Difference between Advertised and Measured Compression Ratio - looking for advice

[polizei]

Guys,

I have an 87 Carrera engine that I'm bumping to a 3.4l with twin plugged heads. I purchased a set of mahle 98mm cylinders and 10.5:1 compression pistons from Andial - intended for a 3.2 carrera with a 74.4 mm stroke (AND 103 986 98 S - ANDIAL - Performance Parts for Porsche). My engine builder has finished cc'ing everything and measured a compression ratio of 9.4:1 with a deck height of 51 thousandths. The deck height was set in accordance with what he believes is appropriate for street car intended to last a long time. Two shims each measuring 20 thousandths had to be added to achieve this deck height.

I was quite surprised to learn that there was such a separation between the compression advertised by Andial versus what was actually measured. Should I be surprised? I called Andial to request Mahle's range for piston dome volume, but they don't have that information and explained that It's very difficult to get technical support from Mahle. Andial's reaction to the 9.4:1 compression was "something's wrong". They suggested that the twin plugged heads and/or unseated valve seats may contribute toward the lack of compression. My engine builder agreed to chat with Andial regarding the variance in order to track down the explanation.

Being a nube to all of this, I could sure use some direction. Any recommendation on what my next steps should be? Is there any additional information I could provide to help you diagnose the issue? Does anyone know what the piston dome volume is on these pistons?

Thanks,
Andy

[snbush67]

I think I remember some professional advice that you should build the motor first and measure everything then order your pistons. I think that off the shelf pistons are gauged to have a specified compression ratio when installed in a otherwise stock spec motor.

If you had other machine work done such as head grinding, spark plug holes, heads shaved etc. that is going to change your compression ratio.

I think your mechanic did right. If your mechanic reduced the deck height then that could increase the compression, but might result in lower tolerances. Lower tolerances could be bad and lead to bent valves, broken rings and or just an engine that wears out faster.

Your compression ratio of 9.4:1 is not bad, it will allow you to run a decent advance curve with normal gas on hot days and keep you from detonating.

I would recommend that you stick with what you got, sounds like a great motor that will last.

[polizei]

Thanks Shane - I appreciate your input.

My engine builder connected with Pete at Andial who confirmed that the appropriate combustion chamber volume to achieve 10.5:1 compression is 88-90 cc per cylinder. Mine measure over 92 cc. My engine builder communicated that the additional volume is explained by the head work that was performed a few years ago. The heads were twin plugged with too much material removed. The valve seats were also cut short. Both resulting in more combustion chamber volume (less compression).

Aside from starting over with different heads, are there methods to decrease combustion chamber volume?

[WERK I]

Originally Posted by polizei

.................................................. ....snipped...............
Aside from starting over with different heads, are there methods to decrease combustion chamber volume?

1.) Reducing deck height
2.) More piston dome
3.) Shaving heads

[PorscheGAL]

Originally Posted by polizei

Aside from starting over with different heads, are there methods to decrease combustion chamber volume?

If you are set on that CR, then ask your mechanic the risk of changing his deck height choice. Deck height sounds like the easiest variable to change or tweek on what you have now.

[polizei]

Thank you both for your input. The deck height is one of the key constraints as this is a street car intended for many miles. We are currently at .051" dc but we will likely move to .043". This will probably move us to 9.5-9.6:1

Once you are at minimum deck height, I think the options listed above would further reduce deck height. Is that correct? To achieve 10.0:1 or better in my engine, it sounds like I'd need reworked or different heads. Fair conclusion?

[rdimarco]

Hopefully the experts will chime in on what they think the safe minimum deck height is. I have seen 1.0 mm (.039”) frequently posted as ideal, with a range of 0.8-1.2mm. Excess deck height is supposed to increase propensity for detonation. I went with 0.87mm (.034”) to get my compression where I wanted it.

[chris_seven]

I would also re-visit the calculations as you seem to be coming up short.

If an 88cc combustion chamber gives 10.5:1 at the 51 thou deck height

90cc would give 10.13:1 and 92mm 9.8:1

You seem to be losing too much compression for an increase in chamber size of 2cc.


What cam will you run?

[polizei]

I'm not sure what the minimum recommended deck height is to achieve 10.5:1 compression on these pistons. That's a good question for me to take to Andial. My engine builder's preference before speaking to Pete at Andial was to set dc at .051". After he spoke to Pete, he was talking about lowering to closer to .040". So perhaps that's an indication. I will follow up.

Regardless, because of the work performed to the heads, the compression will not hit 10.5:1 at a .040" dc. Perhaps we will hit up to 9.8:1. I'm not sure how this all works, but I will find out next week.

I will be running Elgin super c2 cams. Any concerns here?

[snbush67]

Originally Posted by polizei

Once you are at minimum deck height, I think the options listed above would further reduce deck height. Is that correct? To achieve 10.0:1 or better in my engine, it sounds like I'd need reworked or different heads. Fair conclusion?

I think that the only thing that will reduce the deck height is removing one or more of the base gaskets which will also decrease the size of the combustion chamber.

Shaving the heads will reduce the size of the combustion chamber and lower the clearance to the top of the piston, but I believe deck height and squish height, distance between the top of the head and the piston are two different things.

Different pistons can change everything depending on the volume, position of how it is attached to the rod, and cut outs for the valves.

I don't think you need new heads, I doubt you can get what you want out of new heads if you are going to dual plug.

If you are dead set on having higher CR then you need to order a new set of pistons with the right volume, height and clearance. If you want a set of new heads to play with I can trade you.

[304065]

I thought I read here somewhere (Kenik maybe) that Mahle piston compression is expressed with ZERO deck. That allows a constant standard-- of course we use 1.0mm deck in practice. This accounts for the fact that the measured compression is always lower than spec-- this appears in Bruce Anderson's book.

What is your dome volume? I backsolve 10.5 to 1 compression with a dome of 30.9cc with 98x74.4 with a 90cc chamber and ZERO deck. I then plug in 1.0mm deck and get 9.4 compression. Doesn't seem like a big problem.

Whatever you had to shim, well that's a function of your spigot height, cylinder height and the amount that was machined off your heads, minus any offset in the rod.

Anyway, you really need to know the dome volume, and need a precise figure for your combustion chamber volume, to come up with a reliable measurement.

Good luck!

[polizei]

Quick update. After my engine builder spoke with Pete from Andial, he's comfortable setting the deck height to .041" (1.04 mm). He has the exact dome and combustion chamber volumes - I just didn't get a chance to write them down when we spoke. We're able to get a measured 9.6:1 compression ratio. Every bit counts! The upside now is that we can run more timing and shouldn't have issues with knock - especially with the J&S Safeguard.

[PorscheGAL]

Good to hear you found a solution!