Porsche torque plates

snowman · 03-07-2003, 09:10 PM

This question relates to both the 911 and 912/356 engines, although there are differences, I suspect this forum is the place to ask the quesitions.

I fabbed a torque plate for both the 912 and 911 cylinders, for both boring, and honing. It consists of a 2" Thick Al base plate with a hole in it for the cylinder, plus helical inserts for the "head bolts". The top plate is 1.5 inch Al plate with thru holes for the head bolts and a large hole for the boring bar, hone. On the 911 cylinders I do not have so much concern as the top plate rests on the top of the cylinder, just as the head does. But on the 912/356 cylinders my top plate rests on the step at the top of the cylinder, and not on the actual top of the cylinder as the head does. The reason is that the top is just to small, for 86 mm pistons at least. I think it is to small in any case, maybe it isn't.

I first bore the cylinder to within 5 thou of the final dimension, then I torque ithe "head bolts" in the fixture to the spec for the head bolts, and rough hone the cylinder to within the last 0.001". Next using the fine stone, or 280 stone, I hone to the final dimensiion, and follow that by a plateau hone or ball hone. I notice that the cylinder distorts as much as 0.005" when I first torque it down. BUT the first 1/4 inch at the top and the last 1/2 inch at the bottom of the cylinder, that isn't in compression by the fixture is slightly differen't, by as much as 0.002" as the compressed part.

Will this be a problem?? The ring pac just reaches the top area in question, and at the bottom it never comes close. What do others do for torque plates for the 912 engine? Or do they even use them?

Wayne 962 · Wayne 962's Garage

Wow, it's too late for me to understand what you're trying to do here? Can you try to summarize exactly? Hone and bore under tension? Unrealistic unless the cylinders are also the same temps as the engine, I would assume...

-Wayne

snowman · 03-07-2003, 09:32 PM

Well hone under compression, ie just like when mounted on engine block with cylinders attached. Std practice with high perf engines. Usually not done at temp, but some do. It makes a significant difference. cylinders may be perfactly round, except when torqued down, as these are, they actually change about 0.005 from round. If they are final honed to round under torque, they should be the some in the engine.

Britwrench · 03-08-2003, 08:14 AM

Bearing in mind the longevity of the average 356/912/911 engine and the usual condition of the cylinders, even at rebuild time they are usually within tolerance, I think you are finding the answer to a problem that doesn't, in the real world, exist.
If the cylinders are worn, just buy new P/C kits.

snowman · 03-08-2003, 06:09 PM

The main reason for boring cylinders is you can't get them as a pair for some racing applications. For example cutom forged JE pistons for a 912 use only cast iron cylinders which must be bored out to 86mm.

The reason to finish hone a cylinder using torque plates is to get that last couple of percent performance that the factory usually throws away for cost reasons. It really does make a difference, especially in a race engine.

The factory almost certainly uses torque plates to hone their race cylinders. The porsche machine shops that I have talked to all use torque plates for this purpose.

snowman · 03-08-2003, 06:17 PM

I might as well add the following:
Some people, in addition to using torque plates, also bring the cylinder to actual operating temp for the final honing.

The honing steps also are said to make a difference, ie which stones are used to take the last few thousands off, which stones are used to take the last 1/2 thousand off, the final hone stone, plateau honing, if used, the hone brushes used to clean the cylinder, and a few more things I do not remember off the top of my head. all this stuff is progressing rapidly as of late due to , not performance, but pollution emmission control. Turns out performance also improves as a side benefit.

snowman · 03-08-2003, 11:25 PM

Here is the 911 cylinder in the torque fixture and the Sunnen Hone.

I am trying to improve the state of the art, and am interested in talking to people who have tried anything to improve it. What I mean by that is going a step beyond the nascar guys.

TimT · 03-09-2003, 06:06 PM

Hone and bore under tension?

Actually he is boring and honing while the sleeve is in compression. This mimics the actual operating state of the engine.

The torque plate is used to bore and hone a block so that it will be in spec when it is operating at its designed output.

Ive read that some F1 motorcycle builders hone the cylinders to look like a cloverleaf, and when its at operating temp it is a perfect circle.

Having perfect specs at room temperature isnt worth a fart in a high wind... you need the specs to be right when the engine is actually running..

However the factory specs we build our engines to make compromises, and allow the engine to run. There are many tricks to make more power with these engines....

snowman · 01:24 PM

Some people have even been known to toss the thing into an oven just before final honing or to run hot water thru a coil of copper pipe wrapped around the cylinder, but it dosen't make much difference with these particular cylinders. On Water cooled engines, some do hook up hot water. Does it REALLY make a difference though.

For example on an air cooled engine, I suspect that one side is hotter than another due to how the air is flowing, this is very significant in distorting the cylinder. How to compensate for that?

I am willing to bet that use of the torque plate achieves the biggest chunk of the available gain from predistortion of the cylinder.

911pcars · 01-05-2005, 06:12 PM

Snowman,
I understand some engine builders (motorcycle-type) bore/hone their cylinders to take into consideration the top 1/3 of the cylinder runs hotter and thus expands at a greater rate. What do you know or have you heard about this practice?

Sherwood

snowman · 06:49 PM

Don't know a thing about it.

But it would be an easy thing to do. If the top is hotter it is bigger so the bore should be tappered. Not a hard thing to do, in fact a hard thing NOT to do.

350HP930 · 01-05-2005, 07:10 PM

I too built a jig to hold my cylinders for a little tweaking with a diamond hone.

In the end faced with all the temp and other variables to concider I ended up honing it using just enough clamping force to keep the jug from spinning.

I realized that it would be hard to mimick the very same forces the cylinders would really see in the motor so I decided to not attempt a possibly incorrect skew. My bores are now round and straight when unloaded so I will have to hope that they distort relatively evenly when they assume their final position.

The problem with honing them in a really tight jig is that the bores will not hone straight since the hone pressure will distort the middle of the jug more than the ends resulting in an hour glass shaped distortion of the bore.

snowman · 01-05-2005, 07:22 PM

The distortion caused by the clamping seems to be repeatable in the actual engine as the power does increase when cylinders are honed using a torque plate. Still not as in the real engine, but closer than not.

aigel · aigel's Garage

I have not heard of torque plates on 911 engines but they are real common in the american V8 world. I suspect that these may be two completely different worlds and that the cylinders on a flat 6 may not change shape much from being torked. The geometries and materials, and especially the applied torque values are very different. An american V8 calls for about 80 ft lb of torque on a cylinder head bolt, while the 911 has what? 24?

On the american V8, you are clamping down a single head, across 4 cylinders, with themselves are all connected again. Also, the cylinders have threads for head bolts tapped right next to them. All is riddled with water jackets. I am fairly certain that the stress and the deformation is complicated to predict and may add up in some regions to large displacements.

Car Craft (unfortunately I don't keep the issues) had an article about torque plates about 2-3 years ago. There was a bunch of data taken by a machine shop on small block fords and chevies. They would bore engines with torque plates and measure out of roundess of the cylinders before and after the removal of the torque plates. There were some significant numbers between torqued and non torqued bores, at least for racing applications.

Before I have seen data showing the deformation of a flat six cylinder as it has been documented in the V8 world, I would not give it too much thought. The cylinders are very symmetrical, there is only one head, 4 bolts, no cooling jackets and lots less torque.

It would be great if any of the torque plate users could supply data. Maybe you are onto something and we can find a couple of horsepower -each cylinder.

George

350HP930 · 01-07-2005, 11:10 PM

After doing a lot of honing work on v8s, v6s and i4 with torque plates I can tell you that on most motors the distortion never amounts to more than a thousandth of an inch and its usually only about a half or a quarter of a thousandth if that much.

mpdevelopment · 01-08-2005, 08:51 AM

How much torque are you using on the studs?.005" seems like a lot of distortion from clamping the cylinders. New cylinders from Mahle are absolutely round, some newer 911 cylinders may be tapered (smaller at the top) I'm not sure, but Pratt and Whitney aircraft engine cylinders are tapered. The torque plates used on American V8s are used to compensate for the head bolts that screw into the deck of the block and thus distort the top of the bore. If the studs screw into the bottom of the block (944,928,most Ferrari ) very little bore distortion is introduced.

snowman · 04:52 PM

I have talked to several VERY well known porsche machinest who also use torque plates. The ammount of distortion can be totally scarry, extreem (more than 0.005 to 0.010" in spots) and the first time I wondered if I wouldn't do some damage by honing the cylinder this way. But it seems to work. Differen't types of cylinders distort differently, the thinner ones are worse than the thicker ones. As to Chevy 350 blocks I have personally seen over 0.010" difference with and without a torque plate and know of HP gains of 10 to 25 HP depending on heads used, torque, bolts, pistons, power level. Chech any HP shop and they can verify this.

And why is this SCARY? because there is only 0.002" clearence to the piston. THe predistortion MUST be correct or the piston wouldn't move.

Question to 350HP930, why are you using the torque plates then?

Anyone not beleiving this should bolt up a cylinder and measure it. Its just one more thing that makes one engine faster than another.

350HP930 · 01-08-2005, 11:19 PM

Quote:

Originally posted by snowman
Question to 350HP930, why are you using the torque plates then?

For the sake of perfection, but there are cases of where the extra effort is reaching the point of diminishing returns because the difference is negligible.

I would say that anyone who sees 5 thousandths of distortion needs to recheck their torque plate or what they are torquing because something is seriously wrong in a situtation like that.

snowman · 01-09-2005, 12:20 PM

Suggest you try some 912 cylinders bored out for 1725cc pistons.

Anyway I don't know why this subject seem even slightly contraversal, it been well established by Smokey Yuinc many many years ago and everyone in the racing business does it as almost std practice.

aigel · aigel's Garage

Snowman:

I think it is controversial, because boring porsche cylinders isn't done as readily as it is done on V8 etc. If you have a 911 engine rebuilt by a shop, they will go and buy new mahle p/c for you and install them. Most shops that build engines around here would not offer a rebore at all. I can see where they are coming from. If it comes from Mahle, it is new, fits out of the box and makes a warranty on the builders work much easier.

Cheers, George