Mahle 930264570 piston modification

[67xr7cat]

So been looking at the Mahle 930264570 piston in the 4.27" bore size for the 4.25" stroke.  The dish says is 20cc which with my TFS heads that have a 70cc chamber would put me at 11:1 CR.  I am thinking that will be too much for pump premium.  To get down to 10:1 would require removing 11cc from the piston dish, Removing 8cc would put me at 10.3:1 CR which probably be ok.  Question is if the piston is thick enough to remove 8-10cc from the dish area.  This is a NA engine street/strip.

Since sure will be asked the cam is a solid flat tappet and I attached the cam card. Probably going to just keep the 3.78" stroke crank, but just thinking...

[frnkeore]

I ran your numbers and if your using a piston with the first ring down .25, 6.7 long rod, .000 deck clearance, 1020 head gasket, you will have 10.3 CR if you add 6cc (26cc total) and with your cam, you will have a 7.99 DCR, just about right for premium pump gas. The DCR would be 8.045 @ 25cc, (10.4 CR) should be ok, too.

[blykins]

I would pour a chamber on your heads first.  I've seen them vary from 68.5 to a little over 69 cc.  I've never seen one at 70cc yet. 

Unless you have the Mahle pistons in hand, my guess is that you'll be waiting on them for awhile.  It would be just as easy to just measure and get a custom piston from Racetec/Diamond/etc., that already has the correct volume. 

BTW, you will leave power on the table with the duration split on that camshaft.   The TFS heads want more towards a 10 degree split.

[67xr7cat]

Well guess nobody has used this piston. No I don't have it in hand since if I did I'd be able to see for myself if it had enough thickness to deepen the dish.  Icon makes a piston for the same application which is 28cc but uses a 1/16 instead of the 1mm ring pack and does not have the coated skirts. Cost wise is slightly cheaper too.

Most likely will just keep it a 427 anyway.

Brent if I keep it a 427 still feel that cam be a bad choice? How much be down over ideal?

Thanks guys

[blykins]

Well guess nobody has used this piston. No I don't have it in hand since if I did I'd be able to see for myself if it had enough thickness to deepen the dish.  Icon makes a piston for the same application which is 28cc but uses a 1/16 instead of the 1mm ring pack and does not have the coated skirts. Cost wise is slightly cheaper too.

Most likely will just keep it a 427 anyway.

Brent if I keep it a 427 still feel that cam be a bad choice? How much be down over ideal?

Thanks guys

No sir, it wouldn't be ideal with those cylinder heads, no matter the displacement.   I've some back to back testing with 445's, TFS heads, and different duration splits.  On a fairly mild combination such as this, it was good for 15 hp, just by itself.  I didn't look at the other specs closely, but if it had a good bit of duration, the lack of split could really become apparent. 

[gt350hr]

      Cutting the dish .060-.080 deeper would likely cause a "sink hole" in the center.

[67xr7cat]

Yes most likely no margin of error on that piston's dish thickness.  Interesting they don't list if you can use a power adder...  Maybe someone here has a piston they can measure. As said will likely just keep it a 427 as don't need 600 hp and there is always the Icon piston that is already 28cc's.  Guess am just getting old used to making everything "Fit" LOL Thx Steve

[TomP]

Is 11 to 1 even really an issue with a dish or flattop piston? I wouldn't think so unless this is in a truck or large boat and the gas is pretty crappy.

[GerryP]

Is 11 to 1 even really an issue with a dish or flattop piston? I wouldn't think so unless this is in a truck or large boat and the gas is pretty crappy.

I don't think it's that big of an issue if you have a good cooling system.  You can always manage it through the tune up;  slow down the advance curve, use a cooler plug, richen the mixture a little bit.  And you can always add one of those Snow water/methanol injectors like the old days.

[67xr7cat]

Will it run at 11:1 CR? Yes can be made to work, but been down that road of high CR and pump gas and is just a pain.  I'd rather run less compression and be able to optimize the tune as well as not worry about weather or varying pump gas quality. One of the fastest way to damage an engine is detonation.  On the street is better to give yourself some headroom IMHO.

[blykins]

Will it run at 11:1 CR? Yes can be made to work, but been down that road of high CR and pump gas and is just a pain.  I'd rather run less compression and be able to optimize the tune as well as not worry about weather or varying pump gas quality. One of the fastest way to damage an engine is detonation.  On the street is better to give yourself some headroom IMHO.

I wish more guys would look at it this way.  The difference in hp between 10:1 and 11:1 on a street engine is just not worth the headache that you would have if you got a snoot of bad pump gas, etc.

[cammerfe]

In regard to detonation, I pulled into the local Sunoco station for my usual tank of 260, to go into my almost-new-at-the-time '64 Custom/427. The pump-jockey guy put in a load of 190 by mistake and I put my foot in it going west on Plymouth Road in Livonia just as the 190 got to the carbs. I got IMMEDIATE substantial crackle and began to lay down a fog behind the car. I was about out of warranty at that point, but the guys at Bill Brown Ford were reasonably flexible and gave me a new piston, set of rings and all the necessary gaskets. I replaced the parts myself. One piston had collapsed ring lands that bound up the rings and let enough blow-by to create the fog. All was well again after the replacements.

KS

[oldiron.fe]

                                                                                                                                                               
    yep good street tune for everyday- race gas pricy to cruise with                --                   a strong FE with 10/1 comp. will have traction problems / the few exrta hp may not mean a lot in up in tire smoke-- hope to put my old iron in my 69 mustang this next year --pump gas - iron hi-riser heads/block makes solid 680 hp on barrys dyno -will need traction not more compression !!  old guy is building one last iron hi-riser with some of Jays parts @ 14/1 comp -   add 100/150 hp to pump gas build !  john -old iron    ( i look for good gas with my old iron stuff/the 680 hp is a 482)