Jetting for Reno or 4500' above sea level

[conedodger]

I recently moved from Sacramento (sea level) to Reno 4500' above sea level. My triple Weber carbs are pretty much perfectly tuned and synched for sea level. Anyone have an idea what jetting I should go to for 4500' above?

[conedodger]

Seems like I am not getting a quick answer so I decided to whip out something I have had sitting in the garage on a shelf and install it to help me figure out where I am and which direction I need to go... Though I expect I will need to be a lot leaner!

I had a Innovate Motorsports LC-1 sitting on the shelf that I bought a couple years back. I had intended to sell it because it didn't work with the EFI I put in my 914 but what the heck! I can put it in the 240Z and get tuned a lot closer than I could just guessing or asking you guys here!

First I welded in the bung and installed the sensor, then I decided to hide the gauge and what better place than the ashtray since I have never used one in my life! Now it has a use! And as you can see, my interior remains completely stockish since it is a 'now you see it now you don't install.'

[conedodger]

Kind of quiet around here...

So I finished up the install of the Innovate Motorsports LC-1 gauge in the ashtray today. Check it out. Should make tuning much easier. I already know that I am way too rich at idle.

[coop]

Sorry Rob, I could help you if you had the SUs on but not the Webers. I'll be doing some test drives out in the Palomino Valley soon to re-jet my rebuilt carbs so we are up to the same business. Wish I could help you! We should do some joint efforts out there. Nice long roads to wind it out.

[conedodger]

Coop, my friend Bill Madamba schooled me on who you are and what you do with your Z. I look forward to meeting you. When he said you had an ITS car, I knew you were no help on Weber jetting. 8D

I don't do any wheel to wheel but track days are fun... I look forward to spending a day at Reno-Fernley this coming year.

[d240zx2]

Bill M. has, if I'm not mistaken, Mikunis on his Z. He may be able to help if there are some similarities betwixt Webers and Mikes. Next time you see Mr. Bill, give him my best regards.

[Zedyone_kenobi]

Nice setup! My lc1 comes in this week. Let me know how you powered it. I was thinking the CIG lighter may work

[conedodger]

Bill M. has, if I'm not mistaken, Mikunis on his Z. He may be able to help if there are some similarities betwixt Webers and Mikes. Next time you see Mr. Bill, give him my best regards.

Oh I'm sure Bill has seen this thread... He is the biggest Uber-lurker on the internet. 15 hours a day of internet time but 0 posts. Oh and Bill - Oh yes I did say that! 8D

The problem though is that while Bill does run Mikuni's and his jetting is similar to mine, he is jetted for sea level too and I am looking for 4500' or there about.

[LeonV]

You'll probably have to lean it out just a bit, if at all. I'm willing to bet that seasonal swings in temperature, humidity, and air pressure have a more drastic effect on the carbs than just going up in elevation. Make considerations moreso for the climate that you're in, instead of purely elevation.

e.g. If you're higher in elevation, and it's hotter and more humid, then you'll want leaner jets.

Are you having problems with your carbs?

[conedodger]

You'll probably have to lean it out just a bit, if at all. I'm willing to bet that seasonal swings in temperature, humidity, and air pressure have a more drastic effect on the carbs than just going up in elevation. Make considerations moreso for the climate that you're in, instead of purely elevation.

e.g. If you're higher in elevation, and it's hotter and more humid, then you'll want leaner jets.

Are you having problems with your carbs?

I'd love to hear more about your theory Leon. From what I know about science, I'd have to disagree so far... At sea level, air molecules and ultimately oxygen is under more pressure and thus packed more tightly together. At 4500', much less so. This is why humans and all other animals for that matter have trouble breathing at altitude. Where this comes into effect with carbs is that the gas and air must mix at a ratio in order for things to run well. I like to keep that just under the theoretical magic 14.7:1. A larger first number is lean, less is rich. I like to be just north of 13:1 to protect the motor. I'm not disputing that weather has an effect. In fact internal combustion engines like cooler air and that is sometimes a manipulated variable, e.g. water injection. One need only take a highly strung engine out on a cool summer night to feel how much it likes cooler temperatures. I don't think this is something we can manipulate at the carb for tuning purposes. Bottom line, from what I know atmospheric pressure as it goes up and down are the only thing that drives the need to re-jet for rich or lean purposes.

My car has not been to Reno yet but I know that since it is tuned for sea level it will be rich at 4500'.

[LeonV]

I'd love to hear more about your theory Leon. From what I know about science, I'd have to disagree so far... At sea level, air molecules and ultimately oxygen is under more pressure and thus packed more tightly together. At 4500', much less so. This is why humans and all other animals for that matter have trouble breathing at altitude. Where this comes into effect with carbs is that the gas and air must mix at a ratio in order for things to run well. I like to keep that just under the theoretical magic 14.7:1. A larger first number is lean, less is rich. I like to be just north of 13:1 to protect the motor. I'm not disputing that weather has an effect. In fact internal combustion engines like cooler air and that is sometimes a manipulated variable, e.g. water injection. One need only take a highly strung engine out on a cool summer night to feel how much it likes cooler temperatures. I don't think this is something we can manipulate at the carb for tuning purposes. Bottom line, from what I know atmospheric pressure as it goes up and down are the only thing that drives the need to re-jet for rich or lean purposes.

My car has not been to Reno yet but I know that since it is tuned for sea level it will be rich at 4500'.

I'm a bit confused as you initially said that you disagreed with me, but then your statements agreed with me.

You can manipulate a carburetor to do whatever you want it to do, it's just up to the end user to take advantage of that fact.

Yes, air pressure decreases as you go up in elevation, thus you will run richer with all other variables held constant. This is one factor that goes into carburetor jetting, and fueling an internal combustion engine in general. Many EFI systems use a MAP (Manifold Air Pressure) sensor to correct fueling for air pressure changes.

Here is where you contradicted yourself: you stated that you don't dispute the effects of weather and even gave an example why it's so!

Temperature is a major factor in engine fueling. Air gets denser as temperature drops, and this is why you'll find that engines "like" cooler air. Cooler air means denser air, which means more air is entering the engine, and thus more fuel can be burned along with it. More air+fuel equals more power. I wholly disagree that you cannot utilize this for carb tuning purposes. A car that lives in a cooler climate will be jetted richer when compared to one that lives in a hotter climate. This is the reason why modern fuel injection systems use IAT (Intake Air Temp) sensors.

Your point about water injection brings up the reason for humidity being a factor. By utilizing water injection you are changing the relative humidity of the air coming into your engine! More humid air means a cooler charge by way of evaporative cooling. This effect is a secondary one when compared to air temperature, but it is still there, especially as evidenced with water injection. More humid air means a cooler charge, which leans the mixture for a given amount of fuel.

Thus, your engine will possibly be rich at 4500, but not necessarily. I don't know how it is in Reno, but I do know that Sacramento gets hot as hell in the summer. If your carbs were tuned during the Sacramento summer, then your mixture will be leaner at colder temps. Combine this with your now-increased altitude which makes you mixture richer, and it's not necessarily clear as to what changes must be made.

There is a reason why I asked whether you were actually having a problem with your carbs or not! If it drives the same and gets similar gas mileage then nothing needs to be changed in the way of jetting.

I assume you don't have a wideband O2 sensor installed, because that will show you how dramatically the mixture can change with varying conditions, even without changing your altitude. However, it seems as though your "butt dyno" has already shown how much temperature changes can affect engine performance!

FYI, the true stoichiometric (not magic...) air-fuel ratio for E10 gasoline is about 14.2:1, not 14.7:1.

FWIW,

Leon

[conedodger]

Leon,

I see what you are saying now. I agree that all of these things (air temperature, humidity) have an effect on performance, however since they vary pretty dramatically during the day it is impractical to tune for them. I do think that the difference in jetting for sea level (Sacramento) and 4500' above (Reno) will be pretty dramatic. I have houses in both locations but I will be living primarily in Reno. I have dyno tuned my Porsche 914 at both Sacramento and Reno and the difference at best tune was 13.5% lower in Reno. That is with programmable EFI which controls for IAT, MAP, O2 sensor, and throttle position.

I am confused by your assumption that I don't have a wideband O2 sensor installed since a good deal of the posts after the first one have to do with the installation of the wideband O2 sensor. I will include the pictures here so you don't have to back up and look at them.

Reno is generally cooler by a few degrees than Sacramento, it is dramatically less humid and of course, 4500' higher. If we follow your premise that air temperature and humidity should be considered, I might have to tune several times a day to be right on the best tune. My point is that you need to eliminate those variables. I awoke to 27 degrees this morning and I am presently on my garage computer with the garage door open and it is 55 degrees. When do I tune? What if it rains tomorrow? Do I have to tune again before I drive? Carbs just don't have the adjustability to control for your variables. You simply get your best tune on the day and time you tune with those present conditions.

My original post was a spitball hope that someone was running a similar engine in Reno and could give me a good place to start. My best hope was Coop but my friend Bill tells me he is an ITS racer and the rules require SU carbs.

I love a good discussion. You come up and hang with us in the garage anytime Leon! :classic: