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The shims I've used seem to have some sort of tacky coating on them. I wonder if that would account for the extra thickness Dave noticed?
-Chris
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Old 08-05-2005, 06:40 AM
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I now have a set of all three available shims -- 0.25, 0.5, and 1 mm. The two thicker ones are copper, and were from EBS. The thinner ones are a dark color, not obviously copper, but they could have some sort of coating on them. Anyone seen or have any concerns about these dark colored shims? If I have to stack them, can I mix the dark-colored rings with the copper-colored rings?

I didn't notice that any of the shims have a tacky coating on them, Chris, as you suggested.

I haven't measured the thin ones yet, but the thicker ones are all prettty close to their specified thickness, though there is some variation around their circumference. For instance, the 0.50 shims (which is equivalent to .0196 in), measured from about .019 to .021 in. That's a .003 variation. I'm not sure if that's a real thickness difference, or if it's a slightly deformed edge from the stamping process (that would flatten out under pressure).

So, I get some people saying don't stack them, and others saying it's just fine. I talked to a local Porsche engine builder just now who stacks them "all the time" with no problems. Sure, I'd rather not do it, but I'd also like to have the correct deck height. Has anyone, ever, seen a documented problem from stacked shims? If so, can you describe what happened? Does stacking increase the potential need for sealant?
Old 08-05-2005, 04:40 PM
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I'd use the 1.0mm base gasket, install it dry and be done. If you want more power, play with the boost.

I have also heard that stacking is not a problem from several knowledgeable builders, as long as you don't get crazy.

Ever motor I have installed dry base gaskets on has never had a leak at the cylinder bases.

I don't know if the deck height on a turbo motor with flat top piston is as critical as a motor running a domed piston?

Tinker
Old 08-07-2005, 02:10 PM
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Tinker,

From my understanding, the risk for increasing deck height beyond spec (where "spec" is 0.039 in.) is that it puts a pocket above the piston that encourages detonation. Likewise, the risk in having deck height too low is that the piston may hit the head. Both are bad options. I'm not worried about exactly what compression ratio results from the deck height setting -- I'll live with whatever I get, though if I had a choice, I'd optimize by setting the CR as high as I can get (which means deck height as low as I can get) as long as I don't risk piston/head contact.

I've spent hours measuring. Here's the dilemma: I have two choices; go with two shims that give deck heights ranging from .0315 to .041 in. or put in a single shim that gives deck heights that range from .040 to .0495 in. Does anyone have any guidance that would help me decide?
Old 08-12-2005, 01:11 PM
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my vote:
go for the .0315 to .041 spec.
Is the variance in the above spec based on the combination of shims used or are you seeing that much variance from cylinder to cylinder?
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Old 08-12-2005, 01:57 PM
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Dave,

Thats the variation from cylinder to cylinder (using matching shims). Seems like a pretty large range, doesn't it? But I have absolutely no data to compare it with -- I've never checked it before. I suspect that there may often be a large variation in deck height. It's just that so few people actually check it that we don't see the numbers very often. I could actually reduce the range considerably by putting different shims under two of the cylinders. But I'm not sure that's a good thing to do -- it would raise the top of those two cylinders. And I checked that bank of cylinders with a straight edge (as is) and they're quite close (within 0.004 in) right now, so I hate to mess with it. Any guidance on using different shims on different cylinders to achieve a closer range of deck heights?

I'm leaning toward using the range you voted for too. I've seen specs that for "steet" engines, deck heights can range from 0.8 to 1.2 mm (though it doesn't say anything about the average and standard deviation for cylinder to cylinder variation). That's a range of .03175 to 0.0472 in. That puts me right at the lower threshold. Does anyone know if "racing" engines have deck heights that are typically lower or higher than street engines? I suspect lower, but I'm not sure. Anyone know how low a deck height can you safely have on a 930 engine?
Old 08-12-2005, 02:14 PM
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When I was living in Massachusetts, I often sought the technical advice of Jerry P of European Performance Engineering. I told him during my rebuild, that I had a deck height of .031. He stated that was too close and recommended 1mm. So I took out the 25mm shim and used the 50's.
Yes, I think think there is too much variance in the deck heights. It may not be that you are getting variances in deck height, but measuring discrepancies due to pistons rocking in the bores. Are you measuring above the piston pin centerline? If you measure above the piston pin axis, you'll see less of the effects of the pistons rocking during the up and down stroke. I also try to take my measurements using the same technique. I measure deck height while the piston is approaching up to TDC. That way I know the slop is taken out of the crank to rod journal.
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Old 08-12-2005, 02:34 PM
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Dave,

Thank you for your response.

I just sought counsel from a prominent local engine builder. He said the same thing -- 0.031 is too close for comfort. He recommended going higher, to the next step, which has me at 0.040 in. minimum.

Your question about measurement technique is a good one. I've spent many hours trying to eliminate variations due to my measurements. But I'm not sure I've succeeded. So, there's always a possibility that I'm somwhat to "blame" for the measurement variations. I hadn't thought of making sure that I'm always approaching TDC from the same direction. I will do that and see if it makes the readings more stable.

What I have been doing is measuring at four positions (12:00, 3:00, 6:00 and 9:00). I haven't reported the 12:00 and 6:00 numbers, figuring that they are contaminated by piston rocking, but I record them nonetheless, figuring that they provide a reality check in general. Plus, their average ought to equal the 3:00 and 9:00 numbers (across the piston pin). And it generally is...

But what bothers me is the variation from one side of the piston to the other, and the variation from cylinder to cylinder. I think I'm going to pull the pistons off and have them very carefully measured by a machinist. Same for the cylinders. Then again, there's a lot of stuff that legitimately vary when making these measurements, such as the rod bearing clearance and the wrist pin clearance. I just don't have enough experience yet to know how much variation to expect and to know what is normal or acceptable.
Old 08-12-2005, 04:30 PM
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Maybe you can measure the deck height without the base shims?
Also, are the cylinders the same group number? I think it is the number inside the "triangle" cast into the cylinder. All cylinders per bank must have the same group number.
I'm not sure of your background, so I'm just throwing these ideas out to you. Btw, arew you using Wayne's Porsche engine rebuilding manual? Pretty good. Minor differances with the 930 engine, but they're pretty obvious.
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Old 08-12-2005, 04:44 PM
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I am surprised that with all this talk about deck height, no one has asked if the heads have been modified or stock. To bring the mating surfaces back to a perfect fit and increase CR (previous owner), my heads were shaved. Therefore my deck height needs to be .060 to ensure proper CR, clearance and so on. When I tell folks about my deck height, they fall of their chairs. But that's what is required.
Old 08-12-2005, 05:26 PM
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Quote:
Originally posted by DonE
I am surprised that with all this talk about deck height, no one has asked if the heads have been modified or stock. To bring the mating surfaces back to a perfect fit and increase CR (previous owner), my heads were shaved. Therefore my deck height needs to be .060 to ensure proper CR, clearance and so on. When I tell folks about my deck height, they fall of their chairs. But that's what is required.
In my ignorance, shaving the heads so much that you need to run 50% more deck than spec seems like a losing proposition to me. Is a turbo motor that much different than a normally aspirated motor that this sort of compromise is acceptable?
-Chris
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Old 08-12-2005, 06:51 PM
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Dave,

I've got Wayne's book. It doesn't address this issue in much detail. My cylinders were all reportedly machined to exactly the same height, even though they all came from the same height group (but I'm going to double check them). Also, my rods were sized and machined to be absolutely equal in length with both small and big end bores perpendicular to the rod axis and parallel with each other. The cylinder spigot decks were machined to be within 0.001 of each other. The only variables I haven't checked are thepiston dimensions from the pin to the crown; I'm beginning to suspect that there may be some variation. And then there's the possibility that rod bearing and piston pin clearances are allowing things to move around a bit and increase the variability in my measurements.

Don,

I hope someone with more experience will chime in here, but I don't think it's correct that you should increase deck height when you shave the heads.

Deck height, to my understanding, is to be minimized, as long as there's adequate clearance to keep the pistons from hitting the heads. (But exactly where that threshold is, I don't know.) If you shave the heads, it doesn't change the equation for piston-to-head clearance. (The piston "doesn't know" that the head has been shaved; it just needs adequate clearance from it). Piston-to-valve clearance is another story. Shaved heads bring the valves closer to the pistons, and that has to be watched as well. But to gain piston-to-valve clearance by increasing deck height seems risky. It's my understanding that to do so will create a pocket or space above the piston where unburned fuel vapor can ignite unintentionally, causing detonation. These considerations are critical on a high compression engine, and especially a turbo engine.

If you need to increase piston-to-valve clearance, it would have to be done by increasing the valve pockets in the piston, changing pistons altogether, or changing the camshaft. I don't think you want to increase deck height to gain piston-to-valve clearance unless you're able to do so by staying within the upper limits acceptable deck height (whatever that is). My concern, Don, is that 0.060 in. sounds pretty high.

Problem is, I don't know the range of acceptable values. How little deck height can you get away with safely? How much deck height can you get away with safely? Do these limits depend on the exact profile of your piston and combustion chamber? How much varation in deck height is allowable across the piston pin for one piston? How much variation is acceptable for all six pistons? Is it okay to use different shim thicknesses to acheive more equal deck heights as long as the top of the cylinder deck for all three cylinders on a bank stay within certain height limits? What are those limits?

Looking for answers from an expert here...
Old 08-13-2005, 10:13 AM
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Quote:
Originally posted by ChrisBennet
In my ignorance, shaving the heads so much that you need to run 50% more deck than spec seems like a losing proposition to me. Is a turbo motor that much different than a normally aspirated motor that this sort of compromise is acceptable?
-Chris
The PO, the DSM for PCNA located in Denver, shaved the heads to bump the compression to 7.5:1. Since I now have custom pistons that provide 8.0:1 CR, I must compensate for the reduced clearance between the piston face and combustion chamber. By setting the deck height at .060, I effectively acheive stock piston to head clearance for proper squish.
Old 08-16-2005, 07:00 PM
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Very interesting thread, can I revive it to ask about my measurements?

I'm building up a 3.0 street/track toy with mahle cylinders, JE 9.5CR pistons, standard rods & crank, heads and new valves/springs. The cams are DR-20. Single plug.

I measured my #1 and 4 deck heights today using wax (no acid core solder in the area?!?) and got a consistent, but higher-than-expected 1.55MM (.061") deck height on all four measurements (per Wayne's book measuring on the axis of the wrist pins.)

The heads have been fly-cut. I haven't been able to find a burette and so wasn't planning on cc'ing.

Knowing the cutting brings the valves closer to the piston, I am mindful of wanting to err on the side of having a higher deck height, and will be measuring piston/valve clearance when I have the towers on.

It sounds like from many of these posts above that 1.55MM is out of range on the high side. I have only one .25mm copper gasket between the cylinders and case spigots.

Is it acceptable to run without any gasket? 1.55 seems like way out from what others are recording.

thanks all
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Old 10-22-2005, 03:54 AM
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I'm no expert but......

With fly cut heads, single plug and 9.5:1 P&C's, I would think you should confirm the CR by measuring. I believe 9.8:1 is the max limit for single plug unless you run race gas. You'd hate to put this together and end up with detonation. You can use a calibrated hypodermic syringe available from CVS in place of the burette.

Also, I've never heard of running without the cylinder base gaskets. Doesn't sound like a good idea.
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Old 10-22-2005, 05:02 AM
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Definately CC them. The last 2 set of "9.5:1" JE pistons I got CC'd out to 10.5:1.
-Chris
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Old 10-22-2005, 06:48 AM
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I'd also recommend CC'ing the heads. If the heads were cut and you have a .25mm gasket, how did you end up with so large a squish band? You could skim some off the cylinders to close it up and I would close it up.
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Old 10-22-2005, 06:02 PM
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Thanks guys,
I cc'd my heads and am quite surprised. I am using Jim's calculations as I don't have 'jo blocks.'

Head volume
Cylinder 1= 83cc
Cylinder 4= 84cc

Stroke
7.04 (standard 3.0)

Vapt
Cylinder 1 = 41cc
Cylinder 4 = 41cc

Bore
95mm (standard 3.0)

Height
Cylinder 1 = 6.0cm
Cylinder 4 = 5.96cm

By my calcs, I get:

Cylinder 1 CR = 10.92:1
Cylinder 4 CR = 11.30:1

These are with JE 9.5 CR pistons on standard mahle cylinders for a N/A engine and a DR-20 (Camgrinder's GE20.)

Please tell me I'm way off. I'd rather be mathematically challenged than be forced into twin-plugging.

john
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Last edited by JohnJL; 10-22-2005 at 09:43 PM..
Old 10-22-2005, 07:35 PM
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John

I found the CR measurement to be difficult and it took me several tries to get repeatable measurements. Check this post that details some of my efforts and those who helped me through it.

http://forums.pelicanparts.com/showthread.php?threadid=213504&perpage=20&highlight=head%20volume&pagenumber=1

Reviewing your numbers and comparing with my final numbers, a couple of observations & questions:

1. CR is too high even for twin plug on street gas

2. Head volume is consistent with my twin plug heads that were machined to give 10.5:1 CR

3. What base gasket and deck ht do you have? Adding another base gasket of 0.25mm will drop CR by about 0.3 pts. You need to make sure that adding a base gasket does not take you deck ht up too high

4. I don't recognize the swept volume and dome volume in your numbers. I used the PCNA (Bruce Anderson) method. I used a small carpenters level in place of jo-blocks and the depth gage on my digital calipers to measure the "theoretical volume". The technique is demonstrated pretty clearly in the photo tour of Jerry Woods shop that Wayne has posted on this site. I can send you a spread sheet to make the calculation using this method if you like.
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Old 10-23-2005, 03:49 AM
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Hi Tom, thanks for the reply.

I am using Jim William's method, here:

http://members.rennlist.com/jimwms/Tech/comp_ratio.html

WRT your comments:

1. Yes, worringly so. However I understand that twin-plugging also removes enough material in the head to add several ccs and so bring down the CR a bit.

2. I measured the heads 6 times, each giving the same results. That included flipping the glass plate over, removing and re-greasing the edge, etc. I am pretty sure those are accurate measures. I had a few thous removed from the head at the mating surface to deck everything perfectly flat. That might account for the trade off where my single-plug cc's the same as your twinplug heads.

3. I measured 1.55mm deck height with a single standard .25 copper gasket. That's much higher than I had hoped for, and is another reason why I think I may need to go twin plug. I am concerned about detonation originating at the edges.

4. I'll give the level-trick a shot when I get back tomorrow. Could you send the spreadsheet?

thanks
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Old 10-23-2005, 11:58 PM
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