Printable Version of Topic

Posted by: DanT Dec 18 2010, 10:12 AM

I have the chance to acquire an 2.0L 6 case and have thought about building a small bore 2.0-2.5L high revving 6.
Anyone out there running such a combination right now?

This would be a street/AX motor. Prefer to do single plug.
Most likely use Webers, since MFI parts are getting hard to come by .
What combinations of Pistons/cams are you folks using?

Pictures of motor builds would be appreciated.

Just sliding in a 2.7L CSI motor is also a consideration if the small bore motor becomes too much of a pain.

Posted by: jjackson Dec 18 2010, 10:31 AM

QUOTE(DanT @ Dec 18 2010, 10:12 AM)

I have the chance to acquire an 2.0L 6 case and have thought about building a small bore 2.0-2.5L high revving 6.
Anyone out there running such a combination right now?

This would be a street/AX motor. Prefer to do single plug.
Most likely use Webers, since MFI parts are getting hard to come by .
What combinations of Pistons/cams are you folks using?

Pictures of motor builds would be appreciated.

Just sliding in a 2.7L CSI motor is also a consideration if the small bore motor becomes too much of a pain.

Posted by: messix Dec 18 2010, 10:59 AM

are you sure you want small bore?

a small bore will not make an engine a high rever.

how ever a short stroke engine will.

the short stroke has a slower piston speed compaired to a longer stroke engine per rpm. [distance the piston travels for each revolution, hence stoke]. this puts less stress on the crank and allows for higher rpm.

so for a long stroke 2.5= long stroke x small bore

short stroke 2.5 = short stroke x large bore

Posted by: ME733 Dec 18 2010, 11:23 AM

The ..2.0 six...had a short stroke...66mm.(same as a carrera 4 cam)., most also did NOT have a counterbalanced crankshaft....using a counterbalanced crank is essental for hi revs with reliability....titanium rods were used in the race engines, but carrello has excellant rods also...In my opinion the 911-S- cams are way too "peaky", and a better engine can be made with with other cams...get the bruce anderson book... "Porsche 911 performance handbook".read it, study it, then you can make some informed choises.

Posted by: campbellcj Dec 18 2010, 11:39 AM

IMO you would not want a peaky small six for street or autox. Depending on the build specs and tune you might only make decent power between 4500-7000rpm's or so and regardless the peak torque would be modest compared to a larger displacement.

I have significant (18 years) experience driving 2.2 sixes and while they are gems and sound glorious, it can be challenging to keep them moving at a good clip. Short gears help.

Posted by: campbellcj Dec 18 2010, 11:39 AM

IMO you would not want a peaky small six for street or autox. Depending on the build specs and tune you might only make decent power between 4500-7000rpm's or so and regardless the peak torque would be modest compared to a larger displacement.

I have significant (18 years) experience driving 2.2 sixes and while they are gems and sound glorious, it can be challenging to keep them moving at a good clip. Short gears help.

Posted by: J P Stein Dec 18 2010, 11:49 AM

My old 2.7L would pull 7300 with all Porsche parts.....you want more, bring money.

Posted by: sww914 Dec 18 2010, 12:57 PM

They say the last 800 to get to 8000 will cost you $8000.00
That said, Henry Schmidt, a common sight on the Pelican board and a 911 engine builder for many years with much success says that the 2.5 short stroke is the best engine that Porsche never built.
That's a 66mm 2.0 bottom end with 2.7 90mm pistons & cylinders. You can't really use the 2.7 CIS pistons because the dome shape is designed for the CIS swirl, it gets in the way of the valves if you run a good cam and their compression is too low. You have to use some JE or other A/M pistons to build that engine. It would probably respond really well to twin plugging as well.

Posted by: brant Dec 18 2010, 03:23 PM

I put 100 racing hours onto a 2.0/6 with an aluminum case
it was all S spec, real 67S heads, cams, Counter balanced crank, pistons-cylinders, stock 2.0S rods
it was great and only 9.8 compression (factory)
147hp to the wheels at 5000ft of altitude on a 104F day is not anything to sneeze at for a stock motor

We would usually shift between 7200-7500 and loved it
the quick revs (5lb tilton clutch and flywheel) are awesome
I really like the small bore stuff and its so "right" sounding

new motor is a small spigot 7R case
(these are kinda hard to find, but slightly stronger than the 5R cases)

I haven't driven it yet, but can't wait
66 counter balanced crank, big valves, 81mm pistons of just under 11:1 compression, twin plug, Pauter rods, lots and lots of head work with race cams
shooting for a little over 200hp and probably will shift at 7500rpm
(although the safe zone is somewhere around 8,500 rpm)

builder said that rpm is only dependent on how long you want it to last

I'm a huge fans of the small bore motors
used to have an SC 3.0, and the motors are nothing alike..
really love the small bores.


Attached image(s)

Posted by: GeorgeRud Dec 18 2010, 03:41 PM

The 66mm crank with the 90 mm piston bore does make a great sounding, high revving engine, but it will cost money to do correctly. They were popular in the early 70s as it could be used in the under 2.5 liter racing class.

Though they're usually maligned, the 2.7 does give nice grunt and revs quite well compared to the later, larger engines.

In an autocross engine, I think that high torque should be the primary objective. That is achieved most easily with a larger displacement engine.

Posted by: Mark Henry Dec 18 2010, 05:20 PM

If you're rebuilding all sixes work out about the same price in the end.
Even if you get a cheap core it's a pay it now or pay it later type of deal.

Posted by: MikeSpraggi Dec 18 2010, 05:51 PM

Haven't driven it yet, but I've had a 2.2L six built for my track car I plan to run in vintage racing. Built on a 7R case, 84mm JE 11.3:1 pistons, Solex cams and reworked Zenith carbs. Oh yea, a little porting on the stock heads. Using a 2.0l non-counter weighted crank. Elected not to go with a lightened flywheel since the consensus was that I would not really see any gains.

I'll let you know how it turns out, say in April!

Posted by: d914 Dec 18 2010, 05:59 PM

I had a 2.5 liter short stroke six... loved it for the track,,, I even had fun on the street... auto cross not so much.. 220 at crank with 138 ft lbs.. The only thing that made it go was revs!!!! and lots of them... get caught low in a gear and your dead meat...

Posted by: Cap'n Krusty Dec 18 2010, 07:54 PM

Ahem! I have an aluminum case, a long block, and a core motor. All are for sale. The core comes with a good regrindable counterweighted 2.0 crank. Bring money. I'm probably gonna use Cunningham rods in my 2.5.

The Cap'n

Posted by: J P Stein Dec 18 2010, 08:23 PM

QUOTE(Cap'n Krusty @ Dec 18 2010, 05:54 PM)

Posted by: jfort Dec 19 2010, 07:35 PM

Have a 2.5 short striker with E cam and short gears. Quick and the sound is glorious

Posted by: Mark Henry Dec 19 2010, 08:17 PM

There's a set undersize rod bearings on the bird classifieds right now and I know of another set at a good price. First under size on some of the rods isn't that big of a price, the mains are a different story.
$1200 for set of SC first under mains...cheaper just to find a standard crank.

Posted by: koozy Dec 19 2010, 08:41 PM

Hmmmmmm..... Whatcha doooooin???

Posted by: Eric_Shea Dec 19 2010, 09:56 PM

I have Greg's 2.5 now. It will be twin plugged soon. That said; autocross motor? I would find something with torque.

Posted by: DanT Dec 19 2010, 10:22 PM

Mike,
I have always wanted to try a high revving 6 with the proper gearing to keep the motor up on the cam, plus they sound marvelous...
Just dreaming a bit... Ken has some interesting combinations he has used over the years.
need to check my source and see what he wants for the pieces

Posted by: koozy Dec 19 2010, 11:45 PM

I should have saved the 2, 2.4 engines I had. The MFI could have been reworked for less than what it costs to purchase that stuff now.

I was trying to collect enough parts to try a short stroke ST spec build.

66MM crank
89mm P&Cs 10.5:1
Elgin Mod E Cams
twin plug 46/40 heads
High butterfly MFI

Would have been a screamer. But, alas... I ran out of know-how and flipped it all for the 2.7 I am building. I know enough now to lose sleep over it but not so much any more.

Posted by: turboman808 Dec 20 2010, 12:00 AM

Have a 2.7 with some work that revs to 8000 rpm. Just amazes me a old 911 motor can rev that high.


Makes decent power but once you get above 6k it just rockets like a beast.

Now that the transmission works right I can keep it in the power band really easy and get myself into some real trouble.

Posted by: Michael N Dec 20 2010, 01:38 PM

Dan,

Since you are so close to my house you are welcome to come by and take my 1973 MFI 2.4S out for a spin. I am at Camden and Blossom Hill.

Here is some of my favorite info from google books...Pg 108 - 112. Click on contents, then on pg 113, then scroll back to 108 - 112. This has great info on the older engines.

http://books.google.com/books?id=QJVcgiQhRfQC&printsec=frontcover&dq=porsche+911+performance+handbook&hl=en&ei=aqsPTeGECoyosQPWn6TYAg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDIQ6AEwAA#v=onepage&q&f=false

Posted by: Michael N Dec 20 2010, 01:45 PM

...

Posted by: ONTHEGRIND Dec 20 2010, 03:21 PM

Michael if he doesn't I will lol I am also in San Jose.. lol

Posted by: DanT Dec 20 2010, 05:13 PM

Thanks Michael....
over the years of instructing at AX and DE/TT I have driven just about every size 4 and 6 Porsche ever made, including 4 cammers, 2.0L S motor with 906 cams and a few other fairly exotic builds. 2.7RS motored 6, 2.5 twin plug, 2.4S, 2.2E etc.
I have a pretty good idea how one would run. Just thinking out loud...

Posted by: campbellcj Dec 20 2010, 05:18 PM

For years I dreamed/planned to build (er, have someone build) a really hot twin-plugged 2.5 SS MFI or slide-valve screamer. Time, money and opportunity converged and I ended-up with this carbed 2.7, which I am super happy with although I still think MFI would be cool. It has the pump drive cams and injector ports but I just do not see going to the expense and trouble at this point, unless maybe my Webers totally crap out somehow.