Camshafts for elevation

[Steve.k]

This topic came up in another thread to a certain degree. The area where we live is high altitude with a pour DA. What changes to camshaft would you do to offset this handicap as much as possible? Cams with narrow lsa or is the cam part of the equation?Cams with less overlap? Concentrate on upping compression? Likely all of the above.I have the cams i use custom ground but usually we do not talk about the altitude!

[CamKing]

Since the altitude is reducing the mass in the engine, you handle the cam kinda like you would a restricted inlet engine. Only difference is, a restricted inlet engine, the mass reduction increases as the RPM's increase.
Of course you want to increase the compression. If the air density is down 20%, increase the compression by 20%.
As for the cam, you run the same intake duration you would run at sea level, but with more lift, and on an earlier intake closing point. On the exhaust, you run less duration and lift, with the same exhaust closing point that you would run at sea level.

[Steve.k]

Thanks Mike. I never really thought about making changes to cam profile. This area here really puts a hurt on engines as far as da.

[PackardV8]

The problem was solved more than seventy years ago; WWII piston engines were turbocharged/supercharged; some also used water/alcohol injection.

[Steve.k]

Yes Jack that would help a ton. But requires few more beans out of piggy bank to!

[PackardV8]

Yes Jack that would help a ton. But requires few more beans out of piggy bank to!

Yabbut crutching an existing NA engine with custom pistons for higher compression, custom camshaft timing and then a dyno tune to make it optimum will have one eating beans to pay for that also.

[rp930]

Great question. I’m at 6000+feet.

[Steve.k]

So running a less ratio rocker on exh would help out? 1.73 to 1.6?

[Steve.k]

Yes Jack that would help a ton. But requires few more beans out of piggy bank to!

Yabbut crutching an existing NA engine with custom pistons for higher compression, custom camshaft timing and then a dyno tune to make it optimum will have one eating beans to pay for that also.

Jack I've watched the turbo cars and I'll say its not my thing. Supercharged maybe. Always Nos?

[pastry_chef]

As for the cam, you run the same intake duration you would run at sea level, but with more lift, and on an earlier intake closing point.

Very interesting, I never would have guessed more intake lift.

[PackardV8]

Yes Jack that would help a ton. But requires few more beans out of piggy bank to!

Yabbut crutching an existing NA engine with custom pistons for higher compression, custom camshaft timing and then a dyno tune to make it optimum will have one eating beans to pay for that also.

Jack I've watched the turbo cars and I'll say its not my thing. Supercharged maybe. Always Nos?

No one can say turbo cars should be your thing. I've driven turbo cars since 1978 and it's definitely my thing. It took Ford, Chevy, Mercedes, BMW, et al another twenty-five years to understand, but all their entry level performance cars are 2-liter turbo fours.

All my redneck friends swore off Ford when they went turbo V6 in the trucks, but the world didn't end.

Bottom line - a wise man said, "All piston engines want to be turbocharged."

jack vines

[Steve.k]

Well i do have a 07 gt500 and i do like it no doubt about it. If i was going to do something it would be that style. Rig up a roots style or if you know how to set up turbo car to get around that popping and farting on line im in. Im old school I dont get that even though they do haul a**!

[modok]

The amount of compression increase you can get away with is not directly proportional to density. Not quite that simple.

Being knock limited, how much compression you can run is a function of the pressure and temperature when the plug fires and over TDC.

The air pressure IS reduced, but the temperature of the air, and the temperature of engine, is not necessarily any lower. Although reasonable to assume the internal engine temp is slightly lower.... just by virtue of not making as much power.
So roughly the compression increase possible is only half what would be predicted by just comparing air pressure difference.


I've seen this overlooked by people far smarter than myself.
Mike Jones's ideas on the cam do seem to be in the right direction.
It's a good question. I suspect the cam does not change a lot, but probably does change some.

for turbos it's a darn good thing compression ISN't proportional to density.
If it was then you'd all be running... 5:1?

[modok]

This will sound crazy, but the best way to adjust for altitude IMO, is increase the stroke.....1.5%?? or so, for each 1000 feet up, and leave all else the same.
Everything else works itself out.
Comrpession, heads cam, rpm range, carburetors, intake, all end up about right.
Is there a scientific basis for that? Am I crazy? Not sure. it just seem right.
Maybe you can work that backward and figure out the cam??
It could work

And the other thing is that the cam is not really the first thing to change, a lot of things to change, compression first, then.... wet flow is more of an issue. Airspeed is more precious.
So...if you want to run more intake lift, which may be right, then the heads have to be stable and efficient at that higher lift also or it's not going to work,

[Steve.k]

This is all very interesting Modok thanks. We also snowmobile in the bc rockies so we know lots about hp loss at altitude. Start with 165hp in parking lot and end up with 125 at 7500ft. Huge losses. The current turbo two stokes and four are a huge step.