2.7 Build- 2litre conrods wont work will break

[silly]

Re sticksy post on a 2.7 build using the 81mm crank, 2litre conrods, 325i pistons and a decked block. I am just about ready to start this but I have been given the contact for a company in Christchurch here in NZ who have built 16 of these engines. Some for targa racing and others for road use. Their veiw is that the CI crank should not be lightened at all, that the 2litre conrods will break if 6000 rpm as higher rod speed is used so should not be used , that they only way to build them is to use the 325i rods with special pistons ( which they get in) with a std 320 head valves etc.

So I'm in a quandry about this having got my crank , rods etc jsut about ready to go.

I would like to know if anyone has built a 2.7 to stickys spec and either raced on a track or run hard road use and if its ok and hasnt pooed itself.

silly

[maggspower]

Hello there, the artical written by Andyboy is an excelent piece of work, and lists the most common, easiest, and best ways of building a 2.7 using OE parts. There is always a better way to build an engine as they say, "speed is a matter of money, how fast do you want to go?"

The eta crank is thought of as the weak point in the common 2.7 set up, metric mecanic in america have tested and have never seen one break in their engines, they suggest a 225bhp and 6750rpm limit. I think that should do BMW would not of gone out of their way to make an esspecialy crap crank for the eta motor!!!

The thing with lightening the crank is less is more, it still needs to balance or its not going to last long.

The ideal situation with rods would be to use 135mm, but this means you must use alpina, or custom pistons like your machine shop said. In theory it also gives a better "rod ratio" (look it up on the net). The 130mm rods have been used in loads of 2.7s, I met someone the other day using them in a turbo application with over 400bhp Again BMW spent a few Dm £Â£Ã‚£Ã‚£ $$$ designing a single rod, each component they designed has the company reputation resting on its shoulders. The rods are very simular, the only differences are 5mm and the 135mm has "ears" on the sides of the small end bearing.

My vote would be a 325i head, but for optimum performance the piston needs to compliment the chamber shape, so 325i or custom. some people have used the 320i head but use 325i valves, although the smaller valves would give alot of low down torque, at the expence of top end bhp

We all like the best things, i am building a 2.7 as we speak, I would like custom pistons, custom rods, cnc alloy block, oh and a cnc twin cam head, all of these are possible but may be a little expensive

There are 101 ways to make a 2.7 no one is the best, its all about compromise. Just build the engine you have the parts for, as long as the parts are not worn, are machined correctly and balanced, and run to the correct tollerances, and are treated correctly during assembly, run in and use, you should get a fine engine. Any engine can poo itself given the opertunity.

good luck maggs

[silly]

maggs

yeah cool & I think u are right

I also happened to get in touch with another scource who is here in auck and who has built a few .his veiw was that the info i got was hard marketing and that he has never heard of rods breaking. He also told me about several auck E30 series NZ race cars that have 270hp+ that have been going for awhile now no probs

so I'm carrying on and agree totally with your last para

my only last quandry is what piston type to use with the 325i head- 9:1,11:1,10.2:1,8.5:1/or 10:1?

do you know?

silly

ps I see u are a newbie but you joined in 2005 how long till yer an oldie?

[Gunni]

There is nothing wrong with the B20 rods and revs above 6k,
I used such an engine in a drifting competition, constant revs at 6-7k for about 5minutes and the engine survived
without issues.

[maggspower]

Glad to hear you are not going to only listen to the machine shop. I'm sure that their engines have merit, but saying that a 2.7 can only be built one way is bull shit

As for pistons, Im not sure what OE m20b25 ones you got over there in NZ. If its the same as us it will be pre facelift up to about 1987 high compression 9.75-1, or after '87 low compression 8.8-1. Both Pistons work in a 2.7, the final static compression ratio is determined by how much you skim off the block.

The reason i'm still a newbie is the amount of posts i have, I'm not realy a post whore, I only coment on things that I know about, so nt alot realy

[stonesie]

I thought the standard M20 rods were very strong, they certainly look very beefy compared to the things used in ford zetec or rover K series engines...

[march109]

Sounds like they were trying to squeeze you for more $$$'s.

[silly]

Gunni & Maggs

Well you certainly have given me some very sensible advice so thanks...

You can get pretty well anything from Mahle through the parts distributer here or can use BMW dealers but cos the markets small here & there are only a few parts distributers you pay heaps. The Mahle piston types I quoted above are the range I got a photocopy of from EML who are the aftermarkt guys here. Theres quite a range but it gives all the dimensions and piston top shapes so when i strip the engine down i can see with my builder whats what and we can measure the amount to take off the block accurately which will probably tell us what piston to use so we will see. The pistons here are $1200 plus shipping and vat - ouch! My car is an English import touring which I've had for 16yrs so all should be well.

Stonsie

Thanks

Yes i concur
The 2.3 conrods with the pistons I've got from the Hartge car (for $60) do look very strong - before posting this I shot out to the garage to have a look again and compare them to the ones in Andrew Everitts book - so agree with you they look ok . I will get them polished and balanced with the rest of it just to be carfull tho.
I guess the thing with all of this is not to rush things and do your research properly & cross reference on opinion.


March

Thanks also - yes I think they were trying to pressure me and funny thing was they straight away quoted me a price to supply an engine in the car.


silly in NZ

[Andyboy]

323i rods are the same as the 525e and 320i. There is nothing wrong with them - they are the same design as every other M20 rod. The rule is that the con rod bearing centre to centre length (130mm in this case) must not be less than 1.5 times the crank stroke which on the Eta is 121mm, 99mm on the 2 litre and 115mm on the 323i.
Even on the 2.8 it's 126mm so will still work. The reason is because if the con rod length is less than the stroke x 1.5, you will have excessive con rod angularity, i.e too much side thrust on the piston.

Your BMW expert is talking b0llocks - maybe they should try something else (knitting or gardening maybe) if they can't get a 2.7 with 130mm rods to hang together!

[silly]

Hey Andyboy

Thanks

Yeah totally agree. Panick merchants trying to get my work.

Have done a bit more research since and am confidant with using the 130mm rods/2.7 81mm crank and Mahle 8.8:1 pistons. I will be getting the rods & everything else checked & balanced so should be sweet.

Whats more I got hold of one of the gurus round here who has a BMW workshop and is in the club etc and who,has built several 2.7/ 270hp jobs sucessfully no problems using same spec.So I think I'm ok. Just got to source a 272 deg cam and get hold of Irelands bigger E30 injectors and thats it all done.

Thanks again

Silly in NZ