epbrown01

Hold on: everything I've read says this was a stroke of genius because it cools the air for the turbo more and prevents heat soak, which leads to boost fading over the long run because the turbo grows more and more ineffecient as the air gets hotter. The "solutions" I'm seeing posted here, aren't they better for drag racing rather than the racing Porsche had in mind? Great boost for a few take-offs, rather than good boost all day long?

This is all book larnin' rather than any personal expertise, so let me know if I'm missing something.

Emanuel

Sam Lin

I think you've confused the turbine and compressor side of the turbo. You want the exhaust going into the turbine side as short as possible and you do not want any heat lost before the turbine, so there's max energy turning that turbine. On the compressor side you do want to cool the turbo air as much as possible for max compression and high efficiency.

Sam

mumzer

its a stroke of idiocy combined with the necessity of bad packaging...the turbo uses the kinetic and thermal energy of the exhaust to drive the turbine.

less heat in the exhaust creates fewer turns on the wheel...this is why we boil water to make steam to run electrical plants....either by burning oil or splitting atoms...cooling the exhaust gas before the turbo simply reduces efficiency and retards boost onset.

epbrown01

You're right, I did - now it makes sense.

Emanuel

Matt H

The answer is a complex twin turbo system with a very small primary turbo and a larger secondary. This answer however, is only good for people wanting stock like drive. Oh hell, this isnt the RX-7 forum :>)

Check out 3rd gen RX-7, that is EXACTLY why they tried the twin turbo setup. Wanna know what the first big mod on those is? Toss the twin turbo setup.
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Danno

The turbo lag is also intentionally programmed into the chips... 'By who?' is really the question here...

The other thing is the extremely low-compression for the fairly low boost in the stock car. In off boost, you've got a 8.0:1 NA engine, not good for torque or power. Compared to other cars with similar stock boost, the other turbo cars have much higher compression for more off-boost torque. That higher compression also generates more explosive exhaust to spool up the turbo as well.

BTW, I've measured a difference of 200-F degrees at the turbine housing before & after ceramic-coating the headers & crossover pipes.

hosrom_951

Turbo lag has good side. Max boost comes on at 3,000+rpm, so in the city, the car would use less fuel and oil 9 mentioned above). Thus, turbo lasts longer, todays cars have almost no lag coz of competition with acceleration between manufactures, plus i doubt their turbo's would last as ours since they are always being used.

Songzzz

hosrom_951, you can drive around at 3000rpm with NO boost at all.

Sach951

All the new cars that you speak of, for example the Eclipse and the Scoobies, have the engines mounted so that the exhaust outputs are facing forwards, and feed directly from the manifold into the turbo, which then routes into the intercooler which is but inches away. That in combination with a light turbo... no lag

Best Wishes,
Sach

Sam Lin

Sorry, but the Scooby exhausts downwards and then loops the plumbing to the side and up the back of the engine, the exhaust has a pretty long path before the turbo, about 3 feet for the right bank, similar to our cars. My post was pointing out that they do lag, not that they don't.

Sam

M758

On my stock turbo S there really is little lag in the upper RPM band. Below 3000 RPM there is simply no power. Step on it at 2500 and nothing happens. That in reality is not TURBO lag, but just no power in that band. NA engines also have little power at 2500 RPM. They get more power 3000+ rpm. The turbo engine is no differnet. In order to get power there must be enough exhaust gases present to spin the turbine. At low RPM there is not enough air pumping to get this done. Combine this with little off boost power and you get little hp and apparent lag. Really you are just waiting for the RPM's to pick up.

Size of turbo hot side also makes lag. Larger hot side with larger turbine takes more gases to spin all else being equal. So a small hot side means less lag. Problem is that it will hit the limit much sooner. Also there are issues of turbo friction and drag. More interal drag means more lag. Also there is turbo inertia. That is the combined mass of the turbine, compressor and shaft. Bigger turbo's have in general more mass. Newer technology can reduce the mass of these item for similar flow sizes. Don't forget also that turbo technology has been advancing. Turbo's of the same physical size can not spool faster due to less mass and are more efficient thus making better use of the existing gases. There are plenty of folks running Garrett GT ball bearing turbos. The thing spool quickly and can run at 20+ psi boost levels since they are more efficient (less air heating on compression). Sure length of tubing plays a role. Right side turbo's with shorter intercooler pluming would help. Lets not forget why Porsche put the turbo where it did. Bad experience with 924 Turbo longevity due to overheating caused Porsche to want to move the turbo to cooler place in the engine. They traded some lag and loss of efficicnty for turbo life.

Today automakers make all kinds of trades. For exaple my 2001 GTI with the 1.8T has very little turbo lag. In fact it makes power from 2000 RPM. Reason is a very small turbo that spool quicky. Drawback is that it seems to give out at about 4000 rpm. But they it is maxed out and the car gets its power from the 5 valve heads.

Seems to be the when VW did this engine they figured they would use the turbo to boost the low-mid range power of the car with a small turbo. Consider for comparison of a 1.8L Honda or Toyota motor. Little torque and power in the high RPM band. The 1.8T motor makes at lot more torque down low, but not much more (some cases less) in the upper rpm band.

944 Turbo cars have turbo's built to give power to upper mid range. Boost does fall off at higher RPM. The turbo S hot side was desigend to be a bit larger to keep power at higher RPM, but did increase lag some and raised the rpm of the torque peak.

Simply put turbo's are complex as it their intergration in to road cars. There are alot of things to consider and compromises to be made. Drag cars and road race cars are in some ways easier as there are less compromises.

Duke

Excellent response M758!

A couple of other factors that play a big role here, besides the plumbing issue, is:
* Non variable cam timing. Most of the newer cars have this and it plays a BIG role in low end torque for high power engines.

* Compression (as stated before). Our 8-valve heads simply aren't resistant enough for detonation to use higher compression with high boost. Many new turbo cars use around 9.5:1 and STILL uses around 1.0 bar of boost!

black944 turbo

Also the vain air meter causes a lot of lag. Get rid of that and pop a hotwire unit on it all other things the sam (stock chips and boost) and much of the lag goes away. I never thought that it would make much of a difference. But man it is great! Just thought I would ad that to the conversation.

Danno

" * Compression (as stated before). Our 8-valve heads simply aren't resistant enough for detonation to use higher compression with high boost. Many new turbo cars use around 9.5:1 and STILL uses around 1.0 bar of boost!"

Exactly! With more modern 4V heads with pent-roof combustion chambers and centralized spark plugs, detonation is controlled much, much better. Combusion is much faster and more efficient as well as evidenced by the optimal ignition values of only 1/2 the advance needed in the 8V heads. The 4V heads can easily run 1-point higher compression at the same boost to be on the edge of detonation, thus more power for the same boost.

Waterguy

That used to happen to me too. The night before last I installed wastegate shims in preparation for installing Guru chips; chips go in tonight. Suddenly I found that instead of no boost to 3,000 rpm with my stock K26-8, I was getting 2 or 3 psi at 2,500 rpm. Not a big difference, but enough to result in some power oversteer in first gear on wet streets because of my usual driving habits (lots of throttle to get the car moving at low rpm.) Lots of fun!

My conclusion is that the stock wastegate adds to the "turbo lag", at least on the K26-8 cars. Now I can hardly wait to see how the fuel and timing of the Guru chip effects the rest of the equation.