Friend bought a 9sec Dodge Dart 500 cubes 1150 Dominator carb.
At sea level ran 9.20's.
He lives in Colorado at 6000' or so, roughly how much jet should he pull only ran 10.40 up there.

92-94 jets in Dallas, 86-88 jets in Denver. Every engine is a little different. This is on my 565.

That is a larger than normal drop off in ET.
It may have been fat already at low altitude, and much worse now.

I agree with the advice from Verne (Cashflow).
When I moved from Texas to Colorado 7 years ago, he and others got me set up spot on. (My local track North of Dallas is around 600' above sea level). I went down 6 numbers, and AFR was good. That was with a 1050 on Gas.
My car was .6-.7 slower up there, (Low 11.40's at Bandimere, High 10.70's here).

Never been to Denver. I hear a typical race day is 8000+ feet density altitude. When someone tells me they have a 12 second street car, I say where? In Florida, big deal. In Denver, that's a fast car. It's hard to make NA power at high altitude.

When Calgary Had a track, a typical race day was 6000' DA. I raced in Yakima in the late 1990 and the engine sounded so much crisper at the lower altitude. I never did any jet changes for the lower elevation. I've run 9.0 in Edmonton and technically could run 8.6 if I ever go to Mission.

It's kind of a guess. Plug reading and o2 reading are your best bet. Jet down 2 sizes at a time but since you're leaning it out you need to be careful you don't go too lean.

I have recently had Mark Whitener go through the carb and make some changes and I'm now using BLP jets instead of Holley jets. So far the comparable jet appears to be a bit leaner with the BLP jets.

I decided not to rejet up in Phoenix even though the air was great. Mostly because we got one time run and then straight to eliminations.

I did jet down for the Tucson race, it was close using the corresponding jet size conversion from Holley to BLP. I probably could have fattened it up a little but the car was running well so I left it alone.

I just installed the same jets I ran in Phoenix for the upcoming Dallas Divisional. I'm thinking Dallas will be about 1200 feet DA so I'm hoping to be spot on down there. I'll report back after next week.

Have a great weekend everyone, it's FRIDAY!!

quote:

Originally posted by Cashflow:
I just installed the same jets I ran in Phoenix for the upcoming Dallas Divisional. I'm thinking Dallas will be about 1200 feet DA so I'm hoping to be spot on down there. I'll report back after next week.

Probably a good guess on the DA, Verne.
Last Friday night at the new 1/8 mile track in Ferris/Red Oak, (S.E. of Dallas), DA was around 1530 at 6PM, after sunset it dropped pretty quickly, and fell to below 1200 at about 9:00.
We are in a "weather rut" where a cool front seems to come through every weekend.

Read this article. https://rehermorrison.com/tech...o-live-with-bad-air/

quote:

Originally posted by Curly1:
Read this article. https://rehermorrison.com/tech...o-live-with-bad-air/

Hard to argue the Reher-Morrison, but this time I have too. I can tell you on my stuff, the jetting I was running on the mountain while I was in Denver is dead lean here. My stuff was dyno'd, and I tuned for best mph. That jetting here was lean, real lean......I need to add 4-6 jet sizes here to get right. Car even acted funny with Denver jetting here. Like Vern said, every car will be different. All you can do is add or subtract jet and see what happens.

Not to change the subject but I think most people run too much timing on their motors. When you are running more timing you can burn more fuel.

That basic tune may be a part of the difference in motors at altitude.

When a carb is set up right it is SUPPOSED to siphon the right amount of fuel to the motor for the density of the air that goes past the nozzles.

As racers we tend to think more timing-more power, more fuel- more power-good. (More power is good but more fuel and timing is not always how you get it) On my motor on the dyno we were able to get a little more power with a 28* timing and leaner jetting than at 38* with much fatter. Think about that, my motor runs better with a full 10* less timing and leaner than it did with 38* and best fuel tune up. By the way we tried many different tunes. Now that was on alcohol and with mechanical injection which does not compensate for more dense air like a carb does. There was not much power difference maybe 5 hp if I remember right but I am thinking it may be part of the total tune up and not just the altitude.

I agree some motors are different and you must tune them but I suspect if the tune up was really optimum it would be less than than most think.

Since it's cold outside and the freakin wind is blowing I'll tell a couple of Denver stories.

Years ago we were up there just watching. This is when you could park your car up on the side of the mountain. We were right next to this gentleman and his teenage son. They were huge Ford fans, specifically Bob Glidden. I mean these guys were absolutely diehard Ford guys.

Bob's car wasn't running for crap and he was really struggling. The Dad and I discussed that Bob's car was just running terrible. He said "I'm going down there and talk to him" and away he went.

Well, lo and behold when it was time for Bob to run again the car hauled a-z-z and he went straight to number one. The dad came back up the hill and I asked him what had transpired. He said he had talked to Bob and told him "I don't know what's wrong with your car Bob but the only way I can describe it is it's not taking the gas right if that makes sense."

Bob asked Billy, "when's the last time we looked at those carbs?" and they pulled the carbs off the car and worked on them. Whatever they did fixed the problem, he ended up winning the race.

Next story involves Scott Geoffrion and Johnny Gray. Scott was obviously struggling, the car was a dog. Johnny went over to talk to him and asked him what spark plug gap he was running. Scott said "I don't know, .050 or .060 thousandths, whatever we normally run." Johnny said "there ain't no oxygen up here, regap those plugs to .025 thousandths." Scott thought he was nuts but he took his advice and the car was instantly fixed.

I have seen other guys who went back to their sea level tune-up and the car ran better up there but I can tell you my junk runs the best in Denver if I rejet and put in my 8 inch converter.

On the timing issue I have mine at 34 degrees and I don't change it. Might sound crazy but the car starts and runs fine at that setting and I'm not always screwing with it.

We are bracket racing after all, sure it might pick up a little with a timing change but like I said the car always starts and seems to run fine so I just leave it alone.

I am going to add something else to think about. The squirter nozzles are mechanical and is not effected by weather or altitude. They also have a big effect on how the car leaves off the line. So that may be different at altitude.

The main fuel circuit changes with air density as to how much fuel it siphons off. The Squirter nozzles do not they are purely mechanical. the idle circuit is some where in between. You may have to change squirters and pump cam at altitude. Also may need a slight adjustment on the idle mixture.

Cashflow also touched on something that you do make less power at altitude so the converter will stall at lower RPM which may hurt ET to.

He'll be moving on the average of 6 to 7 thousand feet of da change. If tuned for Da at a sea level track, then at that type of altitude he should have 94 jets in it, run 1.5 to 2 degrees of additional timing, add 400 to 600 to the leave chip and loosen the converter up 400 rpms. Ultimately with these changes he'll only lose .4 to .45 of et.

So I read that RM article a few years ago and lived by it for a while. I race at altitude and usually go down in altitude when going to another track. One summer when going down in altitude I tried leaning the carb the number of jet sizes recommended by Holly and backed the timing off. I had plenty of history at the track I was going to and I picked up more ET and MPH that ever before at that track. Ever since then, I jet down and reduce timing. Works for me.