Octane and timing

[Dirk]

A mechanic friend of mine is forever telling me to run my EA71 at 10 degrees btdc because its an old engine and because of modern fuels.

 

I have tried this but have always reverted back to the subaru specs since it doesn't seem to make much difference.

 

Recently I found this fascinating article which seems to suggest the reverse.

 

http://www.faqs.org/faqs/autos/gasoline-faq/part3/section-1.html

 

As far as I can tell it would appear that low octane fuel is more volatile and so requires a retarded timing and high octane fuel burns slower thus requiring advanced timing.

 

The logic is there but it goes against the grain of everything I have been told to date.

 

Comments and opinions please?

 

A further question follows:

 

Unleaded fuel was introduced in the mid eighties. Therefore one would imagine that engines built prior to then would have been designed to run on unleaded fuel which it would appear had an octane of 97 plus RON by the early eighties.

 

So what does this mean for these early engines that mow have to run on unleaded fuel? What octane is really best and is it politic to start messing with the timing?

[GeneralDisorder]

Yep - low octane ignites easier and burns hotter. More BTU's per gallon.

 

You should run the most advance you can without pinging. That is not an absolute number for a given engine model. Age and condition play a big role here. If you can run with 10 degree's without ping then so much the better.

 

Timing is all about getting the flame-front to arrive at the piston top at the RIGHT time. Ping will tell you when it's arriving too soon. Just below the point where you get that pinging sound under load is where you want the timing. Thus NO ONE should be giving you a number - Subaru's spec is for a new engine and is designed to allow all fuel grades to work properly. After this many years and mileage you should be setting your timing by ear anyway.

 

If you don't get any ping under load at full advance - hell go to 11 or 12. It's only a number indicating how many degree's before the piston reaches TDC that the plug fires and starts the burn. The objective is to get the flame front to meet the piston top at exactly TDC.

 

GD

[idosubaru]

Next thermo lesson covers page 45-48 of your text book. Nice job GD!

[Dirk]

Thank you GD

 

So I won't hear a ping when over advanced until I put the engine under load. Correct?

 

I must say though, I find it hard to recognise pining on my engine. Its very obvious in my wifes Toyota but I can't pin it down as easily on mine.

 

Is this just me or is it a boxer phenomenon?

 

Also is the article correct in saying that you can run a lower octane fuel by retarding the timing? And would I notice any drop in power?

[GeneralDisorder]

Thank you GD

 

So I won't hear a ping when over advanced until I put the engine under load. Correct?

 

The pinging most likely won't start till you reach close to max advance - which is somewhere around 25 or 30 degree's.... Thus you will only hit max advance when the engine is up around 2500 to 3500 RPM. And you won't be able to detect the pinging unless you are under load. Ping is when the flame front reaches the piston top BTDC - the noise is the force of the flame front trying to turn the engine backwards. Think of pushing your kid on the swing - you want to catch them right as they stop or very shortly after. If you catch them while they are still comming at you the force tends to try and knock you down, and if you catch them after they start to swing away you can't get much power with your push..... same idea.

 

I must say though, I find it hard to recognise pining on my engine. Its very obvious in my wifes Toyota but I can't pin it down as easily on mine.

 

Is this just me or is it a boxer phenomenon?

 

Try driving up a steep hill in 4th gear at close to the slowest speed of that gear. Pinging will really raise it's head when you are under a severe load condition - really lugging the engine down at around 2500 RPM will make it more noticeable. Full-throttle = more fuel = bigger, hotter flame front = more noticeable ping.

 

Also is the article correct in saying that you can run a lower octane fuel by retarding the timing? And would I notice any drop in power?

 

That's correct. As for power loss - that depends on too many factors to judge really. Typically you should run the lowest octane fuel you can without pinging and it should give you *more* power to a point of course. You don't want something so volitile that it has to be ignited ATDC.....

 

Premium fuels are generally for use in high-compression engines, or engines that run some kind of forced induction. In those cases the concern is that low octane fuels will spontaneously combust under the high compression and heat. This is called predetonation - it's like pinging but it's not caused by too much ignition advance. It can be massively destructive. Very high octane fuels are used in indy cars, etc where compression ratio's are in the 20:1 range

 

What's important is to use the appropriate fuel for the appropriate engine. Using super premium, high octane fuels in an EA81 or EA82 (non turbo) is just silly. They don't require it and they don't benefit from it.

 

GD

Edited November 8, 2009 by GeneralDisorder

[TheLoyale]

GD, This is a interesting thread. I wish I knew what a ping sounded like under a load - cause I have a noise comes under a load while the RPMs are somewhat low, it also comes if you punch it when the RPMs become high (3500-4000) Would a ping sound like something has play in it (Rocker arm, lifter body) Cause I've just been thinking something has be worn over the years from the PO's and thats what this noise I have is. Could this be a ping? I've never touched the timing, so I assume its still whatever the factory set it at.

 

Anyone have a video where I can hear the pinging? I wish I could discribe the noise I have better. (No, its not a lifter tick) sounds like slop in something in the driver side head.

 

-Tom

[GeneralDisorder]

Ping can sound very metallic, yes. For you (SPFI), the timing should be 20 DBTC at idle. If it still happens then try reducing your timing by 5 degree's and retest. If it won't go away you may end up with a rod through the top of the block because it probably wasn't ping in the first place . Subaru rod knocks sound a lot like pinging and also are most prevalent under load. Don't ask how I know this

 

Sometimes exhaust leaks can sound like ping also. Often just fixing the leaking exhaust gaskets will get rid of the weird noises.

 

GD

[TheLoyale]

Ok, I might give this a try sometime. So retard it to about 15 BTDC and see if anything changes.

 

As for a Rod, wouldn't you hear that just be free reving the engine to high RPMs? Maybe not sinces there theroecticly no stress on the engine.

 

Another question since we're on the subject - Would pinging come from both Heads not just one or the other? And what are the odds of lifters or Rockers having play in them?

 

Thanks for the extra knowledge

 

PS: exhaust gaskets and y-pipe and mid pipe are both new (Mufflers the only thing thats old with holes, Rumble!)

Edited November 9, 2009 by TheLoyale

[GeneralDisorder]

When I had a rod knock it was only apparent under load for about 2,000 miles. Then it got REALLY obvious for about half a mile. :-\

 

Ping can come from one or more cylinder's so it can come from just one side or from both. It depends on many factors, and each cylider will be slightly different and will ping at different timing settings.

 

GD

[TheLoyale]

Ok, so Rod knock is off the list - cause I've put prolly over 7,000 miles on it and it always sound the same when it does start (did alot of joy riding, rustic road finding this summer)

 

The other main time it makes this noise (Much louder and somewhat continues) is when your pushing the engine trying to do 60-65MPH on a slight up-slop on the interstate - throttle is about 3/4 depressed. I figure its not getting worse, so I have time to figure out what it is. One thing I noticed is it seems to act up more after a recent oil change, huh? Thats why I've been leaning towards a lifter.

 

This is all great info GD, thank you

 

-Tom