Anyone know if dual 94's or 81's would work better on a stock flathead at 5000 feet elevation? Or 97's?
I would guess the 97's simply because the 94's use a vacuum operated power valve. I would think as air density decreases that it may alter operation of the circuit.....but really I could simply be full of it. Terry
I agree that al***ude would likely lower vac level and so require a smaller# power valve on a 94 or 92 (the 60 version). I too would dodge the issue and go to 97 or 81... Some Ford stuff, from the bulletins. Ford offered 5,000 and 10,000 foot carbs in most models, na Ford Canada also had a "Nairobi" carb for export to the highlands. Haven't looked up al***ude there. Jet sizes are listed sea level--5,000--10,000 Stromberg 52 (.81) 035-033-031 Stromberg 67 (.97) 045-043-041 Ford 59 (.94) 051-049-047 Ford 922 (.81) 035-033-031
i dont think it would effect it much? because its not a contained vacuum. its not an air tank that would loose pressure, the engine creates its own vacuum. its open to the air and is only regulated through the carbs suction. its constantly making new vacuum, maybe just need more throttle? if that makes sense??.... maybe not? erm it makes sense in my head....
Thinking about vacuum hurts my head...let's see, divide by zero, carry the "H"... I'm thinking the engines is ****ing in the same cubage of air, but there's less air in that volume...then what?? Send out for coffee! The small jets are to compensate for less air in the air, of couse...but do they also compensate for a hypotheticl drop in venturi signal from thinner air, or is there any such thing? Easiest thing would be to move downhill, I think!
For what it's worth, 81's really work best on a mild flatmotor.As mentioned, 94's use a vacuum actuated power valve ( like a late Holley ) and can be harder to tune.Add the fact that most of the available parts for the 94's are questionable and you really have a nightmare.I only offer this advice based on many years of rebuilding carbs and tuning flatheads.You have some great help here on the HAMB so don't be afraid to ask if you need it.Good luck !
Vacuum is down significantly at al***ude - I just moved from 7000' and 15" was about normal, where before I had 19" or so at sea level. I've found the 81s need to be jetted up a bit from stock to work on the bigger flathead - even in duals. For a 97, I found 42 jets and #69 power valves to work well.
Oxygen content goes down at al***ude. Sure, there's still "air" there and the CFMs are still CFMs, but the oxygen content necessary for fuel to burn. The "air" is thinner, so you need more of it to get the same amount of combustion. Since the CFM's are fixed by the engine displacement, p***ages, and carbs (so you cannot increase them) the only thing to do is provide less fuel (leaner) to get back to the optimal combustion mixture. The other added solution is to increase compression, so that the fewer oxygen molecules get squished together into the came concentration as they would have at normal al***udes.
hey FIREROD...TERRY was right about the va***e, va***e operated just means amount of fuel that can p*** at sea level. go higher elivation and tha air is thiner, therefor you draw less fuel. terry you just might be a genious NAA maby not! HA! call me POP...or the old FART!
so..... the amount of fuel that needs to go into the motor needs to be more on high alt? but the air is less, but you need more air to p*** over the vaporizers to get more fuel..... but then you might have to much fuel going in to fast? so you lean the carbs out? which in turn defeats the purpose of bigger jets???? kjsdnfkjsdf
You use smaller jets, leaning the mix, at high al***ude. Carb venturis meter fuel by the v0lume of air going through the venturi...but a given volume of air contains less actual air, so smaller jets meter less fuel to maintain proper ratio between air and fuel. This is expressed in terms of air volume, though the oxygen fraction of the air is actually what is of interest here.
Hey all, On the Ford flathead techno site are a series of flathead articles by "rumbleseat". He has a vast knowledge and is located in Colorado so I think he has covered the topic of tuning dual carbs at at altiude before- might be worth a look:http://www.btc-bci.com/~billben/flathead.htm
On a stock Flathead I think 81s are the best. I would run them with .045 jets with jet adjusters, you can really dial them in and stock power valve. Air, who drives at one al***ude, find the happy medium. Thinner air, less fuel, thicker air more fuel..Easy formula !!!!!!!!!!! Duane.. max, I'm getting a veggie burger with less vacuum and thicker air..that should fill me up..
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