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

[tom1394racing]

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?

[Jeff Alton]

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

[tom1394racing]

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??

[ChrisBennet]

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

[tom1394racing]

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.......

[tom1394racing]

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?

[ChrisBennet]

Originally posted by tom1394racing
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.......

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

[camgrinder]

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.

[Carrera3.5L]

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

[tom1394racing]

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 ??

[Eagledriver]

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

[tom1394racing]

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?

[jase007]

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

[David]

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.

[cstreit]

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.

[Rob 930]

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?

[tom1394racing]

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

[Rob 930]

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!

[David]

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.

[WERK I]

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.