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Unread 11-24-2010, 01:04 PM   #46
Charles
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2008 JK Wrangler 
 
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Quote:
Originally Posted by piratius View Post
Actually, you're wrong here. Turbochargers use a wastegate to keep maximum boost pressures constant. A wastegate set at 12psi will produce 12psi boost at sea level, and 12psi boost at 10,000ft elevation. The "speed" the turbo is spinning will change depending on altitude, and doesn't matter to the wastegate, which is actuated by springs and manifold/turbine pressure, not by a set rpm for the turbocharger.

I've only built a few naturally aspirated cars into turbo cars

:teacher:
You're right that the engines have wastegates... an engine that produces 180 HP at sea level is allowing MAXIMUM boost to achieve that number. Take the same engine and allow it to produce maximum power and it's going to drop X percent of it's power at Y altitude. The turbo isn't going to make up for the power loss, it's already giving 100% to produce the 180hp.

I'm sure you're going to argue until your blue in the face so I'm going to go digging around the internet. I had a twin turbo Audi and a single turbo VW here in Denver. There was a NOTICIBLE power loss at altitude.

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Unread 11-24-2010, 01:23 PM   #47
Charles
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OK, here is the NHRA altitude correction chart. Turbo charged and supercharged cars use 1/2 the correction. Anyone reading this that goes to research it will realize that it's very complicated. A FI car will not lose as much power at altitude but it certainly will lose power in a similar way.

NHRA altitude correction factors

Supercharged and/or turbocharged cars (i.e.: AA/A, AA/AT, BB/A, BB/AT, A/PM and AA/PM) use half factor; TA/D and TA/FC cars are not factored.
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Unread 11-24-2010, 01:50 PM   #48
mikeinminn
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I notice that your Wrangler is supercharged (very nice...and great videos). Have you taken it to enough different altitudes to see how much the difference can be felt?
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Unread 11-24-2010, 03:39 PM   #49
Sunburn
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I like to think of it this way, when you are boosted, for every 14 pis you add, you can double the amount of air and fuel in the combustion chamber at sea level. At altitude, the air which is pressurized does not have the same amount of O2, so to get a proper AFR, you can't quite use 2x as much fuel.

Ideally the 3.8 would have such strong internals that we could all get a twin tubo setup that would be capable of pushing 42psi of boost! Imagine burning 3x as much fuel and air and essentially turning that 3.8 into a 11 liter beast!

BTW - The ECU monitoring your AFR through the O2 sensors immediately corrects the amount of fuel need to produce a correct AFR according the fuel maps loaded into the ECU.
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Unread 11-25-2010, 07:42 AM   #50
Charles
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Quote:
Originally Posted by mikeinminn View Post
I notice that your Wrangler is supercharged (very nice...and great videos). Have you taken it to enough different altitudes to see how much the difference can be felt?
The supercharger makes it a pleasure to drive. It's got enough power now that it doesn't really make a difference. Well, of course it makes a difference but it's less noticeable now because I'm not pushing it to the limit.

I think that's one of the reasons turbo guys think their cars are so much less affected at altitude. It's not that the turbo is making up for the loss of air, it's that by nature turbo cars are usually a little more high end and powerfull so they don't even notice it to begin with. It's kinda like taking a NA V8 up there. Yeah, it's got power loss but it's so ridiculously overpowered to begin with the driver can't tell.
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Unread 11-25-2010, 08:19 AM   #51
mikeinminn
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To the OP: This is the real solution. The supercharger is $4000 or so, but you can probably skip the regear ($1300-$2000) for a while, get better gas mileage(3-6 mpg according to RIPP), and enjoy the trip to and from the trail a lot more. It will cost you $10000 to $15000 less than a hemi, weigh less, and provide you with what you need where you need it. Check out cbloyer81's videos. I was over some of the same highway in Colorado this summer and would have appreciated a "bump draft" from a VW bus.

cbloyer81: Did you install the supercharger yourself? Have you checked the mileage yet?
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Unread 11-25-2010, 09:08 AM   #52
Charles
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Quote:
Originally Posted by mikeinminn View Post
To the OP: This is the real solution. The supercharger is $4000 or so, but you can probably skip the regear ($1300-$2000) for a while, get better gas mileage(3-6 mpg according to RIPP), and enjoy the trip to and from the trail a lot more. It will cost you $10000 to $15000 less than a hemi, weigh less, and provide you with what you need where you need it. Check out cbloyer81's videos. I was over some of the same highway in Colorado this summer and would have appreciated a "bump draft" from a VW bus.

cbloyer81: Did you install the supercharger yourself? Have you checked the mileage yet?
You are correct on skipping the re-gear.

I did install the kit myself. It wasn't too bad. Getting the intake plenum off was the hardest part. If you're comfortable with that and removing and replacing injectors you'll be fine.

I'm not sure on the mileage yet. I will say that it is down and I will also say that I haven't run a tank without stomping it a few times. It could take months before I know for sure if it will go up, stay the same, or go down during normal driving.
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Unread 12-03-2010, 08:32 AM   #53
testrat
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I installed 5.38 gears in my 2008 JK/auto as I pull a trailer around for work. They are about right with the 33" tires I have (LT315-70rX17 Dick Cepeck Crusher tires/dc1? wheels). Here in NE Okla, I'm about 700ish elevation, may be some places I can break 900ft.... Local anyway. I'm lookin to install a Ripp SC - I hope next year. With these gears... Oh man... It outa really scat for as heavy as it is. Also in process of getting a snow plow for it... With these gears and tire size - when OD kicks in, @ 70mph - I'm doing about 2200 rpm. Pullin the (utility) trailer with the 4' loading up - forget Over Drive. With the stock 4.10 gears, it wouldn't shift out of 2nd. Goes into 3rd fine now - which is I think about 3000 rpm @ 70mph. Don't know if any of this is helpfull... but thats what I've done to get the power up for what I'm doing with mine. Also hoping to get the Ripp's long tube headers to help some before I SC the motor (and a free-er exhaust).... By the way - is anyone out there running the Ripp Headers? Am wondering how they are liked....
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