Advice on Deck Ht, Comp ration and Shims for my 3.0
 

I am in the process of the assembly of my RSR Clone motor. Since this is my first 911 engine rebuild, I would like some advice on my build clearances so far.

As background, the motor is based on a 1978 SC case that had about 80,000 miles on it before I started. I am building it with new Mahle 10.5:1 95mm pistons and cylinders to retain the 3.0 liters for vintage eligibility. I am using twin plug heads that have been "cleaned up" but not opened up (stock port sizes). I had my Bosch dizzy modified for twin plug by Gerry Woods. I am using the DRC-60 cams (GE-60 grind) from John Daugherty. The motor will be fed by 46mm PMO's and will exhaust through RSR headers with a Porsche style ralley muffler.

The car will be used as a combo street hot rod and track car with Vintage racing a possibility down the road.

The crank, rods and bearing clearances were all confirmed to be within spec before I assembled the bottom end. I just completed the deck ht and compression ratio checks on cylinders 1 and 4 using the standard 0.25 mm cylinder base gaskets. I've included all my measurements below. The first set of numbers is for cylinder #1 and the second set of numbers is for cylinder #4. I measured deck ht both by using my digital calipers and using the trapped solder method. Knowing the trapped solder should give a more accurate deck ht, I used that measurement to calculate the deck ht volume for my compression calculation.


Cylinder #1 Cylinder #4
Caliper deck Ht1 (mm) 2.2 1.55
Caliper deck Ht2 (mm) 1.9 1.43
Caliper deck Ht3 (mm) 2.2 1.75
Caliper deck Ht4 (mm) 1.7 1.62

Solder deck Ht1 (mm) 1.56 1.7
Solder deck Ht2 (mm) 1.54 1.7

The 4 caliper measurements are at 4 spots around the piston perimeter 90 deg apart with the piston at TDC.

The two solder measurements are at either end of the solder piece that I extended across the piston between the valve pockets from cylinder wall to cylinder wall.

Compression Ratio Calculation

Bore (mm) 95 95
Stroke (mm) 70.4 70.4
Deck Ht (mm) 1.55 1.7
Head CC 84.5 83.75
Jo Block Ht (mm) 54.89 54.88
Jo Block to Dome (mm) 61.06 61.27
Jo Block to Deck (mm) 77.75 78.25
Depth H2 (to dome mm) 6.17 6.39
Dome Ht (mm) 16.69 16.98
Cylinder CC 121.5 124.2
Theorectical Volume 162.04 165.65

V1 - Swept Volume 499 499
V2 - Deck Ht Volume 11 12
V3 - Cylinder head volume 84.5 83
V4 - Piston Dome Volume 40.54 41.45

Comp ratio (0.25mm shim) 10.08 10.31

0.50 mm shim
V2 - Deck Ht Volume 12.76
New Comp Ratio 9.80

My deck ht at 1.5 - 1.7 mm is right in the range recommned, so I should not need to go to 0.5mm shims. My compression ratio calculations show that I am on the low side but safe side of 10.5:1. I calculate that the thicker shims would drop my CR to 9.8:1 (too low).

My plan at this point will be to use the standard base gasket thinking that compression will only go up with any carbon build-up in the combustion chambers.

What do you experts think of my measurements and strategy?

This is more proof that Mahle has a little trouble stating accurate CRs. Ralph had the same issue with his 10.5 P/C but they were down in the low 9 range.

Are you happy with the CR or would you like it a little higher? All seems to look okay to me, but I am not an expert.

Jeff

I think I will be happy with something in the range of 10.1 to 10.3:1. The car will need to run on 93 octane pump gas for the street. I suppose I could flycut the heads a few tenths to get to 10.5 but that would be my upper limit.

Anyone want to try and predict the flywheel HP for the motor??

Tom,
I was talking about deck height with Steve Weiner once and he advised me to try to keep the deck height between 35 and 40 thousandths(.040"=1.0mm). I was talking with him in the context of a 3.2->3.4 conversion so this may not be applicable. I think the last two motors I did had deck heights of ~1.2mm (964) and .98 (3.2SS).

If you haven't decked your spigots yet that will raise the compression a little. It's my understand that decreasing the deck height around the edge of the piston into the 1mm range will actually reduce the tendency to knock even though it's raising the compression. Something to do with reducing the volume of the "pocket" out at the edge the piston.

-Chris

Chris

What do you mean by "If you haven't decked your spigots yet"??

What do you mean by spigot deck?

I though that anything under 1.5 mm or so for deck ht would risk piston to head interference. Remember I'm using the standard 0.25mm copper gasket. How would I reduce the deck ht?

I'm learning as I go.......

Another question I have... I'm planning on using the Curil T sealant on the copper cylinder base gaskets. What about sealing the cylinder to head interface? I know that at standard CR Porsche did not use any type of seal. My new P/C set from Mahle looks like it should have some type of seal?

By decking the spigots I mean flycutting the top edges of the holes in the case (where the copper gaskets sit on a pre-964 motor). This makes for a nice flat sealing surface and makes sure all the spigots are the same height.

The classic case (no pun intended) where this is necessary is the situation where a broken stud or two has allowed the cylinder to pound on the case deforming the spigot mouth.

Even when the case hasn't been damaged, decking the spigots gives you a nice square foundation for the cylinders+heads stack.

I would ask some experts about the deck height thing. It may be one of those things where 40 thousands is fine in the context of say sub-7000rpm street motors but insufficient for other motors. I personally don't know.

-Chris

I also agree, the deck height should be set at 1mm. If your turning 8000+ rpms you might leave a little more room, say 1.2 mm. Combustion efficiency goes up with a tighter piston to head clearance.
Henry from Supertec would be a good person to ask as well...

With twin plugs the engine will run fine at 10.3-1 compression on 93 octane fuel.

Some of the top engine builders here in SoCal suggest deck height on a street motor to be between .8mm-1.2mm. FWIW, my deck height is at .75mm with a .5mm base gasket, on the tight side of the spectrum but my Porsche Motorsport buddies all said no problem. I agree with John and Chris and try to keep the deck height under 1.2mm if possible.

Your CR ratio is also lower than you may have anticipated because you twin-plugged the cylinder heads, which I'm sure you know changes the volume of the heads. My buddy milled my heads .020" after our original measurements yielded 9.2:1 on a set that is advertised at 10.3! In Mahle's defense, lots of machine work and parts replacement took place on this 3.2L based motor which changes the parameters and I guess they assume that the motors are also going to remain original single plug when they advertise their CR claims?

Milling the heads up to .020" (and maybe a tad more?) seems to be common. I was able to gain back .3 of compression and remeasured 9.5:1 which is a bit lower than what I wanted (9.8:1) but turned out okay I guess with the 91 octane challenged gas we have here in CA. No use crying over spilt milk. I agree with you, CR probably will change with carbon build-up and other factors and in my opinion it is always better to err on the conservative side unless you want to blend race/street gas on a regular basis. It sounds like you are building a nice street hot-rod motor where you won't require every last available horsepower anyway, especially at the expense of long-term reliability. But, 10.3:1 if you can get there with twin-plugs and 93 octane shouldn't be an issue like camgrinder says.

Everyone has their favorite base gasket to case sealant, I (and others) use Permatex High Tack Sealant but use what you feel comfortable with or what pros you trust use as they build these motors on a regular basis (although it sounds like Chris has a great motor rebuild business going and is fast approaching expert status!:))

Ralph

Thanks for you assessments.

I'm leaning towards staying with the current clearances/CR.

Do I need to do something to seal the cylinder-to-head interface or do I assemble without any sealant or gasket/o-ring?

How about HP/Torque predictions?? Anyone ??

I thought that the 3.0 motors used a CE ring for a head gasket. There should be a groove in the top of the cylinder for the ring. The 3.2 didn't use a gasket so if you have cylinders from a 3.2 then there is no gasket. Just put them together dry. I do what your doing as far a putting a non hardening sealer on the copper gaskets.

-Andy

No one wants to take a stab at estimating the power for this motor??

The guys at Auto Associates say I should expect 275-300 HP.

Does this sound realistic?

If the guy in "guys at Auto Associates" is Jim Newton, then yes, I agree with his estimate.

You are talking 91-100 hp / litre. On a twin plugged high-comp motor, easily doable.

Jason

Too much squish, you could have more detonation problems with lower compression than if you lower the squish to 1mm, JMHO.

I used K&W copper coat on my base gaskets.

With the motor you mentioned I would guess the HP would be closer to 265-275 at the crank.

I'm running a very similar motor. 78 SC heads that have been cleaned, Cams similar to the GE-60, twin plug, 10:1 compression, 46mm webers with tall matched intakes, 93 octane, open exhaust, etc turning 7200 RPM and am getting around 240HP at the wheels shown on my data aquisition.

I'm going to hijack this thread, if you don't mind, because I have a related question for my specific application. I have a 3.4 930 track engine that I'm assembling, and I want to set the deck height correctly. I did a "dry run" assembly that suggested I'd need a 1 mm shim under the cylinders. After installing the shim, I find that my deck height is actually 1.19 mm -- higher than the 1 mm goal. The question is, would I be best going with that, or should I bring it back closer to 1 mm? To bring it back to 1 mm requires stacking two shims (0.5 +0.25 mm) to give approximately a 0.94 mm deck height. Would that be better than leaving it where it is? Are there better ways to get it dialed in -- such as with custom shims? My machinist said leave the shims dry -- don't use a sealer. It seems that others here like to use various sealants mentioned above. Anyone have direct experience with leaks caused by *not* using sealant at the cylinder base rings?

Rob

I'd suggest that you do the compression check to see where your CR is relative to your target before you decide on the shim and deck ht. Deck ht volume will have a significant impact on your CR

Tom,

Actually, I've done that. I cc'd the heads and piston domes. According to my calculations, if I had a 1 mm deck height, the CR would be 7.31:1. That's fine. If it's 1.19 mm, it will clearly be a bit less, which is probably also fine. This is a turbo car, so my power won't be limited by CR as much as it would if it were not turbocharged. I'm trying to walk that delicate line between having enough deck height to protect me from the danger of piston to head contact (such as during at mild overrev) and not having so much deck height as to increase the risk of detonation (which could be a real concern on with the turbo). If I drop the deck height one notch (0.25 mm), I will probably still be okay on the tightrope between these two opposing parameters. But it also means either going with a stack of two shims or getting a custom shim. Of course, if I go custom, I can set deck height it exactly were I want it, which would probably be 1.0 mm.

These questions remain: what is the optimal deck height for (essentially) a racing engine? What are the pros and cons of stacking shims? If I stack shims, does it increase the need for a sealant? What sealant is best? Some of these questions have sort of been answered for other people's applications. I'm curious what people have to say about my situation.

Thanks!

I'd stack the two shims. I don't have a lot of experience with these engines but I know in two strokes you can have detonation problems and loose hp with too little squish. Stacking is pretty common. I used the copper coat spray just to be safe. It's a thin, tacky, copper spray that allows the cylinders to be taken back apart pretty easily.

Has anyone measured the thickness of these shims before their installed with a micrometer? I found the thickness to be greater than the advertised value. It was only a few thousandths, but it was there, nonetheless. Do these shims compress to their stated value when the heads are torqued down?
I personally, am against stacking shims. The squirming these barrels go through in the spigots under load, thermal expansion/contraction, etc., make a perfect seal that much more difficult.

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

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?

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

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?

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?

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?

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.

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.

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.

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

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...

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.

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

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.

Definately CC them. The last 2 set of "9.5:1" JE pistons I got CC'd out to 10.5:1.
-Chris

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.

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

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?s=&threadid=213504&perpage=20&highl ight=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.

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