Michael Benno

I seem to remember a TSB in relation to the correct clocking of the radiator insert on the upper right side of the radiator in S4/GT/GTS cars.

Can someone help me find the correct TSB or provide guidance on whether it should be removed or not?

I seem to remember a recent threa@Speedtoys how about this issue. Hoping someone can give me a hand with this



here is how the insert is clocked on my car right now

Rob Edwards

Here's Speedtoys' thread: https://rennlist.com/forums/928-foru...t-problem.html

TSB:

Michael Benno

Thanks @Rob Edwards. Super helpful. I can confirm that the insert is properly clocked. Porsche seems to think its better to leave it in there. Anyone think there is a good reason to remove it?

DHS928

So years ago I did some research on oil coolers, and found some info on a phenomena called “channeling” in radiators, it seems in certain designs the fluid can flow directly from the inlet to the outlet in a path of lease resistance and bypass a lot of the cooling tubes. So certain design changes were made to direct the flow through all the tubes to increase heat transfer. It looks like the insert forces coolant down into the lower tank, so possibly Porsche thought that was occurring? also the slower the flow through the radiator the greater the heat transfer.

mkhargrove

believe it or not, too much flow can cause overheating. it seems like more flow would help, or certainly wouldn't cause heat build up, but unless you have some wicked strong fans, the increased an increased flow rate doesn't allow the coolant to adequately have heat removed while passing through the radiator. i think that intuitively many might think that this would be neutralized by the fact that more coolant is flowing through the block, so more heat could be removed...but it builds up in the system....unless another variable (air flow rate or radiator size) is changed. deleting the part would probably increase flow rate...maybe not significantly, but the systems are engineered to have an optimized intersection of coolant flow rate, air flow rate, heat load, and a specpfic radiator configuration... the difference in flow rate from deleting the part might not be significant....in fact if you have deposits the in the system that are impeding flow, maybe it could be deleted and it would be a good thing. but, i personally tested the impact of deleting my thermostat in the middle of the texas summer and the heat build up was very fast. in a mild climate, maybe it's not an issue. but if you're in an area with a scalding summer coming up I wouldn't delete it...just clock it correctly.

PorKen

When your car is running... try to determine if you see a difference in cooling, with or without the baffle!

If you want to slow the coolant through the core for better heat transfer would it not be better to put a restrictor in the outlet (hose)?

I researched this a bit and seem to recall that deflectors are primarily installed to increase low rpm cooling. On my S4 auto with a radiator blocking centrifugal supercharger, I chose to remove the insert and increase the idle rpm to that of the GT, 775 (725 in gear).

-

Notes:

Not sure when they made the change but early radiators are thicker - the width of the end tanks - with fewer widely spaced copper tubes. Later radiators are about 1/2" thinner with more aluminum tubes packed closer together.

Aftermarket aluminum radiators have the same thin core as the later radiators and do not have the baffle. They also have a simpler box shape around the inlet tube as opposed to the odd shape of the plastic end tanks.

FredR

Fortunately we do not have to rely on our membership for radiator designs!

Take a look at the bottom of that design bulletin Rob copied- it is dated 1983 then take a look at Ken's post wherein he advises that the radiator design changed from an initial copper core design to the one fitted to most 928's- never heard anyone comment on such previosuly. A "SWAG" [scientific wild *** guess] suggests that Porsche noted a problem/concern with the original design of radiator as planned power output increased, changed the radiator design completely and then noticed a "small problem" with their new design that was subsequently "enhanced" with fitment of that insert.

When Jeff [Speedtoys] posted about his issue I was astonished at what he found as in how on earth did the thing get installed backwards? Clearly it had a major detabilising impact so that tells one just how sensitive the cooling system can be.

Heat transfer design is quite a complicated affair in that there are a number of factors that have to be taken into account- this creates many transient conditions that the solution has to satisfy. Ironically the easiest condition to satisfy will be the maximum design case- if there was an issue with this the design would have changed but the Behr radiator with plastic end tanks survived all subsequent iterations.

The last thing one wants to do in a heat exchanger to improve heat transfer is "slow things down" and therein [most probably] lies the answer to the insert conundrum. The real problem in this system design is inevitably going to lay with the water pump. This is a centrifugal pump with low NPSH characteristics and an overhanging impeller design that means it is ripe for inefficiency given that depends on the clearance between the impeller and the crankcase volute. Slight changes in clearance will make significant differences to the efficency of the design thus why metal impellers that gouge into the casing when there is a bearing failure are such a bad idea. Ideally Porsche should have built in a removable insert [known as a wear ring] but cost factors probably eliminated that possibility. Centrifugal pumps have an effective rotating speed turndown ratio of 3:1, when operated outside that range they become highly inefficient- our water pumps have a speed turndown ratio of 10:1!

Porsche would have well known this and thus would have tried to ensure the system was stable at idle [675 rpm's] and thus maximum efficiency probably occurs around mid range rpm's. The problem is that as speed increases flow is proportional to the square of the speed and head developed is proportional to the cube of the speed. Thus taking the pump from from say 2k rpm to 6k rpm [a three fold increase] develops a 9 fold increase in flow and a 27 fold increase in developed head and the power absorbed by the pump is proportional to the product of the head times the flow a factor of circa 250 in this case and then we are talking about hydraulic horsepower that does not consider efficiency that has to be considered in order to know what it will take to drive the pump- peak efficency will likely be no more than 70% at the most efficient speed and consideraby less everywhere else . Although not related to the point at hand, this is exactly why an electric driven water pump on temperature control is a far superior option efficency wise if the alternator can support such. My estimates suggest a saving in the region of 15 bhp at peak rpm's may be possible

Needless to say the obvious question anyone with an double digit IQ will be asking at this stage is 'what is the relevance of the above?". The answer is quite simple in that heat transfer in the radiator at low flows will likely be erratic to say the least. The flow in such will likely be across the top rows of the radiator at idle conditions [path of least resistance] and thus I suspect [but cannot be sure] that the insert may be intended to help improve flow across the radiator a little bit in these conditions and has absolutely nothing to do with peak performance. Maybe the oil cooler coil in the outlet end tank was suffering a bit at idle? One wonders whether the later [1990 +?] external oil cooler may have evolved because of such concerns?

Speedtoys

"Anyone think there is a good reason to remove it?"

Yes, the TSB tells you why it's there...

PorKen

Porsche issued the bulletin for the thick core radiators. Did they bother re-testing on the thinner core?

Speedtoys revelation compares a baffle inserted backwards restricting most of the flow, versus inserted correctly, restricting some of the flow and redirecting it.

I have a near new replacement Porsche radiator with a 1986 date code that came out of an early junk-yard 1982. It is a low density thick copper core design. I am surprised Fred did not touch on this but a thinner core with higher row density is more efficient. The thinner core would seem to fit better with the lower draft potential of the 87- electric fans versus the engine driven clutch fan.


It would be interesting to convert an aluminum radiator to a 3-pass design by (cutting open and) inserting baffles in the end tanks...

Michael Benno

hey Ken, thanks for the interesting insight. I didn't even know about the number of passes in a radiator but it sure makes sense now. Is the OEM radiator (new or old version) single pass radiator and the CSF is a triple-pass? I looked on the CSF website and they do not mention this but they do mention other cooling benefits:Maybe @Mark Anderson can provide some details on the number of passes and how the end caps are different on the CSF unit. People who use this CSF report on cooler peak temps.

I remember having a 928 MS radiator in my previous S4-5spd (which leaked and I had repaired). Its core was about 1" thicker than my OEM radiator. That cooled very well and never noticed many temp spikes as compared to my GTS. Both cars used only one end tank cooler. I wonder if that radiator is 3-pass.

I found some pictures illustrating how the insert redirects coolant. Also very interesting to see the cooler in the end tank.

Detail of the OEM radiator rows

FredR

Ken,

Until you mentioned it I had no clue of the existence of an early model radiator variant and I still have no clue when the final design variant was introduced but I presume it was somewhere around 1982. However that is all academic as this thread is about whether the insert as fitted to Michael's GTS can be removed without detriment. My point is that whatever the reason for its inclusion, that insert is not listed as a separate part in PET and the only detriment noted was in Jeff's thread when it was somehow fitted *** about face and thus gave his cooling system all kinds of grief.

As for converting the radiator into a three pass configuration why would one want to even consider such? You run a supercharger on your S4- is the cooling system struggling? GB delivers motors making in the region of 600 bhp and he still uses a stock radiator as I recall. The cooling system limitation is not the radiator it is the ability of the water pump to transport coolant around the system and the specific heat of the mix is about 3.5 kj/kg/decgree C and the only way that can be improved is by dumping the glycol and running with 100% H2O.

Whereas thermal performance of a radiator can be improved by a modified pass arrangement to have any kind of chance of additional performance in a 928 it would need a completely different water pump. By going to a three pass configuration the cross sectional area for flow would be a third of the conventional setup and the specific pressure drop would iincrease 9 fold, add to that the flow has to travel 3 times further and the pressure drop across the radiator would increase by a factor of 27 and the water pump could not possibly cope with that or anything close to it. Also trying to fit a pass partition baffle with the cooling coils in the end tank, although not impossible, would be a nightmare to implement.

Kevin in Atlanta

Another data point. This is the sleeve in an 86.5. original radiator.

PorKen

Note also full thickness, widely spaced core.