Hey guys, looking for some collective wisdom. It is time for me to choose the turbo for my 13B-REW -> Miata project. I decided on the GT35R- I feel it will do just what I want. Here's the thing- I live in Colorado at 4600', and I travel up even higher. For example, the race track in Aspen is over 8,000'!

So.. I understand why normally one would choose the 1.06 hotside. However; given that it is going in a 2100lb car (less load to spool wth), I live at high elevation, and it is to be predominantly a street / autocross / a little road racing weapon, do you think the .82 could possibly be a better call? Increased backpressure and reduced top end does concern me a little, especially if the 1.06 spools anywhere near the .82. The plan is to start out around 12 psi since the engine is stock, then turn it up in the future when I get around to porting / strengthening it. I don't really want to compromise the stock apex seals.. Also, I will be limited to our pump gas which is 91 octane, however it acts like the 92 octane in the midest. (West coast 91 is worse- I used to live there.)

Ideas or experiences?

 

Look into water methanol injection. =It makes intake air more dense!!

 

I have a GT35R with a .82 hotside and I live in Colorado Springs (about 6,000ft) I did notice a bit of lag and power loss when I brought the car up here after it was tuned with a PFC at around sea level. I wanted great response over max power so I was told to go with the .82 instead of the 1.06 and I was very very happy with it at sea level. Puts down 350rwhp @ 15psi on conservative tuning on pump gas (93). Given the weight of your car and if you get your computer tuned right for the altitude I would imagine the .82 would make a pretty nasty car for what your looking for. Good luck with whatever you go with, Jerry

 

bump for interest

 

If you come down to the city i could give you a ride in a 1.15 a/r and a 70mm wheel on a half bridge. Or my freinds gt35/40 which feels similar to stock twins. i could also put you in contact with a 60-1 i think thats about a .90 a/r not 100% sure. So we have the range of a/rs in the city

Marc

 

Buellmd- Thanks for the idea, but I prefer not to use WI because I like to keep it as simple and internally sufficient as possible.

DJK- Thanks for the insight At what RPM do you start to make boost, and at what RPM do you make full boost in the Springs? Do you feel like it tapers off at high RPM at all?

mmonaco- I certainly would like to check up some setups next time I come over the hill. I have friends in Denver and Boulder so I get over there every couple months or so. If any of them are on the board and could post to the above question that would be great

 

So then, anyone up high with the 1.06 hotside?

When I lived at sea level, I had a TII with a street port and a T66 (P trim, 1.15 divided hotside). Didn't make 15 psi till 5000 RPM. Too laggy for me. I'd like full boost more by 3500...

 

I start to make boost at about 2000 RPM and get full by around 3400-3500 RPM up here. I would say it tapers off at around 7000 RPM or so. All this is with a PFC that was tuned at sea level (Philadelphia area), so I would imagine things would feel a little better if I had it retuned but I'm not to sure how much longer I'll be here. The Turbo just performs better in every single area at sea level compared to altitude ( I think it reached full boost, 15psi earlier also). But that's not saying it's not a good turbo. The best way I can say it is I like it a lot up here in the Springs, but I love it at around sea level. Hope some of that helps

 

Thanks DJK.

Anyone else out there who lives in the mountains with any other turbo that can chime in on spool and top end qualities?

 

I wouldn't say it feels like stock twins, but maybe non-sequencial stock twins!

 

this isn't completely accurate, but from what i've noticed with spoolup, it seems that a lot of the turbo cars i've driven tend to spool about 150-150 rpm slower at my elevation (~4200ft)

you could generalise that to say VERY ROUGHLY about 50 rpm/1000 ft elevation.

if you do port the motor, i'd do a late closing, but NOT early opening if you drop the a/r on the hot side. the late closing port will let the turbo cram more air in the combustion chamber without the extra reversion (which is already a problem with the t3's) associated with the extra overlap from an early opening port.

 

As for your hot side the changes are marginally if you get your cold side right. The general laws are still the same..larger hot side more top end more lag. What you really have to adjust is your cold side!!! You will make less PSI at higher altitude. So if you compare compressor maps, 2 PR will aquate to more like 10 PSI rather than 14. So you need to spool the turbo harder to accomplish the same level of boost. You will build boost slower, so you will also run into surge problems becasue of that. As a general suggestions increase your exducer wheel and keep the inducer the same compared to a sea level turbo set-up. This will give you higher velocity of the intake charge, building boost faster. This is what we are missing at higher altitude.

 

Claus....you are a smart mother f*@ker!

 

can you explain that? why is 10 psi any different at higher altitude than at sea level? map sensors are absolute, as far as i know.

 

Ok let me try.....this is not very scientific but I think will get you in the ball park.

Lets compare sea level and 6000feet
Atmospheric pressure: 14.7 12.5 (the estimates for AP vary)
Pluming loss, etc: 2 2
Air filter pressure drop: .5 .5

If you compare compressor maps you are looking at a graph that charts PR (pressure ratio = (compressor outlet pressure/compressor inlet pressure) vs flow
That PR isn't very helpful if you don't adjust it to real life. We like to compare intake plenum pressure to atmospheric. To get there we need to consider pressure drops before and after the turbo. (see above)

So, lets calculate the PR for an absolute manifold pressure of 10 PSI, you want to reference to on a compressor map.

Sea level
(14.7+10)/14.7=1.68

This is the PR you will find on a compressor map not considering any pressure losses with your set-up.

Adjusted for that you will look more like
(14.7+10+2)/(14.7-.5) = 1.88 at 15 psi 2.23

So now lets adjust for 6000 feet
(12.5+10+2)/(12.5-.5) = 2.04 at 15 psi 2.45

As you can see a turbo needs to create a lot more PSI at altitude to give you the same absolute pressure reading at your manifold.

In other words, if you see 15 PSI on your gauge at altitude the turbo actually produces and equivalent of 21 PSI.

Hope that helps

CW

 

ok, so roughly, at high altitude it does say 15 psi worth of work to produce 10 or 12 psi.... right?

but aside from the turbo working harder, wouldn't the actual power be the same for the same manifold pressure (other than the AE loss from the turbo working harder)?

 

If you have a N/A, supercharged, or turbo car with manual boost control, you lose MAP as you increase altitude. If you have electronic boost control, you can maintain your MAP with the only difference in power being the rise in boost threshold and the reduction of timing based on the increased air intake temps from the turbo having to work harder due to the reduction in ambient pressure. (You are correct that the MAP would remain the same.)

From the tuning manual I wrote for the Link ECU:
Elevation (ftX1000) 0 1 2 3 4 5 6 7 8 9 10
Pressure (kPa) 101 98 94 90 87 84 81 78 75 72 69

An equation that does a pretty good job approximating the altitude vs. pressure is: P(kPa) = 101.304 x e-Z/26216. P(kPa) is the pressure in kilo Pascals, Z is the altitude in feet. Using a little algebra we can re-arrange the equation so that it returns altitude based upon pressure: Z = -26216 x ln( P(kPa) / 101.304 ). This second equation comes in handy when you are trying to determine altitude based upon pressure.

Note- your boost gauge is delta over ambient and really useless for any type of absolute measurement- this is why standalones use something absolute like kPa.

(I tune turbo Miatas for a living so I know a thing or two about all this. What I am really looking for is how this theory applies in practice to the 13B.)