Calculating deck height with machined heads on 3.3SS.
 

I'm rebuilding a 3.3L SS engine based on a 78 911 SC 3.0. The heads have been machined 1mm deep to accept the larger 100mm pistons.

The original build used .75mm of base shims which produced a deck height of about .3mm based on the distance from the edge of the piston to the top of the cylinder. However, when utilizing the solder method of measuring deck height, I am getting a result of 1.25mm which would account for the machined head.

Am I good sticking with the .75mm of shims and keeping the deck height at 1.25mm even though the piston is only .3mm from the top of the cylinder? I've read that 1.0mm of deck height is more desireable but that puts the piston basically level with the top of the cylinder. Are there issues with that scenario?

I will have to check valve to piston clearances as well when I get to that point.

Here are a couple of photos showing what I'm dealing with.

http://forums.pelicanparts.com/uploa...1688135103.jpg

http://forums.pelicanparts.com/uploa...1688135103.jpg

http://forums.pelicanparts.com/uploa...1688135103.jpg

The distance from the piston to the head at the closest point should be~1mm, could be a little less if you know what you're doing. .3mm or zero is way too small.

I think it's a little unusual to have a step between the cylinder mating surface and the roof of the combustion chamber?

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You're measuring piston to top of cylinder. Usually this is the same as piston to head, but your heads look like they may be cut in an unusual way (the step mentioned in Mixed76's post). And you mention that you get 1.25 mm piston to head. This is usually what matters. You don't want the piston coming within a mm of anything it can hit while moving. 1.25 mm is in line with conventional wisdom.

Do you know why the heads were cut that way?

Consider welding up the first fin on the sides of the combustion chamber/ head. Your machined groove for the bigger cylinder creates a very thin spot that may fail.

Sorry I don't have a picture handy.

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The strange machining looks like a crude way to generate more compression. With a slightly longer rod (perhaps from a previews build) the pistons goes deeper into the head creating higher compression. This modification could allow for longer rods with a static stack height.
That said, that/those are some pretty ugly heads.

It looks like a stock rod small end in one of the pics. I wonder if the cylinders were cut shorter.

The rods are stock.

Would the heads have needed to be machined because of the 100mm piston diameter? Stock diameter would be 95mm. The piston fits perfectly in the machined opening in the head.

My main concern is whether the deck height calculation should be using the 1mm step machined into the head.

The last build was a bit of a frankenbuild with modded heads, 100mm pistons, 906 cams and RSR MFI. It had sat for 15 years before I picked it up and decided to try and get it sorted. I've gone with a milder cam, ITB EFI and an electronic engine management system.

The issue you are having appears to be with the technique used for measurement. Those type of pistons are very hard to measure the deck height of unless use a technique like the solder method. It's almost impossible to place a caliper or a dial gauge in the correct spot to measure your deck height. I would be more likely to trust the solder measurement.

Try measuring the deck with no base shims. If the deck is .3mm like you say then the piston edge will be proud of the cylinder at TDC with no base shims.
Once you confirm deck height you should also check valve clearance when you get the cams timed. You're in uncharted territory with a modified engine design.

If you're trying to determine deck height, all you need is chamber volume, piston dome volume, bore, stroke and desired compression ratio. Base gasket will be reveled with a simple calculation. Once you have this base gasket number you have to check piston to head clearance and piston to valve clearance.
It's pretty hard to build a custom engine without these specifications.

1mm deck height is at least notionally better for combustion efficiency.

It is probably better to keep 0.3mm clearance to somewhat ensure the piston edge can't hit the step in the head. Who knows what that joint looks like when all is torqued in place and heated up; might push into the path of the piston, result in contact. If it isn't perfectly concentric with the cylinder bore it may protrude on one side and lead to contact.

1.25mm deck height is still in the acceptable range.

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Thanks Bobby, I did measure the deck with no shims and the piston was proud of the cylinder at TDC. I will stick with the .75mm shims to keep the piston below the top of the cylinder and maintain the 1.25mm measurement using the solder method.

I'll be doing my volume calcs next to determine my CR and I'll try the formula for determining the deck height. I just wanted to make sure that the machined heads were not going to be a problem.

Thanks, I think we are on the same page. :cool:

My guess is that your compression ratio will be lower than you were hoping for.

The combustion chamber area in those heads look far from optimal.... What is the history????

I remember buying a set of heads years ago that looked like that and sending them for free to another forum member to use on his mock up motor for his header construction....

Maybe it is just the pics....

In American V8 engines, most builders do not want to go below .030" deck height, and prefer closer to .040"/1mm. Much more than that and you lose the squish turbulence that comes from tight deck height. OE Porsche 3.0 engines did not have tight squish zones. In order to get the minimum clearance between the piston and head, I deleted the copper base gasket. The minimum clearance point between the piston dome and head in my engine is a tad more than .030". I say a "tad" because the piston does rock a little bit, and that makes measuring the clearance difficult.

Hi Jeff,

That doesn't instill a lot of confidence in my build :). I don't have a lot of history. The engine was build by a respected engine builder in Calgary back in 2001. It was built for a 914 track car. It sat for 15 years after encountering an MFI/ignition issue and the owner just never got around to dealing with it and I picked it up last year.

I think the closeup photos make things look a little worse than they are. They were rebuilt at a machine shop and everything was given the OK for the rebuild.

Wouldn't the heads have needed to be machined due to the larger 100mm pistons? I am just a newbie so I defiintely don't have the experience around these matters. I'm enjoying the challenge though.

Dave

Those heads will work fine. The pitting is damage from water sitting in the engine at some point in time.

They might not be the prettiest, but it won't affect performance.

If it matters to you, someone like Bill from Xtreme can weld them up and remachine the chambers if it bothers you.

You haven't said what your plans are for this engine. Street, mixed street/track, or race car? What fuel will you run? What CR are you aiming for? Higher lift/duration cams? What RPM? Etc. That all influences your choices here.

I agree with the others that 0.3mm static clearance between the edge of the piston and the step in the head are too close. I built a 914 engine many years ago that had .020"/0.5mm deck clearance on one cylinder and when I took it apart, it had sings of the piston kissing the flat deck of the head.

I'm aiming for a street focused build with the occasional autocross or lapping day. I swapped the 906 cams for DR Camshaft DC-35-108 cams. The RSR MFI will be replaced with a modern ITB EFI setup and electronic engine management. The valve springs have been upgrade to Aasco racing springs with the titanium retainers. RPM limit will be about 7000 rpm and gas preference is 91. I have the twin spark heads to help with that. I don't have a CR in mind, the pistons are rated at 10.5:1 but I will confirm true ratio.

I'm really not trying to squeeze every bit of HP out of this engine as it will mainly be for Sunday cruises in the Rocky Mountains.

I wonder if the machined recess in the head was an attempt to lower the compression in order to run racing pistons on street gas?

Doing a few quick calculations:
If your pistons were truly 10.5:1 [generally Mahle is pretty optimistic] for a stock deck height (1.0mm) and chamber volume, adding the second plug should shield around 10.1:1.
Now taking your 1mm recess into account, you can expected a further reduction of
nearly 1.0 compression points, yielding a static compression ratio of around 9.1:1.
The DC 35 (108) should boost your dynamic compression but you are still looking at a pretty low final number.

With the racing valve springs and titanium retainers, you should be good to 8000RPM. I don't like running big pistons that fast, but I prefer low to mid RPM torque on the street.

Thanks for doing these calculations Henry, that is helpful.

I'm assuming the 1mm stock deck height would be measured from the top of the cylinder to the piston edge. Currently my deck height is at .3mm to the cylinder top but adding the 1mm for the recess in the head brings me to the 1.3mm. This would obviously increase the CR over your calculations but my question is, it safe to do so? Should I be keeping a 1mm deck height and not consider the 1mm recess into the head or would this be creating too large a pocket around the outside of the piston.

Other than a decrease in performance, would the 9.1:1 CR cause any other issues? The engine is going into a 914 so I'm not concerned about getting GT3 power. :)

Compression in these old engines is over-rated. With your cam choice and desired red-line you'll be fine.

Because your heads were cut in an unusual way, honestly I'd cc the heads and do the calculation. It doesn't take much to alter the head volume by a few cc, and that could easily change the CR by a few tenths.

It sounds like your heads were cut rather than the more conventional way of getting additional clearance (adding gaskets or spacers under the cylinders). So the combustion chambers, though a little larger now, are also sitting a little "lower" relative to the cylinder. It's all going to come down to the combustion chamber volume, and setting the piston the head clearance where you want it.