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I recently completed a heads+cam project on my car to increase horsepower. I ended up falling about 50hp short of my goal, and am looking for every little knob I can turn to eek out that last little bit. I've learned some things recently that have led to some conclusions, which I would like validated (or corrected!)

I had previously believed running more timing advance (delivering spark ahead of TDC) was desirable - just before the point of knock. However, that does subject a portion of the mixture to burn during the last part of the compression stroke - effectively not contributing to the power stroke, and providing a little extra resistance during the top of the compression stroke. The reason this is still done in practice, is because the flame speed is finite, and you need to start the burn early enough so there's max pressure just a bit after TDC.

So conclusion #1 - I can actually make more power by running less timing, IF I can get a faster flame speed. Basically I would like to minimize the amount of the charge that's burning during the compression stroke, as long as I still get peak pressures just after TDC.

Ok, how can I get a faster flame?

1) Larger spark gap, so long as the coil is sufficient.

2) run closer to stoich, which is hard to do at WOT, since typically extra fuel is added to lower temps. But if I add a significantly cooler thermostat (say 30 deg), then maybe I can go a little leaner, from say 12.5 to something much closer to stoich.

Does this sound right? Does it work in practice?

  • The problem is, you can't change physics. The flame front will only propagate as quickly as it's going to propagate, so there's no real way to speed it up. Cooling things down will help with knock and will help with producing more power, but ultimately you need spark timing in order to make power. You want it to occur prior to TDC or else you'll leave power on the table. – Pᴀᴜʟsᴛᴇʀ2 Mar 8 at 23:58
  • Also, stoich is 14.7:1. Tuners usually shoot for about 12:1 or richer to make more power with less chance of damage due to knock. This is with straight octane, as you can run not only higher compression, but more spark advance with an ethanol blend or E90. You can run it quite a bit more rich with ethanol. – Pᴀᴜʟsᴛᴇʀ2 Mar 9 at 0:01
  • I've read a few things that suggest the flame speed is dependent on a number of factors, was wondering if there's any real world experience to back up those claims iopscience.iop.org/article/10.1088/1757-899X/160/1/012044/pdf sciencedirect.com/topics/chemistry/flame-speed – user2864293 Mar 9 at 0:02
  • Some of the “bug” names in SI and also CI engines cover flame speed - Ricardo, Judge, Chapman, Cummins etc - Lots to read snd enjoy. Article you link to covers points relvant to engines though. – Solar Mike Mar 9 at 3:30

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