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Something I noticed about the new 997

6.8K views 54 replies 26 participants last post by  Slatfatf  
#1 ·
In reading through the specs I noticed that the bore and strokes of both the 3.6L and 3.8L engines are different from in the current generation 997's. The bores are larger the strokes are shorter but of course they come out to about the same size as before and get listed as 3.6 and 3.8L motors. I also noticed compression has gone way up from 11.3 to 12.5 I think it was. So this tells me that Porsche is making significants changes to its engines for the direct injection and that these changes will likely mean changes in the Cayman's engine as well. I believe shorter strokes mean the motors rev faster correct?
 
#2 · (Edited)
Shorter stroke can lead to higher revs if maximum piston speed was the limiting factor. If the limiting factor was valve float (or some other limitation) then the shorter stroke would not, in itself, allow higher revs, until the other limitations were addressed. But in general, you're correct. Short stroke engines tend to rev faster.

BTW - is the new model a 998?
 
#3 ·
Yes you are correct about the short stroke and faster revs. The bump in compression ratio is due to the cooling effect of DFI. The direct injection of fuel into the cylinders cools the pistons and the combustion chambers and that allows the higher compression ratio without detonation (ping). Gary
 
#7 ·
I would really like a Carrera S with PDK as a daily driver/kinderhauler.

385 HP

My 911lust has returned.
 
#11 ·
I have a DSG in my 08' GTI and I wouldn't swap my 6spd for one any day of the week. Its the best application of an automated gearbox, but it still doesn't have the involvement of a manual. Spend the time and learn to drive a stick, the more time you put into it the better it gets. Then again if you have to commute and you can't deal with the manny tranny then its certainly a better option than the tiptronic(no offense meant to tip owners).

G.
 
#12 ·
I have a DSG in my 08' GTI and I wouldn't swap my 6spd for one any day of the week. Its the best application of an automated gearbox, but it still doesn't have the involvement of a manual. Spend the time and learn to drive a stick, the more time you put into it the better it gets... G.
Whew! I hoped that all the time that I've spent learning to heel/toe downshift wasn't for naught! ;)
 
#13 ·
I also noticed compression has gone way up from 11.3 to 12.5 I think it was.
And Garyinnm nails it with:

The bump in compression ratio is due to the cooling effect of DFI. The direct injection of fuel into the cylinders cools the pistons and the combustion chambers and that allows the higher compression ratio without detonation (ping). Gary
That's how DFI gives us more power, via compression. I am quite suprised at the change in bore and stoke though.
 
#14 ·
none taken!!most of us have learnt to drive a stick a long time ago and just dont feel the need to stir a stick around in traffic and would prefer to chill with a tip.simple really-just like a tip !!most of the fun on track /open road and none of the hassle in traffic
 
#19 ·
I guess you could say that DFI provides some sort of cooling, but the main advantage is in it's ability to handle multiple injection pulses in a single stroke, due to the position of the injector tip in the combustion chamber and the speed of the injection event.

More importantly, the DFI technolgy (at least for Audi) uses pizeo-electric valves, which have no moving parts and can operate at extremely high rates of speed. We'll have to see if the Audi DFI technology wafts it's way into the Porsche motors, but it's a safe bet it has.

With DFI the fuel distribution can be metered using multiple discrete pulses in a single stroke.

The engine designers can now control the flame propagation across the combustion chamber with great accuracy, virtually eliminating detonation. In fact, DFI engines almost start to behave like compression ignition engines in a crude sense, as there is much more flexibility in injecting fuel in concert with the valve overlap on the compression stroke. The bottom line is better control of the flame, cooler combustion chamber temperatures, and most importantly, better volumetric efficiency of the engine due to the substantially increased compression ratio.

More power, better economy, lower emissions.

I suspect the bore and stroke have been changed to optimize the potential for DFI to fundamentally alter the timing of the fuel pulses.
 
#55 ·
After reading the detailed Press Information guide posted to the CC today, The Cayman Club - Downloads - Porsche 997 Generation 2 Press Pack, it is clear that a lot of the stuff posted here about how DFI works was in error:

I guess you could say that DFI provides some sort of cooling, but the main advantage is in it's ability to handle multiple injection pulses in a single stroke, due to the position of the injector tip in the combustion chamber and the speed of the injection event.

With DFI the fuel distribution can be metered using multiple discrete pulses in a single stroke.

The engine designers can now control the flame propagation across the combustion chamber with great accuracy, virtually eliminating detonation. In fact, DFI engines almost start to behave like compression ignition engines in a crude sense, as there is much more flexibility in injecting fuel in concert with the valve overlap on the compression stroke. The bottom line is better control of the flame, cooler combustion chamber temperatures, and most importantly, better volumetric efficiency of the engine due to the substantially increased compression ratio.
Multiple injection pulses only occurs in high load low rpm situations (see p. 17 of the press guide). So, although this post repeatedly cites multiple injection pulses as the "main advantage" of DFI, in fact this only occurs for a fairly limited range of operational parameters for the 997 DFI engine. That stuff about controling flame propogation and eliminating detonation is not true for the 997 DFI. The comments about the improved cylinder cooling leading to higher compression ratio are spot on.

With direct fuel injection, higher revolutions to attain the same torque that an engine with equal displacement (but a lower bore/stroke ratio) might attain are likely now possible with the use of less fuel per combustion cycle. Therefore, engines with higher bore/stroke ratios driven at higher RPM levels need not necessarily be less frugal in terms of overall fuel consumption because the primary combustion is now centered in a cone emanating from the injectors, and the fuel-air mixture around the chamber's circumference region (which doesn't burn off so efficiently anyway and only increases the cylinder wall temperature) is much lower than in port injection engines.
Except in a few circumstances, like the 1st few minutes of warm-up, the 997 DFI injects fuel during the intake stroke of the engine. Thus, the combustion is not "centered in a cone emanating from the injectors", the combustion process will occur after the compression stroke, long after the fuel injection is finished.

The dynamics of the combustion process are indeed the dominant benefit of DFI, not the bore/stroke ratio, although this does contribute. I suspect that the ratio has been altered specifically to take advantage of the new fuel delivery process.

I think what should be added to this discussion is more detail on how DFI affects the combustion process. For the first time we now have the injector nozzle downstream of the intake valve, and this fact fundamentally alters the operation of a 4 stroke spark ignition engine. Because of the location of the injector nozzle we can now inject fuel during the compression stroke, whereas as before the only way to get fuel into the combustion chamber was during the intake stroke. This is a huge difference.

Injecting the fuel during the compression stoke now allows control of the fuel atomization and resulting combustion in the time domain, which is to say we can control the speed of the combustion process by using multiple injection events with varying fuel volumes. This has the effect of spreading the combustion event over a longer time period, which creates more torque. The combustion pressure acting on the piston is now longer in duration, as well as more evenly distributed over the surface of the piston.
This post draws a lot of conclusions from the incorrect view that fuel injection occurs during the compression stroke. It does not (see p. 16, for example, of the press guide). Fuel is injected during the intake stroke, and the combustion process will not be fundamentally changed.

The key point is, that cylinder temperatures in general are now somewhat lower (permitting a higher compression) as before under identical compression levels because combustion can be more focused toward the center, rather than also around the edges, and because more heat is extracted to vaporize the fuel inside the chamber.
The higher compression ratio is the key point. Again, there is no fundamental change in the combustion process, and no "focusing towards the center" that is in any way different than a conventional engine, at least for the 997 DFI.

BMW's 3.0 liter gasoline turbo engine, which recently won the Engine of the Year Award and produces roughly as much torque at 1300 RPM as the new Carerra 3.8 DFI engine does at higher revolutions, doesn't deploy vario-cam.
I don't see how you can compare the tubo BMW engine with the Cayman engine??? With forced induction lots of things change...

An excellent post - thanks! However, I think that Variocam will still be valuable. The amout of fuel that can be burned is limited by the amount of air that can be drawn into the cylinder. The intake system and cams of the engine are "tuned" to operate at a resonant frequency to pack more air into the cylinder than would otherwise be drawn in. The Variocam allows this resonant frequency to be varied with engine rpm, allowing for excellent volumetric efficientcy over a wide rpm range. DFI does not change this in any way.
I would say that my view of the continued importance of intake resonance tuning control through runners, variocam, flapper valves, etc. has been confirmed by the 997 DFI engine. Porsche has added additional resonance control features in this engine - not removing them as obsolete, as several posters suggested would occur. The new engine has resonance control flaps in the air filters!

It’s radically different, and we have to rethink the way 4 stroke engines operate, because they do not work the same way with DFI. When the fuel is injected in the compression stroke as opposed to the intake stroke, it’s a whole new ball game.

In a DFI engine, we have no fuel in the intake charge whatsoever. Fuel is not introduced until the valve is not only closed, but the piston has traveled virtually all the way (but not quite) to TDC of the compression stroke. At this point, the dynamics of the intake charge behavior is completely changed, the gas (air) is now under significant pressure as well as heat, and of course the volume is reduced to 1/12th of the original static volume.

In a DFI engine, optimizing of combustion is done with multiple bursts of fuel spray, timed to coincide with the piston travel and spark events. In effect, the fuel/air mixture is ignited progressively, and flame propagation and the resulting cylinder pressure can be controlled fairly precisely. Remember, the injection events can vary not only with time, they vary with volume as well, and usually, the first “shot” is quite small. In a sense, this is a stratified charge phenomena, albeit highly controlled.
Again, alot of conclusions based on the incorrect view that the DFI system would have multiple pulses of fuel in the compression stroke, near TDC. Niether of these are true for the 997 DFI.

2.) Fuel is injected after the valve closes and downstream of the valve in the compression stroke, not the intake stroke.

3.) The mixing of fuel and air is now totally de-coupled from the intake event.
Same comment.

I'm not trying to be prissy here - just setting the record straight. :cheers:
 
#22 ·
#29 ·
Two questions for those out there:

1. Does PDK still use a clutch on all or any models for initial launch? I assume not.

2. I assume one of the biggest initiatives for using DFI is reduced emissions/increased fuel mileage - the side benefit is more power? In other words, legislative needs drive the technology and we benefit from those awful side effects?;)
 
#31 ·

Anyone noticed that the Carrera S will have 295 rears with 235 fronts?! Talk about understeer :D

________________________

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"Sir! We're surrounded!" - "Excellent! We can shoot in any direction!"

A good car will get you from point A to point B. A great car... will just get you into trouble!
 
#34 ·
Anyone noticed that the Carrera S will have 295 rears with 235 fronts?! Talk about understeer :D
For a mid-engined car, yes... but not as much as you might think for a rear-engined car which is prone to oversteer anyway, right out of the box - one of the great advantages (among many) you can exploit with a rear-engined 911 is the amazing traction you can get out of the corners with all that weight over the rear wheels, and big tire patches to put all that traction down onto the road.

I believe that the 235/295 set-up is standard on the current C2S.

brad
 
#35 ·

Ah you're probably right. I've never driven a 911 but from the reviews I saw, think the rear engine does make it prone to oversteering. Thx for clarifying that ;)

________________________

Image

"Sir! We're surrounded!" - "Excellent! We can shoot in any direction!"

A good car will get you from point A to point B. A great car... will just get you into trouble!
 
#37 ·
The new crankcase sounds very good:

Weight down, stability up – the engine block
Despite their more sophisticated and complex technology, the new power units are approximately six kilos lighter than before. A two-piece crankcase with integrated crankshaft bearings is perhaps the most significant change in technology, replacing the former four-piece block with its separate crankshaft bearing housing. The advantage is not only a reduction of weight, but also a smaller number of individual components. At the same time Porsche’s
engineers have enhanced the thermal and mechanical stability of the engine, making it even more robust by converting the design and configuration of the cylinders from open- to a closed-deck layout.
This means that the cylinder liners so far freely exposed around the cylinder gaskets are now connected with the housing by a top plate comprising the coolant sleeves. In particular, this helps to give the cylinders even greater stability in their shape and design, always remaining perfectly round. Two further advantages are the reduction of oil consumption and, thanks to less friction, even greater fuel economy.

Fewer moving parts – new timing chain drive
The new engines no longer require the former intermediate shaft serving to drive the timing chains. Originally this shaft had the task to reduce the loads acting on the camshaft drive.
Now, through the use of new, highly resistant timing chains, there is no longer a need for such an intermediate shaft, moving masses and the weight of the engine being reduced accordingly.

:banana:
 
#44 ·

Apparently there's much more than meets the eye in modern combustion engines, overpacking and swirling are totally new concepts to me. Thanks for the informative posts, guess I'll hang around this thread for a while!

________________________

Image

"Sir! We're surrounded!" - "Excellent! We can shoot in any direction!"

A good car will get you from point A to point B. A great car... will just get you into trouble!
 
#50 ·
Wow - great, informative posts with some terrific technical information and opinion. When I was a graduate student, I actualy did some of the first computer modeling of finite-rate chemistry in combustion chambers for early fuel injection (in both cars and rockets - a long time ago...). I am by no means an expert in DFI, but can hopefully offer some useful perspective on combusiton chamber chemistry in layman's terms.

The 'stochiometric ratio' is the perfect ratio of fuel to air for absolutely complete combustion - no extra fuel or air left over. To get this 'perfect situation' throughout the cylinder volume is the trick, since the mixing is always imperfect. Some areas will have too much air, and some will have an excess of fuel and that's one of the major causes of pollution.

Certainly, having the ability to directly inject fuel into a small volume of compressed air will give you significant benefits. The injection system knows by the time the intake valve is closed, exactly how much air is in the cylinder, a big advantage over previous systems. You are also 'mixing' the fuel over a much smaller volume, so the mixing will be more uniform. It would make sense to me, that the higher the turbulence (Reynolds number) of this compressed air volume, the more complete the mixing will be. The holy grail of cylinder combustion is complete mixing of fuel and air.

It sounds like DFI introduces an initial 'charge' to blow up the mixture and radically increase the turbulence, then shoots the main charge in to be mixed completely. Very clever.
 
#51 ·
Certainly, having the ability to directly inject fuel into a small volume of compressed air will give you significant benefits. The injection system knows by the time the intake valve is closed, exactly how much air is in the cylinder, a big advantage over previous systems. You are also 'mixing' the fuel over a much smaller volume, so the mixing will be more uniform. It would make sense to me, that the higher the turbulence (Reynolds number) of this compressed air volume, the more complete the mixing will be. The holy grail of cylinder combustion is complete mixing of fuel and air.

It sounds like DFI introduces an initial 'charge' to blow up the mixture and radically increase the turbulence, then shoots the main charge in to be mixed completely. Very clever.
The PDF file from Porsche states that multiple shot fuel injection is only used below 3500 RPM.
Multiple fuel injection is used once again in the higher load range at engine speeds of up to approximately 3500 rpm
So they don't appear to be using an initial burn to provide a higher Reynolds number when operating at higher RPMs. That would imply they are relying on the Variocam to change the flow to raise the Reynolds number at higher RPMs. Though I wonder how much "swirl" from the intake stroke is still present when the compression stroke finishes.

Though the next paragraph is phrased oddly.
Under all other operating conditions fuel is injected in one single process, multiple injection serving additionally when driving under load in the conditions described to make the entire combustion process even smoother and more reliable.
This could be read to say they don't do multiple injections at higher RPMs or that multiple injection is only used under load at higher RPMs. (I hate language translations like this -- I'm sure it was perfectly clear in German. And I don't remember enough German to figure it out even if I had the original document.)


- Dan
a wireless engineer discussing fluid and mechanical engineering -- never a good sign...