Date: Tue, 29 Dec 1998 00:26:44 GMT From: djtcznopsamanet.com (Dan Timberlake) Subject: Re: Turbocharged Engine pings under boost
>I retarded the static timing to 5º BTDC which is 5º below factory >spec, put some octane booster in the tank and even cranked up the fuel >pressure a bit but it still pings. The centrifugal curve at higher engine speed (unless its and electronically generated advance curve) may be what is hurting you. You may need to limit the total full power timing to 30 degrees or so. > >I installed a $400 J&S knock sensor to prevent this but I can't seem >to get it adjusted properly. I bet they can help.> >If I turn the sensitivity dial all the way down, it doesn't pick up >the ping (I've got a meter that tells me how much the timing is being >retarded). > >if I turn it up even just a bit, it picks up all kinds of weird noises >under slight acceleration and starts backing off on the timing upto >20º long before I reach any boost. > >So either it's picking up false noises or my engine pings under even >slight accleration and I really have problems that I don't know about. If a normally aspirated Volvo has borderline loose valve adjustment the knock sensor gets fooled by the noise. If you have loose (or cracked) pistons, or loose valve clearances, you may have to experiment with the knock sensor location, to get it to a point the adjustment is useable . >Here's what I want to know. > >What kind of C/R and boost pressure do Saab and Volvos run? I do >realize that these cars use a knock sensor. > >According to several sources, an engine with a C/R of 8.1:1 using 92 >octane gasoline should be able to handle 7 pounds without an >intercooler with no problems of detonation. > >From what I remember, the engine doesn't start to ping until I hit >maximum boost which means I can turn the wastegate down to 4 pounds >(it has a range of 4 to 7 pounds) and it would probably be OK. > >But I really want to run 7 pounds because more pounds = more power but >on the same token I don't want to burn my engine up. > >Then again it's been in the 50s today so 4 pounds be OK in this >weather but in the summer when it's 95ºF, I don't know what is going >to happen. > >I've got an adjustable (0.5 to 24 pound) pressure switch, which I am >thinking about using to trigger the cold start valve which will dump >fuel into the engine under boost. > >I've got it already mounted after the throttle body. > >I did a test before the turbo installation and triggered the cold >start valve under wide open throttle with my fog light switch and it >tended to flood the engine out but it may work fine under boost. > >I also have an air fuel meter guage and can say that the ping is not >caused by an overly lean mixture. The mixture is around 14.1 under >maximum boost. > >Seems like I read somewhere that under boost, a mixture of 12.1 to >13.0 is the right range under boost so maybe that is the trick to >getting rid of the detonation. Also normally aspirated for max power. >
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