Peak power RPM change with air density

[Truckedup]

I'm planning to take my vintage bike racing junk out to El Mirage in the near future...Here in the east I run the bikes on a chassis dyno and simply gear it run the MPH I want based on the peak power RPM seen on the dyno...This has worked well enough to set numerous speed records..The bike literally comes off the trailer and runs it's best..
The usual air density when dyno testing is 1200 ft...El Mirage air density is around 3500-4200 feet...
Question, obviously I can never regain all the power lost to air density by tuning... But what about peak power RPM? Will reduced air density lower peak power RPM? I'm thinking about gearing changes before hand to minimize changes at the track ...Thanks

[Brian P]

Intake tuning effects are dependent on the speed of sound. The speed of sound in a gas (air) is a function of absolute temperature, but not pressure.

The torque curve will be the same shape at the same engine speeds, just less of it everywhere ...

Your carbureted engine will want different (leaner) jetting, of course.

[pcnsd]

What carbs are you running? I have a RAD spreadsheet I can post that can suggest changes to jetting based on changes to RAD from a saved tuning day jetting standard. It is set up to work with Keihin and Dellorto carburetors, but not Mikuni. Mikuni uses multiple jet sizing methods that do not translate fuel flow areas as simply as the others.
I can say altitude makes the greatest impact on jetting and HP output, more so than temperature (#2) or humidity (#3 and almost none). I can also say you will be down measurably on HP at 3500Ft and I would expect that to impact your available run RPM.

[Roundybout]

The peaks will be around the same, just less. Possibly a gear change if you can't reach the RPM peak HP coincides with along with the usual tuning for that air density change.

[Truckedup]

What carbs are you running? I have a RAD spreadsheet I can post that can suggest changes to jetting based on changes to RAD from a saved tuning day jetting standard. It is set up to work with Keihin and Dellorto carburetors, but not Mikuni. Mikuni uses multiple jet sizing methods that do not translate fuel flow areas as simply as the others.
I can say altitude makes the greatest impact on jetting and HP output, more so than temperature (#2) or humidity (#3 and almost none). I can also say you will be down measurably on HP at 3500Ft and I would expect that to impact your available run RPM.

34 mm Mikuni TM flatslides...I have the Mikuni chart showing suggested main jet changes for altitude changes.. As you say, the bike engine does not seem to be sensitive to changes in air temperature or humidity ....On the dyno best power is a #220 main jet, the reality of the track needs a #230..

[pcnsd]

What carbs are you running? I have a RAD spreadsheet I can post that can suggest changes to jetting based on changes to RAD from a saved tuning day jetting standard. It is set up to work with Keihin and Dellorto carburetors, but not Mikuni. Mikuni uses multiple jet sizing methods that do not translate fuel flow areas as simply as the others.
I can say altitude makes the greatest impact on jetting and HP output, more so than temperature (#2) or humidity (#3 and almost none). I can also say you will be down measurably on HP at 3500Ft and I would expect that to impact your available run RPM.

34 mm Mikuni TM flatslides...I have the Mikuni chart showing suggested main jet changes for altitude changes.. As you say, the bike engine does not seem to be sensitive to changes in air temperature or humidity ....On the dyno best power is a #220 main jet, the reality of the track needs a #230..

The attached is protected but no password. You would need to convert your jetting to mm diameter and back to make use of it. Both the Dells and Keihin jetting system is a straight metric diameter. A 157.5 main jet is 1.575mm in diameter. A 52 pilot jet is .52mm in diameter. You also need to know or best guess the RAD for your tuning day baseline. I hope the sheet is self explanatory, if not I will try to answer questions, but am soon to travel so there may be a delay in reply.

RAD and Jetting Calc.xls

[Truckedup]

Thanks, I believe Mikuni jet sizes refer to cc's of fuel flow per whatever...

[mk e]

Here's a dynomation 6 sim of the same engine at 1200ft and 4000 ft. 1200 is 859hp ar 9000rpm, 4000ft is 763hp at 9000rpm...so as others said the output just shifts with the air density.

[Truckedup]

I suppose the power loss is not uniform on all N/A engine types..?

[mk e]

I suppose the power loss is not uniform on all N/A engine types..?

They should be pretty uniform....you're going to lose about 11%.

In the example I posted there is a 11% hp loss and a 10% air density change for the 1200 to 4000ft elevation. I suspect the hp loss is a bit higher than the air density change due to friction losses being about constant so a high % at lower net hp numbers..

[Roundybout]

That's quite a substantial difference in output from 1200ft to 4000ft. Explains why the kids dirt bikes really suffer going from the nice cool morning here in NH at 1100ft and by the time afternoon arrives they're at 4000ft in the mountains on the trails. For some reason the 2-strokes seem to fair better. They like to run richer than the 4-strokes and don't become a complete blubbering mess I suppose.

[Firedome8]

Does a lower density affect smaller displacement engines disproportionately?

[mk e]

Does a lower density affect smaller displacement engines disproportionately?

Don't know why it would.

[Roundybout]

Does a lower density affect smaller displacement engines disproportionately?

I don't think it does but it sure feels like it. I think when you're dealing with small 8-25HP engines it's just really obvious the loss of HP. Taking a 600HP engine and losing 75 or so HP, well you still have 575 lol.

[Brian P]

1. The loss in power and torque would be expected to be slightly greater than being absolutely proportional to air density, due to certain frictional and oil pumping losses being independent of air density. In engineering terms, the IMEP is substantially in proportion to air density, and the FMEP has a significant component which is independent of air density.

2. Little engines tend to have greater FMEP than big ones.

3. If you have a carbureted engine jetted for sea level ish and you take it to high altitude without rejetting, the loss will be greater than proportional to air density owing to it now being too rich.