In thinking about the altitude power losses and available fuel here, I was told by a local engineering student that the reason for the power loss is that cylinder pressures drop. He suggested that a disproportionate increase in cylinder pressures by increasing static compression a lot would work fine and restore a lot of the lost power.

I noted my car which was tuned on the line with 93 octane actually runs without knock retard here at 6000 feet with 87 octane. I was blown away by this. full point in compression is 14rwhp and 20rwtq and would run on 91 octane.

Any input on this from the fuel/mechanical engineering experts?

I'm bored and looking for a new project. A 12.5:1 stroker with a good sized cam would have the same cylinder pressures as my car had at sea level with 93 octane.

Talk me into it.

 

 

OK...I dont think you can build a stroker..... prove it, and I'll fly out to see it.

 

Spin--I am at 3000 feet above sea level. I can run 9.5 compression on a 12psi turbo car on 91 octane because the atmospheric pressure here is 13.1 psia instead of 14.7 psi. Power is also down, however due to the lower air density. Less power, less pressure. Knock is dependent upon absolute cylinder pressure at the time of ignition. Since the compression ratio is constant, but the suction pressure is lower at altitude, you can stand more compression ratio before you run in to the same cylinder pressure that you see at sea level. I can't remember what the absolute air pressure is at 6000 feet, but let's say its 12 psia. 12/14.7 is 0.82, so you could run approximately 1/0.82 = 1.22 times the compression ratio at sea level before you run in to knock problems. 10:1 at sea level, 12.2 at 6000 feet. Roughly.

This is why the octane rating required at higher altitudes is less than at sea level. The cylinder pressure is lower at high altitudes due to the lower ambient air pressure so you can run lower octane without knocking. But fill up with 90 octane at 5000 feet and drive to sea level, and you will knock.

Does this explanation help?

Joe

 

He was correct... look at http://www.engineeringtoolbox.com/ai...ure-d_462.html to illustrate atmospheric pressure at various altitudes.

As your altitude increase your cylinder pressure decreases, thus a reduction in power (or lower octane requirement).

Mike

 

You can regain some of your lost power with an increase in compression ratio, but you won't be able to drive the car at sea level without really high octane fuel. That's the nice thing about turbos, they are so adjustable.

Also, the main reason you are down on power is that you just can't pump as much air (and thus fuel) into the engine with lower density air. Again, the turbo can make up for this at altitude.

 

This is an excellent point. With a high comp build your pretty much limited to a high altitude tune. I'm sure you can make some adjustments depending on how much you actually bump compression but if you plan on taking any long trips where there's a significant change in altitude, you better stock up on some Torco. I think your original S/C idea will suit you better. A pulley change and retune and you're set to go where ever you like. Un;ess you don't plan on leaving CO with it.

 

Couple of more points. A piston engine is essentially a constant volume or constante VE machine. It will process about the same amount of air on an actual cubic feet/minute basis at sea level as it will at altitude. But the pounds of air per cubic foot changes with altitude because the pressure changes with altitude. In my example above, the ratio of density at altitude versus sea level was 0.82. The engine produces 82% of the power at altitude that it did at sea level. Power is proportional to absolute air pressure or density.

A waste gate controlled turbo car power is limited by the waste gate setting, which is a gauge measurement, not absolute. An engine with a 10 psig waste gate setting will make less power at 6000 feet than it will at sea level. This is because the actual manifold pressure dropped from 10+14.7 = 24.7 psia at sea level to 10+12.0 = 22.0 psia at altitude. Raise the waste gate setting by 2.7 psi to get the same manifold pressure. But the pressure ratio across the turbo compressor increased, so the turbo may not even open the waste gate at 6000 feet. Also, the compressor discharge temp increased, the load on the intercooler increased, the inlet temp to the engine increased, and this decreases charge density and reduces power. Plus the intercooler efficiency itself decrease due to the less dense cooling air. You can't quite get there from here without a turbo and set up designed for the altitude (and turned down at sea level)

Blower cars also have to be pullied up, larger intercoolers, etc. to make up for the increase in altitude and decrease in air density.

Now......nitrous is different! A 100 shot at sea level is a 100 shot at 6000 feet. This is a problem when dynoing nitrous cars--since the air density correction from the dyno is always applied to the entire measured horsepower and it should really only be applied to the normally aspirated portion of the horsepower. Nitrous cars dynoed at altitude have an unjustified correction applied to their measured numbers.

 

unfortunately it might be a real challenge to get that car to run anywhere else but 5000ft up!

 

RaceGAs!

 

Like you Spin, I'm at 6,000 feet, but the DA is usually even higher, try 7 to 8,000 feet.

On my C5 we kicked the CR from 10:1 up to 11:1 and still could run a bunch of timing with 91 octane gas. When I do the heads on my LS7 I'll get the CR up to around 11.7:1 to try and make up for some of the thin air.

BTW: 911 Turbos monitor absolute pressure and reset their wastegates to allow more boost with altitude changes.

 

That is the correct way to do it!

 

For the average person that may be a realistic issue but for me its a 6 hour head swap to lower the compression using one of the four sets of heads I have on hand at any time. For the one shot trip, hiway driving at 13:1 compression wouldn't cause knock and a few cans of torco would save the day(s) on the trip.

Blower cars running here with 10 pounds boost with 91 octane going on a road trip would be calling to order torco too, nnot to mention the new higher boost level that may exceed the safe limits of a bottom end. Please dont say a FI car coming off the mountain is immune to cylinder pressure increases.

Further the distortion of what happens when you have a lot of compression is way too widespread. A friend of mine bought a set of cylinder heads that was supposed to be 72cc and what came was 64cc. His stroker was running a 236 cam at 12.4:1 compression and we didnt know it until the tear down later on. The manifested result of running 12.4:1 at sea level was that it would only run 21 degrees peak timing instead of the common 24 degrees.

If I came off the mountain with 12.4:1 tuned to 91 octane and ran it on 93, it wont knock and if it pulled a degree or two, thats still not knocking.

The truth is just about every car on this board can run 1/2 a point more compression with no down side. I ran the stock cam in my C6 for 7 months at 11.4:1 with no timing being pulled. It was a free 7 hp.

I dont know what the big hang-up about compression is. With the country having 93 available just about everywhere, I dont get why just about every H/C car runs 11:1 compression when the DCR would still be an acceptable 8.7dcr at 11.7:1 and run fine on 93. With the L92 heads being a difficult compression head, I understand, but the rest of the public, compression isnt bad. It helps even out driveabilkity, adds to fuel economy and adds power, all for free. Since its often the result of thinner gaskets, detonation is also better controlled. The fastest track cars back in the east, the compression ratios are the max you can run. I wasnt talking about running a compression ratio that requires race gas. 12.5:1 on a stoker sized cam will likely run fine on 93 octane with a 2 degree adjsutament to the tune. It will not be unable to drive on 93 at sea level.

The above math demonstates that a 93 octane H/C car tuned to the max will be able to run 22% more compression here beofre that would require 95 octane. The math of it is that the average car on the street can run 93 here on the mountain with 13.5:1 before it would be an issue at sea level....even then the car would simply pull timing to prevent detonation. Too many people think the average H/C car is running the max of compression at 11:1 where I find most of the cars runninga 232+ cams can run near 12:1 on 93 with no issue at all. It would manifest itself as a degree or two lower timing. The fastest track cars from a NE tuner we all know for building fast cars runs 11.7 on most cars even with cams under 230 intake duration. 12.5:1 on a stroker tuned for 91 will run fine at sea level with 93. Thats why I wasnt suggesting 13.5:1. I guess now I have to build it to prove the point on a road trip.....

Summary: a 440cu inch stroker running a 240 cam with 12.5:1 comrpession on 93 octane will not have any issues at sea level with optimum timing. It will run an 8.6 dcr with a 117 LSA cam.....thats not even the limit for 93. Here at this elevation, it will run 91 with no issue.

With 1/2 a can of torco at sea level, 95 octane, it will run fine at 13:1 static compression. The average guy with a G5X3 from LG can run about 12:1 if they got the 114LSA on 93. The target is usually about 8.7 dcr on 93 octane but I have seen mid 10 sec stock displacement cars run as high as 8.8-9.0 dcr on 93 with no detonation. Thats 12:1 on 93 with no issue.

 

This was exactly what I was looking for as far as the math. I wasnt looking for people to tell me what compression to run. I wanted to quantify what I already understood with a number.....and even I was shocked at the real drop in cylider pressure that the math shows.

Once the blower cars pulley up as you state ablove, they are then likely over boosted at sea level. Stock bottom ends dont hold much past 600 forever. There is no one fix that works best at all elevations but come on people, 12.5:1 is hardly the max here at 6000 feet for what I proposed. I would have expected that reaction if I said 13.5:1.

On the nitrous car...keep in mind the motors output is donw 17% here so the nitrous shot is still building cylinder pressure on a motor that is sharply down on it.

 

Exactly. Juice is good.

 

With a 232 cam which is small for your stroker, 12:1 at sea level is the norm so here at 6000" you would be at 12:1 with 91 no issue.

 

I really like the way you look at things from a different perspective than most people. Most (myself included) would simply point out that you could run your engine under boost (preferably from an absolute pressure referenced turbo) and make the same power at altitude as you do at sea level. Your high compression idea is sound though, and you are correct in pointing out that you shouldn't (and won't) have any problem running more compression at high altitude.
However, let us consider just one single power event for two engines, one running stock and one running raised compression, all other things remaining the same. We will begin observing the motors when the intake valve opens at the beginning of the intake stroke: The throttle is fully open and the exhaust valve is just about to close; we have some partial vacuum here from exhaust gas scavenging, but for all practical effects the pressure inside the cylinder is close to atmospheric.
As the piston travels down it produces a vacuum on the intake and so atmospheric pressure pushes air into the cylinder. Both engines have identical stroke lenghts and cam duration so they will have the exact same amount of time to take in air and the same piston travel to pull air into the cylinder with. Somewhere at the bottom of the piston travel the intake valve closes and you are now left with a fixed air volume inside the cylinder. The volume of fresh air you have (I used the word "fresh" because there is always some exhaust gas dilution) is always, by definition, a fraction of the volume displaced by the cylinder. Volumetric Efficiency for your average 2 valve per cylinder 4 stroke engine is in te 80% range. Your high compression engine will produce a more powerful exhaust pulse, so exhaust gas scavenging will go up some, but it would be unrealistic to expect massive improvements in VE from just that. Plus you are already running cam, ported TB, intake manifold, headers, etc, so VE for your engine may already be past 85%, leaving you with even less room for improvement before you run into the theoretical limitations of what is possible for that particular engine configuration.
Now the two engines from my example compress their very similar intake charges and we see a difference: The high compression engine ends up with a much higher final charge density. When ignited, this denser charge will burn hotter. Since PV=nRT, T has gone up and so P will go up as well. With more cylinder pressure there is more force on the piston and so the engine produces more torque. Because it makes more torque the engine makes more power as well. Perfect, right?
Welll... If only it were that simple In reality the engine IS making more power but it is also consuming more power because it also takes a lot more effort to compress the air/fuel mixture to those higher compression ratios. Theoretically the net result just looking at the engine as an air pump would be 0 gain. In practice the maximum Thermodynamic efficiency of a 4 stroke engine is determined by the Carnot cycle; n = 1- (Tc/Th). Because you have raised Th, the combustion temperature, efficiency will go up. The engine makes more power not because it is running at a higher compression, but because it is burning the air/fuel mixture at a higher temperature and so operating at a higher level of efficiency. That is (in part) why you see high compression engines getting better gas mileage. Incidentally, you can achieve similar results by leaning out the AFR and advancing ignition timing in a lower compression engine as well, but there is another advantage to a high compression ratio that no adjustment of IGT and AFR will make up for; the amount of energy extracted from the air/fuel charge is work, and work = force x distance. With a higher compression ratio you have a higher average force throughout the stroke. But don't forget that the piston across from the one doing work is also requiring additional force to compress its intake charge...

The problem, however, is that this increase in efficiency will never be enough to offset the reality that the engine's volumetric efficiency hasn't changed very much, and it is still operating at reduced pressure; ultimately the engine size, times the RPM it is operating at, times its volumetric efficiency, time the intake manifold pressure is going to determine how much air it can take in. And how much air the engine consumes determines how much fuel it can burn; that, in turn, is the maximum power you can possibly make, assuming the fuel is burning efficiently. An engine operating at high altitude performs very much like a smaller version of that same engine... Now, you can hot rod the hell out of a small motor (Honda guys do it all the time), but it will always be at a power disadvantage when compared to a bigger motor.
And this is why forced induction has such massive potential for power gains: By pressurizing the intake manifold, you can run an engine at MUCH, MUCH, MUCH higher levels of Volumetric Efficiency... Somewhere around 16PSI your VE would be over 200%. Compare that to a heads/cam/headers/intake manifold engine that would be lucky to see a gain in VE of 5-7% and you can see why, given enough boost, FI will always produce more power than N/A.

Finally, for the sake of not trying to re-invent the wheel, let us not forget that us automotive enthusiasts are not the first ones to struggle with trying to make power at elevated altitudes; Back in World War 2 all engines were piston powered, and if you were in an airforce base in Britain you would have a few minutes between hearing the alarm indicating that the Germans were coming from full cruising altitude, and having to get there yourself to avoid being shot at from above. Those engines were designed to produce absolutely as much horsepower as possible at sea level for a rapid takeoff, and continue making as much power as possible at high altitude, and they had to do it under full throttle for hours at a time, with zero failures because people's lives depended on it.
Add to that the fact that the Germans had to contend with very poor low octane fuel during the war and you will see that the innovations spanned from that era are exactly what we need today for our own hotrods. Nitrous Oxide injection, Supercharging, Turbocharging, Water Injection, were all tried with varying degrees of success.
Ultimately, most aicraft from that time ended up supercharged. As power levels rose towards the end of the war they were using water injection and two speed drives; the engine would take off under reduced boost using water injection as a detonation supression, and then boost would be brought up via some kind of supercharger gearing once at altitude. A more applicable variation of this is to set the boost at a sane level at 6000 feet, and then simply run higher octane when you are at sea level...

My 2 cents worth, and with apologies for not being particularly innovative this time

Unrelated: BTW no luck yet investigating LPG conversions

 

come on spin you can do it

 

Powerlabs: Lotta good words but you are still incorrect, power and efficiency will go up with higher static compression ratios. The only time it doesn't help is when you lose control of the combustion burn and get pre-ignition or detonation.

 

I remember reading about some of the pikes peak engines. Huge compression and/or boost.

I don't know if this helped

Randy