Two plug or not two plug... (kinda long)
07-18-2001, 09:59 PM
#1
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Thread Starter
Two plug or not two plug... (kinda long)
I sense considerable disagreement between experts on the topic of twin-plugging a 930(whether to and how). Would anyone be interested in chiming in (again!) on this often-discussed topic? A lot of what I've read has been directed toward 911s; hence I'd like to be sure to focus on the merits relative to 930s.
I get answers ranging from "absolutely do it" to "absolutely don't waste your time and money on it." Without any convergence on this topic, it's hard to proceed with a clear conscience.
The process is expensive. It likely costs on the order of $2500-$3000. The technology appeals to my sense of neato zowie, but does it really improve matters noticeably enough to justify the cost?
I'm interested in defining exactly what benefit one receives by twin-plugging a 3.3 930, and in particular, whether such a mod will specifically benefit my application. Maybe the reason for such diverse opinions is because of what I have and what I'm doing with it. So here goes...
I have a 78 3.3 Euro engine (currently apart), which will receive significant internal and external mods for use as a track car (relatively high speed, high rpm). The engine will retain stock displacement and compression, but will have 964 cams, mild head and plenum porting, a larger intercooler, headers, an improved turbo, and likely an electronic boost controller. I expect never to run more than 1 bar boost, and I may be convinced to keep it at 0.8 or 0.9. I'm hoping to avoid using race gas, but I'm ready to use it if need be. If twin plugging will negate the need for race gas, then I'd like to know.
Am I really, truly going to have a detonation risk that can be averted by twin plugging? Are there any other real benefits to power/driveability/throttle response that will justify the cost of this mod? The flat-topped pistons in the combustion chamber of the 930 strike me as being less sensitive to ignition difficulties than the domed pistons in high performance 911 engines. True?
And then, what is the best way to do this on a 930? There are at least three basic ways I'm aware of, with many permutations each. One is a crankfire system, such as the Electromotive HPV system. I'm aware of some of the concerns with that system (Steve W, thanks for your constructive criticism on this system in a recent posts on the 911 board; do the same concerns apply to the 930?). Then there's the modified distributor with the oh-so-expensive RSR cap and rotor. It sounds like this is the "tried and true" way to go at the moment, but I find the expensive parts distasteful. I've heard that some people are using a much cheaper Jaguar distributor cap. Anyone had any success? I've also heard of Hall effect chips being used instead of the (more expensive) CD boxes. True? Finally, there's the modified 3.6 twin-distributor route. The parts are cheaper intitially, the caps more plentiful and replaceable, but I'm concerned about whether the darn thing will even fit in the turbo engine compartment, not to mention the all the details (read: costs) to modify it properly to work. Has anyone done this conversion to a 930?
Pardon the length of my post. Your thoughts are appreciated.
Rob
I get answers ranging from "absolutely do it" to "absolutely don't waste your time and money on it." Without any convergence on this topic, it's hard to proceed with a clear conscience.
The process is expensive. It likely costs on the order of $2500-$3000. The technology appeals to my sense of neato zowie, but does it really improve matters noticeably enough to justify the cost?
I'm interested in defining exactly what benefit one receives by twin-plugging a 3.3 930, and in particular, whether such a mod will specifically benefit my application. Maybe the reason for such diverse opinions is because of what I have and what I'm doing with it. So here goes...
I have a 78 3.3 Euro engine (currently apart), which will receive significant internal and external mods for use as a track car (relatively high speed, high rpm). The engine will retain stock displacement and compression, but will have 964 cams, mild head and plenum porting, a larger intercooler, headers, an improved turbo, and likely an electronic boost controller. I expect never to run more than 1 bar boost, and I may be convinced to keep it at 0.8 or 0.9. I'm hoping to avoid using race gas, but I'm ready to use it if need be. If twin plugging will negate the need for race gas, then I'd like to know.
Am I really, truly going to have a detonation risk that can be averted by twin plugging? Are there any other real benefits to power/driveability/throttle response that will justify the cost of this mod? The flat-topped pistons in the combustion chamber of the 930 strike me as being less sensitive to ignition difficulties than the domed pistons in high performance 911 engines. True?
And then, what is the best way to do this on a 930? There are at least three basic ways I'm aware of, with many permutations each. One is a crankfire system, such as the Electromotive HPV system. I'm aware of some of the concerns with that system (Steve W, thanks for your constructive criticism on this system in a recent posts on the 911 board; do the same concerns apply to the 930?). Then there's the modified distributor with the oh-so-expensive RSR cap and rotor. It sounds like this is the "tried and true" way to go at the moment, but I find the expensive parts distasteful. I've heard that some people are using a much cheaper Jaguar distributor cap. Anyone had any success? I've also heard of Hall effect chips being used instead of the (more expensive) CD boxes. True? Finally, there's the modified 3.6 twin-distributor route. The parts are cheaper intitially, the caps more plentiful and replaceable, but I'm concerned about whether the darn thing will even fit in the turbo engine compartment, not to mention the all the details (read: costs) to modify it properly to work. Has anyone done this conversion to a 930?
Pardon the length of my post. Your thoughts are appreciated.
Rob
07-19-2001, 10:12 AM
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Rob,
I understand your frustration and I agree that the camps are quite far apart on this issue. The best I can do is relate some of the information I have gathered and possibly impart some of what I know about internal combustion engines and how ignition systems work.
Porsche uses twin plug heads for two completely different reasons.
The first application was in their high compression race motors with high domed pistons. The combination of domed piston and offset spark plug prevents optimal fuel burn on the non spark side of the dome. This makes perfect sense given the variables.
The second application was in the C2/4 street motors. Contrary to what you might hear this wasn't for performance. it was for emissions. The second spark occured slightly delayed from the first and was used to burn off the last of the unburnt hydrocarbons and thus clean up the exhaust.
In the Turbo you have certain factors that make twin plugging much more optional than required. First off the flatter combustion chamber. Here you've removed one of the variables to the proper flame front by removing the dome. You still have the issue of an offset spark position but this can be worked with using better intake flow patterns and exhaust port scavenging to reduce or eliminate dead spots in the combustion chamber. You can also modulate or play with the spark duration to better ignite the charge. the key here is to ignite as much of the charge at the same time to prevent detonation, i.e. the explosion of portions of the charge at the wrong time.
Based on your mods (simular to what I'm planning to do eventually) I don't see a "need" to twin plug. Keep the intake charge rich, cool, at or below 1 bar, and create a powerful and consitant spark and your motor should make good power and live to see many a track event.
You're flowing the heads, cleaning up the combustion chamber to improve the fuel burn, dialing in a cam that promotes a dense charge, adding more efficient intercooling and not increasing compression significantly. Absolutely all of the right things to make reliable power. Think about flowing the fuel head and going to european spec injectors and fuel lines and I think you've got a real sweet package.
If you do decide to go twin plug then I vote for the electonic crank fire systems. I'm not all that keen on the electromotive but there are others as well as a system under development by Permatune that's programable from a palm pilot at the track! You'll find a lot of resistance to these newer system from many p-car mechanics because Porsche didn't design it. I understand where they are coming from and even agree, up to a point. Electronics, done right, beat out mechanical every time IMHO. For me electonic crank fire ignitions and electronic boost controllers are great addition to the 930 package.
I understand your frustration and I agree that the camps are quite far apart on this issue. The best I can do is relate some of the information I have gathered and possibly impart some of what I know about internal combustion engines and how ignition systems work.
Porsche uses twin plug heads for two completely different reasons.
The first application was in their high compression race motors with high domed pistons. The combination of domed piston and offset spark plug prevents optimal fuel burn on the non spark side of the dome. This makes perfect sense given the variables.
The second application was in the C2/4 street motors. Contrary to what you might hear this wasn't for performance. it was for emissions. The second spark occured slightly delayed from the first and was used to burn off the last of the unburnt hydrocarbons and thus clean up the exhaust.
In the Turbo you have certain factors that make twin plugging much more optional than required. First off the flatter combustion chamber. Here you've removed one of the variables to the proper flame front by removing the dome. You still have the issue of an offset spark position but this can be worked with using better intake flow patterns and exhaust port scavenging to reduce or eliminate dead spots in the combustion chamber. You can also modulate or play with the spark duration to better ignite the charge. the key here is to ignite as much of the charge at the same time to prevent detonation, i.e. the explosion of portions of the charge at the wrong time.
Based on your mods (simular to what I'm planning to do eventually) I don't see a "need" to twin plug. Keep the intake charge rich, cool, at or below 1 bar, and create a powerful and consitant spark and your motor should make good power and live to see many a track event.
You're flowing the heads, cleaning up the combustion chamber to improve the fuel burn, dialing in a cam that promotes a dense charge, adding more efficient intercooling and not increasing compression significantly. Absolutely all of the right things to make reliable power. Think about flowing the fuel head and going to european spec injectors and fuel lines and I think you've got a real sweet package.
If you do decide to go twin plug then I vote for the electonic crank fire systems. I'm not all that keen on the electromotive but there are others as well as a system under development by Permatune that's programable from a palm pilot at the track! You'll find a lot of resistance to these newer system from many p-car mechanics because Porsche didn't design it. I understand where they are coming from and even agree, up to a point. Electronics, done right, beat out mechanical every time IMHO. For me electonic crank fire ignitions and electronic boost controllers are great addition to the 930 package.
07-20-2001, 06:53 PM
#3
RL Technical Advisor
Hi:
FWIW,....Rob and I had a long phone conversation late last night on this subject. My fingers do not have sufficient endurance to discuss this subject in the detail required to cover this properly; especially in his particular case.
The subject is very near and dear to my heart as I've seen far too many engines, Turbo or otherwise, that have suffered broken rings and worse, unbeknownst to their attentive owners, from detonation. To that end, I've posted a Technical Column on our website on this subject that I updated this morning.
Its not the last word by any means and just touches on this very complex subject but for anyone who wishes to learn more, I'd be happy to share the benefit of our 26+ year experience with racing & street 911/Turbo/935 motors and dual ignition if they care to call.
I take no pleasure nor satisfaction in repairing engines damaged from detonation and I see this as totally preventable situation. In most cases, its cheaper to buy the proper hardware, than to buy new Mahle P/C sets. I would much rather spend time making an engine more powerful or more reliable, then repairing damage that was unnecessary.
You may see this article by looking at Detonation, Gasoline, and Twin Ignition
FWIW,....Rob and I had a long phone conversation late last night on this subject. My fingers do not have sufficient endurance to discuss this subject in the detail required to cover this properly; especially in his particular case.
The subject is very near and dear to my heart as I've seen far too many engines, Turbo or otherwise, that have suffered broken rings and worse, unbeknownst to their attentive owners, from detonation. To that end, I've posted a Technical Column on our website on this subject that I updated this morning.
Its not the last word by any means and just touches on this very complex subject but for anyone who wishes to learn more, I'd be happy to share the benefit of our 26+ year experience with racing & street 911/Turbo/935 motors and dual ignition if they care to call.
I take no pleasure nor satisfaction in repairing engines damaged from detonation and I see this as totally preventable situation. In most cases, its cheaper to buy the proper hardware, than to buy new Mahle P/C sets. I would much rather spend time making an engine more powerful or more reliable, then repairing damage that was unnecessary.
You may see this article by looking at Detonation, Gasoline, and Twin Ignition