SC Detonation Problem

RV Flyer · 11-14-2006, 08:46 AM

When getting on the gas in my SC there is normally no problem with detonation, even if running regular pump gas (87 octane) as opposed to 91 octane premium. But, if I get on the gas hard in 2nd gear, there is a very brief transient rattle as I pass thru 5200 RPM. It only lasts a split second, but it is definitely there, and loud - and leaves a noticeable cloud from the exhaust. Obviously I try to avoid this, but it is a repeatable, consistent occurence under these specific conditions.
Any suggestions on how to track down the problem? Timing is set per factory and there are no other driveability issues that (at least to me) would suggest a distributor problem, etc.

Gunter · Gunter's Garage

Take the valve covers off and check the head studs.
If you find any barrel nuts, that's your problem.
Check the torque: Long 10mm Allen, set a reliable 3/8-drive torque wrench to ~20 ft-lbs, check the torque on all barrel nuts.
If any of them feel mushy, the studs may be pulling out.
Generally, avoid 87 octane, go at least 89-90.

beepbeep · 11-14-2006, 09:02 AM

yeah. If car was leaning out, it should ping in higher gears as well. Motor doesn't know which gear he's in. It has obviously something to do with loading on the tranny/engine combo.

RV Flyer · 12-30-2006, 01:36 PM

Head studs check out OK - I will have to gather some additional road test data on this when I get the car back on the road in the spring.... maybe it does ping in other gears under those conditions but I am not often in that setting and perhaps have just not observed/recalled it

Steve@Rennsport · 12-30-2006, 02:29 PM

RV Flyer:

If I might offer,.......

I would tell you that if you are going to rely on your ears as your defacto "knock-sensors", you will soon be buying new pistons & cylinders.

You might take a look at http://www.rennsportsystems.com/2a.html for some useful information on this subject, but I'd kindly suggest that you do NOT use 87 octane fuel in your SC unless you are predisposed to rebuild your engine.

RV Flyer · 12-30-2006, 03:19 PM

Steve, much as I would enjoy spending $$ on the car, like maybe one of your twin-plug setups, I would rather do so for reasons other than "duh, that was stupid of me..." So I will try both 89 and 91 octane (R plus M/2) this spring and see how things go. If the collective experience is that these SCs are not happy on 87, then that is good enough for me - I'll start buying Talisker for mine rather that the old Cutty Sark 87.

My original thought was that either timing, fuel, or both are not quite correct; the head stud suggestion mentioned by others as a cause had me concerned.

Your post raised a question for me, though: as a practical matter, what other means do I have of detecting detonation in real-time other than the Mark 1 ear?

John (wish I was flying the RV today, but working on the 911 ain't bad either)

Steve@Rennsport · 12-30-2006, 04:30 PM

I would strongly recommend using 91 in your SC based on what I see with these engines over the long term. For sure, its cheaper than new P/C's.

Set the timing to factory specs and you'll be OK.

Unless you have the resources to buy, install and tune something like the J&S Knock Detection system, you'll be better served by dealing with any detonation proactively by using good gasolines.

(Your RV sounds fun as those are made a few miles away)

RV Flyer · 12-31-2006, 01:01 AM

Thanks for the advice, Steve, I will plan on just using 91 and consider it cheap insurance. And compared to what I have to pay for 100LL for the airplane, it is not too pricey at all (took a trip to MN in the RV last August when gas prices were still up there and was paying $4.00 to $4.50 for 100LL - OUCH!)

Yes, Van's Aircraft is just down the road from you in Aurora. Van's has done a heck of a job with his designs and company, I'll bet he never dreamed that first prototype would lead to where he is today. Maybe weather will be good today and I can get one last flight in for 2006

I really like the greater Portland area - nice country, tons of airports around.... and my impression is that there are plenty of 911 (and Porsche in general) fans in the area, too.

John

Rick V · 12-31-2006, 03:30 AM

JUst check the owners manual. you need to run at least 91 octaine. I can't get that here so I have to run 93.

Gunter · Gunter's Garage

Quote:

Originally posted by RV Flyer
Head studs check out OK

What exactly did you do to check the studs?
What torque did you use to check?
Just because you don't find barrel nuts under the valve covers doesn't mean that the studs aren't pulling out of the case.
Unless you re-check the torque, it's still a possibility.

Aside from using higher octane gas, I would also check the curve on the distributor.
With a timing light (Stroboscope), what degree/range on the pulley do you see at 5-6K RPM?

Happy NY

Jadams1 · Jadams1's Garage

I run 87 octane gas in mine but my 1980 ROW SC has a low compression ratio, 8.6:1.