing better (like with a more free flowing exhaust system) boost pressures seen will go down. So the name of the game is maximizing blower output (aftermarket blowers, bigger TB, more free flowing intake air via filters and systems) and getting the motor to flow better (ported and polished heads, headers, bigger exhaust system, bigger valves etc etc etc). True to make the extra power you need to be able to inject the fuel required to make it with all that extra air.

You can see this aspect at work if you accelerate your car with a set amount of throttle. When you make the step change in throttle position boost will jump up. Assuming throttle position stays the same boost will slowly drop as the car accelerates and RPM build until acceleration stops for the given throttle setting. (NO WOT RUNS HERE GUYS.....PART THROTTLE ONLY)
 
So ,then the brand-new free-er flowing exhaust I have on it will tend to allow the engine not cause as much boost to be made at the same rpm as before, with all things else being equal?
Would it be safe, or advisable to put a slightly different pulley on the blower to help produce more boost, or would that put too much strain on the (stock) head gaskets.
 
Kit,

All things being equal and the only change being made is to open up the exhaust...boost pressure will drop. I saw it on my own car. I opened up the exhaust (more) on a modified engine and exhaust system and I no longer peg out the boost meter as I once did. (I run and S Model Blower with 10% OD puulley, extrude honed IC Tube, double IC and a custom exhaust on a Coy Miller motor with shortie headers) In fact I saw this when I made the intermediate step of replacing Dynomax Turbo Mufflers (I think that is what was on the "original MN12 Performance Exhaust system) with Borla mufflers.

Now the concerning the question about changing blower pulley's. You would certainly be safer doing this mod. after opening the exhauset system compared to doing it before. Can I say you will not cause a problem???? NO given I have no idea what the conditon of your motor is at present. Have others done this mod with a stock exhaust and "lived"...YES. Have others done the mod with an exhaust system opened and "lived"??? YES. That is the best I can advise you. Modify at your own risk I guess is the bottom line.
 
What are the options for differnt pulleys for mine? (its a 1990) Are there several different sized pulleys avaialable, or just a few..or just one?
I would probably want the mildest upgrade available...something that I would notice in performance, but would put the least amount of added stress on my 'seasoned' motor.
By seasoned, I mean: 70,000 miles, very well maintained, not abused or raced over the years, and everyhting works well. It runs great. I recently installed a new radiator, new hoses and belts. I put the radiator in becuise the stock one was looking a little green around the top tank...who needs that? But it was working fine before I changed it.

It seems that after putting this new cat-less, free-er flowing exhaust on, the car doesn't have the same 'snap' it did off the line as before. I guess that is attributable to the less restriction/backpressure means less overall boost...which means: less performance. But at least the head gaskets have a little easier time of it, which is what I was after.

How much are the diferent pulleys? Does the belt need to be changes also? How easy is it to change the pully?
 
5% and 10% OD pulleys are pretty much the standard. Spinning Wheels (seel Links Page for url) sells pulley set-ups. Personally if I was doing the mod myself I would use a modified Jackshaft Pulley to get the "better" drive ratio on the blower. Some who have installed 10% OD pulleys on the blower itself have experieced belt slippage due to the reduced surface area on the modified blower pulley. Some have also had problems getting the old blower pulley off without damaging it. The Jackshaft Pulley is a couple of bolts. Victor should be able to advise you as to what belts to use with his product.
 
I don't know what a jackshaft is, but I assume it is the other pully in the SC drive belt system.
Why are they better than just changing a pully? Or is it just easier?

I looked on the spinningwheels site, and they only show a pully, no jackshaft.
 
The jackshaft is on the passenger side of the engine on the accessory bracket. It is a water pump style bearing which supports the shaft. The pulley mounted on it is driven by a belt coupled to the crankshaft pulley. The Jackshaft Pulley then drives the blower pulley. If you change the diameter of the jackshaft pulley you chage the effective drive ratio (how many turns of the blower pulley you get for each turn of the crankshaft). So if you want to overdrive the blower you can change the diameter of the blower pulley (make it smaller) or change the diameter of the jackshaft pulley (either the part coupled to the crankshaft...or the one coupled to the blower). I am not sure which diameter is changed on the available aftermarket Jackshaft Pulleys. Perhaps others have something to add.
 
I agree that changing the jackshaft pulley is a better option. Why....because you are able to overdrive the blower without reducing the surface area where the belt contacts the pulleys. You actually increase the surface area.



All the overdriven jackshaft pulleys I've seen maintain stock diameter on the driven portion (small side) to allow use of the stock belt between the pulley and crankshaft pulley. The large portion of the pulley accepts the belt that drives the supercharger. To overdrive you simply make that portion of the pulley larger in diameter. This is a picture of a standard diameter jackshaft pulley compared to a 10% overdrive jackshaft pulley. The one on the right is the overdriven unit....both are made special to accept a 10 rib supercharger belt. The large portion of the pulley on the right is 10% greater diameter than the one on the left. The diameters of the small portion of the pulleys remain the same.


JackshaftPulleys.jpg



David
 
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Spinning Wheels had them listed on their site when I looked the other day and I think you said you saw them as well. Other vendors may have them as well.
 
How much boost is the stock set-up supposed to make under full throttle?
My 1990 (according to the boost gauge) makes about 9-10 at full throttle, and never goes beyond that. Is that normal?


The factory gauge is nothing but a general indicator of vacuum or boost. If you want to know your real boost levels, you need to look into an after market gauge.
 
By lowering the back pressure you move the power peaks up a bit, it may feel slightly less powerful down low but it will pull harder up top. Since most of us drive around on the street and typically stay in lower rpms it will feel slower. The pulley corrects that. If you go too small like I did (10% on a 90 SC w/ exhaust) the intercooler can't cool the air as well and up higher in the RPMS the computer pulls timing. I noticed this at the shootout as my car was strong in the lower power band but up top it layed down. My trap speed showed that it wasn't pulling as hard at the 1/4 as it was at the 1/8th mile mark.

It feels stronger around town with the 10% pulley, it really jumps up and runs away from a stop light, but it shifts around 3000-3500 with moderate throttle. At those speeds the heat isn't so bad, but the faster the engine spins, the faster the blower spins, and the more heat is generated. I have a fan on my IC but it's not enough. The IAT sensor tells the computer the air is too hot and to prevent detonation the computer turns back the timing a bit which in turn isn't making as much power as it could. I have colder plugs in the car so I should be safe, but the computer doesn't know that and it's set in its programming at what temp to start retarding the timing.
 
So, with the 5% jackshaft pully (I found them of the spinningwheels site eventually!) there isn't as much heat generated, right?
Will this still give me a noticeable improvement in around-town driving? Is there a big 'seat of the pants' difference in the 5% and the 10% as far as performance, or is just a 'race-track' difference?
 
So, with the 5% jackshaft pully (I found them of the spinningwheels site eventually!) there isn't as much heat generated, right?
Will this still give me a noticeable improvement in around-town driving? Is there a big 'seat of the pants' difference in the 5% and the 10% as far as performance, or is just a 'race-track' difference?

No matter where you overdrive the blower from, it's going to generate more heat. Yes the 5% will make enough extra power that you will feel it. Especially when nailing the throttle from a standstill :)

David
 
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