wicks

Have read that lag is party caused by the control system of the CIS rather than the general reasons for lag, and something to do with pressure inside the intake. Wondering if there are alterations to said control that people have done to improve this, or that P prescribed earlier in their lives that my car may not have had.

Interested in anything apart from serious parts modifications (exhaust, turbo, etc.)

urquattro20Vt

Quickest, easiest, cheapest way to reduce lag is do a "cat bypass" (Fabspeed, B&B, etc) - a straight pipe that replaces the catalytic converter. Easy swap. That reduced lag by a small amount, and improves sound, too.
I'm not aware of anything (pretty sure there isn't anything) you can do with CIS to improve this....Other than make sure you are running optimal AFRs via an adjustable WUR.

P.S. can improve throttle response and urgency with aftermarket ignition - but that's not cheap or easy.
Brandon

wicks

What about the sport cat from cargraphic - does it really make an improvement similar to cat delete? In California so removing/reinstalling cats doesn't want to be in my annual schedule...

bweSteve

just a sidebar sidenote....

How did you come to the conclusion that turbo lag is a bad thing? ... If it comes from your experience, and you don't like it,... then I get it. I get the desire to reduce it.

But for me,... it is part of the charm of the car. I have other NA cars (like many do here too), that give me the instant torque gratification,... & yes I do enjoy that.
But I also really enjoy the 1991 Porsche Technology,... all of it,... and everything that comes with it.

So I embrace the lag. It's part of the fun of driving it on twisty roads. It really makes you work to control the power band.

And strange as it may seem,.... some of that enjoyment comes from my background in motocross from the 1970's. I had several late 70's 2-stroke RM125's (Suzuki), and YZ250's (Yamaha),... and back then it was called "STAYING ON THE PIPE". It was very difficult to race those bikes back then, because the power-bands were sooo short, and so we were constantly shifting gears so you didn't "Bog Down". It was a blast. Raced all the way into the early 90's. All 2-stroke. My brother still has a mint condition 1993 Honda CR250 that was "the" choice by Jeremy McGrath when he won many National outdoor and supercross championships. It was the last kick-a$$ 2-stroke bike that we ever owned & raced (I had one too). The power band was much larger after decades of improvements. .... anyway I digress.

Just my .02. I love the power delivery on these cars just the way they are.

..... but then again, I have no Cats on my car either, and a aftermarket GHL exhaust.... and a 1 bar wastegate spring.
=Steve

wicks

Generally I feel the same Steve, but I was out with a GT3 in Malibu last weekend so... I had that CR250 back in the day as well, was truly amazing how wide the band was! Still have the DT400 which does pretty well on the band as well, but yes interesting comparison to T-lag!

I don't really want to change much, hence asking if there are smaller changes one can make to bring the boost into a wider band. I don't mind paying attention and shifting often during a spirited run, but when I am feeling lazy and just want to make a pass without downshifting first, it would be nice if the spin up was a little more available.

Curious if anyone knows about this CIS-is-to-blame concept regarding lag in our cars.

urquattro20Vt

CIS may be partly to blame (small portion is my guess) but largely its the actual turbocharger itself/technology that's the core of the....I won't say "problem"...but let's just say...lag. Newer ball bearing turbo and such can spool much faster. Then I'd say the headers/exhaust come next. Oh and we can't forget it's just a 2 valve head.

I don't have firsthand knowledge of the Cargraphic sport cat - I hear they make nice stuff - and a sport cat must flow better than the OE cat....so my simple logic tells me it will flow and sound better than factory, but not as well as the "bypass". But I guess we are trying to live in a greener world so I get where you are coming from - and have contemplated going to as sport cat myself. But I haven't. There isn;t a huge night/day difference between the bypass the factory - so going the middle ground might be a pretty subtle improvement. I don't know...

Brandon
'91 911 Turbo
'00 Audi S4 highly mod'd DD

urquattro20Vt

And there's the crux of the matter. We love the period technology and performance...if we can improve it a little :-)

Brandon
'91 911 Turbo
'00 Audi S4 highly mod'd DD

matt33

Tighten the gear ratios 2-5th. Amazing difference and can change back to stock if you ever needed to.

If if you are near San Francisco you could experience my setup (albeit 3.3)

Matty

bweSteve

Yep, you guys nailed it. Brandon highlighted my crux, & he's right. We love the period,.. tech from the day,... but we all love to tinker,.. put our own finger prints on it.

So it really boils down to how far you are willing to go (to be happy with your own creation / improvements). Make it your own.

Seems the only governing rules we all live by,.. is to ensure it can revert back to stock original. Check.

I've hit the easy ones w/ mine. But have not gone down the route of adjustable WUR, or any CIS mods. Although I did spend time & $$ on Dyno runs to ensure my AFR was spot on through the rev range. I was running rich in the early revs just before the Turbo was to spool up,.. so leaning that out a bit made a BIG difference in how quickly mine spun up. That was well worth the $600 I spent with the Dyno that day.

So the only "other" thing I've considered but haven't pulled the trigger on,.. is a maybe a better turbo. But that just felt like I would be traveling down the slippery slope.

=Steve

Vince964T

I had the cargraphicts 100cell cat on mine and it made a significant difference with stock K27
Pair it with short headers and it's even better
Also make sure your blow off valve works correctly. If not replace it you will see an improvement to.
That's the easy stuff

Metal Guru

Remember, if you have no lag, you have no turbo...
Having said that, CIS in of itself doesn't cause lag. It restricts mass flow into the engine and isn't very linear in fuelling.
Lag is a function of turbo inertia and exhaust system design.
Theoretically, a straight pipe from the exhaust ports to the turbo would be the best system but the exhaust has to be packaged into the car so that's out. What is desired is a system with the biggest bends as possible and the fewest number of them.
The stock heat exchangers, j-pipe, cat and suitcase muffler are the worst possible system for turbo lag. The exhaust gasses change direction 3 times before it reaches the turbo and three times on it's way out of the tailpipe.
Headers are the best for reducing lag. A free-flowing muffler is also helpful as restriction behind the turbine impeller should be kept as low as possible.
The turbo unit itself (especially the turbine impeller; it's made of Inconel and is heavy) has to be a compromise between low inertia, low back pressure and good mass flow. Too small and while it will spool up fast, it will run out of steam quickly. Back pressure will be high also. Too big and hits too hard and at too high of an engine speed.
The factory did a decent job of picking a turbo that has reasonably low inertia but could still deliver mass up to 5000 rpm. When I switched over to a K27 hybrid turbo with a K29 compressor, I didn't really notice and increase in lag. I didn't expect to as the compressor exducer is made from aluminum.
Sorry Wicks, when you fight physics there is usually $$$ and effort needed.

cobalt

Agree with what Paul says. I have the B&B headers which are just OK and not the best build quality the HF K27 1 bar spring, billet BOV, modified air box, magnaflow muffler sans cat I see full boost at 2600 rpm's but there is still lag in 1st gear. Once there is air in the turbo it is a rocket. Power builds to the point that I can spin my rear tires at 5000 rpm in the first three gears under WOT. The 996GT2's were very similar in feel and although far less lag had limited power until the turbos spooled in first. It is the nature of a low compression engine with single larger turbo to see lag. My twin turbo low boost 993 engine with 11.5 compression runs very much like a N/A engine with all boost in the upper rpm range. Although there is slight lag until 3000 rpms it is nothing compared to these cars and far more linear much more like the newer turbo cars. I am currently installing a Haltec on that car and will be doing some dyno tuning this week which should improve things even more.

One more thing when on spirited drives I usually drop down one gear and keep the rpms up and drive around 4000 rpms making for instant power

There's no Sub!

Metal Guru

I guess you have to decide how much do you want to spend. There are many possibilities at a lot of price points.
From what I've read over on the Pelicaparts board (where there a lot of hot -rodders building stuff), people seem to gravitate to a similar configuration:
* 7.5- 8 compression ratio (stock cr is 7/1)
* Early SC heads or ported heads (something bigger)
* Hotter cams (usually GT2 but there are other variants)
* Twin plugs (gets the flame kernel burning from two directions so it works better with the high compression ratio; less chance of pre-ignition)
* EFI
* KKK K27/K29 or the 7006 or Garrett turbo
* Custom headers
* Dual wastegates
Such an engine would be well in excess to what you are looking to get at the rear wheels. It might be well in excess of your budget too
I don't recall if anyone really increased displacement; it really isn't necessary with this kind of build unless you want to run the Texas Mile You might want to discuss it with Chris at Turbokraft.
When you say you want more useable power, where do you want it to be, at the low end or top end?

I went with SC cams, lightweight flywheel and the K27/K29 turbo and headers. I'm happy with that setup; it's not on the edge of reliability like the build I referenced above. The car is a lot better at low engine speed, which makes it fun to drive.

There's no Sub!