Weber Jetting For Altitude

[type26owner]

Rohan,
I'm in no hurry. Appreciate you plunking in the numbers though and seeing what to expect. My plan is to keep it dead simple. Just introduce NO and not any supplemental fuel or water. With an increase of 20-30 hp I'd be happy. That should be enough so I can draft onto him and let him tow me down the straights. It's $500 which I' won't have spend only to discover it only bumps up the hp by 10 or less .

[Matt7c]

I have a book entitled "Weber Carburetors" (which fortunately is about Weber carburettors, despite the poor spelling) written by Pat Braden and published by HP Books in 1988. On page 166 it has an "Altitude Compensation Table" with a jumble of numbers which I fail to understand. Send me your fax number and I'll fax it through.

[type26owner]

Matt,
I have that book and have read cover to cover a few times several years ago. IIRC, it does a fair job at explaining how-to service the carbie. However, the tuning part was exactly one paragraph long. I'll have a look at that section later on tonight.

Unfortunately, I've got bigger mechanical problems lurking at the moment. My gearbox has started making growling noises when loaded just recently. It's covered 100k miles since I rebuilt it the last time though. I'll be freshening it up this weekend since the trackday is just three more Fridays away.

[Matt7c]

Yep, you are right about the tuning section. :angry: Not even an especially good 1 para either! Somewhat annoying when I had bought the book because it was subtitled "select, install and tune..." Not bad on the theory bits, though.

[type26owner]

Got up to 4300 feet and the engine was surging and would barely pull at WOT. Changed the main jets from 110 to 100 and it made the engine come back alive. Spent the day learning the Fernley track and got in over 100 laps which were 2.3 miles in lenght IIRC. Estimating that's about 16 gallons of fuel consumed.

What was interesting was I filled up at the track with racing fuel which was unleaded and the octane rating was at least 100 maybe 110. The engine temperature had been running at 100C (perfect at WOT) on the 91 octane fuel I arrived at the track with intially. As soon as I had completed the first lap with the racing fuel the temperature rose to 110C and stayed there even though my dual electric fans were on continuously. That is too close to the boilover limit and cause for concern. My brother suggested I turn on the cabin heater to add cooling capacity and that did the trick the temperture went back down to 100C and stayed there. That's the last time I'll make that mistake, no more racing fuel goes in my Elan!

The most surprising development was I left that jetting in and returned to sea level and found the new jetting made the engine pull even harder and run better allround. I had tried going one size smaller before and it ran like crud but by going down two sizes I've hit the jackpot and have gotten into a two-phase flow domain that is super stable. I've not measured the lambda at high rpms yet by seat-of-the-pants feel it's not leaning out and laying over on power. The minimum rpm flow test was astonishing though. This is the lowest rpms I've ever seen for any combination of jets by at least 500. The main jets system starts providing the mixture at only 1300 rpms. Looks like maybe I can increase the chokes from the stock 30mm to 33mm ones and not create the usual problems afterall. :) I'm feeling lucky!
-Keith

[rgh0]

Keith

It very unusual for higher octane fuel to increase engine temperature. The higher octane fuel burns slower. This is equivalent to retarding the ignition which normally causes an engine to run cooler not hotter.

Racing fuel is different in density and viscosity than normal fuel and could have up changed the mixture sufficently to affect engine temperature but apart from that I cant see why it went hotter.

Rohan

[type26owner]

Rohan,
That's not the understanding I've got of racing fuel. My impression is more in agreement with this technical article. Where the amount of waste heat becomes really critical is in a 2-cycle racing engine, put in a higher octane fuel in one of those that has the timing advanced for a lower octane fuel and it will sieze in a very short period of time.

http://www.eric-gorr.com/tech/Fuel_Basics.htm

I did not retard the timing to compensate for the faster burning rate of the racing fuel. Total is 34 degrees, mechanical advance is 34, static is zero. Pretty sure this observed temperature spike effect correlates with reality.

p.s. Hey Steve, the recommended engine temperature in this article is for the dunces that don't understand detonation. Extracting the most energy requires it achieve the highest temperature thus cylinder pressures but without cracking the lighter hydrocarbons and having them auto-ignite and create a second flame front. You have to dance with the devil to do it. I'm out there tap dancing as fast as I can go. :P

[rgh0]

Keith

In Australia all so called "racing fuel" is just avgas. Unless you are a rich racing team blending your own jungle juice ( which is not legal as all fuel used must be commercially available). The demand for racing fuel is much to small for an oil refinery who does final fuel blends in batches of thousands of tonnes to bother with making racing fuel blends.

In the US some true high octane fast burning fuels may be available but I would be suspicious if commercially available fuels are really any different from Avgas.

In reality the burn rate or flame speed basically varies with molecule size. hydrogen has a very high burn rate the larger the molecule from that the lower the burn rate. Lower burn rate fuels have more bigger molecules.

With 34 degree max advance I suspect your problem is that your regular emissions formulated gas you use is a strange blend with very slow burn characteristics for emission reasons such low evaporation and low nox production. This is why you get away with 34 degrees advance which is high and you would never use for any fuels in Australia in a twink. The "racing fuel" you put in is a normally burning avgas style fuel. I use 30 to 31 degrees total advance on avgas ( slower burning than normal Australian gas ?) with 12.5 compression ratio ( increases pressures and burn rate). A weber twink or road compression and normal premium unleaded in Australia runs around 25 to 28 degrees max advance.

Rohan

[type26owner]

Rohan,
At $7/gallon that racing fuel stuff was the real thing. IIRC 34 degrees total advance is the low end value the Lotus manual states is proper for the stock twinkcam running a 23D dizzy back when it was new. From memory the static is to be 10-12 and the mechanical is 24. That adds up to 34-36 degrees for the total advance. We run 40 degrees on our race engine on racing fuel. The burn rate for any fuel stays a constant but the piston velocity is changing so the goal is match up the rpm powerband with whatever burn rate the fuel has in order to get the highest cylinder pressure phased correctly so that happens at about 15 degrees ATDC. The total timing must be increased normally, the higher the rpms. I wonder what the total advance is on an engine that revs to 19K! Maybe around 80 degrees?

The reason I use 34 degrees of mechanical and 34 of total advance is to eliminate the mild but very annoying detonation the pump fuel here in California is prone to do below 4000 rpms. Above 4000 and the problem goes away. Having a knock sensor installed makes this determination a lot easier for the hearing impaired. Because the idling advance is zero I must apply a fatter mixture to keep the Webers from spitting back. They never ever spit with a lambda of 0.7. Good thing I don't have to pass a smog test though. If I ran only 24 degrees of total advance here the engine would down on power by about 30 horsepower in the sweet rpm spot of the cams. Your fuel must burn much faster than ours!

Lots of reasons why Avgas sucks in a race engine just do a google search. Unless you're racing above 10,000 feet I would not use that stuff here in the USA.

[type26owner]

Using the smallest air corrector jet possible on the mains makes good sense if you consider the combined auxillary and main venturi chokes vacuum signal is easily extinguished by high flow rates from the main jet stack. If the flow rate is too high at the lower rpms then when the rpms go to redline the mixture leans out and the engine lays over in power. The trick is to match the flow rate of the mainstack to the vacuum signal so the mixture stays constant independent of the rpms. Since two-phase flow has specific velocity domains this is where is gets a bit tricky. The major volume of the two-phase emulsion is by far the air because it's density is so much less then the liquid fuel. If you put too big an air corrector jet then it's just the vacuum signal which regulates the rate the emulsion that is delivered. By using the smallest size air flow then the flow curve of the vacuum signal can be fairly well matched up with the flow curve through the air jet so the ratio of liquid volume is prone to change more rather than is just providing more air which it will always try to do. If the air corrector jet effectively runs out of breathe at the right time the mixture will be remain fairly constant over the whole rpm range. It also appears if you stay at the smallest size of air corrector possible then the likelyhood of remaining within a single domain is much greater. The vacuum signal may not be linear relative to the rpms, I can't measure that value with my funky test setup lacking a real load. I'd need time on a rolling road to really do the measurements properly. It's a knife-edge balancing act though.

Most of this is purely speculation on my part because I don't have the proper means to test this with much precision. Have achieved near perfection tuning the carbies largely by dumb luck so I can now kind of reverse engineer the situation and ponder the true nature why this is possible.

There is a downside to doing this though. The delivered mixture is not atomized as well and when the throttles are abruptly opened the change of pressure causes the mixture to instantly form droplets and fall onto the walls of the induction system for a fraction of a second causing a hesitation. Not all the time and only at the lower rpms. I just don't stomp open WOT below 2000 rpms to avoid this natural effect. However, the overall gain of smooth performance can be stunning and compares closely to what is normal for fuel injection.

[type26owner]

Applying oversized main and air corrector jets allows the total vacuum signal to be degraded due to the high volume of emulsion which gets sucked down the passageway leading to the auxiliary venturi. This airflow is not in the preferred direction to achieve the highest combined vacuum signal from both the main and auxiliary venturis. In fact it swamps it and reduces the straight through total flow which actually creates the low pressure area. So the degraded vacuum signal ends up dominating the mixture strength rather then the jets.

The better way to size the jets is to have them combine into a stable two-phase flow domain were their maximum flow rates are closely matched to the mixture required at redline and the amount of air is limited to the minimum. This restricts the flow to deliver only enough low air volume emulsion and allows the highest pressure gradient across both jets in the mainstack at all times. By limiting the emulsion's airflow contribution the vacuum signal is actually enhanced in strength. Which in turn enhances the responsiveness of the jets and makes the flow errors less of a factor.

Perhaps the traditional depiction of bubbly two-phase flow is the desired domain but who knows since it's not published anywhere I can find. My curiosity is piqued and I will soon try to determine what type of flow is best.

The published formula for choosing the main jet size and air corrector based on the main venturi size is a cruel joke as far as I can determine. That's some engineers' fantasy which is just not true.

[type26owner]

Just incase someone is so inclined to try these jets settings. My carbies are DCOE40_18s and the engine is stock. Inspite of the title these are sea level settings.
main venturis=30mm (stock)
idle jet=40F9
main jet=100
air corrector=120
Enjoy! Let me know if you can tell a difference too.

[rgh0]

Keith

You ignition advance settings continue to intrigue me as they are so different from my own experience, I can only put it down to radically different fuels

I presume you have verified your 34 degree advance setting on a dyno or equivalent for your Elan engine. I have never seen an advance like that work on a road weber twin cam on any sort of fuel here. I believe the original lotus quotes for the early engines with that setting are wrong and the later settings quoted by Lotus in the 25 to28 degree total advance range are correct. There was no real change between the engines that warranted the significant reduction in advance, that combined with my own dyno tests make me prefer the later settings. If 34 degrees works for you then it must be your funny fuels

40 degree in a race engine may be possible with an engine that does not breath well but has a high overlap cam, with a slow burning fuel and 9000 plus rpm power band. But it still seems high to me especially if you are using supposedly fast burning racing fuel. hard to be perscirpive about what works in race enginess as many more desing variations but again I have never seen this sort of advance needed on a large number of race engines here running on avgaas and supposed race fuels. However if 40 degree works for then again it must be your funny fuels !

Rohan

[type26owner]

Rohan,
The total advance was arrived at by doing the standard power timing procedure without a dyno. Wish I could afford dyno time but that's not possible. Where does one obtain a copy of the Lotus timing update article?

The fuel here in Cailfornia is a rather special brew. The vapor pressure is kept low so vehicles that are stationary don't pollute. Makes the tuning of the inefficient Webers all the more challenging. Getting 27mph with a 3.9 diff and city driving now though. Kept having to back off on the straightaways because I was all over the tailend of Dave Anderson's Elan up at the Fernley trackday.

Not sure if the 40 degrees of total advance is going to work on the 41 FB racecar anymore since I've added 'whistles' on the nozzles of the T-J fuel injection system and changed the induction system so the fuel is highly atomized now just as it leaves the nozzle. Before it was a solid stream of fuel which splashed off the head of the intake valve going into the cylinder. Suspect I might have to reduce the total timing a bit now but until the car is track tested I won't know for sure.

If anyone besides myself is watching the Ebay item that is a Tecalemit-jackson fuel injection system from Oz but it's not the racing version from what I can tell. They had a roadcar kit too. This unit appears to be one of those. I have one sitting in a box that I've been slowly doing the improvements on so I could but it on my Elan eventually and not get stranded on the side of the road once a week by it. If you buy it let me know and I'll talk you through all the issues to fix.

[type26owner]

Rohan,
Have realized why my engine ran so warm after filling it up at the racetrack with 110 octane racing fuel. All our pump fuel here is actually gasohol. E10 it's called because 10% of the mixture is ethanol. That stuff burns cooler and reportedly lowers the combustion gas temperature by several hundred degree F. The waste heatload going throught the cooling system is significantly reduced. My dual fans on the radiator can handle the heatload at WOT from the E10 but not from the hotter burning racing fuel.

One of the consequences of burning E10 is the boiling point of the ethanol at atmopheric pressure is fixed at about 78C. So in a Weber which is running lean and not getting cooled enough by the vaporization of the fuel and the body is hot to the touch this seems likely to negatively impact the idle circuit mixture. At the idling induction manifold pressure the boiling point of the ethanol is reduced and likely boiling is occurring in close proximity to the idle mixture screw's needle and seat area.

My carburetors burning E10 gasohol remain at room temperature even after the engine bay has fully been heat soaked. Do the carbies burning just gasoline stay about the same temperature?