This page has been reproduced from Fritz McKellar's site with his permission.

Non-Sequential Conversion [by Fritz McKellar]

Update: Having now driven non-sequential for several weeks, I can say that I will never go back to sequential! The amount of torque available from 3000 RPM up is refreshing as is the smooth delivery of power. For street purposes, I find the factory gearing is short enough that no turbos on the first gear is still adequate (I have actually done this by capping off the turbos when one was blown). For performance purposes, its just a matter of selecting the proper gear. On the highway, travelling 60 MPH puts the power band right near the non sequential's benefit of torque gain. With the S-AFC as means of recording, I have seen positive pressure as low as 1900 RPM, significant boost by 3000 RPM, and peak boost before 4500 RPM. All this is done with a gutted-out precat and the factory midpipe. Updates to follow.

Having driven the car for many years without any problems in sequential mode, why would I consider non-sequential?

1. I was quite weary of the power delivery, and found myself letting off in a corner at the track when the turbo transition was very close (learned to listen for the hissing sound).
2. Intimidation on solving power delivery problem because of the high dependency on vacuum lines and solenoids.
3. Reliability and power issues. Having looked at the stock factory manifold, I don't know how our cars can make the power that they do!

            What convinced me for good that I would be happy with the non-sequential was when I did an analysis of three dyno runs: Images of these runs can be viewed at the end of this section (single turbo was also included for kicks). The three runs were Brian Goble with a non-sequential setup, Brad Barber on a sequential setup, and Brooks on a sequential setup as well. All three runs were approximately in the 12 psi range with similar engine modifications which made the comparison even more interesting. I really didn't care about the peak numbers (though Brad's were impressive), and was more interested in the 3000-5000 RPM behavior which should shed some light on daily driveability. Consider the table below which contains data I extracted from the three runs (all numbers are approximate and were obtained by graphing out the charts).

    Basically Brian's got some really sweet torque numbers. His chart has got over 250 foot pounds of torque from 4000 RPM to 6600 RPM in a flat, predictable nature. In horsepower and torque numbers, Brian plays catch-up with non-sequential until about 3500 RPM when his turbos start to really contribute, and the flow of the second turbo ends up benefiting his numbers in an impressive fashion. Note that Brian's boost start to effectively build at around 2650 RPM (and that's with a cat) which is really not too bad (internet research indicates approx. 5 psi at 3000 RPM and full boost by 5000 RPM). I figure that the removal of the cat (increased flow will be addressed with an external wastegate) in a non-sequential setup will move the numbers up and to the left somewhat. To follow the conversion process, select my Exhaust Project link.

Brian     Brad     Brooks     Single turbo