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I've lurked on this board intermittently for several years and remain intruiged enough about the 8 to still consider a purchase. I recently moved to New Mexico, live at about 7000 ft and am wondering about the engine performance of the 8 at high altitude. I currently drive a 6-spd manual Xterra with the big 287 or so hp motor. However, at this altitude in 6th gear on the interstates, I cannot maintain speed going up gentle grades w/o downshifting. I recently drove to Colorado on I-25 on a windy day and could barely keep the thing moving on hills-with all that power. So, How does the 8/Renesis handle the thin air? Is there anything intrinsic about the rotary engine that allows it to be less effected by altitude? Thanks very much.
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I live in Phoenix, but Flagstaff 140 miles north of town is roughly 7,000 feet. The 8 only has so much to begin with, but the elevation is like having a full load of four people and the trunk loaded down. It is suffice, but you spend more time around 8,000 rpm than on flat elevation!
I live in Wyoming at 6700 feet and have really good luck. I can hit red line and never miss a beat. I went in the mountains of Colorado on I70 and had to drop to fourth on the steep climbs to maintain 70mph. I don't think you have anything to worth about. I can leave my cruise control on and hardly ever drop a gear to maintain speed. Hope this helps
Where I live it is between 5500 - 6000'. It does just fine for me. I haven't really spent much time with it a lower altitudes to be able to compare much though. I'd say that the lowest elevation is has seen is about 3500'. Hopefully this summer I will get over to Seattle. I will be curious to see if it performs any differently.
I was wondering if metering No2 into the fuel stream in small minuscul quantities at high altitudes would be possible or is that even something relevantly possible,in other words, am I full of SHIT in bringing it up
I've read in aviation circles that the accepted figure for power loss in a normally aspirated engine is 3.5% per 1000 feet (density altitude). I imagine normally aspirated automobile engines are affected similarly, if not exactly the same. So, the 238 HP Renesis would produce approximately 177 HP at 7000 feet ASL (above sea level). That's nearly a 25% loss in power. Higher than normal outside air temperatures will cause an even higher loss in power. And of course, all of this applies to normally aspirated piston engines as well.
This reminds me of a vacation in the Sierras many years ago when my dad managed to drive our '58 Volkswagen bus on some remote roads to over 10,000 feet. Let's see, that reduced the 36 HP engine to about 23 raving horsepower!
Another thing that would be a good idea above 5,000 feet above sea level would be to bump the octane. Normally-aspirated piston engined aircraft still can be found burning 80 octane gas at lower altitudes, although 100 octane has become something of a standard. But at higher altitudes, such as Jackson, Wyoming, you find 130 octane gasoline (a pretty shade of green). I recall a relative who used to drive on the "middle" gas rating in the flatlands of Colorado, who bought the lowest octane during drives down to sea level, and premium at higher altitudes.Filling up with premium at 7,000 feet would seem to be a good idea.
Another thing that would be a good idea above 5,000 feet above sea level would be to bump the octane. Normally-aspirated piston engined aircraft still can be found burning 80 octane gas at lower altitudes, although 100 octane has become something of a standard. But at higher altitudes, such as Jackson, Wyoming, you find 130 octane gasoline (a pretty shade of green). I recall a relative who used to drive on the "middle" gas rating in the flatlands of Colorado, who bought the lowest octane during drives down to sea level, and premium at higher altitudes.Filling up with premium at 7,000 feet would seem to be a good idea.
Read the following (paragraph 7.11, about 3/4's of the way down page) on how altitude affects fuel requirements:
All the small aircraft that I've flown had updraft carbs. I thought that I mentioned the relative who switched octane levels was driving a car with a carburetor, but see that I did not. Modern engine controllers have many advantages over mechanical marvels such as carburetors, to be sure.
All the small aircraft that I've flown had updraft carbs. I thought that I mentioned the relative who switched octane levels was driving a car with a carburetor, but see that I did not. Modern engine controllers have many advantages over mechanical marvels such as carburetors, to be sure.
I've also flown lots of updraft carburetor aircraft (and own one), but I still fail to understand why you seem to think that going to a higher octane as the aircraft/automobile increases altitude is a benefit. The link I provided seems to be stating the opposite.
Here's an even better link that describes the effects of altitude on octane requirements:
Another thing that would be a good idea above 5,000 feet above sea level would be to bump the octane. Normally-aspirated piston engined aircraft still can be found burning 80 octane gas at lower altitudes, although 100 octane has become something of a standard. But at higher altitudes, such as Jackson, Wyoming, you find 130 octane gasoline (a pretty shade of green). I recall a relative who used to drive on the "middle" gas rating in the flatlands of Colorado, who bought the lowest octane during drives down to sea level, and premium at higher altitudes.Filling up with premium at 7,000 feet would seem to be a good idea.
First of all, this is 100% the opposite of true. You need less octane at higher altitudes since there is less oxygen in each cubic inch of air. This is why in Colorado, our Octane ratings are 85, 87 and 91 for each type of fuel. Octane is a measure of a fuel's resistance to ignition. Higher Octane will prevent a cylinder (or rotor in our case) from causing fuel to ignite spontaneously due to the compression of the air in the cylinder before the spark from the plug. When air is compressed it heats up and will ignite--this is how a diesel engine works. So, pre-ignition knocking is also called "dieseling."
So in summary, at high altitude there is less O2 to burn so the fuel isn't as likely to ignite before the spark sets it off. You can get by with lower Octane.
Airplane engines use high-octane fuel (which is a lot more expensive--it is essentially the same as race car gas) because they have higher compression engines. Basically they have to make a lot of horsepower at high altitude, so they need to compress the air very tightly to produce enough power. Also many planes are turbocharged.
Turbos are able to compress a lot of air in the piston, so they will also tend to make the fuel ignite prematurely. It's important to run keep the fuel/air mixture rich or use high octane fuel to keep it from knocking.
howstuffworks.com explains all of this with pretty pictures I think.
As for the original poster--I sold my Miata and bought an RX-8 because the altitude took all the fun out of that car. The RX-8 on the other hand is quite capable at high altitude. A turbo car might be a little better, but I think the RX-8 has plenty of kick. You can drive from Denver to Vail in 6th gear the whole way if you choose. When I go back down to Kansas I notice some increase of power, but it's not really huge. The Renesis will get a little bit wimpy at 11,000 feet but the motor revs so well it's not really a problem when passing. Of course your best alternatives would be a WRX STi, Mazdaspeed Miata, MazdaSpeed 6 or some other turbo car. They can't really match the RX-8s handling though.
Thanks everyone for all the great info and sharing of your experiences -and the interesting octane lesson. I'm encouraged by what you all said about the RX-8's ability to function at altitude. If I do go the turbo route, I may also consider the new VW GTI; sounds like a well rounded fun machine -although I doubt the handling will approach the 8's.
It will feel less powerful only if you are used to how it drives at lower elevations. Since you do not yet have an 8, you will notice any issues with less hp at the altitude. You will, however, notice an increase in power as you get into lower elevations. My current car is a 98 BMW 323is (178hp) and it has no problems drive 85mph through the mountains. The car before that was a 92 Maxima (190hp) and it also had no issues either. Since the 8 is lighter and more powerful than both of those cars I am confident when I purchase my 8 it will have plenty of power.
First of all, this is 100% the opposite of true. You need less octane at higher altitudes since there is less oxygen in each cubic inch of air. This is why in Colorado, our Octane ratings are 85, 87 and 91 for each type of fuel. Octane is a measure of a fuel's resistance to ignition. Higher Octane will prevent a cylinder (or rotor in our case) from causing fuel to ignite spontaneously due to the compression of the air in the cylinder before the spark from the plug. When air is compressed it heats up and will ignite--this is how a diesel engine works. So, pre-ignition knocking is also called "dieseling."
So in summary, at high altitude there is less O2 to burn so the fuel isn't as likely to ignite before the spark sets it off. You can get by with lower Octane.
Airplane engines use high-octane fuel (which is a lot more expensive--it is essentially the same as race car gas) because they have higher compression engines. Basically they have to make a lot of horsepower at high altitude, so they need to compress the air very tightly to produce enough power. Also many planes are turbocharged.
Turbos are able to compress a lot of air in the piston, so they will also tend to make the fuel ignite prematurely. It's important to run keep the fuel/air mixture rich or use high octane fuel to keep it from knocking.
howstuffworks.com explains all of this with pretty pictures I think.
As for the original poster--I sold my Miata and bought an RX-8 because the altitude took all the fun out of that car. The RX-8 on the other hand is quite capable at high altitude. A turbo car might be a little better, but I think the RX-8 has plenty of kick. You can drive from Denver to Vail in 6th gear the whole way if you choose. When I go back down to Kansas I notice some increase of power, but it's not really huge. The Renesis will get a little bit wimpy at 11,000 feet but the motor revs so well it's not really a problem when passing. Of course your best alternatives would be a WRX STi, Mazdaspeed Miata, MazdaSpeed 6 or some other turbo car. They can't really match the RX-8s handling though.
Great post! However, I must disagree with some of your statement about aircraft engines. Compression ratios in most normally aspirated aircraft engines are no higher than automobile engines. In fact, many are actually a bit lower. I own an airplane that has a very common 4-cylinder Lycoming engine and the compression ratio is only 7.5 to 1. On the other end of the spectrum, there are some aircraft engines used in helicopters and experimental aircraft where the compression ratio is 10 to 1. While this is at the high end for aircraft, it seems to be quite common in today's automobiles.
As to turbo-charged aircraft engines, there are 2 schools of thought: some turbos are designed to only allow the engine to maintain its sea-level horsepower to hight altitudes, but not boost the normal sea-level power rating. There are a few others where the boost is designed to increase the horsepower right from the get-go (sea-level) and then maintain that HP rating to high altitudes.
Aviation fuel is formulated a bit differently than auto-gas and includes better long-term storage properties and more resistance to vapor lock. The most common av-gas is 100LL (low lead). 80/87 av-gas is becoming more difficult to find simply because many of the engines that were designed to run on it are slowly fading away. 100LL is sort of a "one size fits all" design; the older engines will run on it, but even though it is billed as low-lead, the lead content is still 4 times that of 80/87 av-gas. Some older aircraft engines can eventually develop problems running on 100LL such as sticking exhaust valves.
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