2.5 mid size six..... What is needed to build??

[NC_Colfax]

What are the pieces needed to build a 2.5 off a 2.0/2.2 case and 66mm crank.

Looking to build an engine for my 914

Thanks All

[Flat6pac]

2.5 is built on a 2.7 or there was the race engine of the time, 1972, with opened cylinders to about 86mm with a 2,4 crank.
Bruce

[tctnd]

Bruce Anderson covered this in depth a few years ago in his Excellence tech column. Someone might chime in with the correct issue. I seem to recall that there were two versions, short and long stroke.
regards,
Phil

[Flieger]

90mm cylinders like an RS and a 66mm stroke crankshaft. This version can rev higher for longer.

The other version uses the 70.4mm stroke. That was the first on the 911ST used, with Biral cylinders. They had issues with the flywheels coming loose due to a torsional vibration harmonic in the crankshaft that would hit at a bit over 8000rpm, which was where the racers like to rev them.

The short stroke version came out next when the factory applied their Nickasil technology from Mahle that they had developed for the 917. This allowed 90mm and larger cylinder bores.

The short stroke crankshaft has the harmonic torsional vibration shifted up further in the rev range, beyond where it is a problem.

[NC_Colfax]

Thanks for the information

I wonder if I could use 90mm cylinders from a standard 2.7
Is there a difference between rs and regular 2.7 stuff?
What connecting rods?
Compression ratio?
Please excuse all the questions.....I am new to building flat sixes

[Flat6pac]

The rods are designed to match the crank as the crank throw, rod length and the location of the throw on the crank all help get stroke.
2.7 RS are only 8,5 in standard form so you would need custom pistons to meet your application.
Bruce

[NC_Colfax]

OK so 2.0/2.2 case/crank/matching rods with 2.7 cylinders and maybe the JE 10.5 pistons to bring the compression back up correct?

What would be a good head to use? I would be running a set of carbs.

[Henry Schmidt]

We recently built a 2.45 70.4 x 86 with 9.0:1 compression.
We used DC Mod "S" cams and 40mm Webers.
Heads were ported to "S" specs 36mm.
SSI exhaust and an M&K muffler.
We also added twin plug to add to the wow but hardly needed with 9.0:1.
We chose this combination because we wanted to use the matching numbers case(2.2T) and I really don't like bore the spigots to fit 90 mm cylinders.
The project made 180 RWhp @ 6500rpm without tuning. We just built it, used the jetting recommended by PMO and set the timing.
Once it's broken in the owner will dyno it again with the intent of running it at a higher RPM and tuning to achieve the best running conditions.
This is one of the nicest street performance engine I have ever driven. Great low end and plenty of stamina on long straights.

[Flat6pac]

The easiest to find heads are the 2.7S heads are ported near the early S heads.
Bruce

[NC_Colfax]

Henry Schmidt.....that would be the 2.2 case/2.7 cranks/2.7 rods/and +2mm 2.4 P&C's

That sure is a great looking engine. I don't think I would need the twin plug setup for my car.

[Henry Schmidt]

Originally Posted by NC_Colfax

Henry Schmidt.....that would be the 2.2 case/2.7 cranks/2.7 rods/and +2mm 2.4 P&C's

That sure is a great looking engine. I don't think I would need the twin plug setup for my car.

The engine is in deed a 70.4 x 86. The factory 84 mm biral barrels will not allow for 2mm overbore. We use the AA Performance 86mm Biral cylinders. The cast iron has a Rockwell hardness slightly softer than we desire so we are experimenting with both heat treating and Cryogenic treating to improve longevity.

[NC_Colfax]

Thanks !!!

I will soon start looking for parts. I need to finish work on the chassis and paint then on to the engine build

[Emery G.]

you need to find a set of these

[NC_Colfax]

Yeah that would be nice.....

[Henry Schmidt]

Andial 86.7 cost $4,000 + ?
AA cylinders, JE pistons, Cryo treatment $ 1,800.
I know for some $2,200 isn't much money but for some it is a bunch.
.

[marlinaness]

Henry,
Definitely not comparable to your beauty, and not trying to steal this thread, but could I get good results using a 2.4 T engine (but using E cams), webers, and using 2.2 T pistons and cylinders (with a resulting 9.0 compression ratio)? I would think that could get me about 175 hp. This would be a cheap build for me since I have all the parts except the cams. Any idea if I would have clearance issues? I looked but did not see this combination listed elsewhere.
thanks in advance
Marlin

[Steve@Rennsport]

Just for grins,....here is another twin-plug 2.5 with Mod-S cams, good heads, and all the right innards for this application.



Its now completed and made 260+BHP on the dyno.

Properly done, these can make good power with excellent drivability (using the right cams).

[swade]

I have no immediate need for a rebuild or an upgrade but this motor really looks like a nice build. I would like to start gathering parts over the next couple of years looking to build a similar motor.

I have a 2.2E that is currently stroked to 2.4, engine case marked 3R ( serial 6210154). Will this case work for this motor?

What heads would be right to start with for this build? I cannot make out on Steve's engine if it is twin plugged or not.

Thanks for your help,

W

[Steve@Rennsport]

Originally Posted by swade

I have no immediate need for a rebuild or an upgrade but this motor really looks like a nice build. I would like to start gathering parts over the next couple of years looking to build a similar motor.

I have a 2.2E that is currently stroked to 2.4, engine case marked 3R ( serial 6210154). Will this case work for this motor?

What heads would be right to start with for this build? I cannot make out on Steve's engine if it is twin plugged or not.

Thanks for your help,

W

Wade,

You can use your 3R case but I would have it shuffle-pinned if you intend on 7K operation.

We started with 2.7 heads for the above engine and they have been extensively modified for this application as thats where the power is,...

We also machined them for twin-ignition as thats required for higher compression ratios and/or compatibility with pump gasolines.

[Mr9146]

My newest toy:

76 Mag case (machine work by Ollie's)
66 stroke X 91 bore (Mahle Ps and Cs)
10.8:1 CR
906 Cams
Heads massaged by CR Axtel
Factory Marelli Twin-Plug Distributor
40 IDT carbs freshly rebuilt and modified by Eurometrix

I'll be happy with 210-220hp at the wheels.