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Ross, just to clarify
did you mean "32 deg. total timing" as 32 deg BTDC - or - 16 BTDC plus 16 ATDC (point of peak pressure) = 32 deg. total timing? (Which would come within the low end of the scale of Mikes dyno numbers) Remember Ross's application is a boosted engine. Also, Mike's dyno results are at near sea level 100% power levels. Conditions at lesser % power levels with variations of density elevations, MAP & RPM should have a varied effect on optimized timing point, & should be viewed only in their specific context. A question to the masses - A small engine with a small piston & combustion chamber compared to the typical Lycosauris with it's big old piston & combustion chamber, plus lets rev the small engine twice as fast as the the big guy. Should the timing be the same in both the small and big engine so the flame front delivers the peak pressure on the piston the same, at the theoretical optimal point of 16 - 18 deg ATDC? -Assume variables like fuel type, A/F ratios and MAP are the same for both engine. -Factors I see playing out in this scenario - small combustion volume vs bigger one, smaller piston area vs bigger one, flame front speed same, basically half time slot for combustion event in small vs big due to rpm differences, and you want to deliver the peak pressure at the same point in both engines. |
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We've run a bunch of tests on Les Kearney's RV10 with regards to timing vs. MAP vs. AFR as have other customers. Nobody has reported seeing any gains in TAS (power effectively) running any more than 33 deg total even LOP and below 15 inches MAP. Revs, counter intuitively, don't seem to make much difference in required advance for best torque. Timing required on most engines I dynoed liked to be the same from about 3000 rpm to redline- up to 9000. The thought is that chamber turbulence increases with increasing rpm and this may increase flagregation rates. |
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Fuel is an interesting subject, often not well understood.
Probably the best web resource on gasoline has been around a long time. http://www.faqs.org/faqs/autos/gasoline-faq/part1/ Returning to topic...timing. Mike's data is best power, ROP, with a ported parallel valve head. Chamber turbulence and swirl are key factors to increasing flame speed, thus reducing the ignition advance requirement. We don't know the details of Lycon's port work, but I imagine they try to improve both. It's possible that an unmodified head might like a little more advance. Theory, no data in hand. Reported torque here is less than 1% difference, 20 vs 25 BTDC. I do have some data from another dyno running a highly modified parallel 360. I do not know exactly what was done internally, but it was a showpiece hotrod; 430 lbs ft is 201 HP at 2456. The reported torques were... 20 411@2424 25 430@2456 30 422@2461 ...which is still only about 4% between 20 and 25. |
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Mild detonation still produces much higher peak pressures than normal combustion events, even ones started say 10 degrees early. Have a look at some cylinder pressure plots to see the differences. A good education on this subject can be had from the old texts by Taylor and Liston. Back to Mike's data, running mogas, as he plans, with decreased advance, will certainly increase detonation margins at almost no cost in power. That's a good thing to know. |
Timing Sweep On A Dyno - Interesting Numbers
Since detonation has been mentioned several times in this discussion, it should be defined. It is the abrupt combustion of the END GAS in the cylinder. In normal combustion, as the flame front propagates from the spark plug it travels through the mixture until all of it is combusted, normally at the cylinder walls and piston crown. Higher octane fuels (lots of iso-octane, lots of tetraethyl lead, alcohol fuels) have a higher resistance to this abrupt combustion of the END GAS. In a combustible mixture, higher octane fuels are not harder to initiate combustion than lower octane fuels.
For a more complete discussion see the Lycoming article: https://www.lycoming.com/node/17607 This article has more depth: http://www.contactmagazine.com/Issue...ineBasics.html |
540 inches of SDS/Ly-Con AWESOMENESS!
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Finally found some time to study real time engine data off the Carl Goulet engine test facility at GAMI in Ada OK.
Engine was a TIO540 Chieftain engine at 29.5"/2220RPM and full rich. Sorry, I know it was not your O-320 :) First Question was Avgas Vs Mogas. 100LL Vs 91R+M/2 or 95 mogas. (98RON for the Aussies watching.) Horsepower remained constant at 193GCHP/182BHP. BSFC=0.561 BMEP 120.4 and Toque at 431 ft. lbs. Theta PP averaged about 18-20 degrees ATDC on Avgas 100LL but decreased as you would expect by around 2 degrees to a range of 16-18 degrees. This value bounces around a lot due to cycle to cycle variability. Peak cylinder pressed ranged from nominally 600 PSI to 650-660 PSI which makes sense with the Theta PP change. Interestingly when run on the mogas, and left in this condition, and with fixed spark timing of 20DBTDC (on spec) as the CHT's warmed up to what many consider acceptable 395-400dF and running a less rich mixture (BSFC 0.52-0.53)but still rich enough for Avgas, some cylinders started to knock (detonate). You cannot assume that acceptable CHT means no knock, it might be minimal or low intensity, but it could be there and you would not know. Part of this test was to reduce the ICP and correct for the fuel by using PRISM, and the result was 13-14 DBTDC and the knock vanished. I know of an engine with all 6 cylinders that were run on 28+ degree timing and premium mogas that have valve recession to the point it had to be overhauled. (Ross, you might recall this one). If you consider the above observations with ICP being the key factor, you will not view spark timing the same way ever again. Please note the Chieftain engine is normally a 20 degree engine compared to our typical RV engines being 25 degrees. Also note the spark timing I am quoting is actual plug sparking, not the static mag timing which has nominally 2 degrees of lag. Lessons from this: If you want to run Premium Mogas, be very aware of vapour pressure problems, and these are not insignificant. Also spark timing needs retarding appropriate amounts, and EI's typically do not have the lag a regular mag has. Last point I should make is and Ross has spoken about this a bit in previous posts, there is no gain in running much advance except when LOP and high altitudes / low MAP. And even then 28 degrees is more than enough. Hope that is useful. |
More data
The numbers in the OP were a subset of the full dyno printout and provided in an email with the premise that the particular run was pretty rich, and when leaned to best power it would make more. That said, I now have the full dyno sheet in hand and can say the 320 HP figure (and timing sweep) was done pretty fat - the FF was 29.8 GPH for a BSFC of .5. MP was 29.38. As a way of comparison, best power was found when leaned to 24.3 GPH and it turned out 331HP (.42 BSFC). And when leaned further, to 22 GPH even, it made 330 (.38 BSFC). Not much to do with ignition timing here, but thought the BSFC numbers compelling. |
I am curious but that HP number, was it Gross corrected horse power not BHP?
If so then I am going to stab at it that is still in the 310 BHP range :eek: |
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