knocking - pinging - pre detonation

mikedsilva · 01:58 PM

I'm trying to learn a little about pinging.

From what I can tell, the general consensus is for a single plug 3.0 engine is not to go above 9.5:1 compression.. Maybe 10:1?

(I understand that Porsche made pistons that were rated at 9.8:1 but get the impression most folks do not believe they are actually that high. Also I think there was a range of cars that came with 10.2:1 pistons as well.. )

On a car with programmable injection and ignition, if you were to run the higher compression and subsequently get pinging, is it just a matter of pulling a few degrees timing?

OR if you build an engine that is on the limit and it does ping, does this mean that you would have to pull a LOT of timing out or subsequently decrease the compression?

I guess what I am asking is; should pinging be able to be dialled out with the map?

2.7RS · 11-09-2020, 01:29 PM

Race fuel

Reiver · 11-09-2020, 06:16 PM

The Euro 930-10 has 9.8-1 compression...Porsche usually under represents not over. The difference results in a 24 hp increase over the 930-16 US model. I've had both. The '83 US and an '83 Euro and the difference is very evident.
There are different advance programs for each and different AFR's and plugs etc.
The Euro has a different advance curve with less advance at idle, cooler plugs and a richer afr to deal with detonation.
I've never had an issue and run 91 octane (only thing readily available in Az) with ACES 4 for a little octane boost...and I run it a little fat in the AFR range for safety...this controlled by the fuel head and WUR (no lambda).

mikedsilva · 11-09-2020, 06:39 PM

Quote:

Originally Posted by Reiver

The Euro 930-10 has 9.8-1 compression...Porsche usually under represents not over.

Are you suggesting that the actual compression ratio would be greater than 9.8:1?

Either way, I was trying to understand whether 'too high' compression can be compensated for with less timing.. or whether if an engine has too high compression, then it is going to knock no matter what, and pulling timing will not help.

Reiver · 11-09-2020, 07:28 PM

I'm not suggesting it is more but like they do with HP (unlike US makers) they low ball not advertise the highest number made.
I'm suggesting they report the compression they've designed and made.
I'd also suggest that you go no higher than that...mine may be a tad higher as I've had a top end done...that usually, or can, increase compression if the heads were slightly shaved/cut for a flat surface.
The 930-10 timing is set at You can compensate with the setting but only to a point.....I am no expert so if I were you I'd go to the engine building thread and ask the same question.... they discuss this often.

Arlo911 · 11-09-2020, 09:52 PM

Hi Mike, there are some interesting threads by the late great Grady Clay about pre detonation and pinging. Grady mentioned that a good squish or quench band helps a lot. Beware, it's a huge rabbithole but I love reading his old posts.

OSC911 · 12:08 AM

When people talk of compression ratio, they usually are talking about static compression but that is only half the story. The most important compression ratios metric is that dynamic compression ratio. You can have an engine that has 11.8:1 static compression that will run nicely on pump fuel with little or no pinging or detonation. Toyota Yamaha headed engines (engines with GE in the engine code e.g: 4AGE 20v) have been using these high compression engines for years. Obviously these are 4 and 5 valve engine which helps, but they have low dynamic compression. The camshafts are designed to close the inlet valve quite late - quite a ways up on the compression stroke past BDC. This allows for a lower dynamic compression - usually 8.0 to 8.5 is ok for pump fuel.

I know I should be comparing apples with apples but this is a factor that is often overlooked in most engines - not just a 911. I know that the piston/cylinder design on an air cooled 911 engine isn’t ideal with higher compression - hence twin plug heads. Static compression is fixed measurement when combining pistons, rod stroke, combustion chamber volume. Dynamic compression is a more accurate measurement when thinking about detonation and pinking. Calculate your dynamic compression also when determining fuel type and or twin plug head requirements and go from there with your decision.

mikedsilva · 11-10-2020, 02:29 AM

Quote:

Originally Posted by OSC911

When people talk of compression ratio, they usually are talking about static compression but that is only half the story. The most important compression ratios metric is that dynamic compression ratio. You can have an engine that has 11.8:1 static compression that will run nicely on pump fuel with little or no pinging or detonation. Toyota Yamaha headed engines (engines with GE in the engine code e.g: 4AGE 20v) have been using these high compression engines for years. Obviously these are 4 and 5 valve engine which helps, but they have low dynamic compression. The camshafts are designed to close the inlet valve quite late - quite a ways up on the compression stroke past BDC. This allows for a lower dynamic compression - usually 8.0 to 8.5 is ok for pump fuel.

I know I should be comparing apples with apples but this is a factor that is often overlooked in most engines - not just a 911. I know that the piston/cylinder design on an air cooled 911 engine isn’t ideal with higher compression - hence twin plug heads. Static compression is fixed measurement when combining pistons, rod stroke, combustion chamber volume. Dynamic compression is a more accurate measurement when thinking about detonation and pinking. Calculate your dynamic compression also when determining fuel type and or twin plug head requirements and go from there with your decision.

Very helpful, thank you.
Yes, I changed from a camshaft with slightly more intake duration, to one with slightly less.. when tuned it did not ping. Drove for a few months and it has a really good amount of punch down low which I like. The top end is not as strong but that's a compromise I'm willing to make.

But now it has decided to do ping. Not sure why it's doing it now and not before. Only between 2000 - 3000 rpm it seems.

Octane booster to the tank seems to have lessened it quite a bit. But this is not a solution.

OSC911 · 11-10-2020, 02:42 AM

I remember tuning old points cars and just knocking a bit of total advance out of them if they were pinging. Probably not the ideal fix but it worked. What’s your total advance?

mikedsilva · 11-10-2020, 02:49 AM

Quote:

Originally Posted by OSC911

I remember tuning old points cars and just knocking a bit of total advance out of them if they were pinging. Probably not the ideal fix but it worked. What’s your total advance?

I'd have to check my other laptop to verify, but max is around 33 degrees.

In the 2000-3000 range, before I added the octane booster, I reduced timing from 28 to 20 and it still pinged... if I went all the way to 15, it did not ping, but it also ran a bit iffy...

OSC911 · 11-10-2020, 02:58 AM

Not just a bad batch of fuel? Even with the octane booster it might be down on the rating. Just a thought?

mikedsilva · 11-10-2020, 03:02 AM

Quote:

Originally Posted by OSC911

Not just a bad batch of fuel? Even with the octane booster it might be down on the rating. Just a thought?

Unlikely.. have tried different fuel stations and only buy the best rating.