Roush Supercharger real world PSI at altitude?

[Dane]

That does seem about 1# low. What’s the actual DA on days ur seeing the boost before and after. I was a solid 8# at 5500ft on the 69 pulley stock headers, tb etc. When I swapped the lower jumped to 10# but think may have been 11# lots of spin and gauge maxed out if I recall. So before headers and smaller pulley you made 6# and now with smaller pulley and headers your at 6#?


How does it feel? Any faster, more torque etc? Weren’t you going to have a lower out on like a year ago or am I thinking of someone else?

DA?
I'm at about 6# with full headers. Everything is saying that headers should drop 2psi (a good drop), so that matches. My calculated boost on my SCT is matching my gauge, so thinking my gauge is correct.

Yes, I stayed at 6# going from 75mm and stock exhaust (well, a cat back) to 69mm pulley and full Kooks true dual.

I've talked about doing a lower, but haven't committed to it.

I reviewed all of my fuel trim and lambse numbers with JDM this morning, and they're saying everything is perfect.

 

[EricM]

Boost is just a measure of back pressure in the intake tract. It's truly meaningless in a positive displacement blower setup, other than being a judge for the required octane. PD blowers are unique in the way they move air, and what they are sensitive to.

With a PD blower, the air is going to be fed into the engine based upon the pulley ratio and displacement of the supercharger. There are minor secondary affects, but the blower size and pulley ratio is BY FAR the controlling factor.

If a PD blower sees some downstream restriction (ie "boost") it really doesn't care much and just crams it into the engine anyways (which is not the case at all for centrifugal blowers or turbos). Decreasing back pressure (adding headers for example) will generate slightly more power, but it's only because the compressor is operating more efficiently (less downstream restriction). The same amount of air is going into the engine either way, since the PD blower 100% controls the airflow rate. Therefore, just adding headers and leaving the pulley ratio the same will yield very little power gains. You have to "pulley up" to get more air into the engine if you want more power with A PD blower. Since we are generally octane limited in street vehicles, the headers can allow you to run a higher pulley ratio (more air) and still have the same boost, and similar cylinder pressures. The cylinders pressures will go up some though, since you are making more power, there's no way around that.

What does affect a PD blower big time is inlet restrictions, they are very sensitive to that and any bottlenecks in the intake will cost you big power/airflow. Run the biggest filters, pipes, and throttle body you can get away with to maximize the potential of the PD blower.

 

[biturbomg]

DA is the dense Altitude so for example when I test or race here at 5200 feet the actual DA will be 6800ft. Thats why some of the best tracks you see great times are at MIR or ATCO and they run on negative DA days. Sounds like you checked everything and all is fine. Do you have a bigger TB? I know that can increase boost some depending how big you go. On my Shelby KR with the TVS 2300 going to a twin 67 made 25whp similar torque. Aside from that you can do the lower. Thats why I did the lower her in Albuquerque cause just wasnt making enough boost.

 

[Dane]

Boost is just a measure of back pressure in the intake tract. It's truly meaningless in a positive displacement blower setup, other than being a judge for the required octane. PD blowers are unique in the way they move air, and what they are sensitive to.

With a PD blower, the air is going to be fed into the engine based upon the pulley ratio and displacement of the supercharger. There are minor secondary affects, but the blower size and pulley ratio is BY FAR the controlling factor.

If a PD blower sees some downstream restriction (ie "boost") it really doesn't care much and just crams it into the engine anyways (which is not the case at all for centrifugal blowers or turbos). Decreasing back pressure (adding headers for example) will generate slightly more power, but it's only because the compressor is operating more efficiently (less downstream restriction). The same amount of air is going into the engine either way, since the PD blower 100% controls the airflow rate. Therefore, just adding headers and leaving the pulley ratio the same will yield very little power gains. You have to "pulley up" to get more air into the engine if you want more power with A PD blower. Since we are generally octane limited in street vehicles, the headers can allow you to run a higher pulley ratio (more air) and still have the same boost, and similar cylinder pressures. The cylinders pressures will go up some though, since you are making more power, there's no way around that.

What does affect a PD blower big time is inlet restrictions, they are very sensitive to that and any bottlenecks in the intake will cost you big power/airflow. Run the biggest filters, pipes, and throttle body you can get away with to maximize the potential of the PD blower.

This is great information. Thank you.

 

[Dane]

DA is the dense Altitude so for example when I test or race here at 5200 feet the actual DA will be 6800ft. Thats why some of the best tracks you see great times are at MIR or ATCO and they run on negative DA days. Sounds like you checked everything and all is fine. Do you have a bigger TB? I know that can increase boost some depending how big you go. On my Shelby KR with the TVS 2300 going to a twin 67 made 25whp similar torque. Aside from that you can do the lower. Thats why I did the lower her in Albuquerque cause just wasnt making enough boost.

I have the upgraded 65mm TB.

 

[MTF]

Before going to a smaller SC pulley you may what to consider the ATI 12% oversized Harmonic Balancer.
Just a thought.

http://www.atiracing.com/products/dampers/charts/damford.htm

They will make a 6 rib if you ask.

 

[Dane]

Before going to a smaller SC pulley you may what to consider the ATI 15% oversized Harmonic Balancer.
Just a thought.

There is no smaller pulley to go to... I'm at 69.

 

[MTF]

Well than the ATI oversize would be the next step

 

[Dane]

Well than the ATI oversize would be the next step

Yeah, that's an option.

JDM is saying if I do that I should swap all my pulleys to different sizes to counteract the extra spin (other than the SC obviously). That sounds like a major pain.

 

[biturbomg]

I ran my 10% IW on my Shelby KR and on my Raptor and didn't swap any other pulleys and had zero issues. I understand the logic but just never did it and on of my SC setups and was fine. I have the last remaining IW pulleys made for the raptor if you want one I can get you one at cost. Just PM me.