Deck Height
 

In checking my deck height, the machine shop said between .065 and .070 is what I need to see to ensure clearance and proper CR.

I am measuring tonight and get different readings all around the piston. It ranges from .048 to .063. There can be a difference of .010 within a 1/4 inch of the two readings.

How should this be handled? Also, is this due to the ceramic dome coating?

I am using 1mm shims. Can I stack a .25mm shim with a 1mm shim to get proper average deck height?

Thx

According to Bruce Andersons book the deck height of 911 engines is .030 to .040 and the minimum clearance from piston to cylinder head should be .039 inches. These two measurments can be different due to the shape of the pistons and heads. I have found the deck height ranges because the piston slops slightly in the bore. I think your readings look good provided the piston dome doesn't slope up to quickly and get too close to the head "squish" area.

-Andy

Thanks

This is for a 930 rebuild using custom JE 8.0:1 CR pistons and a 964 crank.

Rgds
Don

It seems like most people think stacking base gaskets is OK. If you have to go over 1mm, you don't have much choice. Here is a old discussion on the topic:

http://forums.pelicanparts.com/showthread.php?s=&threadid=124956&highlight=shim+s tack

Good luck,

JP

My opinion. What ever engine you build you will make it more efficient by substantially reducing the deck clearance.

If you set this at the minimum for each particular engine design, the engine will be less likely to detonate, the fuel mixture burn will be more complete by generating more combustion turbulance. The engine in general will be more responsive and generate more HP and TQ. This is especially true when using cams with aggressive duration and centerline.

Short stroke engines with steel con rods and relatively mild RPM like the Porsche that run tight piston to cyl clearance can go "way close" How close?? .010-.012" on street motor. This is assuming that the pistons are new with proper side clearance.

Remember that the case, cylinders, heads are aluminum and 'grow' slightly with heat as the engine runs.

Just an added note to my above post. Final measurement would include compressed head gasket thickness,

I recommend laying a piece of solder across the piston held in place with some Curil-T then bolt on the head. Make sure the solder length is the same as the bore so it reaches all the way to the edge of the squish area. Turn the engine over and then disassemble. Check the thickness at the two ends of the solder. There's such a tight area at the edge of the piston it's really hard to measure with just a depth measurement off the top of the cylinder.

I should be getting my new 3.4, 7.7:1 JE pistons tomorrow and will be doing this all over again. I'm shooting for 0.035".

I checked with the machine shop today and they confirm .065 to acheive 8.0:1 CR, taking into consideration the head work that had already been done to the head, as well as the valve jobs. They also said that for every .010, I would be increasing the compression by .1.

I set all the decks to .65 using 1mm shims.

The problem with 0.065" is that you're not getting as good a squish so you're not getting the best combustion possible. If you measured with a depth gauge, you might be missing the very edge of the squish band so you may be alright. It would only take a little more time to check with the solder and you'd probably sleep better knowing. :)

That's good advice, but he's building a Turbo motor, which will probably have flat-top pistons, which are much easier to measure against...

Stacking the shims is an acceptable alternative...

-Wayne

Some people may disagree here. I've spoken with many experienced engine builders (non-Porsche) Suprisingly some have said that there is no extra HP in the basic short block other than doing spec clearances. All extra HP is in the top end.

I dis-agree. My experience is that exta HP is everywhere in the engine. Starting from the filter all the way down to the exhaust tip. Minimum deck height is one area that is often overlooked or set at "safe standard".

If that rotating assembly is done correctly and piston/ring measurement is where it shoud be why are people setting this @.035-.070?? -That is a huge 'dead space' and wasted HP.

For some reason .035" for deck height seems to be 'the magic number'. If you look at a performance builders spec for a small block Chevy that number is the same. A Harley Evo engine deck spec .035-.045 -See a pattern here? I think the majority of builders reccomend that because most people won't or don't take the time to properly set up the basic short block. So that minimum number is a safe spec.

Recently I've seen two street builds on this forum that were excellent in every single way except (IMO) comp ratio and deck height. In both cases I feel that some HP/TQ was just given away by not really optimizing the compression and the squish area to the fullest extent per their build plan.

I'm a European tech by trade and have built many non-P-car engines. Most of my late model engine experience is with Volkswagon. The 1.8 Turbo engines they use have the pistons hanging OUT of the block!!....slightly. They typically run less than .020" deck height (with compressed head gasket)! In this engine the stroke is longer than the Porsches' flat motors. They also run @200PSI cranking cylinder pressure -which is close to 11.8-1 static compression and 8-9lbs of boost on pump gas.

So how can they do this?? The answer is in the technology of the combination. And part of that equation is the close deck height. I mean .020" or less in a mass production engine? This engine is rated @190HP. I've seen chip/intake/exhaust mods easily take this to 250. I've also seen the stock short block reliably handle up to 350 HP.

BMW also sets their deck height on the late engines as close or closer. I believe that .035" is an out-dated spec in todays engine build. There has been significant improvement in tooling, piston design to allow closer pisiton side clearance to reduce noise and improve ring sealing and thermal expansion. Why not apply the rest of that technology in your Porsche build?

Hi

In order to get this amount of power/boost without detonation you need a piston head design with a lot of squish. a 2 valve hemisperical head (911) has almost no squish and is therefore limited in this respect. The squish area cools the parts of the combustion chamber that could initiate detonation, The downside of squish is unburnt fuel and therefore emmisions and this is probably the reason for tighter clearances. If you look at the heads of these engines, they are usually 16 valve which is not for aspiration (They are turbo motors) but allow a design with a large squish area (959). As for 0.040", yes this seems to be a std for V8's and having seen one revved to 10K hit the head then its about right, for type 4 VW ive been to 0.060" is required, It really depends on the engine design, rev limit and how much flex you get, but remember the most force on a conrod is at the top of the exhaust stroke so it is 'stretched' significantly

Neven


Neven

It's true looking at the 911 combustion chamber that there is very little squish area, but IMO that is why it is so important to maximize the limitations from the original design.

Most performance street engines are done @7000 RPM's. I don't care if you are talking about a small block Chevy or a Porsche. The hardest thing to build is a performance street engine. I swear it is easier to build either a full blown race or just a stock engine because both have one objective; Race very narrow power band. Stock....well you know. Stock.

The hemi-head design is as old as dirt, but it was at the time it was current technology. With that said, I believe that the importance of minimal squish coupled with the highest cranking cylinder pressure that the OE design will allow will yield a very efficient engine.

Personally, I have never seen any production-based engine go to 10 grand unless it was a small displacement class-rules engine built for drag or road racing.

If the rods are steel there is virtually no stretch. that being the case the only reason the piston hit the head (in your case)is because of component failure. To run .060" is giving away the farm!

Being that the Porsches block, cyl and head are made of aluminum, they 'grow' with engine temperature. Therefor the deck hieght would INCREASE slightly as the engine runs. If the rotating assy was properly set up there is no reason not to run at the minimum. It's free HP!!

There is NO squish area.

Again false, if there was no strain (elongation) they would never fail, the engine I was quoting had not failed but the pistons and heads were marked

Neven

I believe that in a street engine steel rods don't elongate. Pistons rock in the cylinder, Cylinders become egg-shaped, rod bolts stretch, crankshafts can flex through harmonic distortion. Connecting rods (I-beam) don't just fail, there is always a contributing factor.

As far as no squish, anywhere on the very outside of the piston crown even with a hemi-head there is close proximity to the edge of the combustion chamber. If you have the piston set below the deck the exposed cyl wall becomes part of the combustion chamber leading to 'dead areas' in the flame travel.

With deep hemi-head design the piston has to have some kind of dome to have a reasonable compression ratio. Problem is that further inhibits flame travel.

I'm a drag race guy. Of any type of racing, drag racing has the highest horsepower and the fastest speeds of any wheeled sport. Period. It takes a lot of efficient power to run the class rules. A lot of things you learn quickly in the engine build process is maximizing the combustion process. You run less ignition timing, the heat is concentrated more towards the center of the piston= more HP.

We agree to dis-agree