seege

Thanks guys.

nman413

For another data point, I'm getting misfires with my car using Iridium plugs. Not sure what the problem is yet. I'm planning to switch to OEM FR6 plugs when I get a chance to do so.

K24madness

FWIW I got brand new OEM plugs and still have the misfire.

Geoffrey

The issue with misfiring has much to do with the factory ignition system. The factory system is an inductive style ignition which is not necessarily a bad thing. However, this particular coil require about 3-3.1ms of charge time to fully charge. This is fine up to about 6500 where the coil would be operating at 100% duty cycle. At 6500 it takes 9.2ms to make 1 crankshaft revolution and there are 3 ignition events which means that with a coil dwell time of 3ms there is only .2ms where the coil is not being charged. This is very hard on the coil and will eventually burn it out. So the ECU has been programmed to reduce the dwell time as the engine RPM goes up. What this means is that the coil is not being fully charged and the spark energy is less than it would be with a fully charged coil. Now, cylinder pressure is highest at peak torque and in the 993tt, the coils are being fully charged at peak torque of about 4500rpm or so. As the car falls off on torque the cylinder pressure also is reduced and you MAY not need the full spark energy to properly light the mixture. This is what appears to be occuring in a stock 993tt. Now, increase the boost pressure and you increase the cylinder pressure and you may be exceeding the limitiations of the stock ignitions sytem. I would first try to reduce the gap on the plug. However, recognize that reducing the gap means the flame kernl will also be smaller and the combustion process will take longer (and require more timing) but I think in this case it is minimal.

The Crane ignition system is a CDI and I would not run it with an inductive coil, but would rather see it used with a proper CDI coil that has a low resistance value.

Jean

K24,

I don't know what the state of tune of your car is, but judging from the boost you are putting, you must be seeing some very decent numbers. I don't think the misfire comes from the spark plugs as you would see that happening across the RPM range normally. Most likely it is your ignition and Geoffrey's explanation says it all. On the other hand you might need to try NGK spark plugs rather than Bosch for high performance, since you can find better heat ranges to suit your engine at those hp levels. You might need one heat range lower if you are seeing 500hp+.. Iridium is a good choice too, great spark for high performance but not long lasting.

Good luck.

K24madness

Thanks Geoffrey... Your knowledge is well documented. The coil I am using comes with the CDI kit so I assume its the proper one. I also call Crane to varify this and they agreed.

We must have the same background. My next action was to reduce the plug temp one step and reduce the plug gap by .005, this is what I have done on high compression situations in the past with good results.

Todds expertise is also unquestioned. The Crane HI-6 was his recomendation. He has had very good success with this setup. Considering how expensive everything is for this car this was a minor investment. I will keep you posted.

Jean the state of tune on my car is top notch. It has 20,000 miles on it with updated hoses etc. I have also had the injectors cleaned and checked. We also checked the FPR to make sure it was set at 5 bar. Problem was there before and after plug change.

It is definity a spark problem. It occurs at peak torque RPM in higher gears. Then clears up somewhat to redline.

FWIW I put down 461whp & 474 tq on a 2wd dyno jet at 1.1 bar Sunoco 100 oct fuel. I did not have the problem till I turned up the boost to 1.2-1.3 bar and added VP 103 fuel.

Geoffrey

K24madness. "reduce gap by .050"? That must be a misprint as you probably need somewhere around .020" - .025" total plug gap.

K24madness

Opps my bad.... I fixed it. I meant to say I will run 25-30 thousands

Geoffrey

FWIW, the Crane LX-92 is the coil (2 of them) I will probably run on my new RSR engine along with a pair of M&W single channel CDI units capable of 700hz.

K24madness

Could not find the difference between the LX-92 and PS-92 on Cranes web site.

nman413

Have you gotten your car dynoed on a 4WD dyno?

K24madness

I am waiting for Jason at streetwerks to install his. He is in Palo Alto on Lambert street. It will be a 4 wheel mustang dyno. This is the same one as GIAC. It has a solid link front and rear so it does not stress the center diff. because the front and rear rollers turn at the same speed.

Jason and I go back a ways. I will not only be able to just do runs but I will have the freedom to do some tunning.

I have been using the local 2wheel dyno jet for a base run to check fueling and to compair the other porsche runs they have had on hand. FWIW this type of dyno cannot be compaired to Vishnus. Shivs dyno dynamics dyno will read much lower. After my tunning is complete I will do a pull over at Vishnu to see how it compaired to my previous Audi S4.

355 whp and 360wtq on shivs dyno with my Audi S4

http://pictureposter.audiworld.com/20878/355hp.jpg

Cut and paste URL to a new web page.

Not bad for a 2.7 ltr. Imagine what I will do with this car! I have not disclosed all of my current mods that account for the high HP output I am getting.

K24madness

UPDATE to ALL

I did the install of the crane CD ignition

This has cured my misfire. I have not had a chance to change the plugs to the Denso IK-22 yet. I am currenty running the stock plugs (new).

There have been lots of folks that have struggled with misfires on the 993tt. I suggest that if you have stock ignition system that you install the Denso IK 20's and reduce the gap to .030 or if you push the limits with larger turbos the CD ignition upgrade is a good idea.

This type of install is best performed by a stereo installer.

Geoffrey

With the CDI, you can now probably INCREASE your plug gap. This will create a larger flame kernel which will help improve the speed of flame propagation.

K24madness

I agree with you on that Geoffrey. The problem is to do it properly you will need to do back to back dyno runs to measure the difference. Since the 993tt is a PITA to get to the plugs I will set them at .035 and be done with it.

It will be interesting to see how the CDI affects the tunning. I think the reason some of our programs run on the rich side is because it it easier to ignite than a slightly leaner mixture. I will do some dyno runs and play with the fuel ratios. I always err on the side of caution and go slightly richer to help cool things but some of the 10:1 fuel ratios I have seen seem excessive.