Jean

Interesting discussion. I have been dreaming gear ratios for a week now, therefore it is very top of mind for me

Maybe I missed it from my reading above, but does the Turbo R keep the same gear ratios as stock TT? I know the CTR does not, it has taller gears across 1st. to 6th. and this is a MAJOR driving force behind its fast acceleration.

Many experts agree that twin plug is not needed or really useful, you will find as many different opinions as there are experts. It is not a simple thing to do with a Motronic, rather quite difficult, and even the best tuners are hesitant to take you there unless you are really in need. Twin plug is more important with a Tag, or Motec and similar programmable aftermarket ECUs, and is great insurance since they have no knock sensors, however on Motronic, it might not really be needed. Remeber that twin plug allows you to play with about 5 degrees less of advance, which is a good insurance policy against detonation for the same output.

Bill, thanks for the picture, they do not drop the engine to do the conversion?

LAT

FYI, short article I found that describes the single/dual issue.

"The potential benefits of dual spark plugs depends upon the configuration of the pistons and combustion chambers. Engines with high-dome pistons respond well to dual plugs because, near TDC, the piston dome tends to separate the mixture charge, part of which, may not ignite at the proper time from a single plug.

The optimal settings for ignition timing and advance curves are passionate subjects among tuners. Due to the multitude of variables involved, dyno-testing is the most effective method of determining ideal timing and advance curves for a particular engine. The word "curve" is inexact and not actually a smooth curve, but rather a stepped slope comprised of several sequential timing increments".

Woodster

60-130 mph!!! let's post some numbers (like bill s. suggested)
stockers, hybrids, viper bob specials, ruf's, etc. please show us some
numbers!!

Kevin

As long as we are stuck with lower grade fuel, timing is being pulled. The only way to maximize this for the US is to twin plug our heads.. So that timing can be increased.. Either that or you can run 100 octane..

Bill S.

That's correct. All work can be done by removing the rear bumper.

JJayB

"However, he has a very developed CTR2, as JJB mentioned. So, I'm still not sure how much dual-plug helps with 91 octane. JJB, any thoughts?"

Bill,

We didn't run the twin plugs on the dyno at over .9 bar with 91 octane. Why gamble a $30k motor to see how hard it runs on 91 octane. Regardless of what fuel the twin plug produced a minimum of 30 lbs ft. of torque on the 3.6. On the 3.8 it was substantially more, a lot more.

The proof is the euro GT2's that raced in the FIA events killed the Vipers before they were restricted especially on the shorter circuits. However, on the long high speed tracks it was a different story. Its
not the horsepower, its the torque, and thats why RUF has such a good package. Other tuners like Andial, understand its useable power, not just a number.

Jean,

FYI, not all RUF CTR2's run the same gear boxes. The guys that track the cars run lower gear set than the factory twin turbo's. Who knows what gearing the magazines used to get there numbers.

As soon as it stops raining in SoCal, I run some 60-130 on my favorite toll road.

Bill S.

Jim, looking forward to your 60-130 time. From what Amir has told me, those Andial 3.8s are rockets.

P.S. A speedo video with sound would be awesome!

Geoffrey

The hemi head design of the Porsche engine places the spark plug on one side of the head. This means that as the spark plug ignites the mixture, it does not do propogate uniformly. The flame front propagates at an average speed of about 35-50 cm/sec in a gasoline engine and pressure builds at an average rate of about 20-30psi per crankshaft degree and depending on the engine characteristics it needs peak pressure at about 12-14 degrees ATDC. Larger bores require more time for combustion to completely occur, having a spark plug at one end requires more tiem to completely ignite the combustion mixture. THIS is why single plug Porsche engines require more timing. Additionally, if the piston has a dome, the dome can partially block the flame front which is why Porsche went with twin plug on the N/A engines. If you think that twin plug does not provide benefits and allow for more performance then you don't understand the fundamentals of how engines work. It is true that if the engine was tuned on 91 octane and timing maximized for that fuel, then running fuel with higher octane will not increase power (with the exception of the difference in the specific gravity and other characteristics of the fuel). However, with higher octane, you can advance the timing and make more power. Since a single plug engine REQURES more timing than a twin plug, it is more prone to detonation and you CANNOT run as much timing as you need. Twin plug in the Porsche engine is much better. The 996 engine has a pent roof 4 valve head design the places the spark plug in the center of the combustion chamber, so the flame front propagates at a uniform rate within the chamber.

This is not to say that you cannot achieve reasonable performance with single plug, take RUF for example. The largest issue with desigining twin plug for the 993 is the ignition system. Depending on if you choose distributor, waste spark coils, or direct igntion, inductive or CDI, you can end up with a large amount of money invested.

LAT

Geoff
does Porsche not place the spark plug on the squish area of the piston, if so would this not be the area of greatest pressure and thus provide the best ignition.
Have you had the opportunity to built a twin plug motor then test it and then retest the same motor with all else identiacl but with a single plug set up. All timing issues adjusted to maximize performance, if so, I doubt you find the differences you claim.
I have done this and so have others with Harley Davidson engines, these are basic pushrod twin cylinder air cooled motors which we rev to about 6,500 rpm. After much testing of single plug versus dual, only when compression ratios were raised to above 12.5:1 did it prove usefull and then only to prevent detonation with poor gasoline. We are able to get just over 70 HP/Litre and 70 FT LBS/LT of reliealbe long engine life power. All power mearsured at the rear wheelson a Dynojet dyno. This is better than a 3.6 normally aspirated engine on a Dynojet Dyno.
I guess I come from the school of combustion is combustion and to try and get 110% combustiion is just not possible, a little smoke and mirror perhaps.
Niced argument though, you could get some believers.
No flames about the Harley I used this example because I am intimately familiar with the motores and have built and tested many. I have over 200 Dyno runs on Harley engins ove the past 8 years.

Regards

Geoffrey

The difference between a Harley head and Porsche head is that Harley uses a squish area to push the combustion mixture towards the spark plug. The Porsche head is uniform with no squish area. The pistons likewise are different with the turbo Porsche being for all purposes a flat top piston (it does have a very slight dome), but again it is uniform. So, in the case of a Harley, the head and piston design overcome some of the issues of an off center single plug engine. I gather then in your work with Porsche engines, you've modified the heads or pistons to provide for a squish area similar to your work with Harleys? How did you make out?

While I've not had an opportunity to run back-to-back tests on the same engine, I have done over 50 turbo Porsche engines with MoTeC engine management and have enough data to support my statements. This is both from the ignition maps I've developed in the software as well the dyno results.

FWIW, I can get reliable 100-105hp/liter with a N/A Porsche engine

Geoffrey

Jean,

Regarding the issues with knock sensors. OEMs use knock sensors primarily as a guard against the wide difference in fuels out there. This allows them to tune the engine for a specific fuel, usually a lower grade fuel, and then protect themselves from the deviations. Knock sensors do not gaurd against detonation, they detect detonation occuring and will begin to alter the engine timing until it stops. It is never a good idea for engines to detonate. When installing aftermarket ECUs like you mention, the ignition timing is optimized for peak torque on the dyno for a specific fuel. As long as that fuel or a higher grade fuel is used and the programming accomodates temperature changes, the engine will not detonate. You do not need knock sensors to have a reliable, powerful, highly optimized engine.

AdamT

I timed the CTR from 60mph to 130mph and i could get between 6.5 and 6.7 each time.

Dont know what the R turbo gets as i have lost the video of its speedo to 190mph

all the best
adam

Jean

Geoffrey

Thanks for the great explanation. I was aware that knock sensors detect detonation and immediately pull engine timing to avoid it and that is what I meant with prevention, only you use much better words

When I was thinking about the Motec route for my 993TT that was one of the things that made me think about it twice (Stephen K. and you, I believe, mentioned it to me) as there are no good Motec tuners here. I am a big fan of aftermarket ECUs, and believe that you can get more optimized tuning if you find an expert...I would use them anyday if I lived in the right country.

JJB, I meant a stock CTR2 gearing vs stock 993TT ratios. The difference in acceleration is very considerable all else being equal. I have a hard time believing those numbers driven by twin plugging. If you say you witnessed them it must be true. Don't get me wrong, I am going 3.8, Motronic and Twin Plug too, only my tuner does not think I will get anywhere close these numbers because of twin plug.

Twin Plug is beneficial for higher compression engines requiring better combustion under different octane levels and quality of gas, which is why Porsche used them on the Carrera and because of piston design and flame distance travel. On a TT is is usually said that below 570-600 hp, you do not really benefit, it is just "good insurance". That is what I heard from different sources.

Adam, those 60-130 are sick fast.

Rassel

Oooh, another thread I must follow!
This is why I prefer rennlist instead of "Check out my new bling-bling Wonderbaum"-forums.

Bill S.

Adam, that is very impressive! Do you know the octane you were using and if your car is single or dual plug?

Also, how much does the CTR weigh?

I'd love to see the speedo video if you have it.

Best regards,

Bill