What's wrong with a 2.9?
 

The consensus seems to be that increasing the displacement of an air-cooled 2.7-litre engine to 2.9 litres will yield to an engine that is too stressed, and which will tire of sustained, high-rpm use sooner than if it was a 2.7. According to Bruce Anderson, "... the crankcase has to be bored out even more, further weakening the material in the area of the spigot bases."

Wayne Dempsey doesn't like 2.9s, a few fellow members of the Porsche Club of Victoria think they're bad news, and Bruce Anderson warns against converting a 2.7-litre to a 2.9 unless you're using an Andial 2.9-litre conversion kit, which Bruce says "... addresses all of the problems ...."

So what's the deal? How does the Andial kit "address" all the problems? If I can't confirm whether a 2.9 has been built with an Andial kit, should I be worried? Can you make a 2.9-litre engine out of a 2.7 without an Andial conversion kit?

Matt,

I don't know all the ends and outs of the 2.7 vs 2.9 but I did just have a conversation with a wrench about taking my 2.7 to a 2.8. One racing guy said "There is no substitute for displacement." After talking with the guy doing my machining he convinced me to stay with my original plan. A 2.7 RS specs P&C w/ webers.

The key to me was that my machinist listened to how and for what I was going to use the car. He said the jump from CIS P&C to RS P&C w/ webers would be big jump in HP and "a durable & fun motor". I got the sense that the racing crowd is a lot more comfortable dropping and repairing and spending on the motors. It goes with the territory. I want to experience a re-build and then take care of it and not touch it for years.

I plan on using mine for street, DE, & maybe a little track time. (after all it is a targa, and therefore kind of worthless:D )

See Ya, Mark

I don't know how the Andial conversion differs.

However, the only 2.9 I've been exposed to was built by Andial. It lasted about 1 year before it pulled head studs. It was being tracked regularly.

I think Mark is giving good advice on this one.

Is it the 2.9 that wound up with ultra high compression and could not run on any pump gas? BA discusses this in his book somewhere.

i can give you my experiance on the andial 2.9 conversion as the engine chuck is referring to was mine.

i had my 2.9 built for my 74 off the 2.7 and retained the cis. i built the car to be a fun street car and once the engine was complete it was very responsive and great fun to drive.

i did an auto cross with my local pca and got hooked hard and soon found my way to the track as often as i could. looking back, i made a huge mistake that i believe ultimately lead to the premature end of life of my 2.9. while every time i went to the track i came home and modified suspension, tires, racing seats, harness, etc i never addressed oil cooling. i regularly ran the car on the track with oil temps hitting 250 and the head studs started to pull from the case even though it had been time serted. i believe that if i had proper oil cooling i would still be running my 2.9.

in the end, andial took care of me big time and the andial built 3.0 that is in my car now has been bullet proof not requiring a single thing except regular service and it is drivven at the track 90% of the time.

my advise is you really need to know what you are getting into to make an informed decisions, be clear as to how you are going to use the car, know exactly who built the engine and if their reputation will back it up and never cut corners on oil cooling, even for a short time.

Can I ask a STUPID question?

What the heck was Andial doing building you such a great motor and not insisting that you cool it properly?

When I bought my 2.7 MFI '74 my local mechanic told me to immediately put a spoiler mount oil cooler on because I was tracking the car so often.

as i said in my description the car was built to be a street car and with the 28 tube cooler in the fender i never had a heat issue on the street. as i spent more and more time on the track i made the decision to spend my money on other upgrades and a nose mounted oil cooler always fell to the bottomn of my list of to do's. as anyone nows that has started to track a car that is mostly stock you can't have enough money or time in the first year or two to make all of the upgrades most of us desire so somethings have to wait.

now that i have a good amount of track experience i see newbies come to the track all the time just like i did and not show any concerne for the temps they run, especially important here in california when most of our tracks are in the desert. i try to share my knowledge now to hopefully save others from making the same mistakes i have.

Jim- Beyond the pistons, cylinders and case work were there any other modifications necessary to run the 2.9 with CIS? For instance, did it require twin-plugging the motor? Could you run on pump gas? I've been contemplating going to a 2.9 and am curious as to what is involved.
Thanks,
Jon

I'm going to buy this car as soon as I sell mine. It's a '74 RS replica with a twin-plug, 2.9-litre MFI engine. My father bought it last week to tie it up for me.

<img SRC="http://www.holscope.com/74Carrera2.7/RSR_replica_04.jpg" height=375 width=500>

<img SRC="http://www.holscope.com/74Carrera2.7/RSR_replica_02.jpg" height=375 width=500>

The engine in this car was built by a legendary Melbourne-based Porsche wrench -- my late mechanic, John Gregory. It's a great engine; it's done less than 15,000km since it was rebuilt.

I plan on participating in a few club sprints per year with this car. It has a front-mounted, horizontal cooler, which, I suspect, will provide more than adequate cooling. If this 2.9 doesn't run hot, should it last as long as the 2.7 that John built for me?

Matt,

Sounds and looks like a great car. Maybe there's more comfort in your car becuase you know exactly wha twent into it. The only other thing I've heard about the 2.9s is that the cylinder walls are very thin. I would guess that they could go out of spec easier than the 2.7.

Either way, you have an interesting experience on the horizon. Why was the car sold in the first place? Assume the PO put a lot of Aussie-backs into it.

Tristan

You might as well find a Euro Carrera case and make a short stroke 2.8 or a 3.0 for the money you'll spend on a 2.9. With the aluminum cases, your engine will be bulletproof...

-Wayne

Tristan,

The previous owner spent over $200,000 building this car. He's selling the car because he has to sell off some assets.

Out-of-spec cylinders = increased oil consumption, yes?


Wayne,

The '74 RS replica (check the above photos) that I'm going to buy is powered by a twin-plug, MFI 2.9. I'm just trying to determine if I should leave the engine alone or reduce the displacement.

I'd leave it alone until you know for fact that it's a problem that needs work.

With that big oil cooler, I doubt you'll have any issues. I was astounded at the difference my fender-mounted Carrera cooler made. Realistically, it doesn't get very much airflow compared to that center-mount unit. Your car probably won't be doing much heavy traffic grind, either.

Even off the track, imagine the efficiency with roughly 2-3 times the airflow!

Adam,

I love the look of you car. Very sweet. Can I ask you about your Fuchs? After you took off the paint (as you show on your site) how did you re-paint? Thanks and sorry Matt for hijacking but I thought I'd squeeze that one in :)

Matt: Get a black ducktail for that 2.9 and it'll be one grumpy, bad-arse looking 911. Nice. :D

Tristan

Of course, you could always use an early aluminum case ('67 vintage) for your 2.9, and it would also be bulletproof. That's what I'm doing with my current 2.8L racing engine rebuild.

HoHum,

Thanks! I just sent you a PM. :cool:

This is good advice as a ducktail will also aid airflow toward keeping the 2.9 cooler.

David,

Yes, a ducktail is mandatory for this car! Having said that, the IROC tail is as good as having Robert De Niro riding shotgun with his don't-f-with-me expression.

LOL Matt - I like to think Mr. White from "Reservoir Dogs." :D

David,

Yes, a much better deterrent!

http://www.holscope.com/74Carrera2.7/Mr White.jpg

Matt,

Please tell me about the throttle body and intake system on your 2.9. I haven't seen that before. It looks VERY intresting, as in VERY COOL!
Looking forward to you posting.
All the best!
Roger Grago
R Gruppe #27

Matt,
I'd love to see about 100 more pix of that car. Please post some or PM me.

Roger,

All I know about the intake system is that the previous owner spotted them in the workshop and told the mechanic that he had to have them! I'm guessing it's a custom intake system.

Hi Matt

What case is the engine built on - early aluminium or 7R?

Cheers - Ryan

Ryan,

It's built on a 7R case.

that's good news matt - i think that means it should be eligible for targa tas, classic adelaide etc... (as 7R cases were used in 74 right?)

i got my entry for targa tassie confirmed yesterday # 448

Ryan,

I think the 7R cases were introduced in '73. All the 911/83 2.7s were built on them.

Good luck with your '04 Targa campaign! I'll be keeping track of your progress online!

Speaking of aluminum, what about a 2.9L motor on an early aluminum sand cast case, or even more interestlingly, the same pistons in an early case w/ a 66mm stroke?

The main problem with the 2.9 conversion is with the cylinders not the case. The cylinder walls are too thin. If you want reliability then the 2.8 cylinders are a better option.

0.1L increase in displacement froma 2.8.... result is 3% more hp

That answers the whole thing, doesn't it?

Agreed. At what cost/benefit ratio is this upgrade? And at what strain on the case and componentry?

Alan, owner of the Stable in San Francisco seems to like the 2.9's. Has built a few for his customers. You should talk with him. We bought a '74 rsr clone that came with a 2.8 twin plug, but pretty mild compression as it runs fine on pump gas. Has been converted to a race car. I havn't taken the motor apart so I'm not sure what's in there. I notice a lilttle improvement at the track with 100-102 octane, but not significantly.
'74 rsr w2.8 (255 h.p. @ flywheel)

I don't particularly agree, but everyone has different experiences.

FWIW, the 93mm 2.9L Mahle pistons/cylinders that were developed by Andial were to alleviate case problems because at that time there were alot of people building 3.0L's from 2.7L's using a 95mm bore. When these 3.0L conversion motors were being built, the case spigot bores also had to be opened up from 97mm to 100mm which meant that casesavers were no longer adaptable. THAT IS WHEN PEOPLE WERE REALLY HAVING PROBLEMS WITH PULLED HEAD STUDS! These 3.0L conversions were just not smart to do if long-term reliability (like most street engines are built for) is a significant factor.

In response to these issues, Andial's solution was to just increase the bore to 93mm which meant that the case spigot bores did not need to be opened up and thus remained at 97mm. This allowed the use of casesavers which greatly helped with the reliability aspect. The only head machining necessary was a small chamfer for the slightly increased bore.

There is nothing wrong IMO with a properly built 2.9L. It will be just as reliable as a 2.8L will. YRMV I guess but we built alot of them at Andial back in the day and didn't have issues. Whether the cost difference (if there is one) versus power gain is justifiable is a different story and is up to the purchaser to decide. If I wanted to build a motor from a 2.7L rather than going the 3.0L/3.2L/3.6L transplant route, I would have no qualms building a 2.9L. To each his own.SmileWavy

Ralph

Cylinder wall thickness aside, the magnesium cases are a very weak design. They will never be able to handle large power engines. Not only do you have problems with head studs pulling out, but the entire case tends to twist and deform over time. The 74 RSR engines were aluminum for a reason. If you want to build up an engine greater than 2.7L, then don't waste your time - build it on an early sand-cast aluminum block, and you'll be miles ahead when it comes to longevity. These cases run about $750 these days. Be sure to get one with the intermediate shaft, as the shafts are difficult to find these days...

-Wayne

Gee, this kinda supports what I was saying.

The case is not the issue.

The cylinders, however, are thinner. They just aren't as strong and in some cases have failed.

If I had to have more than 2.8L displacement then I would sell off and buy a 3.0L to start with.

Good thread, and many good points were brought up here,...:)

Everyone has their own experience with these things and I would
simply contribute some of mine. Although there is far more to this than I have time to address, Please bear with me as I would make two points about large-bore, mag-cased engines.

1) 90mm-92mm-93mm cylinders are thin. Thin-walled cylinders do NOT maintain concentricity when hot and leakdown figures always show that. One gets compression loss and increased oil consumption.

One of my favorite ways to demonstrate this to a customer is to hand him a Mahle 90mm cylinder and have him hold it tightly between the heels of his hands. Then, I slip a piston into the bore (sans rings). The piston will not fall out the bottom until he relaxes pressure on the barrel and then, voila',....out the bottom in a flash. Not very damned rigid and thats at room temperature!

2) 2.7-2.8-2.9 litre engines operated continously over 7000 RPM develop cracks in the case around the main bearing journal bulkhead behind # 3. It doesn't seem to matter whether its all shuffle-pinned, line-bored, or not; its doesn't matter how precisely the crank, rods and piston assemblies are balanced. Its a likely occurrance and tough to swallow after you spent $ 2K in case prep and machine work. Maintain reasonable rev limits and you'll be fine. Run the things hard, and don't be surprised to find cracks after a few years of that. Its a combination of lack of case rigidity and crankshaft harmonics. The Factory figured this out in '73 and even made some (rare) 2.8 litre 7R aluminum cases. After that, all race engines used the special aluminum RSR case since they had the 70.4mm crank (and 95mm P/C's).


All of these cylinders are thin and so we all make compromises. IMHO, the 93mm (2.9) ones are just too damned thin and hot leakdowns always show the less-than-ideal ring sealing. Compression is HP and for my money, these ultra-thin things give away too much to the 90-92mm ones for power and durability. In short; its not worth it. (IMHO, of course)

Bottom line; its your money but I'll not build anymore 2.9's for these and other issues. There are MANY products made to satisfy a market-driven demand, but that doesn't make the purchase of such things a smart decision,....:) 2.8 litres is max for me.

For those with very deep pockets, one day I'll tell you about a 2.8 we made from an SC engine using custom cylinders with modified 935 heads that made 327 HP at 8200,....:)

I do hope this helps,

you tease...