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Discussion Starter #1
Been working on a turbo setup for my crx for a few years, just put motor in, finally ready for the turbo.

I was wondering on the effects of altitude on a turbo setup.

I spend 1/2 my time in Denver, about 5000', and the other 1/2 up in Frisco, my driveway is at 9200'. Some of the mountain passes, I like driving on, are up there-like 12000'. Obviously, NA sucks up here. At 12000', the pressure is about 40% less than sea level.

I built a Z6, Vitara, FJ rods, ported head, L3 with Mfactory, RC550's, Neptune RTP, Thermal R&D 2.5", Random Tech HIflow cat, Acton 1MS 6puck.

My plan was 250 to 300 hp range, fun to drive on the mountain twisties. (I haven't driven a turbo crx, so I don't know what 250-300 even would feel like, just figured that was a good HP# to build the engine and tranny for, if I was gonna do it...)

I need to be able to start producing some boost pretty quickly, so I figured I'd go with something like Autoworks setup with the t3-60, or something like a t3/t4 45 trim(but that seems kind of big).

I'm know the turbo is going to have to work harder at the high altitudes to start making power(40% less atmospheric pressure at 12000'), so seems it would make sense to go with a slightly larger turbo, but then I'd be hurting the spool, which I really need cause of the altitude. Again, not dragracing, twisty mountain roads.

Anyone have any experience at the high altitudes, and some advice for me?

Thanks
 

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the nice thing about a turbo is that it will generate 10 psi for example, no matter the elevation. it will not stop spooling unitl it reaches boost cut at 10 (again, example)


NA and superchargers, that is where they suffer in the thinner air.


your off boost driving will suck regardless.
 

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Discussion Starter #4 (Edited)
Right, but at sea level, there is about 15psi, at 8000' it drops to about 10psi.

So, say with a given turbo, I need 20 psi to get the volume of air I need for 200hp.

At sea level, the engine already has 15psi, so the turbo only needs to boost that up 5psi to get to our 20 psi imaginary pressure/volume goal.

At 8000', the engine is sitting at 10 psi, so the turbo has to boost that another 10psi to get to our 20 psi pressure/volume goal.

If I'm thinking about this right, I'd need to compress a larger volume of air at altitude, so I'd need a bigger turbo, but that would spool slower, but I need a fast spool, cause of the altitude, so I need a smaller turbo... lol

I guess I'm wondering what the guys that live at the higher altitudes use on their rides, for auto-cross/hill climb type driving, not drag racing. (If I go to big, there is probably gonna be two black marks, leading to the edge of road, where I ran off the mountain when the turbo finally spooled-I figure 200-250 hp at the wheels is probably about all I could put down anyways?)

what are some other models of turbos that are close to a T3-60? I see some of the guys sell garrett, precision, bullseye, etc, but I don't know which models compare. Guess I need to spend some more time learning to read compressor maps...
 

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The only thing i can say about this is It will be hard to tune. Since your going up and down the tune will be off when your high and on when your low (or the other way around).

And your right about your power. The less o2 will cause less power.
 

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Discussion Starter #6 (Edited)
hadn't really thought about that. I know the ecu has a barometric sensor, but I don't know how much it can correct for, or what it actually does. There is less O2, cause there is less pressure. The ratio of the O2 in the air doesn't change as altitude increases.

Will it effect the tune, or will I just need more psi the higher I go, to get the same power?

Driving around NA, up higher now doesn't seem to mess up my AFR's, at least not enough that I've noticed.
 

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hadn't really thought about that. I know the ecu has a barometric sensor, but I don't know how much it can correct for, or what it actually does. There is less O2, cause there is less pressure. The ratio of the O2 in the air doesn't change as altitude increases.

Will it effect the tune, or will I just need more psi the higher I go, to get the same power?
I would just make 2 maps for the different elevations, and when you travel to the other location just load up the 2nd map.

Honestly I dont htink its going to be a HUGE difference on the turbo. if you are shooting for 250whp, I would go relatively small on the turbo, like a gt2871. It will spool quick, and make the power you want plus some. I think more over with altitude and DA changes, your going to need to worry about managing your timing/fuel adjustments at altitude rather than focusing so much on your turbo. The fuel/timing will effect your drivability more directly, and also effect the way your powerband and turbo kick in. So if you get a way to manage those (separate proms/tunes or what have you) at altitude, I think your time would be better spent
 

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play along

elevation A is 1200, and makes 10psi off the turbo, turo spinning 13,000rpm.
elevation B is 4900, and still makes 10psi off turbo, but turbo is now spinning 20,000 rpm to make that boost.

get it? turbos will make similar or same power regardless of the elevation. some elevations will over workthat turbo, and of course off boost and partial boost (spooling) power will be effected dramatically.


ifyou buy a current issue of D Sport, there is a civic in there making 971whp at nearly 5k elevation. and on 12.5:1 compression. his build runs 12.5 in place of the normal 7.5-8.5:1. solely for offboost driving, and so he can actually spool his bigass turbo
 

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Discussion Starter #9 (Edited)
sounds like we are on the same page.

Interesting about the higher compression ratio. I did read somewhere how increased altitude reduces detonation.
 

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play along

elevation A is 1200, and makes 10psi off the turbo, turo spinning 13,000rpm.
elevation B is 4900, and still makes 10psi off turbo, but turbo is now spinning 20,000 rpm to make that boost.

get it? turbos will make similar or same power regardless of the elevation. some elevations will over workthat turbo, and of course off boost and partial boost (spooling) power will be effected dramatically.
Both conditions will have 10 psi of GAUGE pressure, so the trapped air mass will be reduced for the higher elevation case, and he'll make less power.

To make the same power at high altitude, you need more gauge pressure in the intake, which means a higher pressure ratio and hotter intake temperatures.

It would be good to look at compressor maps for both cases to make sure your turbo is robust enough to perform well at different altitudes.

EDIT: If you have an electronic boost controller and you're regulating boost pressure based on a target absolute pressure, you'll make about the same power in both cases, but the tune will have to be robust.
 

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Discussion Starter #11 (Edited)
I had my ECU set up for PWM boost control through Neptune RTP.

I think I'm going to stick with the t3-60 size turbo, and just plan on the power decreasing as I go higher. I was getting to hung up on having the same power all the way up the hill.
 

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I live in fort collins, and often head to the mountains to mess around in seeing as they are 5 minutes away from my house.

There is a slightly noticeable difference, but nothing that is going to be terribly worrysome. Make sure you tune your AFR's rich in boost and set lean protection on all the time, i've found that it can vary day to day around here. The biggest help is setting it in closed loop for normal driving purposes, and to tune it rich is just a safety measure for days that are hotter/colder than others.

I'm just running a greddy kit, and I honestly think that anymore turbo lag would be miserable in the canyons. I see full boost around 4k rpm in the mountains. You WILL notice a lot more turbo lag at higher elevation. Peak power will be minimally affected IMO, the temperature is going to have a higher effect on that.

Here in foco we get 85, 87, and 91... OH, and E85 :)
 

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Discussion Starter #14
So, finally got the turbo in, tuned at 6000'. Went with a S252 so I'd have plenty of "reserve" at altitude.

Dyno print out shows 16psi, boost gauge shows 14psi, using 40% duty pwm. without pwm, just waste gate spring, is 9psi on the gauge at 6000'.

Up at 9500', with the engine off. my map reads -4.5psi(or 8.7inHG). without pwm, the boost gauge reads just over 6 psi full spool. I didn't get a data log with the pwm, but it was also about the same amount lower.

I was expecting the full spool pressure to be the same, regardless of altitude, just the turbo having to work harder. Turns out it didn't work that way. looks like I'll have to turn up the pwm duty cycle to compensate. I did a pull at about 7000', full spool psi followed the trend, and was a bit less than 6000'.
 

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Discussion Starter #15
Turbo works great going up and over the continental divide. No need to downshift anymore, pulls 5th, no problem. Turbo will boost a few psi in 5th, at 60mph. Downshifted to 4th at 10000' and 65mph, floored it-thing pulled like a freight train. Life is good. :)
 

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the nice thing about a turbo is that it will generate 10 psi for example, no matter the elevation. it will not stop spooling unitl it reaches boost cut at 10 (again, example)


NA and superchargers, that is where they suffer in the thinner air.


your off boost driving will suck regardless.
Thats not true, the wastegate it using the atmospheric pressure as a refference, so it will only spool to 10 psi above atmospheric.

This is why you see rally cars with "sealed" wastegates when operating over a wide range of altitude. It will seem to increase in boost as the atmospheric pressure decreases with altitude. The problem with this is that at very high elevation you can overspin the turbo if youre not careful.
 

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Discussion Starter #17
Makes sense after I thought about it for a bit. Without pwm active,the top of the waste gate is just vented to atmospheric, so at higher altitude there is less force working with the spring. So the turbo is making the same total boost, its just shifted down on the boost gauge.
 
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