High altitude jets
Racer
Joined: Feb 2007
Posts: 271
Likes: 25
From: Buena Vista Colorado
High Altitude Tuning
Lars Grimsrud, THE Quadrajet guru, posted this about 2 years ago:
I'm at 8,100 ft elevation and rejetted my stock 78 L-48 Quadrajet last year to match the OE spec for 78's delivered to altitudes above 4,000 feet. The car runs great, but it also ran great with the stock sea level jetting; so any improvement was marginal at best.
Given difficulties you'll have in finding Quadrajet metering rods, and if your car's running OK as-is, IMHO you would be wise to leave well enough alone. If your car is not running well, you likely have problems a jetting change will not solve.
LOTS of info out there on our Quadrajets. Here's a place to start:
http://www.florida4x4.com/tech/quadrajet/
Good luck!
Jerry
If the carb has been correctly set up for sea level, the general rule of thumb is that you can drop the jet size one size for every 2000 feet elevation. You also want to bump the timing up a few degrees - here in Denver we run 36 to 38 degrees total timing instead of the 34-36 commonly used at sea level.
Most stock carbs are set up a tad on the lean side at sea level. If you use the stock jetting here at altitude you actually get pretty close to a good performance setup.
Your car will run like garbage at 10,000 feet no matter what you do with tuning. If you tune it optimum, you will be down 20% on horsepower in Colorado Springs versus what you had at sea level. At 10,000 feet, the power loss is dramatic, and there is no recovery for this in a normally aspirated engine.
Most stock carbs are set up a tad on the lean side at sea level. If you use the stock jetting here at altitude you actually get pretty close to a good performance setup.
Your car will run like garbage at 10,000 feet no matter what you do with tuning. If you tune it optimum, you will be down 20% on horsepower in Colorado Springs versus what you had at sea level. At 10,000 feet, the power loss is dramatic, and there is no recovery for this in a normally aspirated engine.
Given difficulties you'll have in finding Quadrajet metering rods, and if your car's running OK as-is, IMHO you would be wise to leave well enough alone. If your car is not running well, you likely have problems a jetting change will not solve.
LOTS of info out there on our Quadrajets. Here's a place to start:
http://www.florida4x4.com/tech/quadrajet/
Good luck!
Jerry
Racer
Joined: Feb 2007
Posts: 271
Likes: 25
From: Buena Vista Colorado
Some engine gurus (I'm not one) will chime in soon, I'm sure. In the meantime, with the serious plug fouling you have, I would guess you have other symptoms, too. Please help the many really smart folks on this forum help you by providing more information, such as:
I hope this helps...
Jerry
- Exhaust smoke at what times during a drive?
- Color of exhaust smoke?
- Oil consumption?
- Excessive fuel consumption?
- Catalytic converter still installed?
- Results of a compression check?
- Miles on the motor?
I hope this helps...
Jerry
Thread Starter
Heel & Toe
Joined: Jul 2008
Posts: 22
Likes: 0
From: Steamboat Springs CO
Jerry...Thanks again for your suggestions. Engine was rebuilt about 4K miles ago. No exhaust smoke; compression OK; fuel consumption OK;oil consumption seems fine: cat still on. Funky upturned mufflers collect moisture, etc and blow out junk at startup, but no smoke.
I realize the effects of altitude on normally aspirtated engines, just keep thinking the carb is running too rich. Anyway, performance is satisfactory for pretty much a stock engine, so I will keep chugging along with the fun car.
Frank
I realize the effects of altitude on normally aspirtated engines, just keep thinking the carb is running too rich. Anyway, performance is satisfactory for pretty much a stock engine, so I will keep chugging along with the fun car.
Frank
