Chain Tensioners - Race Engine
 

I'm hoping some of the racers on the forum may have a point of view on this. I have a 2.9 ltr full race engine in my 71 911 that currently has the adjustable solid tensioners. If adjusted properly are these the best to use in a race engine. The car is only used for racing with an RPM shift point of about 7200.
I ask as I frequently see the pressure fed tensioners being used on cars I race with.
Thanks for any comments
Geoff

Your mechanical tensioner is only correctly set once during the heat and change in engine temperature. The Carrera tensioner is always carrying proper pressure.
Bruce

Geoff,

I prefer the regular non-pressure fed tensioner (turbo tensioner with wide idler arm) over solid and pressure fed. Use a collar to insure AGAINST failure.

Pressure fed can over pump at high rpm as well as under pressurize during shifts. The IMSA 962's discovered this issue.

I run this setup on engines with 8700 rpm redlines.

I sure hope the word "against" is missing.... ;)

Scott

Chain tensioners are an interesting subject.

The failure of tensioners seems to have been an issue with 911 engines since the very early days and still is being debated after almost 50 years.

I like the idea of a simple mechanical device such as a solid tensioner as there is not much to go wrong and they are very unlikely to break.

I can understand why a spring loaded or hydraulic tensioner is a good idea on a production engine as it avoids the need for each engine to be individually adjusted and does allow the car to be driven away the instant it starts.

On a competition engine with a narrow operating band in terms of temperature I am not sure that this is needed.

The life of a chain is also an interesting debate.

It is often said that chains 'stretch' and although they ceratinly do get longer with age this is due to wear rather than the chain material plastically deforming.

Wear of the roller pins and the links takes place which allows the chain to extend.

the question is when does it wear out and how should this be measured.

With a solid tensioner any wear will be relatively easy to identify as the noise will be obvious.

With a tensioner more wear will occur before any noise shows up and chains may be quite badly worn.

I would also like to know if chain wear affects cam timing to a significant degree?

Ha!, it was a long day...

I would say chain wear can easily affect timing settings. My belief is based upon the use of the solid tensioner to set timing. When you use the solid tensioner, there has to be some judgment used in establishing how much tightness/preload you put on the chains. Depending on your preferred amount of preload, the timing will change a noticeable amount on the dial indicator.

Therefore, since the tension setting on the solid tensioner is similar to chain wear. A looser preload on the tensioner is similar to chain "stretch"

I agree the chain tensioner debate has been going on for a very long time. I've heard others say that the tensioner has often been blamed for problems that are more a function of the chain idler arm that the tensioner bears on. I tend to agree that a lot of chain tensioner failures were based on the failure of the idler arm causing the tensioner to be over-extended.

Seems to me that's why Porsche updated the idler arm a few times. First iteration was a narrow, unbushed arm (which can seize on the pivot shaft and caused sprocket damage- happened to me a few years ago), second iteration was addition of the bronze bushing and third iteration is the wider arm pivot area with bushing.

I also think the non-solid tensioners are designed to dampen noise?

The collars themselves look like they could also be the cause of a problem down the road.. (I bought them but never installed them)

I run the solid tensioners. After three years, about 10 race weekends never had to adjust them. I checked them but the chain tension was good. One of the few pieces on the car that doesn't need my attention.

AAron... can you provide more backgroud and the 962 issues?

Thanks to all who provided comments. This was my first time using the forum and I found the depth of comments on this topic and others I found to be a valuble resource.
Geoff

the carrera style tensioner has a HUGE STIFF spring in it that provides most, if not all, of the tensioning function

it's not at all clear that the oil feed does anything to add to the tension - mostly it just lubes the thing and maybe cushions against a sudden shock

imho - the carrera style tensioner, even if not hooked to oil feed, would be a better choice than a non-flexible mechanical device - you could even use an old used leaky carrera tensioner (i.e. - cheap) & still have substantial control of the chain flex

- and if you want even more tension just disassemble it & stick a washer under the spring - just like we've been doing to boost engine oil pressure on myriad cars for ages

.

As I recall now it wasn't a over pressure issue... the 962 was the first engine to utilize the pressure fed tensioners. Which 962, I do not know. The tensioners would bleed down under low rpms, say a tight corner at the end of the straight. The timing would alter and the valves would make contact the pistons. The engine would then run a bit rough but the extreme heat would re-seat the valves. This happened every time at that particular corner throughout the race. The 962 even went on to win the race.
Upon disassembly they saw the telltale marks on the pistons, but it took some time to track down the what the issue was. I cannot say how accurate this account was, but it's a neat story. If I find out which car(s) and race(s) I'll post it.


I have also heard of damage to magnesium chain boxes with pressurized tensioner, high rpm and big oil pumps due to high oil pressure pumping them up....but I have not witnessed such a thing. It would be similar to hydraulic lifters over-pumping at higher rpms than they were designed to run.

Personally, I do not see the need for the pressurized units. They have the same failure rate as a "turbo" tensioner with the updated idler arm and is another source for a leak. Seems like a solution in search of a problem.