Duke

Nicolas,
what kind of modifications have you done to the head?
Valves, coatings etc?

Gustaf

Nicolas

I think there is a lot of misconception about 968 turbocharged engine:
- Stock 968 pistons are dished and a lot (around 30 cm3)
- Stock 2L7 head has around 60 cm3 chamber volume

I want a C/R of between 8.3 and 9.0 to have a great torque at low rpm and run 1.3 max boost on euro 98 unleaded.

So when you add deck/piston clearance + Compressed height of gasket you will see that the stock 968 pistons will give a too low C/R not speaking of a not very nice dish and valve pockets designed for the 4V head....

So if you want something nice you are better to find pistons with a 951 type dish and with 15-20 cm3 volume.

JE pistons are cheaper but not able to be used in alusil bores without coating on the skirts - other way make a nikasil treatment on the bores and use the JE but with non-stock rings (stock have triple chrome plating)

I like the idea of coated JE or Wiseco pistons but coatings do not last long on forged pistons (main problem of the 928 GTS ...) Coatings are great on cast pistons (porosity is higher).

Any other point of view ?

cruise98

Perry:

I am the one that posted about the 964 rings being compatible with the Nikasil coating for a 951 block. This is what I have installed in mu engine. The stock rings are chrome faced and work well for the stock alusil bores, but they are not the best application in the nicom bores. Ring seating is the issue.

I discussed this issue at length with both Scott at US Chrome and a JE piston engineer whose name escapes me at the moment. I have his name/number in my notes somewhere. It was the JE guy that recommended the 964 rings. I used the original stock pistons, so I had to use a stock size ring, and the 964 is a 100mm bore with the same size rings, and they were used in the nikasil cylinders.

If you are using a JE piston, use what they recommend to go with the nicom bores.

Brian Morris

Nicolas,

I don't *think* your calculations on the CR of this combination are right. I talked to John Milledge (well known 944 builder in US) about this *exact* combination - 2.7 head on a 968 block - he suggested it. He said with this combination you could grind the 968 pistons a little and use a thicker head gasket and get the CR *down* to an acceptable level for turbocharging.

My quick calculations confirm this - this is not exact but using the formula

D = Displacement
CV = Clearance Volume

CR = (D + CV) / CV

Use 90 cc for CV - 60 head + 30 dish - this doesn't include the deck clearance or the head gasket but they're pretty minimal - and this is close enough for a "quick check".

750 cc displacment

CR = (750 + 90) / 90 = 9.33

9.33 sounds about right - with head gasket and deck clearance it would be a little lower - but still too *high* for a good turbo application. Close enough that with a little grinding and a thick head gasket you could get it down to around 8.5.

Am I missing something?

--Brian Morris
89 951

Nicolas

I presume there is an error in your formula:
Displacement is just divided by the CC volume...
And not Added and then divided

Regards
Nicolas

Nicolas

The compressed height of a gasket is not so minimal added to deck clearance (0.8 mm+0.5 mm) it gives something like 10 cm3...

So :
750 / (60+10+30) = 7.5 to 1

I have mesured very inaccurately the dish of 968 pistons with water and weighing the needed water to fill the dish and have obtained 32 cm3 .
Some posts stated 26 cm3.
So for the calculation I took an average of 30 cm3...

Brian Morris

You do need to add the chamber volume to the displacement.

The CR is the ratio of the "starting volume" to the "ending volume". The "starting volume" is the displacement plus the ending volume.

I doubled checked a couple of books as well as on the web - here's one link:

<a href="http://www.poorboyz.8m.com/how_to_calculate_compression_rat.htm" target="_blank">http://www.poorboyz.8m.com/how_to_calculate_compression_rat.htm</a>

--Brian

Nicolas

I am maybe wrong but their formula the 968 is no more a 3 liters engine but a 3.3 liters...

The aspired volume of mixture is equal to the physical displacement of the piston in the cylinder and not to the actual volume of the cylinder when piston is at BDC.

example:
Take a syringe - if you aspire when the piston is not totally pressed the total aspired volume is equal to the positive displacement of the piston and not to the actual volume of the chamber...

Konstantin

from what I know I think brian is right on this.
check the compression on a 951 or stock 968 and you will see it is right.

I think the 2.5L 951 head had 57 cc.

Konstantin

m42racer

Nicolas,

May I offer another alternative. I have a sleeved block, had to because of earlier Piston failure, and the sleeves have thousands of miles on them. This way I went to standard type forged Pistons with rings suitable for boost and the liners. I have seen no problems with my block. This is another option.

PS. Sorry for the earlier remark. It was meant as a joke. Oblivoiusly poor timing and taste on my behalf. My opoligies.

Simon.

Danno

I'm also installing an <a href="http://www.andersonmotorworks.com" target="_blank">Anderson Motorworks</a> 2.8L sleeved and bored 951 engine using JE pistons. I had the pistons coated with a ceramic thermal-barrier top and a teflon layer on the skirts. Our new headgaskets in 106mm bore were designed with this particular engine configuration in mind. Hopefully I'll get my engine before this weekend to install...

Sleeving is done in all types of applications. Some engines use an iron block with semi-wet Alusil sleeves. The trick to getting this to work properly is the type of sleeves used and the correct installation procedure using the right clearance figures.

Turbo Fanatic

I am not sure how accurate this is, but RayHall has a java script compression calculator. <a href="http://www.turbofast.com.au/TFcomp.html" target="_blank">http://www.turbofast.com.au/TFcomp.html</a>

Based on 3.0 liter bore of 4.09 inches, stroke of 3.46 inches, average deck height of .040 inches and 8mm thick compressed gasket or .0315 inches, combined with 57 cc combustion chamber in the head, and 26 cc dish in the factory 968 4valve piston you are looking at 8.567 to 1 compression.

If head is 60cc and piston dish is 30cc then it drops compression ratio to 8.06 all other factors remaining the same. Again I can't verify how accurate this calculator is but I do agree with Nicholas you must factor in the both the deck height and the compressed height of the gasket as it does have an effect on overall compression ratio. All of this does not however address the concern of the dish/valve pocket design of the 16 valve piston. That being said, Mahle is reported to have restarted production of factory 3.0 turbo piston and latest report has them being fitted for the rings after they had been sent out for coating. I will neither deny or confirm this, as after all I am a doubting Thomas and need one in my hand to believe they exist!

James

Konstantin

so!! anybody bought these new Mahle 3 L Turbo Pistons for much cheaper than my original 968 Turbo Pistons?

If yes I would be also interested in one set

Konstantin

gabej45

Hey there,

I am (now two decades later) attempting to do this exact same thing. I have sent my block to Millenium and it is being nikasil plated. I am using clean-low mileage stock pistons. Before I order and install these 964 rings I wanted to ask: How was long term usage of these rings? Did they seat properly? Would you use something else if you were to do it again?

Thanks and sorry for hijacking this super old thread, this post was made well before I was even born (I am almost 20 haha).

-Gabe

michaelmount123

If you have a 2.5L block, the 964 rings are a perfect solution for your Nikasil 100mm bores. They will seal nicely and have great longevity. They are a great choice.
MM