Rebuild 2.7 or start from scratch?

aft911 · 10-26-2003, 10:55 AM

Hi all, I'm a newbie to racing and engine work. Here's my situation and I'd appreciate any input this crowd might have. I bought a very good 1974 911 chassis. The current engine has broke the crank twice in quick succession. Two pistons and the case need to be replaced... that's the diagnosis of the mechanic who did all the work. It was converted from a street to race car just before this happened, with balancing and some other standard "hotting up" to make it competitive, which it was while running.

The overall decision I'm trying to make is: 1) current engine is cursed, I should part it out and start w/ a different engine -- either w/ a local or find a motor shop that sources factory engines and hots them up for a living, 2) the problem is the mechanic and I should have someone else rebuild the current engine.

At the end of either process, I want a reasonably competitive engine which has been balanced (etc. etc.). Any opinions as to which path is most likely to be lower cost? Lower risk? I've seen pricing on rebuilt 2.7s in these forums but assumed that wouldn't give a truly race-ready engine?

Thanks in advance to all!

Rot 911 · 10-26-2003, 11:19 AM

Not sure I would rebuild that engine. Here is a guy that had Otto in CA build him and engine: New Motor!

Wayne 962 · Wayne 962's Garage

The stock engines are pretty much ready for racing - that's the way the factory designed them. The mods and improvements that people install on the engines help, but this is no production street engine - it's designed to take a licking right out of the box.

What are you looking to do with this car? Two busted cranks - I wouldn't trust the mechanic or the engine block. Do you have a copy of the Engine Rebuild Book?

-Wayne

aft911 · 10-26-2003, 03:17 PM

Thanks for the posts! I bought the car off a friend very recently... he was the one who endured all this trouble. If "Engine Rebuild Book" means the build sheet, I don't think he got that supplied by the mechanic, which is a bad sign in and of itself.

I want to do PCA racing, either G or H class. The H would be a rebuild or transplant 2.7. The more I look into things, I'm thinking go w/ a 3.0 (and compete in G) for a few reasons...

First, so far I'm getting an overwhelming response of "don't try to rebuild this dog" from this forum and from knowledgeable friends. Second, the 3.0 SC motor has such a stellar reputation for longevity. Third, out here in TX, G is so much better populated than H. I toyed w/ the idea of F (3.2 engine) but the SC engine has a better rep and doing F gets into more chassis mods than I think I want to invest in right now.

The message I seem to be getting from you and some other friends is "a solid stock SC will be competitive and you're better off not messing with it vs. running risk of trying to hot it up".

vash · vash's Garage

aft911, i think wayne is talking about his 911 engine rebuild book. you can find a link at the bottom of his reply. i am also new to all of this. you are at the right place for answers, these folks constantly amaze me. get that book, i am still waiting for my copy in the mail. alot of people have suggested i get it even if i am going to farm out the work. good luck.

aft911 · 10-26-2003, 05:03 PM

Oh. In that case, I'm happy to state that I ordered it earlier today based on all the great feedback throughout the various forum threads.

Halm · 10-26-2003, 05:06 PM

AFT911, where are you located. PM me tomorrow and I'll tell you my initial inpressions on the the motor Otto is building for me.

Bobboloo · Bobboloo's Garage

The first question you have to answer is which class you want to run and take it from there. I will say this though that a 3.0 core will probably turn out to be cheaper to build on. The initial buy in may be a bit more but you'll make up for it in less machining costs. The 2.7's cases require alot more machine work to make them usable. The 2 busted cranks suggest that maybe the case was out of spec on it's line bore.

aft911 · 10-27-2003, 08:48 PM

Thanks for the input. And thanks for replies to my private PMs, Wayne. Today I talked with quite a few of the shops that Wayne recommends. Thought you might be interested in this feedback.

All are deeply skeptical in the mechanic's statement that "previous owner missed downshifts and that broke the crank" (twice, by the way). A bad shift violent enough to do that would have destroyed most of the top end, whereas selective damage was actually done.

All of them also made statements like "I've been doing this for 20-30 years and have seen a TOTAL of 2-3 broken cranks over my career".

Their candidate diagnoses:

Bad crank – extremely unlikely that the same car has the unbelievably bad luck to have gotten two bad cranks

Bad case – but wouldn't have problems shown up in the first 75k miles w/ 1st owner driving it around on the street?

Bad line bore – nothing obvious from looking at the wear patterns on the bearings, but maybe it happened too quickly for it to show on the bearings? Both breaks happened fairly quickly after the rebuilds.

Bad other machining on the case -- was shuffle-pinned, time-serted before the 1st break. Some probability

Bad assembly – highest probability culprit

aft911 · 10-27-2003, 08:50 PM

Should have added to second sentence above that none believed that missing a downshift could break a crank. One said the most it could do in the bottom end is break off a flywheel.

Halm · 10-28-2003, 06:21 PM

In the "FWIW" category, I have to tell you that my new 2.7 is unbelievable. Very strong through all rev ranges but with the S cam really comes alive above 3500 rpm. And this is before we work out the mixture. It will go onto a dyno before the end of the year.

aft911 · 10-28-2003, 06:32 PM

Hal, are you racing a PCA stock class? Are S-cam's allowed in H?

Halm · 10-28-2003, 06:35 PM

Sorry, I don't race and thus can't answer that question. I guess my point was that a well done 2.7 can be a terrific motor.

aft911 · 11-06-2003, 10:57 AM

As I've done some more research on the original problem that started this post (2 broken cranks on same engine). As mentioned above, I called several gurus (defined as "made Wayne's list") who answered my question of "can a blown downshift break a 911 crank" with "probably not". But after skimming Wayne's and Bruce Anderson's books (just got them in), I'm wondering if I asked the right question.

So I wanted to ask this crowd a follow-up, modified question. Is it reasonably probable that (mechanic's contention) a SERIES of over-revs -- not just one bad downshift, which is more how I asked the gurus -- break a 911 crank? Or would all the damage likely be done only in the top end?

I know exact diagnosis w/o looking at the engine is impossible, so not asking for that. I just want to explore reasonable probabilities, to better gauge where to set the BS filter w/ the mechanic.

Wayne 962 · Wayne 962's Garage

BS meter is reading high with the mechanic - don't go back...

Reflect upon what the others told you (2-3 broken cranks in 20-30 years). Two broken cranks with you is like lightening striking twice. I personally think it would be difficult to break a crankshaft without throwing a rod through the engine case.

If your case wasn't properly prepared, then you will get stress on your crank. Send the case to Walt (Competition Engineering). When you get it back, it will be perfect, and ready to go!

-Wayne