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Old 09-29-2010, 07:25 PM   #1 (permalink)
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Compression vs. ignition timming

As a basic rule more ignition timming is good for FE/low end power and the same for more compression. I am going to bump up the compression on my Metro soon and I have been wondering how far to take it. I know I am going to have to back the timming up a bit with 87 octain and more compression. I am sure it is diffrent for every car but has anyone done any research on this to find the "sweet spot" with the two? My plan right now is to go from 9.5:1(stock) to 10.5:1 on my car. I feel good that a whole point of compression and a few less degrees of timming will make the car have more power, get better mpg and still run fine on 87 octain. I would go to 11:1 or more but I am scared that I would have to pull so much timming out that it would be gutless and FE would drop. What do you guys think?

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Old 09-29-2010, 08:29 PM   #2 (permalink)
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I think you have the right idea, there will be a balance between the two as an optimum. IF IF IF you can keep the engine loads down I think that upping the CR would be the way to go as you take advantage of the superior expansion ratio. Seeing that we are talking about a metro that really isn't possible.
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Old 09-29-2010, 09:09 PM   #3 (permalink)
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Haha yea I don't use a lot of throttle much but there are times where mashing it is the only option! I think I could pull off 11:5:1 99% of the time on 87 octain but on a hot day, going up a steep hill at 55-65mph....probably major engine damage or get run over!

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Old 09-29-2010, 11:11 PM   #4 (permalink)
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singh grooves

http://somender-singh.com/uploadfile...s_Analysis.pdf

test results on a G10 engine , with and without singh grooves
with 87 octane fuel
you will be able to increase compression without switching to mediocre fuel
which is what "high" test fuel really is .

contact 3tech for details on how far you can boost compression
or
for the machine work needed , or both



Quote:
Originally Posted by Turbo Dan-O View Post
Haha yea I don't use a lot of throttle much but there are times where mashing it is the only option! I think I could pull off 11:5:1 99% of the time on 87 octain but on a hot day, going up a steep hill at 55-65mph....probably major engine damage or get run over!

Dan-O
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Old 09-29-2010, 11:27 PM   #5 (permalink)
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i was looking at chevys website out of curiosuity of the new new engine specs, there 2.4 and 3.0 run on 87 oct with 11.2:1 (maybe even 11.7:1 if I remember correct). I don't know if thats useful at all but I found it interesting.

In theory your pulling timing because of pre ignition, as the higher compression ratio will burn faster you need to reduce the ignition point to keep the burn at an optimal point in the cycle. following this theory when you reduce timing to compensate for higher compression your not doing anything negative that would reduce power, just keeping it at the optimal point.

The second thing to consider is detention where the fuel self ignites from another source, as u raise the compression the pre ignition temp in the cylinder can raise to a point where it will ignite without a spark. This higher temp is what calls for a higher octane fuel.

hope that is kinda useful info to you
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Old 09-29-2010, 11:39 PM   #6 (permalink)
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...gasoline only requires about 30 millijoules of energy (Watt-seconds) for ignition, hence it's "high flamability."

...which is *why* things are added to the fuel to increase its octane-rating (resistance to self-detonation).
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Old 09-30-2010, 09:52 AM   #7 (permalink)
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Quote:
Originally Posted by Meph View Post
i was looking at chevys website out of curiosuity of the new new engine specs, there 2.4 and 3.0 run on 87 oct with 11.2:1 (maybe even 11.7:1 if I remember correct). I don't know if thats useful at all but I found it interesting.

In theory your pulling timing because of pre ignition, as the higher compression ratio will burn faster you need to reduce the ignition point to keep the burn at an optimal point in the cycle. following this theory when you reduce timing to compensate for higher compression your not doing anything negative that would reduce power, just keeping it at the optimal point.

The second thing to consider is detention where the fuel self ignites from another source, as u raise the compression the pre ignition temp in the cylinder can raise to a point where it will ignite without a spark. This higher temp is what calls for a higher octane fuel.

hope that is kinda useful info to you
I think you have to be careful comparing new engines to oler engines CR with octane rated .Many new engines use variable cam timing so they can adjust overlap and timing at low rpm/part throttle to give EGR affect and still run higher CR for same low 87 . Also ECU are faster so they have better chance to set up knock sensor to pull back or advance depending on fuel quality .

Here my 08 Accord 2.4 has 10.5 CR for 87 fuel, the Acura TSX 2.4(same engine but with 11.2) runs on 91 .
Both of these (K24Z3) have VCT . note i read some do use 87 in TSX and it does work but not as good (performance) .

I think your at a good value now for octane .
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Old 09-30-2010, 11:01 AM   #8 (permalink)
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another thing to consider is that static compression isn't that important, go online and find a decent dynamic compression calculator and go from there, that will factor in the cam, cam timing, and valve overlap events.. previous guys have hit it right on the head, hell a prius is like 11-1 if u go by traditional standards, but its a modified atkinson cycle so its dynamic compression is pry closer to 7-1 or something crazy low like that.... Good luck, and hope u figure out a happy medium!
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Old 09-30-2010, 01:22 PM   #9 (permalink)
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Yea, this is true. I am also planning to run a lot of cam timing also so cylnder pressure will be high and hot! Wow this is a lot to think about!

Dan-O
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Old 09-30-2010, 01:24 PM   #10 (permalink)
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Oh and I am going to try the "Singh grooves" also. Looking foward to trying that out.

Dan-O

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