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1
- #1
Marquis
Automotive
- May 29, 2002
- 162
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 looked under the bonnet 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 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 looked under the bonnet 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 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.
