I'm at 6800 feet elevation.

My stroker is in with 14psi.

Mods are:

LS3 416 10.5:1
LS3 heads Manley valves
230XFI/234XER 117+0 cam
AR 1 7/8 headers with 3" all the way back to Z06 muffs
Parallel fuel system w Bosch 44 pump in addition to the Z06 in tank
Meth inj w twin 15gph nozzles
Cartek race trans
Cartek Race diff
ECS awesome kit 2200
50 shot (closer to 60 shot actually)
MT ET street 305's
3.73's for trap speed adjustement
Spec Twin Clutch
Ported Intake/TB
MGW 35% short throw

959rwhp 1044rwtq

My question is: the elevation lowers cylinder pressures. What static compression do you think would be optimum? I will want to swap to Trick Flow 235 heads and can adjust compression. This car isnt comming off the mountain and will be tracked at Pueblo at 5500feet.

Local tuner shops that do FI installs say this elevation lowers cylinder pressures by 2 full points in static compression. I was wondering if any brainiacs out there had exact numbers. For comparisons, stock shortblocks show 155-160psi on cranking compression tests as normal. I think its 190-205 at sea level. This can translate into more TQ down low with more compression. If this theory holds then at this elevation, 11.5:1 compression here is like 9.5:1 at sea level.

Any opinions?

 

Wow, great numbers. I should be so lucky to have them.

I remember living in Colorado Springs with a turbocharged car (1993)and when I took it to sea-level it was like night and day.

Hopefully some elevation-smart brainiacs will chime in here.

San

 

I don't know about 2 full points...maybe above 7000ft. .5 to 1 full point wouldn't be uncommon though.

There's less air density, and SC or not, you still need to compensate for the thinner air. I've hear pump gas 12 and 13:1 isn't uncommon either. But that's a SBC and I'm new to LS motors...so take that for what you will.
Perhaps a couple bumps in timing? From everything I've read, you lose about 8-10% compression at your altitude...so that assumes that you could jump your scr to 11.5:1 and be close to where you would be at sea level.

How is the altitude affecting your AFR too?

 

Spin: Will this calculator help?

http://www.wallaceracing.com/dynamic-cr.php

Several more at the home page:
http://www.wallaceracing.com/Calculators.htm

 

12:1 to 12.2:1 is what this cam would call for at sea level n/a to be at 8.6 DCR. To get the same dynamic compression here it would call for 14:1. If a car runs at sea level with 14PSI at 10.1:1, then it would run here on the same octane at 12.1:1.

Since cranking compression is down 20% at my house (155 down from 195-200 at sea level), its down 20% in compression not 8-10%. 20% of 10.9:1 is 8.72:1. Stock cars would run the same octane at 13:1. Thats over two full points just like tuner shops here say it is, not one point as you say. Dynamic compression as per the above post's calculator is also down nearly 1.5 full points. This supports running a static compression 2 full points higher than at sea level on a dedicated high altitude car.

The correction factor here is 28%.

Altitude doesnt affect any car's A/F since air is metered by the MAF and MAP unless they are maxxed out (bad install). My car is a 2 bar MAP operating system with a 100mm MAF for higher resolution. If you have a base kit that maxes out the MAF, you are out of range in fueling by the MAF. A 2 bar MAP is a good idea if your car supports it. Then you get a boost fueling table.

Im asking what static people would run due to the boost level. I can calculate the altitude difference.

Please dont tell me 9.x something. There are guys running stock compression with 14psi and I dont want an off-boost slug. I have lots of octane both with VP fuels and meth injection.

 

Congrats on the numbers...959rwhp 1044rwtq

 

The first one shows a loss of 2.65 points in dynamic boost pressure. Its even more of a loss than I thought.

Its 2 full points of static compression and 1.5 of dynamic off boost.

 

I remember reading a few article's where really aggressive FI setups ran about 8:1 static CR, and with just over two bars of boost. 2.2 bar or 32 psi.

The lower static compression, the more boost you can safely apply, which will give max power up top. The trade off being not all that much power down low, off boost.

I think with a stroker though, you should be fine in the lower rpm's with the cyl. pressure having such a leverage advantage on the crank.

I'll ask around tomorrow, a good friend of mine does a lot of Motec tuning on FI applications.

 

Sorry, it's late. I was figuring for MAP. You have about a 10% loss in MAP compared to sea level. (Actually 11.5%) ...and that has little to do with you're asking.

 

Reducing static compression to compensate for boost is a futile science. An increase in 2PSI is equal to 1 full point in static compression in cylinder pressure. So if someone says some arbitrary number then if you reduce boost by 2 psi You can increase 1 full point in compression and maintain the same cylinder pressure.

EX 1: Someone says 9.5:1 for 15psi. The same exact cylinder pressures would be there for 10.5:1 at 13psi and 11.5:1 for 11psi and 12.2:1 for 9 psi.

The formula is what I want not some guess. I want to model cylinder pressure. If some guy at sea level runs 10.1:1 with 17psi, then you can run 11.1 at 15 and 12.1 at 13....here that would be 14.1 at 13.

The way to tell if you have the right or wrong compression is by the timing curve it runs without detonation.

When someone wants to run 5 more PSI and he drops static 1/2 a point, he didnt compensate for cylinder pressure gained from boost. If he is able to run that, he was way too low on static in the first place.

I remember years ago when people said that you cant run more than 6psi on a stock LS6 (10.5:1). Today, our LS2's at 10.9:1 have been holding together with 12psi+. I ran 16psi at this altitude with 10.9 and it could have run that on 89 octane up to 14 degrees peak timing.

It seems to be about 1/2 point for every 1700 feet in elevation over what you run at sea level with the same octane.

When you have guys in California running 91 octane at sea level on the same boost as the east coast guys run with 93 octane, that should tell you something.

 

My MAP when keyed on instead of reading 5 volts, it reads 3.7 which is a -26% loss.

Here is the multitester hooked up to my hummer (N/A motor 1 bar MAP). I did this when I did the Meth thread to activate the Meth system on an N/A car under WOT. This is also why you can use a MAP to do the fueling calculation by changing altitude readings....hence the car would be immune to A/F changes if you use a 2 BAR MAP.



This would be the scanned reading for the truck floored and its N/A. Cylinder pressures are down by this amount. The reason cranking compression isnt as great a loss is because its dynamic due to the intake valve not closing allowing the reading to be by full stroke sweep of the piston.

Again, I'm not looking for these numbers. I wanted input on the static to run for my boost pressure and my altitude.... a formula since it varies by altitude which varies by 1000feet in my daily travels.

The kicker for my car is that the nitrous shot is immune to altitude changes because it is its own air supply. Since the 60ish shot makes 125hp and 250rwtq, its got to be figured in here.

Think about it tomorrow when youre not tired.

 

How much boost were you planning to run, 14?

And what is the goal? Max hp, or drive ability with ave. power?

 

Its 14psi and built already.

 

@6800ft? WOW That puts Spin about number 1 on the forum!

 

I assume those are corrected, still very strong! Guy, how is your 60 shot making 125hp/250torque? w/o it numbers are ~825/800? Curious what rpms does it peak? Glad to hear your gonna get some track numbers, what's the goal?!

 

I'm still tired and can't sleep, but I've been thinking about this (and a flat plane crank SBC build elsewhere. lol! )

If I'm understanding what you're asking for, you want a formula for boost CR, corrected for a given altitude...which I have.

"X" being the boost CR
"Y" being boost in psi
"Z" being final corrected CR
"A" being the given atmospheric pressure at the specified altitude (charts can be easily googled "atmospheric pressure table".)
"B" being the specified altitude.
"C" being engine static SC.

(Y/A)+1) x C = X-[(B/1000) x 0.2] = Z

You should just be able to plug in your current and/or desired CR and boost #'s at whatever atmosphere/altitude you want.

...somebody else can figure how to account for nitrous. lol!

 

Sorry I'm do not have any of your elevation answers but...


"959rwhp 1044rwtq"

 

We got thrown out of a race at 14.9 to 1 in Denver We should have been at I think it was 12 to 1 was the rules.

At 1000 to the wheels, you shouldn't worry much about it.

Randy

 

There are performance shops specializing in turbocharged as well as supercharged vehicles up in Colorado, right? Lots of subarus and such. Perhaps you could ask them what they have done?

 

Spin,

Air pressure equals:
P = 101325 (1 - 2.25577 10-5 h)^5.25588

where

p = air pressure (Pa)

h = altitude above sea level (m)

At sea level the standard pressure is 101.33KPA.
6800feet = 2070meters = 79KPA = 26% air density loss
5500feet = 1680meters = 82.7KPA = 22% air density loss

In other words, at 6800feet, you would have to compress any given air mass 26% more to achieve the same final density you would have at sea level. If your target compression ratio at sea level was 10:1, you would want 12.6:1 at 6800 feet. Not that this does NOT imply the car would make the same power at 6800 feet as it would at sea level just because you raised the compression; with the intake sucking in stratified air, you are still taking in less oxygen mass with every power stroke, so you burn less fuel and the amount of energy released is 26% lower; all the higher compression is doing for you is keeping the combustion efficiency the same, so your flame propagation velocity and thus ignition timing requirements and knock characteristics should be similar to what they'd be at sea level.
Also even though your gauge shows 14PSI boost, that is 14PSI above atmospheric pressure. Since atmospheric pressure is 11.45PSI at 6800 feet, the effective boost you have is 3.2PSI lower, or, 11.8PSI.
This is all further complicated of course by the fact that with the inlet at lower presure, the supercharger efficiency curve is shifted up, so in all likelyhood the blower is operating at lower efficiency as well, meaning it is not a simple "multiply by x" calculation. The numbers might be accurate but the conclusions you can draw from them are still best guesses unless you take a ton of other factors into account.

Of course the real catch is that if you really built your car to perform at its max at those altitudes it will blow up at sea level...
Hope that shone some light on it?