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Posted by: zfisher6164 Dec 31 2022, 06:13 PM

Engine rebuild on a 2.7 weber carbed engine and the pistons used 8 years ago are fuel injected pistons and I have gotten a few opinions of the type of piston to use by none are
definitive. Any ideas?
thanks
zac

Posted by: sixnotfour Dec 31 2022, 07:14 PM

2.7 Mahle MFI.. only 8.5-1 thou or 2.9 mahle motorsport... 10?? -1 slip ins ..built a few cheater engines with these..

Posted by: zfisher6164 Dec 31 2022, 07:36 PM

thanks the pistons in the engine now look like this and was thinking keep these or get new ones since the engine is open

Posted by: zfisher6164 Dec 31 2022, 07:37 PM

another picture


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Posted by: Superhawk996 Dec 31 2022, 08:22 PM

Pistons for the CIS fuel injection are not a great choice for carbs.

As stated previously - Mahle is the gold standard and bump the compression ratio. You should also ditch the mild CIS cams - your leaving HP on the table there too.

Posted by: zfisher6164 Dec 31 2022, 08:49 PM

thanks this is the info I need the previous engine build came with these pistons and I was thinking they went with the wrong pistons so now I will go with the best ones
thanks

Posted by: zfisher6164 Dec 31 2022, 08:50 PM

not sure about the cam but will find out soon

Posted by: flyer86d Jan 1 2023, 11:55 AM

The engine that I built for my 1968 911 started out as a 1975 911S core. I used a set of 1973 Mahle 2.7 RS pistons and cylinders that I bought 30 years ago and never used and a set of 911L (911E) cams with Webers. Like your CIS pistons, the compression ratio is only 8.5-1 but they have enough valve relief to run 911S cams which the CIS pistons do not.

Today, there are other choices like using JE pistons in your stock nikasil jugs and higher compression ratios available. Have fun!

Charlie

Posted by: brant Jan 1 2023, 12:03 PM

You really need a copy of the Bruce Anderson book

You should have Nicki cylinders
Lots of options. All expensive

But you have one of the least desirable pistons for carbs. And also the least desirable cams

Much horsepower will be gained with the right combo and carbs

Posted by: JmuRiz Jan 1 2023, 02:04 PM

Budget is key, I was going to keep my cis pistons and run sc grind 2.7 cams with carbs. Bought to cams, and tore the engine down finding it needed full rebuild. Figured I’m in there so went DC30 (mod S) cam and mahle/nickies 2.8 combo. $$ but ~ 230 instead of 170hp will be nice.

I think I still have those cams if you’re interested, but might as well grab some new pistons with it apart.

Posted by: 930cabman Jan 2 2023, 04:03 PM

Thanks for posting, I am in the same boat, building a 2.7 former CIS soon to be carbed. Also trying to run from the prices of the new Mahle sets. Probably the smart move, but I was not that good last year.

Posted by: zfisher6164 Jan 2 2023, 11:53 PM

thanks for the replies the CIS pistons are what I found from a previous rebuild I am looking for a reliable street engine and was wondering if the nicki mahle piston and heads is needed.

Posted by: Superhawk996 Jan 3 2023, 12:15 AM

There are other options

Nickies (brand) cylinders / Mahle pistons are top of the line. Nickes are not exactly the same thing as OEM Nikasil. Nickies are CNC from billet as just one differentiator. However the OEM 2.7L Nikasil are great cylinders and were in existence long before there were Nickies (the brand)

JE makes 90mm pistons that you should be able to use if your existing OEM Nikasil cylinders are in good shape.

The 2.7l heads pose some challenges for an acceptable compression ratio perspective but I’d assume JE has that worked out with their dome and valve relief. I haven’t researched the 2.7l pistons much so I’m not an expert. I only know enough about 2.7s to be dangerous. Any info I know is from the Bruce Anderson book and what I found as I was going back and forth on 2.4l vs 2.7. I went 2.4l - part of that being that the 2.7l is somewhat unloved for a multitude of reasons one being the lack of clarity on good, affordable, buildable 2.7l variations.

Keep digging. You’ve got homework to do!

https://auto.jepistons.com/pistons-381250s

Posted by: brant Jan 3 2023, 12:33 PM

it should work with stock pistons.. and low compression with weak cams
but 165 hp with CIS
may or may not be less with carbs.

why not leave the CIS in place?
at least you have easy starting and Fuel injection efficiency then...
plus less hassle than carbs which are hard starting, and altitude influenced.

but the assembly cost of the motor is the same
so an extra 75 hp while your already in there is a big big difference.... almost 50%

seriously... buy the bruce anderson book and read it twice... then research more, based on what you learn from the book/bible.

Posted by: zfisher6164 Jan 3 2023, 12:54 PM

I am reading the book and this was a 914-6 with a 2.7 engine with webers that ran great for awhile then popping and fire from the carburetor horns, so after taking it apart and finding many problems with the build I found CIS pistons as well so it never had the CIS fuel injection, I like the JE piston idea
thanks

Posted by: campbellcj Jan 3 2023, 02:48 PM

My 2.7 presently has Mahle cylinders w/ JE pistons and DRC cams and that's worked out well. I wanted to upgrade to Mahle Motorsports pistons and get the C/R up around 12:1 vs 10.5:1 but that would've further blown my schedule and budget at the time, as the existing P&Cs were OK to reuse in the last rebuild.

Posted by: flyer86d Jan 3 2023, 04:17 PM

That is good to know. Yours is a track car and it is holding up well.

Charlie

Posted by: pek771 Jan 3 2023, 05:46 PM

Are these Nikasil or Alusil cylinders?

Posted by: rudedude Jan 3 2023, 08:33 PM

CIS with those pistons are most likely alusil. They can be replated with nikasil and use the JE pistons. I did it years ago and havent had any issues

Posted by: targa72e Jan 5 2023, 02:31 AM

While the 2.7 RS pistons and Cylinder do not have the best compression they can still be used to make good power. This is a dyno is of my friends engine. It had the larger port 2.7 heads and used 2.7 RS P&C that while used were still in spec. Cams were MOD S. Intake was Weber 40's and headers for exhaust. I bit more compression would have helped., but not bad. Blue lines are 2.7 red are 2.4 with E cams same port heads with 9.5 compression, same car, same dyno. I think most would want the 2.7.




john