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View Full Version : help me explain afr variations from low compression to high compression



trinydex
05-08-2006, 01:15 PM
it seems in my limited research on the subject that higher compression na vehicles like a higher afr (leaner). high 12s and even 13s are common in na tuned vehicles.

our low comp boosted engines like low 11s to be safe and high 11s to be daring.

boosted s2ks like around mid 12. and they run crazy compression plus boost... and still so lean...

short question, i don't get it.

RhYZiN
05-11-2006, 10:52 AM
if we're talking about a turbo car on-boost, cylinder pressure is higher than that of a high compression NA engine. a turbo car will run a hotter combustion temp due to compressing so much air into a tiny space. the more compression, the more heat. less compression, less heat so you can run a leaner AFR on a NA engine. a turbo car will run a richer AFR to bring temps back down to prevent preignition. if we say VE is 100%, a car running a comp ratio of 9.0:1 on 28psi would have an effective comp ratio of 27:1, where an NA engine only sees what it's rated it, 12.0:1?

if we're talking high compression and high boost in a s2k, it's low stroke would help make this happen. the flame front is distributed along a higher area of piston than higher stroke/higher torque engines. the heat and pressure is distributed over a larger area. the lower stroke also gives less load on an engine overall since it's not producing as much torque. it depends on the high revs to produce it's power.

looking at VE on a high rev engine as well, if it's VE is low over it's last few k of rpm, higher boost will strain the valve train instead as boost is a measurement between the turbo and when the valves are closed. boost isn't that much of an indication of how much air is in the combustion chamber, although flow rate is a better indicator. high compression + high boost doesn't mean much towards destruction if the VE isn't there.

trinydex
05-11-2006, 04:27 PM
so with a poor ve does it them become less possible to heat up the motor towards knock? no right cuz we know that poor ves will usually cap off tunable potential like the 9.8 hotside.

so my next question is does the limited ve of the s2k motor allow for leaner mixtures? or have i missed the point?

RhYZiN
05-12-2006, 02:19 PM
poor VE does make the temperature lower than high VE. adding the hotside into the mix does change things since that variable was a constant in the previous example.

limited VE does allow for a leaner mixture. less air in the cylinder leaves less likely for predetonation. some cases for predetonation is when a lot of air is compressed and a casting flaw (like a jagged edge of a valve relief or casting flaw) is still heated and causes the AF mix to ignite. some predetonation is caused on a high temp charge of air and compression alone.

looking at where torque peak is is somewhat an indication of VE if looking at the NA dyno plots. generally speaking, a turbo car will make torque early and have the turbo keep the torque flat or increasing. based on the hp formula, hp will increase despite torque staying constant. VTEC dynos are indicative of this. peak torque(high VE) early where rpms are low will seem more likely to knock than peak torque in the latter rpm with less VE. if the cam setup for the evo is peak torque at low rpm, and the honda is setup for peak torque later, you can see where the honda is setup for least likely to knock. i honestly don't know what a turbo s2k cam setup is like, so i maybe fighting the wrong fight. :uglystupid2:

i'm not sure i quite understand why you mentioned the 9.8 hotside, but the different compressor housings change where the turbo will be efficient and where it will choke. a smaller housing *usually* will spool somewhat better and choke lower. a larger housing is the opposite. it's just like looking at the AR for turbine housings. either way, a wheel will push the same volume of air. it's exit velocity will be different depending on the housing. having a housing that's too small will just make the turbo retarded. i'm probably answering this part all wrong. i'm not quite sure how you're putting the hotside into the mix.

RhYZiN
05-12-2006, 02:37 PM
i just looked at s2k specs(AP2 2.2L) and they're almost identical to the evo's. just about 2mm bigger in stroke and bore. this sorta throws out the low stroke high bore scenario. but there's still whether they're interference or not, and combustion chamber design, although most modern cars are all pent-roof anyways.

trinydex
05-14-2006, 12:39 AM
the 9.8 hot side has less ve than a 10.5 and we all know that the 10.5 will gain you more TUNEable power, and not necessarily anything if you just bolt it on.

that it appears you're speaking of the ve in the head and not the other portions of exhaust.

RhYZiN
05-15-2006, 12:33 AM
well yeah, you were talking about pistons so i didn't really figure about variations in intake or exhaust components.

i've never really expressed a turbo's flow rate or efficiency as volumetric efficiency, but i'm pretty sure i understand what you're saying. same compressor and turbine wheel. just a different compressor housing, but you're able to get more power out of the damn thing with some tuning. that's what you mean, right?

GokuSSJ4
05-15-2006, 08:51 AM
it seems in my limited research on the subject that higher compression na vehicles like a higher afr (leaner). high 12s and even 13s are common in na tuned vehicles.

our low comp boosted engines like low 11s to be safe and high 11s to be daring.

boosted s2ks like around mid 12. and they run crazy compression plus boost... and still so lean...

short question, i don't get it.

you will be suprise to see even some n/a motors running afr in the mid 14's. running an 11.5 in some n/a would be consider pig rich.