16v timing modification -- your opinions?

pontifex4 · 05-20-2009, 11:11 AM

(This is a cross-post from 944 Online) view the original here
I was able to find a very tidy replacement head for my 944S from a race series car in the midwest, but it comes to me with an unusual modification:

The oil supply line to the tensioner has been machined off, in order (as I understand it) to allow the pad to yield more easily under acceleration, thus advancing intake timing earlier.

Most people I mention this to regard it with disbelief, however the previous owner indicated that it does, in fact work, and causes a fairly smooth change over at around 3,000 RPM.

I haven't installed the head, yet, and I could use your opinions on it: is this dangerous or likely to fail? If you think so, why?

Razorback1980 · Razorback1980's Garage

Keeping in mind that I am not familiar with that model of head....the first question that came to my mind was...how is the tensioner being lubricated? If it's not being lubricated, don't you think it would wear much quicker? The other thought I had was...is the timing chain tensioner yielding easier something you really want...I don't know the answer to that one.

Wipeout · 05-20-2009, 12:59 PM

I could be wrong, but I thought only the 968 engines had any timing changes thanks to the variocam. S and S2 engines only use the chain between cams to keep the cams in sync.

rb101 · 05-20-2009, 01:31 PM

I was under the impression that the oil tube was to supply oil pressure to the chain tensioner to apply tension to the chain. I would be concerned......

pontifex4 · 05-20-2009, 02:32 PM

Rick: you're right about the oil supply being there to keep pressure on the top pad. There is also a spring (which does require a fair amount of force to compress), so the oil pressure is supplemental. What's more, I'm not convinced there is very much oil pressure on the pad, since the line into the tensioner is long and quite thin.

As for lubrication, when the engine is running, there's oil all over the place up there, and I'm guessing that lubricating this particular part isn't a big priority, anyway (no bearings, and the chain guides are plastic pads, along which the chain glides).

Variocam in a 968 uses a solenoid to do what I suspect is more or less the same job: retract the pad at a particular point in the rev range to advance intake timing.

Since it came this way, and the claim is that it works, I'm tempted to try it.

Has anyone actually heard of this being done?

rb101 · 05-21-2009, 10:04 AM

I'll just be worried if there is too much slack all the time without the oil pressure to allow the chain to slip a tooth or two. Is the oil tube blocked off at the tensioner end? Can you send a picture. I'd like to see this modification.

Thanks

pjburges · 05-23-2009, 09:34 PM

Ive got an S motor apart in the garage right now....also curious. Im not sure exactly how this little system works just yet. Sure would be simple to do a couple dyno runs - that valve cover pops right off - J-tube in for oil - jtube out for none...

fast924S · fast924S's Garage

Intresting Idea. I dont think wear would be a issue as the oil pressure is only there to apply force and keep a steady pressure on the chain. I have not played with a 16V head but Im assuming the chain is conneteced between the intake and exhaust cams Correct?? SOOOO if this is true your also effecting the exhaust valve timing. Sounds like it would work but who knows if its advancing or retarding the timing? Also by how much? Now what would be really cool would be to play with the spring tension too and kinda do a Vtect setup. GEt a stronger spring so at low to mid RPM it would be normal, Then high RPMS it would change.

One concern I have is I believe the oil pressure is also there so the Pad/tensioner/chain asembely doesnt start to bounce around and blow itself apart. The spring and oil pressure setup is kinda like a little coilover, Take away the oil and the chain may rattle alot and cause things to wear more cuz it just bounces off the spring.

pjburges · 05-24-2009, 08:03 AM

I agree, I think the oil pressure damps the motion that the spring does not. However on the 968 doesnt the variocam solenoid RETRACT the pad at high RPM's to change the angle of attack on the chain? Maybe at high RPM's in a racing environment there is a centripital force on the chain that makes it easier to run without the pad pressure? Still not convinced this is a good idea...

Endat · Endat's Garage

I bet it would work, but I would be worried about the functions that need the oil tensioner, like the computer timing and knock sensor... I guess keep 91 octane and you wont have to worry about it?

Do you have a pic of this head? I'm putting an S together right now

pjburges · 05-24-2009, 08:35 AM

The S has active knock control and 2 knock sensors unlike the 8V versions.

pontifex4 · 03:37 PM

PJ: What I'm most concerned about is getting into a situation where the chain starts bouncing the tensioner, due to the lack of damping noted above.

In considering it thus far, I have assumed that there is actually quite a bit of chain tension when the engine is running, and given that, in this configuration, the intake cam advances at > 3,000 RPM, I have assumed that the tensioner exerts a fair amount of force on the chain via spring pressure, alone.

By way of background for anyone just tuning in, the 968's Variocam does use a solenoid to retract the pad at a programmed point in the rev range. The configuration of my head (J-tube removed) does something similar by yielding when the chain tension gets high enough.

All this to say that I don't think there will be much slack for the chain to bounce. I'm going on theory, alone, here: how tight are these chains when they're running?

Thanks for all your interest, everyone!

SoloRacer944 · 05-24-2009, 06:08 PM

I just replaced the upper tensioning pad on my S2 last week. Keep in mind that the diameter of the oil line that feeds the tensioner does not have to be large since the tensioner requires very low flow similar to your brake lines. Next, the rotation on the motor is clockwise as you stand in front of the car. The crankshaft is driving the exhaust cam which in turn drives the intake cam. The exhaust cam is therefore pulling on the timing chain on the bottom (fixed) side of the tensioner assembly which is the flat side. This allows the chain to return along the top side (moving due to spring and oil pressure). I don't think that eliminating the oil line will change the timing other than the possibility that the spring pressure may not be enough allowing the chian to wrap around either cam sprocket and cause damage.

I'm keeping the oil line connected on my car.

Jim
91 944s2

pontifex4 · 08:09 PM

Quote:

Originally Posted by SoloRacer944

...This allows the chain to return along the top side (moving due to spring and oil pressure). I don't think that eliminating the oil line will change the timing....

Jim, I think I was imagining this backwards, as I had thought the direction of rotation was the opposite of what you're stating, and thus that the increased tension on the pad from the pull of the exhaust cam would be responsible for advancing intake timing. This doesn't make much sense, though, since the tensioner on any chain should be on the return side, where it doesn't need to deflect the chain at its maximum load.

What's more, I don't think the chain's centripetal force would depress the pad, since all the angular momentum developed at the sprockets would allow the top tensioner pad to move up as the chain attempted to spin in a circle, not push it down.

It may be that this is a sham modification, unless anyone has any other ideas?

SoloRacer944 · 05-25-2009, 06:26 AM

I did not measure the diameterof the piston of the tensioner, but it looked to be around 1/2" diameter. Even with 5 bars of oil pressure that would only put about 14 pounds additional force on the tensioner. The spring force was more than 14 pounds, so i am assuming that the oil feed is acting more of a dampener (shock absorber) to the spring tensioner.

I was glad that to change the upper pad was only a 1/2 hour job and I didn't have to remove the cams.

pjburges · 05-25-2009, 09:08 AM

I heard somewhere that the 968 cams had less teeth on the sprockets? I believe 13 teeth? Even if the tensioner is deflecting the chain on the return (top) side...it should still advance the intake relative to the exhaust...

But yeah Im imagining that below 3000 RPM the chain may pick up a harmonic because the load (valve springs) are pulsed on off and this may cause the chain to slap requiring the damping action of the oil tube. Drive above 3K and apparently we dont even need this tenisoner....?

Dave L · 05-25-2009, 09:20 AM

Quote:

Originally Posted by pjburges

I heard somewhere that the 968 cams had less teeth on the sprockets? I believe 13 teeth? Even if the tensioner is deflecting the chain on the return (top) side...it should still advance the intake relative to the exhaust...

But yeah Im imagining that below 3000 RPM the chain may pick up a harmonic because the load (valve springs) are pulsed on off and this may cause the chain to slap requiring the damping action of the oil tube. Drive above 3K and apparently we dont even need this tenisoner....?

On a race car you would pretty much always be above 3000RPM on a daily driver you do have a lot more stops and starts. I dont know that this modification is good or bad but I wouldn't necessarily say that if it works on a race car it should work on the street.

Dantilla · 05-25-2009, 12:26 PM

Quote:

Originally Posted by SoloRacer944

I'm keeping the oil line connected on my car.

Me too.

pontifex4 · 05-25-2009, 04:08 PM

Quote:

Originally Posted by Dave L

On a race car you would pretty much always be above 3000RPM on a daily driver you do have a lot more stops and starts. I dont know that this modification is good or bad but I wouldn't necessarily say that if it works on a race car it should work on the street.

Dave -- I agree, only I'm not even sure it would work on the track;

Coming back to the point about the chain, if you spin it really fast, shouldn't centripetal force cause it to try to form a circle, rather than a loop whose top and bottom runs are more or less parallel? In other words, wouldn't the chain be inclined to lift up when driven quickly, rather than put pressure on the pad?

If only I had the head together so I could verify this!

mistertate · mistertate's Garage

Quote:

Originally Posted by pontifex4

Coming back to the point about the chain, if you spin it really fast, shouldn't centripetal force cause it to try to form a circle, rather than a loop whose top and bottom runs are more or less parallel?

Armchair physics. There's a tension side and a slack side. The tension side will get tighter as the cams accelerate. You only get centripetal force when there is a change in direction. The tension side is traveling in a straight line until it goes around a sprocket. The tensioner has pads on BOTH sides to keep the slack and the tension sides tight. The bottom (smaller) pad is the tension side, the top is bigger because there's is more slapping around going on the slack side. You MIGHT get some intake cam advance but if the oil pressure feeds both sides then you'll get a lot more slapping around there.
I'm guessing whoever came up with this idea saw one of those tensioner delete kits for a Honda F4i bike motor. Those kits have to have the tension set when the engine is hot or you'll bend valves.
Any one could easily test this in about 20 minutes on dyno, taking off the valve cover, blocking off the oil feed and doing back to back runs. Curious but skeptical. You try it.