Dodge Challenger Forum banner

1 - 7 of 7 Posts

·
Registered
Joined
·
74 Posts
Discussion Starter #1
Ok, guys, I need help on understanding some performance numbers.

I just got back from a dyno/tuning session. Car is an 09 5.7 with an Edelbrock SC using the standard 3.875" pulley being run on a dyno at 6000' altitude. Engine has stock internals, JBA shorty headers and a Modern Muscle throttle body. It did 440 hp, 475 ftlb torque on 6.7 psi of boost. I was expecting better, but tuner did not want to go with a smaller pulley, maybe he is smarter than I am?

Anyway, the machine was a DynoJet, I don't know the model number. I assume that this machine measures boost relative to sea level - or would I be incorrect in that assumption?

I also have an AEM boost gauge inside the car. I've been told that it measures boost relative to sea level pressure. My house is at 8200' altitude and the boost gauge is currently showing -7.1 inHg with the engine off which is about -4 psi. At 6000' altitude the air pressure change is about -3 psi from sea level. So before the dyno/tune session I would run the car at 6000' and see a max boost of 2 psi on the gauge, which I thought was reasonable. The Edelbrock SC was supposed to boost 5 psi out of the box, so the gauge shows -3 psi (-5.9 inHg) for ambient air pressure and max boost is 2 psi (total change of 5 psi), seemed to make sense. By the way the tune that Edelbrock supplied was complete crap, the tuner basically had to start from scratch.

First run the dyno measured boost was 6.3 (where did the extra inch come from???), mixture was way rich and the ECU was pulling back the timing when it had no reason to (explains the surging). The tuner worked several hours to get things under control. We ended up with boost as measured by the dyno at 6.7 psi. My gauge still shows 2 psi so that means to me that the SC is still boosting 5 psi above atmospheric pressure.

Researching the forums, it would appear that the Gen 2 5.7 can handle 7-8 psi boost above sea level, so that is 14.7 + 8 or 22.7 psi max. At 6000' altitude, atmospheric pressure is 11.8 psi so 10.9 inches of boost would be required to get to the 22.7 psi. Going with what I think would be max boost, should show about 8 psi on the gauge (gauge referenced to sea level). I don't know what the 6.7 psi the dyno was measuring really means. If it truly is referenced to sea level, then maybe I could go down one size in pulley. But I still don't understand the large mismatch between the dyno boost number and my car gauge.

Does anyone have some insight? Won't hurt my feelings if someone points out a major flaw in my logic.

Lowell
 

·
Registered
Joined
·
74 Posts
Discussion Starter #2
Just about done with this experiment. Here is where I'm at currently.

I talked with Edelbrock and Dynojet. Edelbrock said I do need to run with a smaller pulley to offset altitude loss. Dynojet said that their dynos measure boost relative to local atmospheric.

I decided to baseline things for the altitude in town which is 5300 ft. That means I have to boost about 2.6 psi just to get back to sea level horsepower. The forum indicates that a stock 5.7L can handle up to 8 psi of boost above sea level conditions, so I used 10.6 psi as a not to exceed for 5300 ft elevation.

I tried a 3.25" pulley first, but then switched to a 3" pulley. Turns out there was a dyno facility much closer than the one I had used for the tuning, so I did several pulls at the closer one. We were getting boost oscillation of about +/- 2.5 psi above 4000 rpm. The SC I installed was manufactured 4 years ago and did not have a spring on the vacuum actuator. I now have the spring and brackets installed, but won't get to do some additional pulls until spring.

I just recently received the drf files from the last pulls (with the boost oscillation) and did some simple calcs to filter out the boost oscillations. The average boost above 3000 rpm was 9.5 psi which gives me a 1 psi safety margin when running the engine at 5300 ft.

It appears that the tune at the first facility is doing ok with the smaller pulley. Corrected numbers are 501 rwhp, 542 ftlb torque, AFR was 12.6, and boost 9.5 psi above ambient. Uncorrected rwhp was 410. My AEM boost gauge was showing a max boost of 6.8 psi, so it is consistent with the boost measured by the second dyno facility (6.8+2.6=9.4). I'm using a Trinity to monitor the knock sensors and so far I'm not seeing anything above 2 volts.

One thing I don't know how to factor in is the lower octane of available fuel in this area. Max octane is just 91 out here. I hope backing off by 1 psi on the boost still keeps the engine in a safe place. And of course, if I ever take the car somewhere the altitude is less than 4000 ft I have to remember to keep my foot out of it. It is so nice to have the extra power here in the mountains. So far intake temperatures have not been a problem.

Lowell
 

·
Registered
Joined
·
3,500 Posts
The 12.6 afr is more of a concern than the pulleys FYI. Be very careful. You need to live in the 11.2 - 11.7 range 12.6 for long will turn into detonation and destruction


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
74 Posts
Discussion Starter #4
Thanks for letting me know that I'm running too lean. Plotting with Winpep7, the boost plot had a line at 13, so I thought that as long as I was richer than that, I was ok. I went back to the tuning plot and see that they had me at about 11.5 with the 3.875 pulley.

Next spring I'll get it richened up. We just got our fourth snow event out here, so it is time to put the Challenger away for now.
 

·
Registered
Joined
·
328 Posts
Air is thinner up there

So for a given mechanical volume you get less pressure. They were invented to pump air into fighters at high altitude to keep the HP up. You won't get sea level pressures at high altitudes, but you will improve on NA runs. Just don't over compensate for the altitude, because if you go lower you will be developing too much boost. I don't know the system or the overboost controls. Maybe it can compensate, but I'd check with Edel first.
 

·
Registered
Joined
·
74 Posts
Discussion Starter #6
I actually reside at 8200'. Just about all of the roads around here run 6000' or higher. The car is probably never going to be driven at an altitude less than 4000' ever again and if it ever is while I own it, I just have to remember to not let it pull very hard. Mt. Evans out here has a road going up it to about 14000'. The SC would easily make sea level pressure in the intake manifold going up that grade, all though intake temps may get out of control. Speaking of airplanes, I also own a Glasair. A hot rod at sea level and low altitude. At the airport where it is currently based, I'm down on horsepower about 25%. Along with less dense air for the wing to work with (21 psf wing loading) I use up a lot of runway getting it off the ground. I've toyed with the idea of turbo-normalizing it where I would be able to get sea level horsepower up to about 7500'. But that is a much more costly endeavor than bolting up a SC to a 5.7L.
 
1 - 7 of 7 Posts
Top