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Old 02-24-2005, 02:47   #1 (permalink)
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Is 13.5-13.6 too lean at wot??

I made a thread a while ago about jets and 750dp's. At one of the dyno's up here i was told i had an afr of 12.4. He said it was too rich. With no changes, i went to the only other dyno up this way, which uses "shootout" mode.
They told me it was way too lean(mid 13's). They had a quick play, but said it need "further investigation". The following graph was with the 750 demon and 76 and 83 jets. Does it just need a bit more jet?
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Old 02-24-2005, 02:57   #2 (permalink)
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Re: Is 13.5-13.6 too lean at wot??

Every man here and his dog will tell you its to lean, mine runs much harder at 13.4-13.2 with 73 - 75 jets than it did at 12.5afr . Any unburnt fuel left in the chamber will cost you HP fact!
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Old 02-24-2005, 03:12   #3 (permalink)
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Re: Is 13.5-13.6 too lean at wot??

Quote:
Originally Posted by northiam
Every man here and his dog will tell you its to lean, mine runs much harder at 13.4-13.2 with 73 - 75 jets than it did at 12.5afr . Any unburnt fuel left in the chamber will cost you HP fact!
There does seem to be big differences in peoples ideas in a safe, but ideal ratio. Another guy up here that has build some "nice" motors commented if i can control the temp, and go no leaner, it is perfect for a Demon type carby.
Oviously there is a point where a lack of fuel will hurt horsepower real bad.
Where is that point
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Old 02-24-2005, 03:23   #4 (permalink)
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Re: Is 13.5-13.6 too lean at wot??

I've ran mine @ 13.2 with no issues. Last setup ran 12.7 and I am going to ask them not to run it so rich this time. As I believe thats the reason my plugs keep fouling.

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Old 02-24-2005, 04:01   #5 (permalink)
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Re: Is 13.5-13.6 too lean at wot??

Speaking to a few engine builders......

COME Racing in Melbourne say that 13.2 is your magic number.........they have extensive dyno research (make their own cams/heads etc).

Too rich, won't give optimum burn. Too lean and it starts getting way too hot, pinging/burning things etc. Safer to run tad rich , than tad lean.

Make sure your dyno guy is running a WIDE BAND oxy sensor. Most should be. They use a 0-5volt gauge instead of the standard 0-1 volt vehicle types.
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Old 02-24-2005, 04:20   #6 (permalink)
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Re: Is 13.5-13.6 too lean at wot??

Stoich is 14.7 from memory, good idea to run a bit richer as fuel is far cheaper than pistons.

A bit leaner than stoich tends to give more power and melt motors. Check out the thread on reading plugs properly and experiment very slowly when your leaning it out however.
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Old 02-24-2005, 04:56   #7 (permalink)
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Re: Is 13.5-13.6 too lean at wot??

Stoichiometric is 14.7:1, ie: the ideal AFR for CRUISE power only. 14.7:1 under full load/power will kill any engine quick smart.

I think some of the new EFI engines drop it even higher than (19:1??) that so they get ultra good economy when cruising, using their knock sensors to control the pinging.
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Old 02-24-2005, 14:53   #8 (permalink)
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Re: Is 13.5-13.6 too lean at wot??

Don't try and go anywhere near 14.7:1

You'll nearly always get more power out of a 'lean' mixture than a 'rich' mixture.

If everything else is good, ie fuel pump is keeping up, not fuel issues like poor pickup under acceleration, etc, then I'd be happy with a a/f ratio of 13 to 13.4

But play around a bit on the dyno, power will drop off either way when its not happy. You may find you can go a little leaner without having to worry about pinging because your running alloys worth a try

Let us know
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Old 02-24-2005, 15:58   #9 (permalink)
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Re: Is 13.5-13.6 too lean at wot??

Your printout shows 13.4/13.5 at the business end. Thats only just OK, and starting to get into lean territory. I'd fatten it up a couple of numbers.

If its not pinging and its not raced, it would probably be ok as it is.
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Old 02-25-2005, 04:57   #10 (permalink)
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Re: Is 13.5-13.6 too lean at wot??

Is this too rich then?

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