cam drive - belt or chain

[michael.fordham]

Hi


I've been a member for 15 years now and just signed up to this as need some advice please.


My 993 has 25K on the clock and does 2K a year on only fast runs other than occassional battery charging / warm up and lub the ac seals etc runs on fine Wx winter days.


The car runs wonderfully and is still running in I think as gets better with age! Mobile one from the factory where I picked up the car in 1995. OIl changed several times and some garage check overs and 2 'light' services incl laser wheel alinemets etc.


Should I be concerned about timing belt/chain issues??

thanks

 

[Mark Elder]

Hi - Welcome to the forum!
Really good to hear from someone who has had their 993 since new - and even with the pick-up-from-factory experience!

Tell us more about your car (colour - spec - etc).

To answer your question - the timing on 993s is by chain - which is designed to last as long as (probably much longer than) the rest of the engine. I've never heard of anyone changing the timing chain. The belts that exist are "consumable" and do occasionally need to be replaced - but their failure is not catastrophic - unlike 944s and 924s where the timing is via belts that do wear out. 993 belts drive the cooling of the engine, the alternator, aircon etc - so important, but not engine-damaging if they fail.

 

[clyde]

Your service book should tell you everything you need to know about what needs to be replaced/checked and when Michael. Would've expected a bit more than a couple of "light" services and "several" oil changes over a 15 year period, even with only a couple of thousand miles a year as most of the maintenance items are on a mileage / time whichever comes first basis [8|]

 

[IndischRot]

ORIGINAL: Mark Elder

993 belts drive the cooling of the engine, the alternator, aircon etc - so important, but not engine-damaging if they fail.

For the avoidance of doubt - if the belt driving the cooling fan breaks STOP THE ENGINE AS SOON AS IT'S SAFE TO DO SO ! The engine won't last long without cooling air. Just as critical as the belt that drives the water pump on a water cooled car.

 

[pse_SC]

important, but not engine-damaging if they fail.

Very potentially, though[&:]! Agree with the previous post[]

 

[Mark Elder]

ORIGINAL: pse_SC

important, but not engine-damaging if they fail.

Very potentially, though[&:]! Agree with the previous post[]

I agree! How would you know if a cooling fan belt had gone? I guess oil temp would increase, but in traffic on a hot day it can anyway. Anyone had this happen ?

 

[sue11sam]

Don't know what the split is between oil cooling and air cooling on a 993 is (must find out) but suspect its 70/30 in favour of the oil, therefore not critical if the belt breaks, but fix it asap, and keep the revs down

 

[clyde]

ORIGINAL: Mark Elder


How would you know if a cooling fan belt had gone? I guess oil temp would increase, but in traffic on a hot day it can anyway. Anyone had this happen ?

A warning light comes on, there's a wee spring loaded sensor against the belt which activates the light when the belt breaks.

I had a new one break a few days after it was fitted by a PC. It had not been tensioned correctly using shims on the pulley and made an increasingly louder screech. I was going to go back to the PC the next day but it snapped just as I was driving into my garage. To their credit the PC sent a low loader to collect the car, fitted a new belt (correctly this time) and delivered the car back to my house.

That was around five or six years ago so I probably should get the belts changed again at the next service [8|]

 

[clyde]

ORIGINAL: sue11sam

Don't know what the split is between oil cooling and air cooling on a 993 is (must find out) but suspect its 70/30 in favour of the oil, therefore not critical if the belt breaks, but fix it asap, and keep the revs down

I wouldn't advise driving very far to find out.

I wouldn't think that the oil plays a huge roll in cooling the engine and if I had to put a figure on it would go 70/30 the other way at best, i.e. air plays the larger part in cooling the engine. They are known as air cooled engines after all, not oil cooled [8|]

 

[sue11sam]

sorry clyde , don't agree, think you will find the principle cooling on this engine is the oil rad up front, would agree on old 356/912 and later less tuned flat 6 motors air was king, will check and advise,

 

[clyde]

That was just my personal thoughts on the matter, I have nothing tangible to base this on. I stand to be corrected and look forward to the outcome of your investigations Suesam

 

[IndischRot]

I wouldn't underestimate the cooling value of the air. Oil cools the bearings and the bottoms of the pistons but the primary cooling of the cylinders and the heads is air as minimal oil gets to those parts. Porsche goes to great lengths to select the correct fan and the correct fan to crankshaft speed.

 

[D Ward]

ORIGINAL: sue11sam

sorry clyde , don't agree, think you will find the principle cooling on this engine is the oil rad up front, would agree on old 356/912 and later less tuned flat 6 motors air was king, will check and advise,

I would not recommend, driving or starting a 993 engine without the air pump running, if it breaks, turn of the engine and coast to the side of the road.

The clue is in the name, porsche engines are either classed as air or water cooled, never oil cooled.

Without that big air pump on the back of ur 993 engine, pumping several thousand ltrs of air to keep it at optimal temperature, your engine will last, if your lucky, between three to six minutes, before terminal failure and your lovely flat six engine becomes useful only as a coffee table or a boat anchor.

Oil does play a part in the cooling, but its primary job is lubrication etc and the oil cooler is their to keep it at an optimum temperature to carry out that function, as oil constantly to hot or to cold (much like porridge for bears) is not ideal for an engine.
Regards

 

[A Ward]

Hi

As stated before the primary function of the engine oil is lubrication - with a dry sump the added advantage is that the oil has a larger reservoir, is sufficiently defrothed (defoamed and dense) and is available under all driving conditions.

The 'air cooled' engine is exactly that - although the oil will indeed carry some heat away in its circulation - the engine fan is designed to be the primary cooling mechanism. That is why the engine is finned to provide the maximum available heat transfer area as the air is drawn across it by the fan. It is also tied in with the operation of the retractable rear spoiler which in deployment at higher driving speed provides increased air pressure and hence more cooling airflow over the engine.

Therefore if the engine fan belt fails you should coast and pull over in a safe manner ASAP and call the 5th emergency service as they like to be known - or one of their many clones. I appreciate that this may not always be practical depending on your location (ie outside lane of the M25) - but it would not be sensible to continue driving for any duration...your oil temperature gauge would become your new best friend in this scenario.

Below is an extract taken directly from the Porsche air cooled engine manual regarding the cooling system.

In line with its predecessors, the Porsche 911 Carrera (993) is fitted with an air-cooled engine. Air cooling is particularly suitable for a compact drive unit mounted in the rear of the vehicle. Apart from ease of maintenance, a particular advantage of air cooling is the high temperature difference between the external surface of the cylinders and the ambient air that allows large amounts of heat energy to be transferred. The cooling air required is drawn in by the engine cooling fan across the screen mounted in the rear spoiler and then passes the finned cylinders, absorbing the heat to be dissipated in the process. Then the hot air is discharged towards the floorpan.

When designing the cylinders and the cylinder heads, sufficient cooling surface and the correct layout of the entire cooling area must be ensured. In addition, a sufficiently sized flow area for the cooling air and correct distribution to the cylinders and cylinder heads must be provided. In order to reduce heating of the cylinder head, the exhaust port is fitted with a ceramic port liner to act as a heat insulator.

The fan has been designed and optimized for a high feed rate at high efficiency and low noise levels. At high vehicle speeds, the fan is supported by the retractable rear spoiler that creates an increased pressure ahead of the spoiler due to the increased dynamic pressure ram effect.

I guess it all depends how much you like expensive repair bills on whether you decide to continue in traffic - but as previously stated you should know if your belt has gone though as the belt sensor (roller mechanism) located in the engine bay will flag up a warning light on your dashboard.

 

[gm997_1]

Oil is lubrication.
Air is ventilation.

The temp of the oil is kept at an optimum level in the oil cooler rad by the oil cooler fan (air blown across it) - also assisted by the distance it has to travel to the front wing and back to the engine. It needs this as the engine is a dry sump design therefore no oil sitting in it as you're driving along as the oil's doing its work horizontally with the pistons, getting hotter and hotter. It can't drop back into the sump for any cooling effect on itself until the car's stationary while running, but even then takes some 15 mins or so to descend in full, so of no major effect.

When the blower fan belt warning lamp comes on in the dash, it can also mean a fault in the sensor, as happened to me. I stopped, turned the moter off, got out, looked at the belt on the fan and it was fine! Only by looking v. closely at the sensor could I tell the sprung arm had failed and was sitting slightly skewiff off the belt thus causing the lamp to illuminate.

 

[sue11sam]

No sorry you are incorrect, the oil within this system contributes considerably to the cooling of the engine, I will concede that it is not the major contributor to the cooling, but for sure it has a major influence, cooling the whole of the 'bottom end' the fan cooling the heads and a major contribution to the barrels.

 

[John Maddox]

I accept the fan is vital,hence the roller that indicates a failed belt to the driver. But so is the oil - we have an oil pressure gauge, an oil temp gauge and an oil level gauge !! Not many production cars monitor the oil like ours.

 

[clyde]

The oil in any four stroke engine, air or water cooled (and probably a rotary motor as well but I don't know enough about them) will provide some cooling benefit. As it moves about the engine it will take heat from hot spots and dump it in colder parts of the engine, as well as obviously loosing heat when in the sump (or oil tank in our case) and/or oil cooler. The 993 is no different in this respect. I don't think that the oil in a 993 runs any hotter than any other sports car and the oil cooler isn't any larger than any equivalent performance car, so I would suggest that the oil removes the same amount of heat as any other sports car, air or water cooled.

I agree with you John and I certainly wouldn't want to run my car too far if the oil cooler switch was not operating as I would imagine that the temperature would climb reasonably quickly. But nowhere near as quickly as it would rise without the cooling fan working.

As before, if anyone can come up with a definitive ratio of air to oil cooling I would love to see it but in the meantime I'll stick with my earlier assessment of the split as 70 min/30 max in favour of air cooled.

 

[sue11sam]

Hmmm thats why you have 12 litres of the stuff slopping about the car, I am not suggesting the air part of the cooling partnership is unimportant, I am saying that the oil part is probably a higher contributor to your cooling capacity than you think. Why do you think it takes my garage 30 mins to bring my engine to temperature with a leaded blind on the air inlet . It would be interesting if you disabled the water pump on a fluid cooled engine.

 

[A Ward]

Ok - How about some data from the Porsche 993 carrera maintenance manual - this is the manual written by Porsche engineers and is therefore the definitive guide to your car and its engine.

Engine cooling

Air-cooled system
Fan drive Via V-belts from the crankshaft
Transmission ratio: Crankshaft to fan approx. 1 :1.60
Air delivery rate 1010 Ltr /sec at 6,000 rpm of crankshaft
-----------------------------
Engine lubrication

Dry sump lubrication with separate oil tank
Thermostatically controlled, front oil cooler in
right-hand front fender, 2-stage electric fan
-----------------------------
Oil cooling

Oil filter in return line
Oil pressure at n = 5,000 rpm approx. 6.5 bar at 90. C oil temperature
Oil pressure indicator 0...5 bar, electric, and oil pressure warning lamp
Oil consumption up to 1.5 l/1000 km
------------------------------
Engine Oil

Approx. 11.5 L (approx. 9 L for oil change)
Determined by measurements with oil dipstick
as per Owner's Manual

As stated previously the primary source of cooling is air (1010 ltr/sec @ 6000rpm)- the oil as stated before will always carry heat away from the engine in its circulation...but its 'primary function' is lubrication.

The reason that the oil cooler plays such an important part is because oil characteristics change with temperature and if the oil gets excessively hot it will thin down and could fail to adequately protect your expensive engine internals. The oil cooler is to maintain a stable operating temperature of your engine oil to provide optimum lubrication charcteristics under all driving conditions.

I hope that helps