ever heard about a 2.5 flat six? Options

[Gustl]

got some info on a '70 six for sale here in Austria
the owner told me, that it got a 2.5 engine installed in 1971 by the Porsche factory - now there's a 2.7 in

well, I've never heard about a 2.5 (IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif)
the seller told me, that is was a very special/rare engine, but it's gone for more than 20 yrs

now I wonder about that engine
does anyone know such an engine? what could you tell me about?
carbs or FI, power, torque, rpm bandwith ...
just try to describe this engine, please

many thanks in advance,
(IMG:http://www.914world.com/bbs2/html/emoticons/pray.gif) Gustl

[Aaron Cox]

i assume its just a short stroke 2.7...

[TravisNeff]

Yep there were 2 versions of the 2.5 engine. Porsche only did it for racing classes, not for production. One was based off the short stroke 66mm crank (2.0, 2.2) with bigger pistons and cylinders, the latter was a larger bore 2.4. I think they put out around 250HP, not sure about induction but I think it was carbs. Short stroke 2.5's are supposed to be real screamers.

[Gustl]

QUOTE (Aaron Cox @ Apr 9 2005, 07:20 AM)

i assume its just a short stroke 2.7...

but in 1971 there were no 2.7 available - as far as I know, thea came up in 1973

would be more likely, that it's a modified 2.4 - maybe for any rac car (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

[JoeSharp]

As I recall one of the 916 had a 2.5 in it.
Joe

[TravisNeff]

The 911 handbook has references to that engine and power output. There is a couple hurdles when making them as I think you use 2.7 P/C's (someone correct me if i am wrong) and the compression can be a little low, so you have to machine just a hair of the heads, but leave enough meat for the cam chains

[ArtechnikA]

QUOTE (Gustl B @ Apr 9 2005, 01:29 AM)

but in 1971 there were no 2.7 available - as far as I know, thea came up in 1973

the production Carrera RS 2,7 was a '73 model year car.

but the factory had access to interesting stuff back then... you do the math:
5,4-liter 70,4mm stroke 12-cyl 917 with 90mm Nikasil cylinders...
2,7-liter 70,4mm stroke 6-cyl 911 with 90mm Nikasil cylinders ...

Frère talks about the E/F Series 2,5 on p 246, 7th Edition. the first were overbored 70,4mm 2,4's but there were serious issues with crankshaft vibration. the 2,4 was produced for the '72 model year for which production typically begins August the prior year - 1971. development parts would have been available to the race shop even earlier than that.

the second series used the 90mm Nikasil 917 cylinders and the 66mm crank.

both produced about 275 HP with MFI.

okay - re-read your original request for technical detail...

stroker: 86,7mm bore, 70,4mm stroke. Bosch MFI only. 275HP @ 7.900rpm 27mkg torque(196 lbs-ft) at 6.200

flywheels came loose and cranks broke due to weaker crank and mag case distorsion.

big-bore: 90mm bore, 66mm stroke, MFI, 'similar' output.

[xitspd]

I have seen a couple of 2.5 six cylinder engines. Paul Bernardo former POCer had one in the mid 90's. They are short strokes that scream like weed wackers if they are sorted, ported and cammed right!

[mskala]

There were 2.5's run in the 911ST, which was made in '70 to '72, but
I don't know when which engines were used.

[d914]

I run the short stroke 2.0 liter version with 2.7 jugs, lots of fun....

[TimT]

There are a few iterations of 2.5's

the factory specials, and the 2.7 jugs on short stroke cranks.. ( low CR) either variation is a nice engine.

[Porsche Rescue]

I once had (and J. Fort in Ohio now has) a 2.5, early 911 aluminum case, stock 2.0 crank with 2.7 P&C's, E cams. Very quick. Way better than my current stock 2.0 six.

[Al Meredith]

I bought a 2.4 T MFI engine about 1 yaer ago. I was told it hadn't run in 10 years. I cleaned a lot of blanket fuz and animal dung off the top of the cylinders. Took the MFI off and installed some carbs ( don't know anything about FI but know I can get carbs to run). My son installed all the plumbing and a motor mount in a 914 and it fired up. On the DYNO it pulled 160 HP at 6oooRPM at the rear wheels......not a 2.4 T !!. We tore it down anyway and found 86.7MM pistons and nicasil barrels on a 70.4MM crank. The problem is when they bored out the 84MM 2.4 cylinders to 86 + the thickness of the metal on the inside of the "O" ring gasket slot was so thin that two of the cylinders had about 1" slivers broken out. NOW what do we do? Go 2.4 high compression; 2.7 high compression and have the case machined ; or go to L&N and have a set of "nickeys" made for the pistons we have. We are going to go with the L&N CNC barrels and keep this a 2.5 L . We are shooting for 200 HP with some torque for autoX.

[d914]

go 2.5 with torque, long stroke, replace only what you have to and then look at gearing. Possibly a shorter 3rd for autocross?? Dyno it and build the gear box to use. HP is nice but I eat up bigger 911's with stock 4th gear at the track.

Someone here also had the second cut off and replaced with a custom gear??? Taller second???? lots of torque 7 k red line might be the only shift you need depending on the autcross course. Around here they tend to be tight...???

[ArtechnikA]

QUOTE (d914 @ Apr 10 2005, 09:46 AM)

Someone here also had the second cut off and replaced with a custom gear??? Taller second???? lots of torque 7 k red line might be the only shift you need...

the 914.6 standard 2nd mainshaft is GA (1,778:1) while a /4's is F (1,889:1)

that with a 7300-ish redline was just about all i ever needed.

there is always the 904 mainshaft... there was talk about remachining the stock shaft but after further investigation it was discovered (again) to be a sub-viable option...

[Eric_Shea]

I'm building a 2.5 liter.

Yup... if you check out Victory by Design, word has it that Redman's driving a 2.5 loaded six as a factory "pratice" car for Targa.

The two versions mentioned were years apart. The only 2.5's produced by the factory were race engines. The early one is simple (if you can find the special 89mm Mahles). They had 906 cams and 41mm intake ports (if'n I recall). They "were" around 250hp and the 911ST (as mentioned previously) competed with a 2.5.

Mine is going to be built off my 2.2 block with 90mm Euro RS (Nikasil) pistons. Rod ends will be rebushed to bring the CR back up to 9.5:1. I'll be using S-Cams and Webers for my race car. That's the "plan" anyway. It will be an autocrosser and sometime DE car that won't see "hours" of spanking time.

[J P Stein]

Back when I was trying to figure out WTF to do with my 2.4 T for MO POWER I did a lot of research on your basic early 911 mag case motors. Some conclusions....feel free to disagree:

Only 2 cases are worth fooling with (for a hot rodder), the 5R & 7R. of the 2, the 76 & later are best....followed closely by the 73.5 7R.

Odd ball displacements are a waste of money unless you're building for a specific class.

Nikisal cylinders are THE way to go.

The compression ratios for stock pistons for the 2.4 & 2.7
are too low and all the "tricks" for raising the compression....suck.

All the post 70 heads have the same size valves, but the ports vary widely.

S cams and Webers are a poor combo for a street motor.

Shopping for "pre owned" parts can be a boon or a bust.

Given enuff money, none of the above means squat.


BTW: Otto's makes (or converts) the 901 mainshaft to a replacable 2nd gear item. I *think* it's around 600 bucks.

[ArtechnikA]

QUOTE (J P Stein @ Apr 10 2005, 11:20 AM)

Only 2 cases are worth fooling with (for a hot rodder), the 5R & 7R. of the 2, the 76 & later are best....followed closely by the 73.5 7R. S cams and Webers are a poor combo for a street motor.

there are some who actually prefer the 5R on the basis that they have not been subjected to as much thermal stress in street service.

i'm working a deal for a 49-state 7R 2,7 that i think will work out nicely.

the bigger the displacement, the happier the S cams are in street service. at least, that's what my research has indicated, and which i'm hoping to demonstrate IRL, because the Webers are paid for, and rebuilding the MFI to 2,7RS specs isn't ...

the rest, no argument at all.

[J P Stein]

The least stressed case would be the 7R from 73.5 T.....logically. The 5R has the big spigots but not the bracing.
Pre7 7Rs don't have the oil mods....but any that are straight & true (or easily straightened) are gud.

My 7R needed a line hone. I had one arrogant dickhead tell me it must have been a shitty job cause I didn't get charged enuff to suit him. I think he is the PCA 930 tech guru.

[ArtechnikA]

QUOTE (J P Stein @ Apr 10 2005, 02:39 PM)

The least stressed case would be the 7R from 73.5 T...

i have just such a case waiting in my garage for the 2,2's rebuild :-)