Jetting for Reno or 4500' above sea level

[conedodger]

A couple more pictures of the car...

[LeonV]

Brainfart on my part on the WBO2.

What I am saying is that your tune is affected by the climate of the area you're in, not just the elevation. I'm not saying that you must constantly tune for each condition. You have moved from one climate (and elevation) to another and you're curious as to how the jetting may change. The point I am making is that you should consider the average climatic differences between Sacramento and Reno, not instantaneous ones. You did go up in elevation which would make your mixture richer, but let's just say (making things up here) that on average Reno is 10 degrees cooler and more humid than Sacramento during driving hours. This would dictate jetting the carbs richer, all else held constant. This is why it's hard to say whether it should be richer, leaner, or stay the same.

The only way to really know what to change, if anything, is comparing data-logs from your WBO2 from Sacramento and Reno. The more sample days the better.

You're right in that corrected dyno numbers will be lower (aside: interesting comparison with the 914 since that's about the % difference in air pressure!). Dyno results are usually normalized ("corrected") to a standard temperature, pressure, and humidity (we've essentially covered why this is necessary, reference the link above for more). What the dyno actually measures is assuredly different! Therefore, dyno numbers are a red herring, since temperature and pressure variations have been taken into account when the data is presented to you.

You have use of a WBO2, so you are your best source of information regarding the jetting changes that must be made to your engine. Nobody can answer that question better than yourself, especially since you don't even have to guess with the wideband. Relying on heresay, "he did this, I did that" is a lot less effective, and more time consuming. If your engine runs fine and you get similar gas mileage then don't worry, all is fine! Since you've just installed the wideband, it makes comparisons difficult as you don't have anything to compare to. Get the AFR where you need it in the weather that you typically drive in and you'll be set.

I'm just saying that it's not exclusively elevation that will dictate what that WB says and what jets you'll ultimately settle for. Day-to-day changes in temperature, pressure, and humidity can make notable differences in AFR that you may as well leave the carbs alone if the AFR is "close" to where you want it and everything is running right. How "close" depends on your discretion, which is controlled by your knowledge of the subject at hand. That was the intent of my original post.

That's a beautiful Z! I would love to hang out in the garage although I seldom make it out to Sac and can't say that I've been to Reno!

[conedodger]

A little rich but not too bad at idle...

[coop]

Hey Rob.

What really affects engine performance is air density. Air density is affected by the air pressure, air temperature, and the humidity. Pilots concerned about the performance of their airplanes use a term called "density altitude" that takes all this into consideration. They get an altimeter setting, get a temperature reading, and then they determine an "effective" altitude which they call density altitude. For example, if you live at sea level in Sacramento and it is 100 degrees outside on a day with a standard altimeter setting of 29.92 inches of mercury barometric pressure, then you will effectively be operating at some higher altitude (say 1000 feet since I don't have a density altitude calculator) where the air is thinner. The effect on air density caused by humidity, is readable but minimal compared to that caused by barometric pressure and temperature so most folks don't worry about humidity. You do, however, need to be concerned about pressure and temperature and their effect on density. You could do as the pilots do by listening to the local weather conditions on an aviation radio, then enter the temp and pressure on a wiz-wheel(calculator) to get density altitude to use as a reference for your jetting or you could do what I do. Use a DENSITY gauge that gives you a density reading to set your baseline. You can come by and borrow it if you want and I'll show you how to use it. What it is good for is once you are in a location on a given day, say up in Reno, you get your car tuned just right, then take a density reading. Then you go down to the Bay area and re-tune the car and take another density reading. Now you have two density versus jetting data points that you can draw a line between on a piece of graph paper. Where ever you go then, you just take a density reading and re-jet your carbs to to satisfy the line. Now this works real cool on my SUs as they are re-jetted with a twist of the knurled knob, but you would have to have a stash of jets handy all the time and it is a bit cumbersome on the Webbers.

Now, all this is important to me since I race, and every HP is critical. I do a quick check of the density and re-jet the SUs as necessary just prior to every qualifying session and race and it works great. You, however, should look at the big picture. Are you going to do track days? Are you going to go cross-country over Donner Pass to the valley and how often? Are you looking for power, or economy on these trips? With the density gauge and your data line, you could jet your car for the conditions you will normally drive it under, say an average tune. Then on days when you head out to the track, you could do a quick fine tune just before leaving home. The possibilities are endless. I don't know why folks don't use this sort of thing more often. When I had Dave Rebello build and dyno my recent engine, he said that I had my carbs "right on." Now if I can only find out what is causing the engine fires....... Doh!

Edited January 5, 2012 by coop
spelling

[LeonV]

That's very good advice, coop!

[coop]

Rob.

I tracked down this cool density altitude site that has a density altitude calculator that is easy to use. You can even input relative humidity to really nail it down. There is even a link there to change over to a relative horsepower conversion.

Here is the site: http://wahiduddin.net/calc/calc_da_rh.htm

I ran some numbers and in Sacramento, a 100 degree day is like being at 2700 feet on a standard day. If I remember right, most of the time up in Reno, my density gauge reads about 85% as apposed to around 99% at Laguna Seca. This web site calculator bears that out. You basically only will put out about 85% HP in Reno of what you'd get in SAC since the air is about 85% thinner. Maybe some Webber folks or some dyno shops will have some pointers on which jets you will need based on what data you pass them. Just a thought.

[coop]

Oops, not 85% thinner but 85% of that at sea level(15% thinner).

[LeonV]

Rob.

I tracked down this cool density altitude site that has a density altitude calculator that is easy to use. You can even input relative humidity to really nail it down. There is even a link there to change over to a relative horsepower conversion.

Here is the site: http://wahiduddin.net/calc/calc_da_rh.htm

I ran some numbers and in Sacramento, a 100 degree day is like being at 2700 feet on a standard day. If I remember right, most of the time up in Reno, my density gauge reads about 85% as apposed to around 99% at Laguna Seca. This web site calculator bears that out. You basically only will put out about 85% HP in Reno of what you'd get in SAC since the air is about 85% thinner. Maybe some Webber folks or some dyno shops will have some pointers on which jets you will need based on what data you pass them. Just a thought.

Coop, you indicate that the difference between your Reno and Laguna Seca is about 14% so I'd expect the differences between Reno and Sac to be less.

I'll use your link for an example. Looking at current (8am) Reno conditions (24 degF, 30.18 inHg, 71% humidity), assuming 4500 ft elevation, your density altitude is just below 3000 feet. If a 100 degree Sacramento day is 2700 ft density altitude then there is not much of a difference here! Back to my original point, you may not have to change much, if anything, at all. Climatic swings affect carb settings just as much as a change in altitude does, so there is no need to worry unless you make a really big change in altitude and/or climate.

With that said, getting your initial tune done right is a whole different story!

[coop]

Yes, both temp and elevation have a big effect. The thing is, when it is 100 degrees in SAC, it often is 100 degrees in RNO. See what you get when you run the numbers for RNO at 100 degrees!

[spitz17]

I too know Bill from track events. He has a contact in the Bay Area that is supposedly a "Mikuni guru" who may know a thing or two about Webers. May want to ask him about that?

[Oiluj]

What really affects engine performance is air density. Air density is affected by the air pressure, air temperature, and the humidity. Pilots concerned about the performance of their airplanes use a term called "density altitude" that takes all this into consideration.

Wow, "density altitude" is a term I'd forgotten many years ago! Thanks for the memory jog and the good web link.

[conedodger]

Carbs are synched within a 'gnat's breath' and were effectively pretty well jetted for Sonoma, CA (Original Customs) restoration shop that I use. I trailered back to Reno yesterday and got her off and fired her up. Still runs nicely, but the A:F ratio numbers moved to the rich side in a dramatic manner. 9:1 range for idle and just on the rich side for acceleration. I didn't dare do WOT testing as I live within a few short blocks of Damonte Ranch High School and I would imagine there is a cruiser or two waiting for the boys to display their testosterone...

All my jets are in a box in a Sacramento storage garage so it will have to run a bit rich for now. I am looking around for the stock choke mechanicals that I can cable up to the Webers with for now. I managed to find a lever and mount with all the electricals for the light on ebay but when I got it I realize it is meant for the right side of the console and mine, a '73 is on the left. Anyone want to trade?