Stock boost is ~5.5#, would I have detonation or other probs. w/ a 7.5# pulley change. Note I have heads milled .030.

 

i would not go 7.5 psi on pump 93 w/o intercooling unless you're down to low 9.x:1 CR. to make that much boost on a chevy ls1/ls6 motor would mean lots of inefficiencies that contribute a lot to charge air temps on top of the adiabatic compression. also i think the magnacharger is at or near its rpm limit with the 5.5psi standard pulley set at the vette's rev limiter.

 

I think the stock set up is about it. By the way, on my ZO6, the max boost reads a bit over 6 pounds at 6500 rpm. Could be the guage is wrong, but I got the info on volumetric efficiency and so forth from the Magnason page and did some claculations. Efficiency increases with RPM and the unit is near its rev limit at 6500 RPM, where my computation predicted it would produce 5.5 at 6000 and 6.20 boost at 6500 -- forced flow grows faster than RPM since vol. eff. increases with RPM, so pressure builds slightly at higher RPM.

 

I've had 2 GTP Grand Prixs w/ a mild overdrive pulley and a friend had a T'Bird S/Coupe w/ same and we both noticed the main improvement was that the boost came in sooner giving more low end performance, however, we didn't notice any appreciable boost on the big end. :confused: The pulley was supposed to increase the GTP boost from 7 lb to 10 lb. Since the car didn't have a true boost gage w/ a numerical readout, I don't know what the boost was.

 

I agree with STAGED, I would have concerns going much above 6 without an intercooler. You are going to push the temps up quite a bit as you push more boost, as you get to 7psi I believe your temp increases ~100% over ambient, and that heat may cause KR to pull timing and lose hp or be more prone to detonation. Just my thoughts.

I know the 03 Cobra guys are doing that, but their 7.5-8 psi has an air-water intercooler, thus they can swap to smaller pulleys to up the boost to a good 12 or so on top of the forged internals and lower 8.5 compression engine and be fine.

ZZ

 

Check out this info from Thrasher's website and let me know if you think I can use a 7.5 lb. pulley.

A Common Question: "I installed a set of ported heads with no other changes and my boost level dropped. What is happening?"

The Answer: In this instance, the improved flow of the exhaust ports on the new head allows the engine to more efficiently get rid of the burned gases. The blower is not building "psuedo boost" trying to push out the remaining exhaust gases. Although the boost pressure is higher, less air is actually being forced into the engine because of the restricted exhaust and power is limited. With a better flowing exhaust, more air is forced into the engine because the blower is not also trying to force out the exhaust byproducts. The boost pressure is lower, but the mass of the air entering the cylinders is actually greater.

Stated another way, boost pressure is a measure of how restrictive your intake and exhaust system is. The more restrictive your system is, the higher the boost number will be. Restrict your exhaust flow and you may see your boost rise, but your power will actually fall off as the blower uses more energy tryng to push against this back pressure.

Trying to spin the blower faster in an attempt help "push out" the spent gases may help slightly, but it may actually hurt power. As a Roots type blower spins faster, its adiabatic efficiency drops. This means that you are heating the incoming air more than compressing it. As the air is heated it expands and although the apparent boost pressure rises, there are actually less air molecules per unit volume and less air means less power.

This point is outlined in the October 1998 issue of Automotive Industries, a highly respected automotive engineering industry trade magazine in an article titled "Eaton Boosts Engine Aspirations" by Don Sherman.

"Compared with supercharged engines offered by the imports, GM's Series II 3800 V-6 uses lower boost pressure and no intercooler, note GM Powertrain engineers Bill Owen and Robert Gardner. The GM engineers regard boost as a measure of inefficiency - GM uses 7.5 psi, only 2/3 as much as Jaguar. Boost is the pressure required, above atmospheric, to push the charge through the engine to make power. Less boost for a given amount of power means more efficient flow, because heat and parasitic losses both rise with boost pressure."

Another important point made in this article is this:

"Superchargers are highly tunable. By adjusting the crankshaft-to-supercharger drive ratio, OEMs can schedule air delivery low in the rpm range, for extra surge from a stoplight, or up-register for enhanced passing performance.

What this means is by changing the size of the supercharger pulley, one can optimize the power output to favor high RPM horsepower, or low RPM torque. We've seen this in testing on the dyno where decreasing the size of the blower pulley beyond a certain point results only in more low end torque with diminishing gains in upper RPM horsepower - the smaller you go, the more the high RPM horsepower is sacrificed. Therefore, the optimum pulley size is one that balances the low end torque and high end horsepower, and is very dependent on how efficiently the exhaust can be expended.

Also: http://www.thrashercharged.com/L67_htm/pulley.shtm http://www.thrashercharged.com/L67_htm/dynodata.shtm http://www.thrashercharged.com/L67_htm/intercooler.shtm

Rhino, what do you think given this info, can we do the 7.5 lb pulley to get more tq and response at a lower rpm while sacrificing some hi-end hp?



[Modified by Steve02C5, 4:25 AM 8/21/2002]

 

I think the marginal boost creep on the Roots that you speak of comes from greater increases in charge air temps as one climbs which is realized through higher boost pressures but not necessarily proportionate increases in MASS AIRFLOW rate.

As for GM's philosophy with using a moderate boost non-intercooled setup, I think (in additon to what's mentioned) that they use that particular setup because of reduced costs and complexity of running non-intercooled, putting a lid of boost pressures limits the driving force one needs, and eliminating the intercooler eliminates further drive losses. Of course with higher dollar euro imports all this is less important compared to the loftier performance targets, hene higher boost pressures and intercooling is used.

Fortunately, for many recent years, GM's 3800 S/C started out with a good foundation and their modest requirements to stay at the top of the class in straight-line acceleration only required moderate nonintercooled boost pressures thanks to a fat blocky boost curve. As competition heats up, they may well resort to higher boost pressures and intercooling.

 

On the GTP's GM stuck a rather big pulley (3.8") as stock so that allows plenty of room to play with and still be within the safe limits of the M90 (I believe that 3.1 is the lowest safe for the SC pulley) I ran a 3.25 on mine for almost a year with no problems. Find out what the max safe spinning speed of the Magnacharger is and what size pulley somes with it. After a certain size you have to worry about boost stacking, KR, and a whole new slew of problems. Ask some of the guys on http://www.clubgp.com about SC's, they know them pretty well over there.