View Full Version : So SC's are the same..high or low elevations??

04-02-2003, 10:03 AM
So there aren't any differences between upper elevations SC's and lower elevations? I have a 35th owner who has moved to Colorado and is having power drop offs. Any inputs appreciated.


Aaron Pedroza
04-02-2003, 11:51 AM
Less air, less power. You will tend to be able to rev higher though.

04-02-2003, 12:08 PM
Just did a little searching on the web...Might spark some thinking on the subject.
Altitude - Due to thinner air at high elevations the combustion pressures are lower because the engine is drawing in less air on each intake stroke. If the pressure is lower there is less heat. Less heat equals slower burning. Slower burning requires greater advance. More time to let the gases get to max pressure. This is why at high elevations low octane gas is 85 or 86-octane. Travel to the coast or below four or five thousand feet and low octane gas is rated 87-octane. The lower octane, used at high elevations burns a little faster and compensates for the lower pressures and lower heat and slower burning that takes place at higher altitudes. Some vehicles have altitude compensation which automatically adjusts the fuel mixture and spark timing. More on this later.
Generally, three grades of unleaded gasoline with different AKIs are available in the United States regular, midgrade and premium. At sea level, the posted AKI for regular-grade is usually 87 and for midgrade, 89. The AKI of premium-grade varies more, ranging from 91 to 94.

The posted AKIs gasoline are lower in the Rocky Mountain states. These altitude gasolines historically provided the same antiknock performance as higher-AKI gasolines at sea level. The octane requirement of older-model engines decreases as air pressure (barometric pressure) decreases the barometric pressure is lower at higher elevations.

Since 1984, vehicles have been equipped with more sophisticated control systems, including sensors to measure, and engine management computers to adjust for, changes in air temperature and barometric pressure (see Chapter 5). These vehicles are designed to have the same AKI requirement at all elevations and the owner's manuals specify the same AKI gasoline at all elevations.

It is difficult for a driver to know whether a gasoline has the antiknock performance the engine requires when the engine is equipped with a knock sensor system. These systems, which temporarily retard spark timing to eliminate knocking, are installed on many late-model engines (see Chapter 5). Retarding the spark reduces power and acceleration. The knock sensor responds so quickly that the driver never notices the knock. Loss of power and acceleration will be the only clues that the antiknock quality of the gasoline does not meet the vehicle's octane requirement.

Using gasoline with an antiknock rating higher than that required to prevent knock or to prevent spark retardation by the knock sensor will not improve a vehicle's performance.

Knock Control
Most new cars come equipped with a device called a knock-sensor that detects knock and provides a signal to the ECM. To eliminate the knock, the ECM retards the engine's timing, that is, it fires the spark plug later in the compression stroke. This reduces the peak cylinder pressure and, as a result, the tendency for autoignition. The downside of timing retardation is a decrease in power. When knock is sensed in a turbocharged engine, the ECM also decreases the amount of boost to further reduce peak cylinder pressure. Boost reduction significantly decreases power. Loss of power due to the ECM's knock-elimination strategies can be prevented by using a gasoline with a higher AKI.

Aaron Pedroza
04-02-2003, 12:12 PM
Ok, all that too.

04-02-2003, 12:19 PM
Yea, it looks like you don't need nitrous injection at higher altitudes, you need an oxygen bottle.
A normally aspirated 230 HP engine at 7,500 feet only generates 168 HP.

Aaron Pedroza
04-02-2003, 12:33 PM
You know that brings up something I have thought a lot about. These monster motors like NHRA Top Fuel and the likes, why don't they make a sealed system where they have a bottle of oxygen to feed in pure oxygen into the intake? I know it would be goobs of power and the whole engine would have to be built to handle it but are there rules that say you can't do that? You figure feeding it in at normal pressure and you get 8 times the amount of oxygen for about the same amount more power. There would be no use for blowers, nitrous, complicated intakes.... If you want more power just crank up the amount of pressure into the system and add more fuel. Just seems like a relatively easy set up to make massive amounts of power.

04-02-2003, 12:38 PM
Great idea! Can you imagine what the HP and TQ #'s would be! The only thing is...I might be wrong but I read that a Pro Stock car moves enough air to fill a hot air baloon in a single pass. Can you imagine what a Top Fuel or Funny Car moves? That would be one big air tank!


04-02-2003, 12:43 PM
The main issues are a reduction in air pressure, and oxygen content.

On a naturally aspirated car, it is air pressure that causes air to enter the vacuum created by the piston moving down. Less air pressure, means that air will attempt to fill the vacuum at a lower velocity. Since there is a limited amount of time for the air to enter the cylinder, a lower velocity means less air enters the cylinder.

Our cars are supercharged by a positive displacement supercharger. When operating, air is pressurized at a constant. So our cars are not noticably impacted by less air pressure at high altitudes.

BUT our cars are impacted by less oxygen content in the air. For a given gulp, less oxygen is available. If there is less oxygen, combustion processes will suffer.

For our cars, the engine computer should see this with the oxy sensors and decrease the pulse widths on the injectors to decrease the fuel to keep the air/fuel mixture correct. This will cause a drop in power. If the oxy sensor is weak, the computer may not be restricting the fuel enough, creating a rich mixture that also impacts performance.

So for a car that moves into higher altitudes and see's power drops, I'd start with new oxygen sensors if they haven't been changed for a long time.

In general, max boost pressure should see a decrease, and performance should be slightly lower than at a lower altitude. But overall, peformance should stay quite snappy. Non-blown or turbo'ed cars have it much worse at higher altitudes.

Aaron Pedroza
04-02-2003, 12:49 PM
Rino, the key thing there is moves. 80% of that air isn't oxygen and is unusable in the combustion process.

04-02-2003, 12:55 PM
Very interesting...you make a good point Aaron. Have there been any studys done on this? Might just have to put on the ole white lab coat and try this on a small engine...don't want to try it on my baby...smoke, running for cover, flames...hot parts a flyin etc. I really hate repairing drywall...don't ask.


04-02-2003, 12:57 PM
Maybe a liquid oxygen system.
Liquid oxygen expands 862 times in volume when it hits room temperature. A few cups of that stuff would fill your hot air balloon.
Liquid Oxygen -- Oxygen is stored as a very cold liquid in a vessel very similar to a thermos. When released, the liquid converts to a gas and you breathe it in just like the compressed gas. This storage method takes up less space than the compressed gas cylinder. Liquid oxygen is more expensive than the compressed gas, and the vessel vents when not in use.
Pressure release valves on top allow gas to vent if contents are warmed. Vaporizing columns allow oxygen to warm to room temperature and go from liquid to gaseous state. Liquid oxygen is maintained at 279.3 F at 1 atmosphere. Liquid oxygen expands 862 times to it gaseous state at room temperature.

Aaron Pedroza
04-02-2003, 01:15 PM
I've been there and done that on to many things already. My last project like that was the alky injection. Cost me a fortune to just figure out how it would best work for me. Figured it out and was going to sell kits but ran out of money, should have just bought a kit for a GN. Don't want to do that again. Besides, I know engines pretty good but there are a lot of guys out there that know a whole hell of a lot more than I do. I'm sure it's been thought of, probably rules in NHRA that say it can't be done.

04-02-2003, 02:41 PM
The other thing about running pure oxygen - it burns really really hot. I think you'd probably melt the block doing that. Somewhere on the web there's video of some group at NASA or somewhere lighting a grill with oxygen. About 2 seconds after ignition, there was no grill. At all. And I don't just mean the little bars you put food on, I'm talking the whole Hibachi. Gone...

Aaron Pedroza
04-02-2003, 02:53 PM
Well that's when you get a ceramic block and internals. That stuff is big time bucks though. I was hanging out in a buddy's shop when a friend of his came in with a piece of something. We didn't know what it was at first. He asked him to see if he could drill through it. Well to give you a better idea of what this guys shop was like, he had been there in business for 30 years and knew his chit. He tried everything to put a hole in it and didn't even scratch the surface. It was a piece of some ceramic material that was about 1/4" thick and about 4" round, real light and sounded like metal when it was dropped. The guy said it would probably cost about $250,000 in materials to make a block out of it. It was pretty cool to watch.

04-02-2003, 06:31 PM
Gentlemen! What a conversation. I appreciate your inputs. I checked with Ford and there isn't any different ECA for SC's except for those for California. I will pass this along to the owner.

A great week all.

04-02-2003, 08:42 PM
a top fuel motor tha burns nitro methane does not need as much oxygen. nitro methane requires only 10% as much oxygen as gas. of course every top fuel car does not run 100% nitro, each team has ther own recipe. i belive you can only run a max of 20% nitro.

04-03-2003, 11:23 AM
Aw come on Aaron, your not going to put the ole coat away for good are you? I'll lend ya mine if you want...it's only slightly burnt :>) He he he...!


Aaron Pedroza
04-03-2003, 11:30 AM
If this wouldn't be as expensive as I know it would be maybe, but I know it would be. Wouldn't that be cool though. Add some way to flip a switch and seal the system and have an oxygen tank in the trunk. Talk about having a sleeper.

04-03-2003, 11:33 AM
And then you could watch the heads come right through the hood, wow what a good show that would be. Pure O2 is WAY to powerful for use like that. But maybe if it was metered in at a certain flow like (injectors) then it might work.

Aaron Pedroza
04-03-2003, 12:11 PM
Well maybe not pure O2 but a good mix for our motors. Maybe 40% oxygen and the rest inert gases. Still, it's just the idea that's cool.

Aaron Pedroza
04-03-2003, 10:35 PM
Just talked with a guru about the O2 setup I was thinking about. He said that regular compressed oxygen was a problem because you couldn't get enough in it. Then I mentioned the liquid O2 thing and he said that it was tried but they couldn't control the burn, it turned the block into fuel for burning. So all that speculation was a fun conversation but a no go. I still think with a intricate way of regulating the liquid O2 it could be done but for now it isn't hapening.

SCgraphics guy
04-04-2003, 12:01 AM
My o2 guys...I live in CO. & run a CMRE Stage 2 SC & NA 383 N/A Chevy motor.

First , from my experience our SCs will not rev higher up here, much to the contrary they rev lower!..I know this because up here power significantly falls off about 4800rpm, whereas at sealevel... I was able to effortlessly rev to 6000 before power fell off significantly.

As for the N/A stroked motor
Suppose your at sea level...your using a 650 cfm carb, and the car runs great.
You move to high elevation & the car is a dog.
In order to even come close to same type of power at high elevation you need to add air...a 750-900cfm carb will really help.

I've seen this proven time & time again! at the track.
This is also the reason the top fuelers run so much overdrive up here & throw belts left and right in order to run the same ets they run at sea level.
I've listened to many of the big boys who come thru here & they either hate it cause they don't know what to do, or they love it cause they understand it & they others don't...SO THEY WIN RACES!

04-04-2003, 01:45 PM
i remember this subject from a fear years back with a buddy of mine who is a fords part guy when i was swapping my auto out for a stick. according to him, there was three computers auto, stick and high altitude. i never took the time to confirm this as i just kept the auto computer but certainly worthwhile checking into