dbf73

Here's a related question - are you better off running 93 octane which has 10% ethanol or 92 octane w/o ethanol? My guess (and I'll happily defer to Kevin, Steve W and the other boffins in this area) is that with the factory ECU, there's enough headroom to use the 92 and you'll get more power because you won't have any of the lower energy density ethanol. If you have a replacement/tuned ECU that is programmed for 93, this might be a closer call - do you let the ECU pull back boost/timing just a bit and still get more hp because of the energy density?

ttAmerica RoadsterAWD

Kevin, so to "fix" the limitations of the 993tt in order to use 1.2 bar boost would be to change out rods/bolts to, say, Pauter rods with raceware bolts/nuts? What about the pistons?

Typically, what is done to the engine if one were contemplating high boost performance with some reliability (assuming no abuse of course)?

Jaime

jrgordonsenior

I'm sorry, that cracked me up. Made me think of the old adage:
"Fast, cheap, and reliable. Choose two."

Air Kuul TT

JRG,

I was just there (The Lodge) yesterday for a meeting.... I didn't want to leave. What a beautiful resort.... Have a great weekend!

Kevin

tt, I failed to mention that one needs to install a sealing ring system, like a nirosist flame ring to lock the cylinder head and cylinder liner together. Most of the racing 993GT2 team actually welded the lower fin of the cylinder head, this practice started with the high boost 935's. One can check out another way to seal the head to the cylinder on this thread.. https://rennlist.com/forums/911-turbo-930-forum/192483-building-new-engine-cylinder-head-diy.html
A sealing ring spans both of the surfaces.. However, for high boost applications I'd run a flame ring setup. Another point that needs to be mentioned, running high boost in an air cooled engine needs the best octane that you can dump in the tank. High cylinder temps and advanced timing can break all of your ring land in less than 15 seconds.. I'd also add twin plugging to the engine build..

The 993TT has very good pistons..

TB993tt

An excellent post and one which will doubtlessly be ignored by many chasing the boost "numbers" holy grail !
I would ask people to consider the 993GT2 racers which rarely used more than 1.2bar at peak power to produce their 620odd hp and they all used the ultra efficient Secan Porsche Motorsports intercooler to control the intake temp generated at these power/boost levels - and they had to stick to rigorous and expensive hours based maintainance program.

Steve Weiner-Rennsport Systems

Hi Kevin:

Kind of an FYI,......People racing 993 GT-2 cars as well as high compression 993 RSR's using 100mm & 102mm P/C's all welded the bottom 2 fins of the heads spanning the distance between the top & bottom head studs for stiffness. This helps prevent the head from distorting on that span and allowing cylinder pressure leakage and burn through (GT-2 only) these big bore engines are subject to.

935's used 92mm, 95mm and 98mm bores and didn't use nor need that. The 935 engine used special alloy flame rings and special head studs to hold everything together. Even so, the 935 motors would not last long (20-24 hrs) at 1.4 bar and above and these contained some very very special parts. Normally, we raced at 1.2 bar and concentrated at improving airflow in & out of the engine for power gains. Now, we see 735 HP at 1.0 bar with 3.2 litres.

I was fortunate to be an integral part of that scene for many years beginning in '76 to '83 with a number of these cars in several (2.8, 3.0 & 3.2) configurations.

I'd agree though, any high boost, air-cooled Porsche motor needs flame rings to contain those kinds of cylinder pressures. Even with our flat fans that provided FAR better cylinder and head cooling over the vertical fans, the real limit of boost pressures were and still are, the mechanical attachment between those two components and cylinder head temps which compromise efficiency and HP.

Having been at this for some time now, I thought I'd add some input here as I've seen a LOT of blown and melted Turbo engines and the consequences of high boost in these things,....

wini

Kevin,Steve thanks for your detailed replys this has made things a lot clearer for me.Does this mean the 996tt Engine has more tuning potential on a standard engine being water cooled and 4 valves per cylinder & variocam timing? On a road odd track day 993tt does this mean at 1bar boost that it's just a matter of time before getting cylinder/head leakage? how many hours very rough estimate would you say a fast road 993tt standard engine with 1bar boost would last before a pulldown for resealing?

Jean

Steve, great insight and it is great to see you on this board discussing hp
I just have a question re your comment above
This kind of hp is equivalent to 400 hp approx at absolute pressure, which means that the engine is more efficient than a 12-13% larger displacement 3.6 Ltr GT3 RSR, while running lower compression and RPMs, is this really possible?
Even with 4 valves and watercooling it seems quite challenging..

It would be very interesting if you could share more (even in private if you wish).. I was only able to get 705 at slightly more than 1 Bar hp from a twin plug 3.8 ltr.with quite a good build.

Outstanding! Thanks.

Steve Weiner-Rennsport Systems

Hi:

The 996 TT engine has several advantage over its air-cooled relative; water cooling (MUCH greater control over cylinder head temps), much stiffer and more rigid head and cylinder assemblies that prevents the distortion that air-cooled engines get, to name a few. Its far better suited to high boost in this regard. Porsche learned that lesson way back when they started building the 956/962 cars.

At 1.0 bar a 993 TT engine (running good gas) suffers no real long-term harm as long as the fuel mixtures are correct and oil temps are controlled with two oil coolers at the front of the car. One would expect to replace the normal amount of parts at overhaul time,....

With good oil change intervals (3-4K), provided all of the above conditions are met, a 993 TT motor should run well over 60K-70K before needing anything aside from valve guides which are the bane of all these engines. Of course usage (street or track) helps determine ultimate engine life.

Steve Weiner-Rennsport Systems

Hi Jean:

I'm always happy to offer assistance and the benefits of some long-term experience as time permits,....

I'm not sure any comparisons between a contemporary engine such as the GT-3 and a 935 are fair; those 4-valve heads are far superior to the average 935 head and are more efficient over a wider RPM range than the 935 stuff is. At absolute (atmospheric) pressures, the GT-3 family of engines including the racing variants, are truly amazing with anywhere from 370 to 550 HP from 3.6 litres without forced induction. Those engines are quite complex in their gas thermodynamics and much more advanced than the older 2-valve turbocharged race cars.

The "secret" to achieving big power at modest boost levels are a combination of many things including:

1) LOTS of time spent on the flowbench with the heads and some custom valves.

2) Lots of hand and machine work in the case, crank, cylinders to reduce internal pumping losses.

3) Custom cams of our own design.

4) Optimized and modified intake and exhaust systems.

5) In some cases, longer rod engines were built to improve cylinder filling. These are VERY time consuming to build with lots of custom parts and components required for assembly.

6) Lots of time spent on our engine dyno to find out what works and what doesn't,....

The rest of the "recipe" is simply very close & detailed attention to the entire assembly process.

I don't know if this answers your questions or not but its a simplified outline of the process required to configure & construct engines that make decent power without relying on the excessive boost pressures that really shorten component life.

K24madness

Steve if I understand your position correctly you look to achive a modest cylinder pressure over a broader RPM band than massive cylinder pressure over a very narrow RPM range.

I share the same beliefs that moderate torque carried over a broad rpm range is a happy motor!

Steve Weiner-Rennsport Systems

I look to achieve the desired power output with the least stress on the engine for maximum longevity & durability.

To that end, I spend a great deal of time improving airflow through the engine without relying on high manifold pressures. The effective RPM range will mostly be dictated by the choice of camshafts, turbocharger A/R ratios, static compression ratio, and of course, displacement.

I strive for "area under the curve" instead of peak HP at high RPM.

Jean

Steve,

Thanks for your detailed reply.. I have to question some numbers however, please forgive me if I am sounding like a real pain, I am trying to understand where I went wrong after spending so much money on my engines, and this being the 993 Tech forum, I cannot have a better source, especially when the explanations are coming from you. (since we got your attention, we will monopolize you )

Some facts: (given to me by a top notch racing tech person, and Rennlister)

The best European 996 GT3 RSRs have an output of 420 hp (they can go slightly more but not reliably), and these are fully prepped racing engines being rebuilt every race or two. The only GT3s capable of going beyond that, have bigger displacement, such as the Koro 4.6 Ltr ( not built by Koro) that is the fastest Porsche on the track today, eclipsing the all mighty Manthey Racing 996TT, fastest Porsche on the Nurburgring. Are US GT3s doing 500 hp+?

My discussion is not about GT3s however, I just used the 996 GT3RSR as a comparison, since, I you said, these are highly efficient engines with the ultimate in head tech work.

My point was that if a 2005 GT3 RSR, with its efficiency, can only yield around 400 hp at absoute pressure, at high compression ratios and RPMs, how can a 3.2 turbo engine yield the same, with much lower compression, lower RPMs and 2 valves per cylinder, air cooled etc..? To get 735 hp at 1 Bar, you have to be close to 400 hp at atmospheric pressure levels.

I spent a fortune on my engine, with all the mods you mentioned for case air flow, work done on the heads was top notch, from air flows to valves, amazing headers and almost free flow exhaust, etc.., I have pressure sensing, not MAF, with intakes as short and efficient as they come, 3.8 ltr Mahle pistons, twin plug, very special camshafts, etc..and I get 705 hp at 1.1 Bar (on an engine dyno as well) whereas a 3.2 turbo engine gets more?.. What did I do wrong? I am not discussing turbos, since I am focusing on efficiency at atmospheric pressure.

As a reference point, I know that the BEST 993GT2 racing EVOs, pressure sensing and 3.8 ltrs, used to yield 640 hp at 0.8 bar.

My engine was built with the same objectives in mind that you discussed, since longevity was the key deliverable, that's why it was expensive. I have I believe a great area under the curve..I can run 1.5 Bars+ of boost with my build if I wanted since the heads are welded, niro rings etc.. (as you mentioned earlier) but we max'd it out at 1 Bar and it will run at 0.8 bar 90% of the times. I could not care less about peak hp

Your insight is highly appreciated. Thanks for your time.
Jean

Kevin

The "Race" engines of the the past, were very high strung and had to be rebuilt often. The size of the turbochargers were the size of basketballs. They were based on low pressure, high flow.. They also didn't make power until 5500RPM (usable power).. The power was like a light switch.. Even up to the race GT1 it strapped on 2 K27's, with very large turbine wheels.. There wasn't power on tap below 5K.. You can't have "the best of both World's" I was talking to a friend in Switzerland, and he basically said that in the US we want torque.. In Europe and the ROW, they will sacrifice the torque to gain the top end HP. Basically it is 2 different tuning applications, and hardware setups.. Todd and most of us today could give you another 100-150 HP for your engine, but would you want to rebuild it every 25-30 hours.. You make the comparison to the RSR engine, which everything is basically Titanium, you'd have to penny up another 20K to build your exact engine using lightweight racing components, from TI rods, to TI valves and TI springs, and TI rockers, the list goes on.. Like CNC'd gun drilled camshafts..