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Porsche turbo: Valve timing

G

George Elliott

New member
Found an interesting piece of info on the 997 Reg regarding valve timing, see below. Thankyou Motorhead - Jeff.

Can anyone give the 951 std cam timing? I am happy to ignore the valve lift at this stage.

"Unfortunately, I don't have the figures for the 997 GT2 but Paul Frere, in his excellent book "Porsche 911 Story", gives the timing for the M96/70 engine used in the 996 Turbo (the GT2 uses the same engine but with slightly larger turbos). These engines have both variable inlet valve timing and lift (Variocam Plus) as follows:

In the high lift mode at 1mm valve lift and zero lash -

"Early" diagram: Intake opens 15deg before TDC; closes 15deg after BDC
"Late" diagram: Intake opens 15deg after TDC; closes 45deg after BDC
Exhaust opens 41deg before BDC, closes 9deg before TDC

In the low lift mode -

"Early" diagram: Intake opens 2deg after TDC; closes 62deg before BDC
"Late" diagram: Intake opens 32deg after TDC; closes 32deg before BDC
Exhaust opens 41deg before BDC, closes 9deg before TDC

Incidentally, the Paul Frere book (in its revised and expanded 8th edition) contains a lot of technical information on the engine which you'll find very useful."

George
944T





 
Hi George,

Not sure how that relates to us?

I can see "951" but none of the data seems to relate to one...

Or am I just being dim?

Regards

Graham
 
Standard 951 valve timing (same as early na cars)

951_ValveTiming.png




VarioCam Plus uses the Lotus/INA/Porsche developed 'switching tappet' system combined with a camshaft phaser to vary timing and lift.

VarioCamPlus.png


The switching tappet system provides the option of two different lift profiles (similar to the original VETEC which used a switching rocker arm), hence the widely different valve timing values.


Are you planning to 'VarioCam Plus' your 951 George?
 
Graham,
the relationship to the 951 is - how does the valve timing of a recent Porsche turbo engine compare to the 951 generation?

Back then, the 930 was the king of the jungle, and the 951 was a problem, with digital engine management, ceramic valve seats, knock control, low friction 4 cylinder layout, water cooling, bearings, crank and driveline that would not look out of place on an earth-mover, the engine had greater potential than the old air-cooled queen. Therefore as we all know, there was a move within Porsche to curb the 951's real potential.

This was done in many ways; boost limitation, even the boost-gauge bluff of absolute versus actual, I/C end tanks, down-pipe restriction, Over-boost detection software in the ECU, Exhaust system spec, and quite rough component finish (look at the waste gate to main pipe connection), the snorkel intake cross section over the pipe to the TB, the complication of the cycling valve, the K26/6 size, It was all intended to conceal the 951's real potential, and it worked.

Unfortunately - in a way Porsche shot themselves in the foot, because had their engineers been encouraged to develop the transaxle layout, and the 4wd achievements of the 911 been applied to the 951, then perhaps the 4wd GTR wonder-car we got from Japan, would have come from South West Germany. Proof that if you are not going forward with development, you are actually going back.

However, many of the original 220hp cars are running reliably today with 350bhp. To achieve that, many mods have been developed, and I have a hunch the cam spec's should be analysed. Being unqualified and busy, my curiosity wants to compare with a known good Porsche turbo engine. A 996TT operated up to 3800rpm is a very swift device. A std 951 (remember them!) is a totally lethargic lump up to 3400rpm. The physics of an internal composition engine running on petrol is the same, so why the vast difference (I acknowledge the 50% capacity difference is a big fact, but I am referring to behaviour not power here)
I am aware that small valve lift assists turbulence & in turn torque, and hi lift assists breathing and ultimate power. With a fixed profile and lift, where does the 951 cam sit in the 996TT range of timings & lifts?
So I want to analyse the figures, consider it too for me if you would...?
chat soon
George
944t
 
Johnny,
thanks for the numbers, need to take time to study them.

I notice the 16v 944s 2.5L head gasket, is the same part number as the 944 turbo head gasket. That set me thinking, would the VVT mechanism from a 968 fit that 16v head?

thx again
George
944t


 
To make any significant changes, with a traditional fixed camshaft on a forced induction engine, you may find yourself 'robbing Peter to pay Paul'.


I venture a key basis for the impressive low speed torque on the 996T and subsequent derivatives was the 9.4:1 compression ratio.

This, combined with an advanced version of Motronic and variable valve timing, gave the engineers the tools they required in-order to pay Paul direct, leaving Peter still smiling [;)]



would the VVT mechanism from a 968 fit that 16v head?
Click to expand...

Not sure, but I would be surprised if it did as the VVT solenoid would require a dedicated oil feed.
 
I seem to recall that the 996 turbo had variable geometry turbos, which wont harm its lower down flexibility either - or was that introduced with the 997?
Tony
 
The cam chain tensioner on the 968 cylinder head has three mounting points against two for the cam chain tensioner used on 944S/S2/928S3/S4/GTS cylinder heads. They all feature an oil feed line to assist the spring.

Some good pictures on this page.

Though it may be obvious to most on here, it's still worth mentioning that on any turbocharged engine the camshaft profile should be considered along with how much gas flow/pressure the exhaust sytem can handle. More exhaust opening duration should theoretically result in higher exhaust gas flow thus increase exhaust back pressure, thus increase the engine's possibly working harder at evacuating a higher quantity of exhaust gases and possibly result in insignificant performance gains, at least with the standard-ish 951 exhaust system

I'm sure the exhaust and turbines on the 997TT are far free-er flowing than on a standard-ish 944 turbo.
 
Though it may be obvious to most on here, it's still worth mentioning that on any turbocharged engine the camshaft profile should be considered along with how much gas flow/pressure the exhaust sytem can handle. More exhaust opening duration should theoretically result in higher exhaust gas flow thus increase exhaust back pressure, thus increase the engine's possibly working harder at evacuating a higher quantity of exhaust gases and possibly result in insignificant performance gains, at least with the standard-ish 951 exhaust system

I'm sure the exhaust and turbines on the 997TT are far free-er flowing than on a standard-ish 944 turbo.
Click to expand...
Exhaust opens 41deg before BDC, closes 9deg before TDC
Click to expand...

I make the 996/997 exhaust duration to be 212° compared with the 951 at 220°... Unfortunately we don't know important details such as the CH flow capacity and valve lift time-areas of the two engines, in order to make a worth-while comparison.


For me, the key driver for the variable intake lift and duration on the 996/997 engines is to allow the engineer freedom to design an intake lift profile that is optimised for just the full load maximum torque condition. The "˜aggressive' profile (in combination with a very high CR and variable timing) facilitates significant gains in the torque curve below ~ 3000rpm.

Without VarioCam Plus, the engine would perform great on track but struggle with cold starts, emissions, fuel consumption and NVH...

 

ORIGINAL: George Elliott

Graham,
the relationship to the 951 is - how does the valve timing of a recent Porsche turbo engine compare to the 951 generation?

Back then, the 930 was the king of the jungle, and the 951 was a problem, with digital engine management, ceramic valve seats, knock control, low friction 4 cylinder layout, water cooling, bearings, crank and driveline that would not look out of place on an earth-mover, the engine had greater potential than the old air-cooled queen. Therefore as we all know, there was a move within Porsche to curb the 951's real potential.

This was done in many ways; boost limitation, even the boost-gauge bluff of absolute versus actual, I/C end tanks, down-pipe restriction, Over-boost detection software in the ECU, Exhaust system spec, and quite rough component finish (look at the waste gate to main pipe connection), the snorkel intake cross section over the pipe to the TB, the complication of the cycling valve, the K26/6 size, It was all intended to conceal the 951's real potential, and it worked.

Unfortunately - in a way Porsche shot themselves in the foot, because had their engineers been encouraged to develop the transaxle layout, and the 4wd achievements of the 911 been applied to the 951, then perhaps the 4wd GTR wonder-car we got from Japan, would have come from South West Germany. Proof that if you are not going forward with development, you are actually going back.

However, many of the original 220hp cars are running reliably today with 350bhp. To achieve that, many mods have been developed, and I have a hunch the cam spec's should be analysed. Being unqualified and busy, my curiosity wants to compare with a known good Porsche turbo engine. A 996TT operated up to 3800rpm is a very swift device. A std 951 (remember them!) is a totally lethargic lump up to 3400rpm. The physics of an internal composition engine running on petrol is the same, so why the vast difference (I acknowledge the 50% capacity difference is a big fact, but I am referring to behaviour not power here)
I am aware that small valve lift assists turbulence & in turn torque, and hi lift assists breathing and ultimate power. With a fixed profile and lift, where does the 951 cam sit in the 996TT range of timings & lifts?
So I want to analyse the figures, consider it too for me if you would...?
chat soon
George
944t
Click to expand...

Ah, ok I think there are probably too many other factors that are different and so what you may glean from the comparison would not be of much direct use to you - beyond increasing your understanding of course.

I'm sure that there are significant benefits to be gained by changing the cam timing but I suspect that they would be mainly at the top end and you would need to sacrifice some torque lower down to release tham. VVT would help get round that, but would be a big development!

Regards

Graham
 
Has there ever even been an 8V engine with VVT?

As for 2.5L engines running reliably 350bhp... cough cough...
 
Has there ever even been an 8V engine with VVT?
Click to expand...

Exactly - moving intake and exhaust lobes together won't help much, other than piston to valve contact.

One of these combined with a cam box modified to accept switching tappets could be rather neat though.



Anyone got a spare 500k to develop it?
 
My very simplistic analysis below shows the 951 timings and the average of the 996TT
Is it relevant? Perhaps....that 500k budget would find out Johnny.[8|]
For me the interesting difference is the intake closing timing, its average is miles from the 996TT. But the "late Hi Lift" figure on the 996 is almost spot on the 951 figure (225 v's 229 deg). Is that why a 951 goes so well from 3800rpm with boost flowing?
The low down rpm Cam profile Porsche use in the 996TT is likely to be "Late Lo" - (agreed??)...and the figure there for intake closing is 148, which when we consider the 951 is stuck with intake closing at 229 deg in the same situation, perhaps confirms the 951's difficulty with Lag low in its rev range.

Cam Timing analysis
Early Hi Late Hi
951 Crank Deg 996 Crank Deg 996 Crank Deg
Intake Opens 1 Intake Opens 345 Intake Opens 15 0
Intake Closes 229 Intake Closes 195 Intake Closes 225 210
Exhaust opens 137 Exhaust opens 139 Exhaust opens 139
Exhaust Closes 357 Exhaust Closes 351 Exhaust Closes 351

Average Degrees Early Lo Late Lo
996 Result Variance 996 Crank Deg 996 Crank Deg
Intake Opens 8 7 Intake Opens 2 Intake Opens 32 17
Intake Closes 191 -38 Intake Closes 195 Intake Closes 148 172
Exhaust opens 139 2 Exhaust opens 139 Exhaust opens 139
Exhaust Closes 351 -6 Exhaust Closes 351 Exhaust Closes 351


Well - at last I have made a rough check on the timings, and am happy about that even though it only explains part of a super complex picture.

Graham, whats a 9R Cam? I love your ref to many other factors, and then I read your Engine mod list...[:D][:D] yep - many many.

TTM, agree some excellent images, thanks. The exhaust science you refer to is a bit beyond me, but the Cam timings show the two engines have very similar Exhaust Valve opening figures. Hope that cough is getting better......[:)]

Tony, the vario geometry arrived with the 997

Johnny, Compression is a significant factor I agree, and I have also ignored lift which is also pretty critical. The reason for ignoring this is, I wanted to single our timing first - it seems more appropriate to curtailing the tendency for a 951 to light up its rear tyres which Porsche had a problem with in 1984/5.
The Mahle Single Cam Vario Inlet is a lovely bit of engineering. Thanks.

One piece of info I would like is the 1984 930 Cam timing data. It would show what Porsche knew at the time...but did they use it on the 951...?

Another is, what are the 944s (16v) cam timing data, and the 968 data

Bottom line is, I am happy to have these figures and all the interesting activity you have all highlighted, I will continue my simple tuning projects for now.


George

944T 1989
K&N Filter, Stage 1 I/C, 55lb Injectors, 3Bar FPR, Knife edge TB butterfly, SPS BB turbo, SPS Perfectbore Block, Hoffman Chips, Tial W/G, 3" DP, 3" Exhaust, 1.2Bar MBC, LSD, AFR measurement, TOYO's, M030, regular Silkolene Fuchs 5w 50, India Red / Black.
 
Some general comments:

1. VarioCam Plus 'low lift' mode:
The particular low lift profiles of the 996 are used for part-load fuel consumption and emissions improvements only.
Use in the 'full load' condition, for any engine speed much above 1000rpm, would be pointless; the time-area of the lift event is far too short.

2. Full load volumetric efficiency (or torque) improvements:
In valve timing terms, the 996 engine achieves improvements by tailored variation of the intake camshaft phaser versus engine speed.

3. Fitting a 16v head with VarioCam to a 2.5 turbo:
Low rpm torque issues for the 2.5L 16v na are well documented. The na has a compression ratio of 10.9:1, even if you raise the CR from the standard turbo 8:1 you are likely to have an engine that is woefully weak at low rpm's. I'm not sure the 15° variation of the 968 VarioCam would even recover the loss from the std 8v head.
 
Though camshaft timing may be similar, cylinder bore and exhaust valve sizes may not, which also needs to be factored in to make a comparison, as what matters is still the amount of exhaust gas generated that has to be evacuated.

George, I would be interested how many 2.5L cars developing a reliable and genuine 350bhp there could be. Not many I bet, or maybe I'm being too skeptical?
 
When a reliable 300 hp costs practically nothing over the cost of a fit, healthy standard engine, there is not much motive for people to spend the significant amount of money to build a reliable 350 hp 2.5; but as WUF's 400+ hp has amply shown, it is there to be had if you put the right parts together.
 

ORIGINAL: George Elliott
Graham, whats a 9R Cam? I love your ref to many other factors, and then I read your Engine mod list...[:D][:D] yep - many many.
Click to expand...

Hi George,

It's the cam from a 924S or late 944 - the standard cam in a Turbo is from an early 944.

The part number is 944 105 155 9R as opposed to 944 105 155 5R - hence the "9R" nickname.

Inlet timing and lift is unchanged but the exhaust has 8° extra duration (opens at 133° and closes at 1° after TDC) and 1mm more lift.

I don't think it will be making a huge difference but maybe helps the top end a bit.

It felt a bit more free at high revs but could just be wishfull thinking of course!

Regards

Graham
 
Back then, the 930 was the king of the jungle, and the 951 was a problem, with digital engine management, ceramic valve seats, knock control, low friction 4 cylinder layout, water cooling, bearings, crank and driveline that would not look out of place on an earth-mover, the engine had greater potential than the old air-cooled queen. Therefore as we all know, there was a move within Porsche to curb the 951's real potential.
Click to expand...

George,
Just going back to an earlier point; although I like your appetite for lateral thinking and company politics, I'm struggling to see any evidence that the 951 deliberately had it's wings clipped at a core engine design level.

Here is my very basic attempt at an objective car/engine performance comparison, for a selection of Porsche Turbo cars:

Porschetubospecs.jpg



Also thrown-in is a current state-of-the-art Turbocharged engine - One stage on from the current 997 generation IMHO (Ducks for cover.... [:)] ).
 
[:D]

Johnny, I am amazed by that data table. An 87 turbo cup car equals the std 996tt bhp/litre.

And betters every 911T pre the 996TT

I would never have guessed that.

Where did you get that bhp/m2 from? its fascinating.

The M5 tt is a great engine I expect, credit where its due

I have just noticed my timing analysis table is unreadable. Will try to resend it.

George
944t
 
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