Engine slows down

[Mario Bellettato]

Hopefully moderators are not going to delete the topic. On a hi perf VW engine I experience same kind of problem I once had on a rebuilt 75HP 616/12. I just finished a hot 1600 VW engine, with counterbalanced crank, Mahle forged P/Cs, 290° Webcam cam, hi-lift rockers, ported heads with 40/35 valves, 8,9 CR, 123 distributor, ext oil cooler, modified doghouse with 914 cooler, everything balanced and tuned. I was pretty happy with result: 107 HP at 5.600 rpm on reliable dyno with more than 13,5 kgm torque from 3.300 to 5.500 rpm.
On a very light bug the engine won its class last Sunday at Bologna-passo della Ratios a classic hillclimb.
However the driver said that at the end of the race (about 7 km - 5 miles) the engine seemed to somehow slow down. Oil temperature never exceeded 95°, total advance was 28° from 3.000 rpm on, carbonation was ok, slightly rich, tested with lambda sensor. Test was performed at sea level, race ended at about 2.500 feet, could height have caused lean mixture and thus cause heating? Your experienced opinion are welcome, grazie!
Here is the link of the video with the second stint
Mario
https://youtu.be/pUFnBSdbKc8-

[David Jones]

Mario,
As altitude increases the amount of oxygen in the atmosphere decreases effectively richening the mixture which will tend to cool the engine. Also with increase in altitude the octane requirement decreases so ignition timing could be increased. Not a problem with an ECU but requires some finesse with a carburetor.
Some very fine tuning is going to be required. Perhaps an atmospheric density meter will help.
I used the analogue version of this instrument for many years.
http://www.longacreracing.com/products. ... %25-130%25

[C J Murray]

If you start at sea level a bit lean you will end a bit rich at 2500 feet. You don't want to start rich. You can probably get away with a tiny amount more timing advance. If there are many turns with short straights you can be less precise than if you have very long steep straights where you hold WOT a long time. If a long straight is at low altitude vs at high altitude then you need to jet for that situation. The driver must give you feedback if he hears pinging on the hard pulls. The rule of thumb is a 6% reduction of jet for every 1000 meters of elevation. I road raced motorcycles in Steamboat Springs CO which is 7000+ feet and There was quite a power loss even with corrected leaner jetting. With sea level jetting the bike was really slow.

[Jim Alton]

A normally-aspirated engine will lose something like 9.9% to 11.5% of it's sea level power for every 1,000 meters of increase in altitude. I don't know the altitude of Passo della Raticosa, but I assume it's fairly high. Could this explain your power loss?