HomeDashboardFAQSearchRiviera Questions & AnswersWrite-Ups IndexRegisterRelated LinksMemberlistLog in
Share | 
 

 The Consequence of KR

View previous topic View next topic Go down 
AuthorMessage
AA
Administrator
avatar

Name : Aaron
Age : 40
Location : C-bus, Ohio
Joined : 2007-01-13
Post Count : 18310
Merit : 239

PostSubject: The Consequence of KR   Tue Oct 02, 2007 6:26 pm

You may have read of the dangers of knock to your engine, and to your car's performance. While it's hard to quantify just how dangerous knock is, it's generally agreed that over 5 knock retard (KR) is the time to start letting off the throttle.

The PCM engages KR very quickly; the result is knock is usually not heard. I've personally observed spikes into the 12 range without hearing a thing. But most of us think we can feel the difference, right? It's true, retarding the ignition timing results in loss of power. Somebody somewhere once said that for each degree of timing pulled we lose approx 2hp. If you're knocking 10, that's 20 hp, surely a noticeable difference. But what about 5 or 2 KR? Can you really feel that?

I did a couple of WOT test runs last week. Both were under nearly identical conditions: same stretch of road, both launched from a stop, both days were dry and 75F, same 94 octane, same mods, same 3.4" SC pulley. The only difference was the tune. One tune didn't command enough fuel (moderate KR), the other tune came pretty close to optimal (minimal KR). Here is a graph of the two runs:



Looking at the curves, you can see vehicle speed and KR for both runs. Pink (mph) and red (KR) correspond to the "bad tune" run, while the teal (mph) and blue (KR) reflect the "good tune" run. Notice KR for the good run is under 1 for the most part; KR for the bad run is a near sustained 2 with spikes close to 5 and 7.

Did the car feel any slower? Not really. Was KR slowing the car down? Probably, but only a tiny bit, as can be seen from the two mph curves (teal is good run, pink is bad). One can argue I had a better launch on the good run, so if I'd launched as well even with 5-7 KR, it's likely the two runs would be very close to a tie. The performance difference would cause small differences in track times, or could change the outcome of a race by a nose, but I don't believe I'm losing tenths in the 1/4 mile from moderate levels of KR.

I think these results are interesting, because I used to think that if my engine was knocking 7, I could expect a sag in power that would slow me down enough to feel it in the seat. If you look closely at the pink mph curve, it does show up barely (teal line is slightly straighter), but just barely... you would never notice this otherwise. A change in head wind could cause a bigger loss in velocity than what we're seeing here. Severe KR levels in the 15 range are another matter. I believe knocking to this degree could be felt, but things should never be allowed to get to that point.

What did I learn? Basically that KR should be minimized more for engine safety reasons than for performance reasons. Knock is still a bad thing, but all else being equal, you must have a lot of it to slow you down. Other things, such as launch technique, wheel traction, even a 10F difference in ambient temp seem to be the more important factors affecting the car's acceleration performance.

_________________
'98 SC Riviera 268k miles 298 HP/370 TQ 0-60: 5.79s ET: 13.97 @ 99.28 4087 lb 20.1 avg MPG Nelson Ledges Lap: 1:30
3.4" pulley AL104 plugs 180 t-stat FWI w/K&N 1.9:1 rockers OR pushrods LS6 valve springs SLP headers ZZP fuel rails
KYB GR2 struts MaxAir shocks Addco sway bars UMI bushings GM STB Enkei 18" EV5s w/ Dunlop DZ101s F-body calipers
EBC bluestuff/Hawk HP plus SS lines Brembo slotted discs DHP tuned Aeroforce Hidden Hitch


'05 GTO 49k miles 0-60: 4.8s 16.9 avg MPG Nelson Ledges Lap: 1:26
Because fun
Back to top Go down
http://www.cardomain.com/ride/657082/4
dreww
Junkie
avatar

Location : Dallas
Joined : 2007-04-10
Post Count : 851
Merit : 9

PostSubject: Re: The Consequence of KR   Tue Oct 09, 2007 12:56 pm

maybe your PCM isnt retarding the timing like it should be...

lol, im just kidding. But I probably got ya paranoid now.
Back to top Go down
AA
Administrator
avatar

Name : Aaron
Age : 40
Location : C-bus, Ohio
Joined : 2007-01-13
Post Count : 18310
Merit : 239

PostSubject: Re: The Consequence of KR   Tue Oct 09, 2007 1:07 pm

Haha, yeah... better check that timing curve.

_________________
'98 SC Riviera 268k miles 298 HP/370 TQ 0-60: 5.79s ET: 13.97 @ 99.28 4087 lb 20.1 avg MPG Nelson Ledges Lap: 1:30
3.4" pulley AL104 plugs 180 t-stat FWI w/K&N 1.9:1 rockers OR pushrods LS6 valve springs SLP headers ZZP fuel rails
KYB GR2 struts MaxAir shocks Addco sway bars UMI bushings GM STB Enkei 18" EV5s w/ Dunlop DZ101s F-body calipers
EBC bluestuff/Hawk HP plus SS lines Brembo slotted discs DHP tuned Aeroforce Hidden Hitch


'05 GTO 49k miles 0-60: 4.8s 16.9 avg MPG Nelson Ledges Lap: 1:26
Because fun
Back to top Go down
http://www.cardomain.com/ride/657082/4
turtleman
Expert
avatar

Name : Codith
Age : 29
Location : Villa Park, IL
Joined : 2007-02-08
Post Count : 3598
Merit : 123

PostSubject: Re: The Consequence of KR   Tue Oct 09, 2007 1:44 pm

Interesting stuff indeed. I think that kind of supports your ideas on the relationship fuel/air tuning has on all out power vs. knock control, Aaron. I can't believe how close those really came out to be. And it doesn't really seem like the bad tune is really making up for the KR by producing more power during the time with less KR.

Following basic theories, the most efficient and effective ratio is 14.7:1.
But I keep seeing people tuning to make it more like 12:1. - richer.
Is it done like that because it's necessary to sacrifice the perfect mixture to keep KR down (thus making smoother more stable power) or is it because the computer just isn't smart enough to maintain 14.7 with mods that add boost and such so we need to try to compensate for imperfection?
Back to top Go down
AA
Administrator
avatar

Name : Aaron
Age : 40
Location : C-bus, Ohio
Joined : 2007-01-13
Post Count : 18310
Merit : 239

PostSubject: Re: The Consequence of KR   Tue Oct 09, 2007 1:55 pm

14.7:1 is the most fuel efficient ratio, but it's not optimum for power.

Since fuel economy is important 99% of the time, 14.7:1 is what you mostly see. But at WOT everything changes. For optimum power, between 11:1 and 12:1 is ideal, and seeing this at WOT is normal. Efficiency is thrown out the window for power, and somewhere there's a sweet spot where your car has no KR, and is producing the most torque. Mine seems to like 11.5:1 at WOT.

The PCM is plenty smart, as it will switch to the special A/F ratio for WOT, and it will even command slightly above or below this for hot/cold engine temps.

_________________
'98 SC Riviera 268k miles 298 HP/370 TQ 0-60: 5.79s ET: 13.97 @ 99.28 4087 lb 20.1 avg MPG Nelson Ledges Lap: 1:30
3.4" pulley AL104 plugs 180 t-stat FWI w/K&N 1.9:1 rockers OR pushrods LS6 valve springs SLP headers ZZP fuel rails
KYB GR2 struts MaxAir shocks Addco sway bars UMI bushings GM STB Enkei 18" EV5s w/ Dunlop DZ101s F-body calipers
EBC bluestuff/Hawk HP plus SS lines Brembo slotted discs DHP tuned Aeroforce Hidden Hitch


'05 GTO 49k miles 0-60: 4.8s 16.9 avg MPG Nelson Ledges Lap: 1:26
Because fun
Back to top Go down
http://www.cardomain.com/ride/657082/4
 
The Consequence of KR
View previous topic View next topic Back to top 
Page 1 of 1

Permissions in this forum:You cannot reply to topics in this forum
Riviera Performance ::   Supercharged 3800 Tech :: Series II Scans, Tuning, PCM-
Jump to: