85mm Mahle piston ID help

[boxer246]

Anyone ever seen one of these? Trying to figure out what it was made for.

It’s a Mahle 80p25+ which according to the charts makes it a 80mm 906 piston. Clearly not a 80 though, it’s a 84.95 as stamped on the dome. It also is significantly different than a standard 2.2S piston in that it is two ring, narrower and shorter skirt, and different dome shape slightly. The compression ring is a thin stepped ring. Never seen one like it.

Weight is 360g and pin is 70g and tapered on the inside.

It comes out of a 1970 2.2S MFI engine with 906 cams.

Next step is to CC the domes take all the measurements and try to figure the compression ratio.

[jmz]

...factory cheater motor.

Remind me to tell you about the cheater slicks and wheels that a friend had from an old 935.

[boxer246]

The only thing this things gonna cheat is my wallet!
I’d love to hear about it.

[ed mayo]

I ran a set of those once, mine were 81mm, an allowed 1 mm over bore, a two ring piston for less drag. The top ring is a Dykes ring, has less tension, because the shoulder on top of the ring allows for more combustion force behind it to seal on the power stroke but have less drag on the other strokes. State of the art at one time. I ran these in Biral cylinders, but the original 906 cylinder used a chrome bore.

[boxer246]

Thanks for the info Ed. Never heard of the Dykes rings. These are also run in biral cylinders. Not sure yet if I’m going to use them, I assume it would probably be impossible at this point to find new rings to match these that’s one problem. End gap measurement shows them to be about half worn according to the max wear limit specs.

[911 Vintage Par]

Best guess....Ed is right, there was an SCCA D production piston, late 70s.
Production classes were allowed .040" (1.0mm). 80 was stock, 81 was .040" oversized.
The two ring design was an attempt to reduce friction/ring drag.
I believe the piston you have is a C production 2.4 .040" over version. 07.4 x 85mm
It had an advertised compression of 10.4 but that number was greatly optimistic.
If the space between the valve pockets was smaller, it would have more compression. The 2.2 version of the 84mm had a larger dome volume to make up for the shorter (66mm) stroke.

[ed mayo]

When I last bought rings for these they were from " Childs and Albert " part # DL-.020 X 3.190. Don't know if they are still around. Seem to remember they were also used on Formula Fords. Might be worth a shot,,,,maybe "Pegasus Racing"

[boxer246]

Thanks for the info guys. I will check into availability of the rings.

[Matt Monson]

It wouldn't hurt to call Don at EBS. They've got some oddball stuff in their stocks.

[boxer246]

Thanks Matt I will do that.

What little research I’ve done on the Dykes rings seems to suggest that they were mostly developed for high pressure engines, supercharged etc, and not usually “recommended” for street use. Apparently work best at higher RPMs.

[stownsen914]

Another option to consider for rings is Total Seal. They also make conventional rings and oddball/custom rings. They might be able to make something for you if it comes to that.

I recently had them make a ring for me when the piston manufacturer made a mistake and didn't cut the ring grooves deep enough in the pistons so the standard rings wouldn't fit.

[boxer246]

Any opinions about the usability of these piston for street use? Assuming I can get new rings, what would the downside be to a Dykes style two ring piston?
From what I gather the rings are normally for high performance/high cylinder pressure engines. They seal better under those parameters and mostly on the compression stroke. And they are classified as a “low tension” spring. Now the oil scraper ring seems pretty standard to me, except perhaps being a little taller than average.
Would this combination result in poor compression sealing at low rpm’s and/or increased oil consumption?
To summarize, aside from being perhaps an outdated design, are these for sure a poor choice for a street motor?

[Matt Monson]

Are you going to keep the 906 cams in it? Those are definitely not street friendly.

[boxer246]

My plan as of now is most likely not to keep them. Probably move to a Mod S or DC44 range cam. General consensus seems to be that they are a somewhat outmoded design and not real useable below 5-6k but I don't know if that's just rhetoric people keep repeating or really true. I'm guessing its a relatively small group who've actually driven a 906 cam motor.

[Henry Schmidt]

Originally Posted by boxer246

Any opinions about the usability of these piston for street use? Assuming I can get new rings, what would the downside be to a Dykes style two ring piston?
From what I gather the rings are normally for high performance/high cylinder pressure engines. They seal better under those parameters and mostly on the compression stroke. And they are classified as a “low tension” spring. Now the oil scraper ring seems pretty standard to me, except perhaps being a little taller than average.
Would this combination result in poor compression sealing at low rpm’s and/or increased oil consumption?
To summarize, aside from being perhaps an outdated design, are these for sure a poor choice for a street motor?

What you will find is hi oil consumption and short service life. These were basically a single season racing piston. Rings were replaced regularly.
Find another option.

As for the cam: 906 cams are definitely not street friendly unless the goal is to keep the engine rpm above 4500. Overlap is so large that at 4-4.5K you can look down the intake and see fuel burning in the port above the valve.

We've played around with them for nearly 40 years and the only way they work on the street is with a large bore 3.2SS and larger and very small intake ports and small exhaust primaries. We did run them on a 3.1 SS (66x100mm) and that engine rocked but street friendly was not how I would describe it. Even then, not a great choice.

[boxer246]

Thank you Henry thats very helpful. I had been planning a call to you at some point to ask your opinion on those points. I appreciate all your inputs on this (Matt and others)....learning as I go