10.5CR twin spark 3.2SS pinging with 95RON
 

my 4000 miles old 3.2SS conversion has 10.5:1CR CP 98 mm pistons, twin spark EDIS ignition run by MS2 and stock SC cams. On 95RON (=91 US octane R+M/2) fuel I have pinging under load, that only goes away if I run as little as 10 degrees@3500 WOT ignition advance or back off on the throttle. Also pinging goes away with 98RON fuel but unfortunately this is getting increasingly difficult to find.

I would have thought this configuration should have been able to run 95RON/91 US ? What do you think

I am not sure what heat range spark plugs you are running, but these configurations are sensitive to that. Try 1 step colder spark plugs.

Got E85 over there? You might consider adding up to 50%.

With those cams, you have far too much cylinder pressure below 5200 RPM for 95 RON fuel. Reducing mid-range & total timing will simply kill throttle response and make the engine run too hot.

I would either:

1) Install lower CR pistons (best choice)

2) Install longer duration cams (narrows the effective RPM range, requiring close-ratio gears)

3) Use 98 RON fuel even though its not convenient

Hey there Steve, glad to hear you are alive and kicking!

What is max compression ratio you would go for with the SC cams or what combination would work?

I am planning on 10.3 cr with DC35 cams on my 3.4 and hoping not to have the issue of the OP.

Must admit I always thought that twin plug was a panacea for pinging on 911 engines....yoiks!

Dennis

Sage advice from an expert. Listen to Steve, he's been there, done it and built it!! He is a wise man

Not saying he's old, but I remember when I crewed on the Santa Maria with Columbus, Steve was the navigator.


FYI, audible detonation is the least destructive. It's a lower frequency which the ear can detect. It's the high frequency detonation the ear cannot hear that is the most damaging. I suggest you install a simple knock detection device as well. I have these if you need.

Hi Dennis!! Guilty as charged. ::) :)

It really depends on what local octanes you have. I would not go over 9.8:1 with twin-ignition, SC cams, and 92-93 octane fuel. Less CR for 91. Timing will need to be optimized for each configuration.

With twin-ignition and 92 or better, you should be OK.

It does work very well, up to a point. :) :)

Local fuels and camshaft profiles both play a big role in selecting all the components and how things are set up.

Engine configurations must be considered and selected holistically, otherwise one can have the same problems that Trond has.

Damn,...that really WAS a rough trip, too. Hardtack & salt pork tastes pretty bad after a few weeks eh? :) :)

Excellent point on sub-audible detonation. I tell my clients that if they rely on their ears as the defacto "knock-sensors", they are going to buy a lot of expensive parts.

This is not a joke, I use a little bit of diesel on my cars (about 10%) and pinging disappears.
I don't have any technical proof but in my case I can feel the difference.
Regards

thanks for the advice. I will go on 98 and octane booster when needed and on next occasion either change to 964 cams or 9.5CR pistons. Most likely for my driving style lower the CR and keep the SC cams I did cc the combustion chambers on assemby and found 58.5 ccm which with 98 mm*70.4mm bore & stroke corresponds to a theoretical 10.1:1CR. I thought at the time that might work with 91 US/95RON but now I think not.

I understand how a hotter cam with more overlap will give less cylinder pressure but thought that in the peak torque area where cylinder filling is best cylinder pressures will be equally high also with hotter cams. If not there would not be much point in hotter cams ?

That is correct. A hotter cam only raises the rpm where peak efficiency is realized. Static compression is still being reached, or if it's a good breathing engine such as the case with ITBs, greater than static compression is reached depending on head flow, cam, and engine rpm. This is why it's called dynamic compression as it's dynamically changing based on flow and rpm.

I would still go one heat range colder on the plugs and see if that helps. With NGK plugs, and if I remember right, each heat range decreases plug tip temp ~50C. Try heat range 8 if you're using 7 now.

Thanks for the advice on plugs. Will see what I have (forgot), change as required and report back !

found Bosch WR-5-DC and NGK BKR6E in the lower set. NGK 6 corresponds to a Bosch 6/7 and is clearly not a match.

I will need to order either one step colder in the bottom and run Bosch5/NGK7 equivalent. Or go to even colder and try a Bosch4/NGK8 equivalent ? Any advice ?

I was adviced by supplier of pistons to use NGK FR5DTC. That is a even hotter plug than the BKR6E I have ?

I have always been a Bosch fan, used them exclusively on my Kremer 3.2 (non-dual plug) and the Kremer guys who built the motor and set it up for autobahn work basically told me to use Bosch W3CS plugs as they were cold and would resist fouling due to the silver electrode. With the CS's they said I could leave the plugs in for a few years but if I wanted to go cheaper they said use the copper equivalent W3CC and switch them every year.

Not sure if they were just a tad anal or not, but I figured why argue with success as the engine was broken in at the Nuremburgring and ran like a clock for the 20 years I had it.

Bosch shows them as being available and intended for earlier performance cars....so they may be the trick for you. Stupid expensive, but figure you don't need to change them out very often.

Dennis

thanks mr icicle. Have ordered those, along with slimmer equivalent for bottom plugs. Car runs good on 98RON as is btw ! I was adviced to move to a more lively cam but decided to first try the SC cams I already had and so far they seem to be spot on for me and the slow roads here. Great bottom end torque