Twin Plug Air cooled Porsche Boxer engines
Discussion
I find it interesting that the last of the air cooled Naturally aspirated Porsche 911 (993 in code) are of twin plug per cylinder lay out. Nothing remarkable about that.
However the Twin Turbo charged variant is single plug only!
It's easy to see why an engine like the 993s would have to be twin plug. It's VERY over square by contemporary standards, it's a hemi with quite a big included valve angle for a modern engine. This doesn't lead to compact chambers and to get the compression up it needs domed pistons, hardly ideal from a surface to volume ratio point of view. None of this leads to a fast burn so the twin plugs will no doubt help while also contributing to Cold Start emissions ignition-retard combustion stability for maximium heat rejection during the warm up phase.
All of the above makes sense, however why would Porsche on their most expensive model at the time NOT to go twin plug.
Just for back ground so we know, The Bi Turbo 993 has twin low inertia turbos that operate individually.The integrated wastegates are electronically controllable and maximum boost presure varies from 1.6 bar to 1.94 bar.
The air cooled Porsches never went to 4 valve per cylinder for series production because of the extra heat rejection of the smaller valves and the air cooling being marginal in that case ( The legendary 959 was 4 valves per cylinder but had water cooled heads!).It's also interesting to note that the water cooled Porsche flat 6 successor the 996 is 4 valves per cylinder far less over square and does away with the twin plug lay out.
So why do we think that the Turbo variant had only 6 plugs while the naturally aspirated models ran with 12?
My theory?
Well I have a few
They are:
1) Packaging- perhaps on the Turbo there is no space to package the twin plug lay out. I haven't been able to look under the intercooler closely-so I don't know. I think packageing is possibly but unlikely.
2)You CAN have too fast a burn!!
If we say that increasing boost pressure is roughley analagous to upping the engine load- we know that the greater the load or the less throttled (or more boosted an engine in this case)the faster the engine burn rate- it's possible when running so much boost the extra fast burn rate actually leads to combustion instability and even knock-I've seen this occur in engines! I myself doubt this in this case, but don't discount the possibility.
3) Flame front shrouding. A domed piston isn't great from your 10-90% burn region as it can shorud the flame front. This is the very reason why the naturally aspirated over square engine ( just like the new Dodge Hemi) uses twin plugs.
Well the 993 Turbo runs an 8:1 Compression ratio and no longer has a domed piston so it's likely it no longer has flame front shrouding as an issue. It's my reckoning that THIS is the most likely reason the turbo is Not twin plug.
However the Twin Turbo charged variant is single plug only!
It's easy to see why an engine like the 993s would have to be twin plug. It's VERY over square by contemporary standards, it's a hemi with quite a big included valve angle for a modern engine. This doesn't lead to compact chambers and to get the compression up it needs domed pistons, hardly ideal from a surface to volume ratio point of view. None of this leads to a fast burn so the twin plugs will no doubt help while also contributing to Cold Start emissions ignition-retard combustion stability for maximium heat rejection during the warm up phase.
All of the above makes sense, however why would Porsche on their most expensive model at the time NOT to go twin plug.
Just for back ground so we know, The Bi Turbo 993 has twin low inertia turbos that operate individually.The integrated wastegates are electronically controllable and maximum boost presure varies from 1.6 bar to 1.94 bar.
The air cooled Porsches never went to 4 valve per cylinder for series production because of the extra heat rejection of the smaller valves and the air cooling being marginal in that case ( The legendary 959 was 4 valves per cylinder but had water cooled heads!).It's also interesting to note that the water cooled Porsche flat 6 successor the 996 is 4 valves per cylinder far less over square and does away with the twin plug lay out.
So why do we think that the Turbo variant had only 6 plugs while the naturally aspirated models ran with 12?
My theory?
Well I have a few
They are:
1) Packaging- perhaps on the Turbo there is no space to package the twin plug lay out. I haven't been able to look under the intercooler closely-so I don't know. I think packageing is possibly but unlikely.
2)You CAN have too fast a burn!!
If we say that increasing boost pressure is roughley analagous to upping the engine load- we know that the greater the load or the less throttled (or more boosted an engine in this case)the faster the engine burn rate- it's possible when running so much boost the extra fast burn rate actually leads to combustion instability and even knock-I've seen this occur in engines! I myself doubt this in this case, but don't discount the possibility.
3) Flame front shrouding. A domed piston isn't great from your 10-90% burn region as it can shorud the flame front. This is the very reason why the naturally aspirated over square engine ( just like the new Dodge Hemi) uses twin plugs.
Well the 993 Turbo runs an 8:1 Compression ratio and no longer has a domed piston so it's likely it no longer has flame front shrouding as an issue. It's my reckoning that THIS is the most likely reason the turbo is Not twin plug.
Not remarkable really, and as you say you have answered it yourself. The turbocharged engine runs a lower compression ratio than the NA and so without the piston domes you have no regioning of the combustion chamber to worry about. The higher temperatures of the boosted temperature (even with intercooling) would also give rise to faster flame speeds. The deUtscherS would be better off reducing their BOre with the NA, sticking proper pistons in & reducing their emissions...
alfredo said:
Not remarkable really, and as you say you have answered it yourself. The turbocharged engine runs a lower compression ratio than the NA and so without the piston domes you have no regioning of the combustion chamber to worry about. The higher temperatures of the boosted temperature (even with intercooling) would also give rise to faster flame speeds. The deUtscherS would be better off reducing their BOre with the NA, sticking proper pistons in & reducing their emissions...
They were forced down the over square route at the time, due to the adherence to air cooling (indirectly) -where they had to stay with a 2 valve layout-so they needed the over square cylinder dimentions in order to obtain the valve area for the specific horsepower requirements.
Someone at Weissach obviously agreed with you, as the subsequent water cooled engines that followed were less oversquare, were able to go 4 valves and more compact combustion chmabers as a result.
I thought the boost pressure was limited to 0.8 bar - this is the figure that one should see on the digital boost readout at around 4.5k rpm in 3rd or 4th as a sign of an engine in normal health (with a 0.5 bar max indicating that the ECU has gone to unhappy mode because of low octane fuel)?
Another reason that they might have gone for a single plug layout is that the increased turbulence due to forced induction aids the flame front propagation and does away with the need for twin plugs.
Another reason that they might have gone for a single plug layout is that the increased turbulence due to forced induction aids the flame front propagation and does away with the need for twin plugs.
And since when would Porscher worry about engine out emissions ? They can stick the cats as close to the engine as they like & charge you over-rich yuppies another couple of grand to add some extra melted down platinum rings to the bricks. Cheaper than giving it to yer bird & then her leaving you !!!!
alfredo said:
And since when would Porscher worry about engine out emissions ? They can stick the cats as close to the engine as they like & charge you over-rich yuppies another couple of grand to add some extra melted down platinum rings to the bricks. Cheaper than giving it to yer bird & then her leaving you !!!!
Ohh, here we go, another anti-German-car pro-proletarian! :wink:
First I'm no yuppie-just a poor engineer. Second, I don't buy Porsches because of the image, I'm an engineer and am able to look past image and look into the engineering without bias.
Even with close coupled cats modern emissions standards are measured from Key on, while the cats are still cold unfortunately. The use of a twin turbochargers means that the cats will ALWAYS have to be somewhat further back then an equivalent Naturally aspirated engine of bigger capacity.
I work on engine design but am currently on secondment in engine management systems- mainly emissions, where as previously my responsibility was performance development. I'm trying to get a broader view of powertrain, which is hard these days, where most engineers are getting specialised and pidgeonholed...
Relying on air-cooling on a 4-v turbo engine is simply begging for trouble. You can pretty much guarantee cracking between the exhaust valves during thermoshock tests. You would be ridiculed for even trying it.
Running a little more boost would easily compensate for the reduced curtain area of a two-vale head.
Running a little more boost would easily compensate for the reduced curtain area of a two-vale head.
Did some more ressearch, reading various technical journals and the excellent 911 performance hand book by Bruce Anderson.
I now think the twin plug layout was abandonned in the 993 TT due to packaging reasons.
An excellent and worthy mod, if I ever get bored of 408 Bhp would be
1) the twin K24 turbo chargers in liu of the K16s. (this gives about 450 Bhp with appropriate remapping on the 3.6 litre).
2) Use pistons and barrels of 102 mm bore, to get the 3746 cc capacity increase. This will boost low speed torque and go some way to reducing the lag of the larger turbos. These are readily availiable
3) Invesitigate package further working hard to fit Twin plug layout combined with a compression ratio increase from 8:1 to 9:1 (assuming you're not knock limited-expect a 3 % gain per unit ratio rise up to 10.5-11:1 when it becomes less). Porsche may not have been able to fit twin plugs due to manufacturing and or service/assembly constraints
4) New mapping with a new ECU system, possibly MoTEC- although it's would be a great shame to abandon the ultra sophisticated Motronic 5.2- I would map it myself to achieve lean running on part load- as engines should (if it weren't for anal Nox rules!)to also get better fuel economy.
With modest levels of boost 470-490 Bhp seems reasonable with unchanged if not better fuel economy.
With over a bar and a half of boost 550 Bhp may be possible. Ideally combined mods with lower loss intake and exhaust. All conservative number as I'm not a fan of zealeot pub talk.
Of course the current levels of power and torque are more then enough for me right now, but there's no harm in fantasizing...
>> Edited by Marquis_Rex on Tuesday 26th October 10:03
I now think the twin plug layout was abandonned in the 993 TT due to packaging reasons.
An excellent and worthy mod, if I ever get bored of 408 Bhp would be
1) the twin K24 turbo chargers in liu of the K16s. (this gives about 450 Bhp with appropriate remapping on the 3.6 litre).
2) Use pistons and barrels of 102 mm bore, to get the 3746 cc capacity increase. This will boost low speed torque and go some way to reducing the lag of the larger turbos. These are readily availiable
3) Invesitigate package further working hard to fit Twin plug layout combined with a compression ratio increase from 8:1 to 9:1 (assuming you're not knock limited-expect a 3 % gain per unit ratio rise up to 10.5-11:1 when it becomes less). Porsche may not have been able to fit twin plugs due to manufacturing and or service/assembly constraints
4) New mapping with a new ECU system, possibly MoTEC- although it's would be a great shame to abandon the ultra sophisticated Motronic 5.2- I would map it myself to achieve lean running on part load- as engines should (if it weren't for anal Nox rules!)to also get better fuel economy.
With modest levels of boost 470-490 Bhp seems reasonable with unchanged if not better fuel economy.
With over a bar and a half of boost 550 Bhp may be possible. Ideally combined mods with lower loss intake and exhaust. All conservative number as I'm not a fan of zealeot pub talk.
Of course the current levels of power and torque are more then enough for me right now, but there's no harm in fantasizing...
>> Edited by Marquis_Rex on Tuesday 26th October 10:03
Hi Marquis - my 993tt has the K24s as the previous owner very kindly had it ferried back to Germany for the 450 bhp upgrade. I'm not sure how much real world difference this makes compared to the K16s as the bigger turbos spool up more slowly, but when they arrive...
.
However, if you you read the info on the Rennlist forums, there is a yank called Kevin who specialises in building flowed K16/K24 hybrids that have the spool up time of the smaller turbos with the flow of the larger ones. Enthusiastic owners talk about turbo power starting to arrive soon after 2000 rpm, whereas I have to wait until 3000 rpm for the wallop to arrive.
As stated earlier in this thread, given the forced induction turbulence I'm not sure how much you'd gain from 4 valve heads. If it was that beneficial without negative reliability trade offs then surely Porsche would have done this themselves, e.g. on the 993 GT2 they stuck on the K24s and re-chipped the engine, but (AFAIK) they stayed with 2 valves per cylinder. If it was win-win they would have given the GT2 4 valves per cylinder.
. However, if you you read the info on the Rennlist forums, there is a yank called Kevin who specialises in building flowed K16/K24 hybrids that have the spool up time of the smaller turbos with the flow of the larger ones. Enthusiastic owners talk about turbo power starting to arrive soon after 2000 rpm, whereas I have to wait until 3000 rpm for the wallop to arrive.
As stated earlier in this thread, given the forced induction turbulence I'm not sure how much you'd gain from 4 valve heads. If it was that beneficial without negative reliability trade offs then surely Porsche would have done this themselves, e.g. on the 993 GT2 they stuck on the K24s and re-chipped the engine, but (AFAIK) they stayed with 2 valves per cylinder. If it was win-win they would have given the GT2 4 valves per cylinder.
nel said:
Hi Marquis - my 993tt has the K24s as the previous owner very kindly had it ferried back to Germany for the 450 bhp upgrade. I'm not sure how much real world difference this makes compared to the K16s as the bigger turbos spool up more slowly, but when they arrive...
However, if you you read the info on the Rennlist forums, there is a yank called Kevin who specialises in building flowed K16/K24 hybrids that have the spool up time of the smaller turbos with the flow of the larger ones. Enthusiastic owners talk about turbo power starting to arrive soon after 2000 rpm, whereas I have to wait until 3000 rpm for the wallop to arrive.
As stated earlier in this thread, given the forced induction turbulence I'm not sure how much you'd gain from 4 valve heads. If it was that beneficial without negative reliability trade offs then surely Porsche would have done this themselves, e.g. on the 993 GT2 they stuck on the K24s and re-chipped the engine, but (AFAIK) they stayed with 2 valves per cylinder. If it was win-win they would have given the GT2 4 valves per cylinder.
I think you misunderstood me, as did Bor, I never advocated the use of 4 valve heads on the air cooled engines. There is a cooling problem with the heat rejection rates of the twin small exhaust valves. I DO know this! It's the reason why 959 4 valvers went water cooled heads.
(I think it might have been the clown Alfredo-wittering on about this- and taking the thread off course)
Twin plugs have been succesfully used in racing Turbos, such as 935s in the 70s. The twin plug layout is to try to cater for the very oversquare design, which causes a nasty shaped combustion chamber- from a surface to volume -ratio point of view, the twin plug layout shortens flame paths and allows a higher CR. I would only pursue this because I like my engines to produce power AND fuel economy and they don't have to be mutually exclusive- regardless of whether I can afford the fuel bills or not- I like efficient engines. Porsche didn't probably brother with this as it thought fuel economy didn't matter to most owners-along with the package constraints already talked about.
The larger capacity would also go a small way to addresing the lag issues.
Marquis,
Dunno if you've ever spoken to Derestrictor but he has had "The Beetle" taken from a stock 408 to 493.
Might be worth dropping him a mail for further detail?
He has invesitgated going further (indeed, a twin spark solution was mooted) but the costs of such work were to put it bluntly, amazing.
DeR?
Dunno if you've ever spoken to Derestrictor but he has had "The Beetle" taken from a stock 408 to 493.
Might be worth dropping him a mail for further detail?
He has invesitgated going further (indeed, a twin spark solution was mooted) but the costs of such work were to put it bluntly, amazing.
DeR?
GregE240 said:
Marquis,
Dunno if you've ever spoken to Derestrictor but he has had "The Beetle" taken from a stock 408 to 493.
Might be worth dropping him a mail for further detail?
He has invesitgated going further (indeed, a twin spark solution was mooted) but the costs of such work were to put it bluntly, amazing.
DeR?
Hi ya GregE240, I'm only fantasizing about this, I won't actually pursue for a while now if at all. Sad engineers like me, like to keep our minds occupied. If I don't need any more power at the moment at all.
If I were to pursue such work one day I would do alot of it myself, it's the only way I can be sure that a good job is done, I don't trust these tuning muppets.
Designing and developing engines for a living and seeing what some of these alledgedly reputable tuners do to our engines, makes you cautious!
Marquis_Rex said:
[I think you misunderstood me, as did Bor, I never advocated the use of 4 valve heads on the air cooled engines. There is a cooling problem with the heat rejection rates of the twin small exhaust valves. I DO know this! It's the reason why 959 4 valvers went water cooled heads.
(I think it might have been the clown Alfredo-wittering on about this- and taking the thread off course)
Twin plugs have been succesfully used in racing Turbos, such as 935s in the 70s. The twin plug layout is to try to cater for the very oversquare design, which causes a nasty shaped combustion chamber- from a surface to volume -ratio point of view, the twin plug layout shortens flame paths and allows a higher CR. I would only pursue this because I like my engines to produce power AND fuel economy and they don't have to be mutually exclusive- regardless of whether I can afford the fuel bills or not- I like efficient engines. Porsche didn't probably brother with this as it thought fuel economy didn't matter to most owners-along with the package constraints already talked about.
The larger capacity would also go a small way to addresing the lag issues.
Yep - reread the previous posts and I have gone off on a tangent!
However, the same argument applies. Did Porsche equip the 993 GT2 with two plugs per cylinder? If it would have given easy power gains then surely they would have done so. Maybe, as you say it was a packaging issue, but equally the logic might have been that the returns would not justify the means....
nel said:
[I think you misunderstood me, as did Bor, I never advocated the use of 4 valve heads on the air cooled engines. There is a cooling problem with the heat rejection rates of the twin small exhaust valves. I DO know this! It's the reason why 959 4 valvers went water cooled heads.
(I think it might have been the clown Alfredo-wittering on about this- and taking the thread off course)
Twin plugs have been succesfully used in racing Turbos, such as 935s in the 70s. The twin plug layout is to try to cater for the very oversquare design, which causes a nasty shaped combustion chamber- from a surface to volume -ratio point of view, the twin plug layout shortens flame paths and allows a higher CR. I would only pursue this because I like my engines to produce power AND fuel economy and they don't have to be mutually exclusive- regardless of whether I can afford the fuel bills or not- I like efficient engines. Porsche didn't probably brother with this as it thought fuel economy didn't matter to most owners-along with the package constraints already talked about.
The larger capacity would also go a small way to addresing the lag issues.
Yep - reread the previous posts and I have gone off on a tangent!
However, the same argument applies. Did Porsche equip the 993 GT2 with two plugs per cylinder? If it would have given easy power gains then surely they would have done so. Maybe, as you say it was a packaging issue, but equally the logic might have been that the returns would not justify the means....
Porsche probably didn't do it, because it was far easier to up the boost, but I would prefer a good fuel economy-performance balance. So it wouldn't be the most cost effective solution for the performance.
I'll do some more ressearch.
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