Nevdog69 Posted 7June, 2019 Report Share Posted 7June, 2019 A local porshe specialist has advanced my timing beyond spec and it scares me a little. He says its fine but if its not, he gets to rebuild my engine. I like the performance but don’t like the idea of possible engine damage. It is a 1978 911sc US spec. It is the lower compression version with a ratio of 8.5:1 (They lowered the compression because they only had access to poor unleaded fuel in the US in 1978). I only ever use 98 Ron fuel. The timing has been advanced to 12 degrees BTDC at 900 RPM (The spec is for it to be only 5 BTDC so he has added another 7 degrees). The only markings on the flywheel are Z and 5BTDC. Therefore I can’t tell how much it is advancing at higher RPM. I have installed SSIs and it pulls well all the way to the redline so I figure it’s not that far advanced it is fighting against itself . I have not heard any pings under load, but then again you may not hear it that well when the engine is behind. Due to the low compression and high octane fuel, I think it probably is safe. I was wondering what others experience is. I love the setup now and don’t really want to wind back the timing, but if its gonna stop in blowing a ring or piston, I would. Any advice is welcome. Quote Link to comment Share on other sites More sharing options...
Harvs11 Posted 7June, 2019 Report Share Posted 7June, 2019 Hi @Nevdog69, I'm also running a US spec. lower compression SC like yours but later model with the O2 sensor (disconnected) and vacuum ignition idle retard (disconnected). From memory I have 5 deg advance at 950rpm idle speed with vacuum retard disconnected and 33ish deg adv. at 4000rpm I only use 98 or higher and my engine performs just fine...all 150kW of it. The factory ignition timing spec. is written for conditions in the US ie shitty low octane fuel. The timing has to be tuned to avoid pre-ignition for the worse case fuel combo. We are lucky to have better fuels. A discussion on Pelican for a similar car running in Norway on 98 RON fuel gives a good insight to the confusion around how to tune for various fuel/engine combo's http://forums.pelicanparts.com/porsche-911-technical-forum/381031-please-help-me-understand-ignition-timing.html the most informative for me is the last post on the page... Hello Ole,No, your car has more advanced timing. The full advance is 38 BTDC, more advance than 30 BTDC. The idle spec is retarded for emission test purposes, the high speed advance is what matters. There are many other factors involved in your comparison. The early car had 9.8:1 compression, you have 8.5:1, a difference of several octane numbers, which would allow more advance. Your car also has a larger bore, which generally means more advance is needed for peak torque. The early smaller bore engine has higher dome pistons, which reduce the amount of advance that can be used before detonation. My guess would be that the early car is more knock limited and 30 BTDC was chosen to provide a comfortable margin of safety for a mfg. The lower compression of your engine, combined with the smaller dome and better combustion shape at TDC allows more advance with a similar safety margin. Running the earlier engine on higher octane fuel with more timing would probably make more power. Running your engine on higher octane fuel and advancing the timing past 38 BTDC would probably yield little benefit. The later SC's had even larger bores, with higher compression, but were limited to 25 BTDC because of the 87 CLC fuel spec. These are the engines that really benefit from more timing with higher octane fuel. Every engine has a sweet spot once all the factors are accounted for. Consider what emissions controls at idle, fuel type, mods. to your engine. distributor type, ignition system type etc. It gets very murky without all the info. My best advice is talk to your mechanic and understand his reasoning for the tune you have. Nevdog69, firstone and Peter M 2 1 Quote Link to comment Share on other sites More sharing options...
Nevdog69 Posted 8June, 2019 Author Report Share Posted 8June, 2019 Thanks, I am gonna stick with what I have. dave Quote Link to comment Share on other sites More sharing options...
Cheshire Cat Posted 8June, 2019 Report Share Posted 8June, 2019 Is that just me who can see an irony in all of that? CR for late cars US market 8.5:1 for poor fuel quality back in the days in the U.S. and yet we use 98 of presumably much better quality and scared to go a bit further with ignition... Nevdog69 and Redracn 1 1 Quote Link to comment Share on other sites More sharing options...
Nevdog69 Posted 8June, 2019 Author Report Share Posted 8June, 2019 Hi Ches Cat, I don’t get it. Are you telling to wind back the advance? Quote Link to comment Share on other sites More sharing options...
Harvs11 Posted 8June, 2019 Report Share Posted 8June, 2019 4 hours ago, Cheshire Cat said: Is that just me who can see an irony in all of that? CR for late cars US market 8.5:1 for poor fuel quality back in the days in the U.S. and yet we use 98 of presumably much better quality and scared to go a bit further with ignition... I don’t get it either. I just know that my car runs like crap on less than 98. Quote Link to comment Share on other sites More sharing options...
Cheshire Cat Posted 9June, 2019 Report Share Posted 9June, 2019 23 hours ago, Nevdog69 said: Hi Ches Cat, I don’t get it. Are you telling to wind back the advance? I`m not suggestion anything. But one of us really needs to hook up a knock sensor and check the theories... But I can`t see why additional 7 degrees can help on idle but can pretty much destroy your engine on WOT Quote Link to comment Share on other sites More sharing options...
Peter M Posted 9June, 2019 Report Share Posted 9June, 2019 16 hours ago, Cheshire Cat said: 8I`m not suggestion anything. But one of us really needs to hook up a knock sensor and check the theories... But I can`t see why additional 7 degrees can help on idle but can pretty much destroy your engine on WOT I agree a bit more data would be good, even knowing the distributor mechanical advance curve would be useful along with understanding the actual vacuum advance/retard setup on the car. The additional 7 degrees would help all through the rev range provided it isn't over advanced at any point in the range and throttle opening combination. I understand for maximum power a total advance of somewhere between 34 to 38 degrees would be expected for this particular engine. Assuming the mechanical advance contributes somewhere around 15 to 20 degrees on this car, even with a static of 12 degrees, the total advance would be 27 to 32 degrees. ie safe but leaving some power on the table. At idle you can put heaps of advance in because the combustion is so slow but is usually pulled way back for idle quality and emission reasons by having the vacuum advance unit only operate at above idle throttle openings. I reckon its well worth exploring further but wouldn't lose any sleep over the mechanic recommending 12 degrees static. Redracn and firstone 2 Quote Link to comment Share on other sites More sharing options...
Cheshire Cat Posted 10June, 2019 Report Share Posted 10June, 2019 5 hours ago, Peter M said: I agree a bit more data would be good, even knowing the distributor mechanical advance curve would be useful along with understanding the actual vacuum advance/retard setup on the car. The additional 7 degrees would help all through the rev range provided it isn't over advanced at any point in the range and throttle opening combination. I understand for maximum power a total advance of somewhere between 34 to 38 degrees would be expected for this particular engine. Assuming the mechanical advance contributes somewhere around 15 to 20 degrees on this car, even with a static of 12 degrees, the total advance would be 27 to 32 degrees. ie safe but leaving some power on the table. At idle you can put heaps of advance in because the combustion is so slow but is usually pulled way back for idle quality and emission reasons by having the vacuum advance unit only operate at above idle throttle openings. I reckon its well worth exploring further but wouldn't lose any sleep over the mechanic recommending 12 degrees static. shit, completely forgot it`s a mechanical stuff which I have no idea of how it works lol Quote Link to comment Share on other sites More sharing options...
Peter M Posted 10June, 2019 Report Share Posted 10June, 2019 On 08/06/2019 at 16:56, Cheshire Cat said: Is that just me who can see an irony in all of that? CR for late cars US market 8.5:1 for poor fuel quality back in the days in the U.S. and yet we use 98 of presumably much better quality and scared to go a bit further with ignition... Probably not irony, just a sign of the times. The '70's was a pretty grim time for the internal combustion engine with manufacturers struggling to met increasing emission standards using very primitive engine control systems. Consequently they used retarded ignition timing and reduced compression ratios to deliberately reduce emissions (namely hydrocarbons and nitrogen oxides respectively in this example) rather than a response to "poor gas". I think the matter of "poor gas" isn't really supported by fact and I think is more of a result of the US changing their advertising standard for octane in 1972 that resulted in the same gasoline posting lower octane numbers on the pump. (Change from RON to AKI which is an average of RON and the much lower MON). Hence the common mistaken belief that fuel quality dropped at this time. Anyway, enough chitchat and history, I hope Nevdog69 sorts out his timing concerns successfully Cheshire Cat 1 Quote Link to comment Share on other sites More sharing options...
Mike D'Silva Posted 11June, 2019 Report Share Posted 11June, 2019 On 08/06/2019 at 07:02, Nevdog69 said: A local porshe specialist has advanced my timing beyond spec and it scares me a little. He says its fine but if its not, he gets to rebuild my engine. I like the performance but don’t like the idea of possible engine damage. It is a 1978 911sc US spec. It is the lower compression version with a ratio of 8.5:1 (They lowered the compression because they only had access to poor unleaded fuel in the US in 1978). I only ever use 98 Ron fuel. The timing has been advanced to 12 degrees BTDC at 900 RPM (The spec is for it to be only 5 BTDC so he has added another 7 degrees). The only markings on the flywheel are Z and 5BTDC. Therefore I can’t tell how much it is advancing at higher RPM. I have installed SSIs and it pulls well all the way to the redline so I figure it’s not that far advanced it is fighting against itself . I have not heard any pings under load, but then again you may not hear it that well when the engine is behind. Due to the low compression and high octane fuel, I think it probably is safe. I was wondering what others experience is. I love the setup now and don’t really want to wind back the timing, but if its gonna stop in blowing a ring or piston, I would. Any advice is welcome. Sounds fine to me. Drive it and enjoy it. Feel free to bring it over if you'd like a second set of ears to listen for pinging while you drive it. Mike firstone 1 Quote Link to comment Share on other sites More sharing options...
Nevdog69 Posted 12June, 2019 Author Report Share Posted 12June, 2019 Thank you all for your advice. This site is wonderful firstone 1 Quote Link to comment Share on other sites More sharing options...
RTP Posted 22November, 2019 Report Share Posted 22November, 2019 Keep in mind that many fuels contain heaps of volatiles like xylene toluene etc. Basically they add paint thinners to boost the octane rating. This stuff evaporates quickly starting from leaving the refinery in Singapore and the octane rating plummets quickly. I had detonation once after fuel sat around for only about 4 weeks. BP premium in Australia only uses benzine to boost the octane and no alcohol. The discounted fuels tend to have the additives. Before anyone mucks around with timing, I recommend to download the safety data sheet for any fuel you are considering using and check out what they add to it. Quote Link to comment Share on other sites More sharing options...
Redracn Posted 22November, 2019 Report Share Posted 22November, 2019 My understanding is that with current bowser fuel in Aus the octane increases as the the volatile components evaporate off. The problem is that starting becomes more difficult as is the case with avgas that has less volatile components as it has to stay liquid at high altitude/low pressures. Quote Link to comment Share on other sites More sharing options...
RTP Posted 22November, 2019 Report Share Posted 22November, 2019 Other way around. Since the closure of the Australian Clyde, Kurnell and Port Stanvac refineries, the fuels destined for Australian discount outlets are refined in Asia to a lower budget and at the final stage they add volatiles to get the octane rating up to the required legal level for the grade. These volatilies immediately start evaporating so by the time they have sat in your tank for a while there is not much oomph left. Particularly a problem for people with historic cars that they drive once a month. Yes AVGAS has no volatiles which cause vapour locks. Drum AVGAS can be stored for a year without risk of detonation. Quote Link to comment Share on other sites More sharing options...
RTP Posted 22November, 2019 Report Share Posted 22November, 2019 Perhaps my last post was an oversimplification of a complex topic. Here is a worthwhile article by article by SUNOCO Race Fuels which talks about how butane is used in the USA to up the octane rating but evaporates easily. https://www.sunocoracefuels.com/tech-article/octane-stability-high-octane-vs-low-octane-fuels Definitely a topic worth researching more. Quote Link to comment Share on other sites More sharing options...
Redracn Posted 22November, 2019 Report Share Posted 22November, 2019 There is no Butane added to fuel in Aus as even our coldest locations are warm by US standards. This article from BP while still having a marketing bent is closer to correct. There is a lot more technical info out there as well.https://www.bp.com/content/dam/bp-country/en_au/media/fuel-news/petrol-life-vehicle-tanks.pdf They do carry on about high rpm when engine is cold being an issue but modern engines do not really suffer from this as the 02 sensor heats quickly to operating temperature to correct the mixture and the knock sensors are very effective these days. Older cars may want to warm the engine up before racing it flat out. The evaporation rates quoted are also for open containers and not representative of a cars pressurised fuel tank even an older one or even a stationary engine tank. This is easily checked my measuring the change in volume but be careful as the volume changes a lot with temperature change. The lack of volatiles in AVGAS helps prevent vapour lock not cause it. I have used 6month old 98 pump fuel in my EVOX race car at 26psi boost and it is tuned to run on the edge of knock. The logging which monitors every knock detection shows no change when compared to fresh 98 pump fuel or even 98 racing fuel. Also no cold start issues. I have also stored fuel in stationary engines with a stabiliser for two years with no issues starting or running. Rob and Peter M 2 Quote Link to comment Share on other sites More sharing options...
RTP Posted 22November, 2019 Report Share Posted 22November, 2019 That is a good article by BP.....Thanks! Which brand of fuel was in your EVOX? Yes I meant to say that the lack of volatiles in AVGAS prevents vapour locks not causes vapour locks. Butane for USA only sounds right. It is very volatile at our temps. BP Premium 98 contains only benzine to up the octane rating. Benzine is a bit volatile but not as much as the toluene, xylene and napthalene that other brands in Oz use. The BP data sheet says it can be stored for 12 months in a drum. This is what I use. Good discussion! Quote Link to comment Share on other sites More sharing options...
Peter M Posted 23November, 2019 Report Share Posted 23November, 2019 RTP, I'm not sure the data sheets have sufficient information to back up some of your claims. The MSDS for BP 95 and 98 are essentially the same from what I can see. ie both say that these fuels contain less than 20% toluene and less that 1% benzine (which I understand is legislated maximum amount ?). Unless you are a petro-chemist currently working in the industry, I don't think anyone could recommend one brand over another based on published information. Happy to learn if you have the data and the facts. Quote Link to comment Share on other sites More sharing options...
RTP Posted 23November, 2019 Report Share Posted 23November, 2019 Yes you are right, I shouldn't make any recommendations as I am not an expert. I can absolutely say for sure that I have has detonation problems a number of times with old fuel. The BP link that REDRACN posted explains this extremely well. Essential reading for anyone enthusiastic about engines. After reading it I realise my knowledge was very incomplete! Quote Link to comment Share on other sites More sharing options...
Peter M Posted 23November, 2019 Report Share Posted 23November, 2019 2 hours ago, RTP said: ... After reading it I realise my knowledge was very incomplete! After reading it RTP, I'm plain confused!😀 The data table is saying to me that detonation should be less of a problem as the fuel ages as the octane increase reasonably significantly and the effective air/fuel ratio actually becomes richer which are usually the two of the three changes commonly used to eliminate detonation! I don't understand their comments about loss of volatiles causing "higher temperatures, pre ignition, detonation and piston damage" during cold starts. Doesn't make any sense to me at all. Surely you ensure the air fuel mixture is acceptable by using the choke? When an engine is cold, I'm just happy if it starts and idles reliably Quote Link to comment Share on other sites More sharing options...
RTP Posted 23November, 2019 Report Share Posted 23November, 2019 Yes, I had to re-read that quite a few times! Basically I think I and perhaps many others have been focusing on straight Octane rating but it seems that many components of petrol have different octane ratings but some are light, some are heavy which effects when the cylinders can actually make use of that octane rating. Light volatiles seem to be important for starting and high RPM and the heavier content more relevant for chugging along. This certainly explains why I have experienced detonation with old fuel at high RPM not mid range hauling along. I found the MSDS for Australian Shell, BP and Caltex which are attached. BP-Premium-msds.pdf Caltex-Vortex_98-SDS.pdf Shell-premium-msds.pdf Quote Link to comment Share on other sites More sharing options...
Redracn Posted 23November, 2019 Report Share Posted 23November, 2019 I think that the term High RPM which is not defined in a lot of articles needs some context. When it comes to Petrol burning engines High RPM will be ones like chainsaws and small bike engines that spin over 10,000rpm even 12,000. These engines often have a carby or are open loop and owners tend not to warm them up before flogging them. This is where you will get into big trouble with old fuel. After starting even though enough fuel is being supplied to technically be rich there is not enough heat or time(hi rpm) to vaporise it and the reality is that the actual mixture at the time of combustion is lean With lots of liquid fuel in the cylinder. There is a point at which adding more fuel (choke) to overcome this leanness becomes impractical. Medium speed (around 7000rpm) engines such as road cars do not suffer this as badly and are not usually run straight to the redline while cold. Low speed such as stationary engines 3000-3600rpm are also usually low compression and should not but still can have an issue given they are nearly always never warmed up before going straight to full load, still have carby and the choke is off as soon as the engine will run without it. There are of course engines that run much slower but usually not on petrol. Rob and Peter M 2 Quote Link to comment Share on other sites More sharing options...
Peter M Posted 29December, 2019 Report Share Posted 29December, 2019 Kevin Cameron does a good explanation as usual: https://www.cycleworld.com/what-gasoline-is-and-isnt-and-why-motorcyclists-should-care/ Quote Link to comment Share on other sites More sharing options...
RTP Posted 29December, 2019 Report Share Posted 29December, 2019 Good Article! I have learned a lot from this thread! Quote Link to comment Share on other sites More sharing options...
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