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BMW Twincam, Which block?
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Author:  kirby [ Wed Sep 29, 2010 11:29 am ]
Post subject:  BMW Twincam, Which block?
So Ive got hold of a BMW head 8v, with all injection components.

But which block to use?
I'm just a student so price is my number 1 concern.

Doing all this work a 1275 is the obvious choice but they are pricey.
998 and 1100 I'm lead to believe are the exact same block just a different stroke, same bore. Which has the better crank?. (Bottom end strength is a concern)

How far can you overbore a 998 or 1100 and how available/economical are pistons?
Author:  sgc [ Wed Sep 29, 2010 12:24 pm ]
Post subject:  Re: BMW Twincam, Which block?
kirby wrote:
998 and 1100 I'm lead to believe are the exact same block just a different stroke, same bore.


Noooo.. the big bore block has its name for a reason ;) Bigger bore, and different bore centres to the small blocks.

The biggest you can reasonably go with a smallblock is around 1220cc with a 1098 crank.

[edit] Oops, I may have misread. As Doc says below, 998/1098 are the same, but not the same as the bigbore :oops:
Author:  drmini in aust [ Wed Sep 29, 2010 12:27 pm ]
Post subject:  Re: BMW Twincam, Which block?
kirby wrote:
So Ive got hold of a BMW head 8v, with all injection components.

But which block to use?
I'm just a student so price is my number 1 concern.

Doing all this work a 1275 is the obvious choice but they are pricey.
998 and 1100 I'm lead to believe are the exact same block just a different stroke, same bore. Which has the better crank?. (Bottom end strength is a concern)

How far can you overbore a 998 or 1100 and how available/economical are pistons?

Yes 998 & 1098 are really the same block, except the number on top.
998 crank will stand over 8000 rpm they are pretty bulletproof.
1098 crank I'd keep to 6500 max maybe 7000 peak, but you do get 100+ more cc for a given bore, = more torque and HP at sensible rpm for road use.

You can bore to +.100" with either crank, also +.120" pistons are available for 998. Using Hypatec or Russell Engineering pistons. Prices around $190-$270 a set.

You can go to 68mm (+.134") with a 1098 crank + Imp pistons, but these are scarce/expensive now.
68mm bike pistons have been used too, ask TheMiniMan which ones...
Author:  Gareth_1330 [ Wed Sep 29, 2010 12:39 pm ]
Post subject: 
i dont want to rain on anyones parade.... but...

have to question if a bmw head on a small bore motor represents the best HP per dollar
Author:  awdmoke [ Wed Sep 29, 2010 1:20 pm ]
Post subject: 
It doesn't in my opinion, and the small bore block will need to be rebated to allow for the BMW valves.
Author:  Molina [ Wed Sep 29, 2010 1:25 pm ]
Post subject: 
I also don't want to rain on parades but if you're that tight on dollars should you be attempting this converson?
Author:  TheMiniMan [ Wed Sep 29, 2010 2:01 pm ]
Post subject: 
It`s "do-able" on a small bore (((& in a few different ways))),,, we`ve found that 100thou oversize (or bigger) is best & you would also need to "off-set" bore it &&& also off-set the head mounting onto the top of the block , to make it all fit nice

If you`re only going to go .080 oversize then you will need some valve cut-outs (reliefs) in the block for the valves to clear

The 1275 block is a far better choice , for a number of reasons, some of which being --->power, ease of build, & strength,,,, but we`ve built a few small bore twin cam engines (all of which were 998 as i`m not a big fan of the 1100 crank) & they do go really really well,,, i`ve never been a big fan of small mini motors until running our small-bore twin cam engines,,, well now i`ve changed my mind some-what

Obviously there is not ever going to be as much torque delivered from a small-bore compared to the grunt from a big bore, so "THAT" is what you need to keep in mind when making the decision as to which block to start with

However,,, if you like a "REAL:LY" smooth, hi-reving engine,,, then a 998 twin-cam is a great choice,,, these things just SING :-)

RS cams would be my choice for the small bore 8v twin cam conversion --- if you can find them

LT cams are smoother but less top end performance

either way,,, they`re a VERY efficient cyl head
Author:  kirby [ Wed Sep 29, 2010 5:32 pm ]
Post subject: 
Whats the going rate for a 1275 and where would I look for a block or short motor.

I would happily sacrifice size for a high and free revving engine.

This engine will also be supercharged, with an SC12 so compression ratio will be a concern, If i needed dished pistons are they available and I assume they wouldn't need to be notched to accommodate the valves.

A jap engine conversion is starting to look like it would have been a better idea.
Author:  Kennomini [ Wed Sep 29, 2010 7:18 pm ]
Post subject: 
1275 blocks can go cheaply $50-$100 but that will only be a bare block in rough condition. $200-$500 for a decent block crank and rods is also a good price but this can go anywhere up to $1000 or more :roll: .
Check in The Mini Experience and on mini club web sites for mini business adds and for sale sections also keep an eye on the online auction sites (ebay and stuff).
This will be cheaper than a Jap conversion by roughly $5000 at least. Plus it's still an A series :D .
Author:  kirby [ Wed Sep 29, 2010 8:05 pm ]
Post subject: 
I was saying a jap conversion would be easier.
That way the engines already engineered but you just have to bolt it up.
But thanks for the advice.
Author:  Gareth_1330 [ Wed Sep 29, 2010 8:31 pm ]
Post subject: 
got to make a custom subframe, custom driveshafts, sort out a computer and wiring, likely using aftermarket gear. polution gear, exhaust, engineering costs.

Maybe you should post a few guys who have done jap engine conversions, people see a jap motor for a few k and think its cheap, but the cost of the motor pales in comparison to the cost of getting it in, running and legal/

I reckon best HP for buck is a turbo 1100, but i like 1275s

Also, i will buy any 1275 blocks people are selling for $50 8)
Author:  kirby [ Wed Sep 29, 2010 8:42 pm ]
Post subject: 
Still have to make custom exhaust, computer, wiring.

But add to that custom water jacket, custom oil ways, head stud holes, valve cut outs, custom timing belt.

Much harder than welding some stock bar together and paying an engineer to look at it.

But that's neither here no there.

I'm thinking an 1100 unless a 1275 block presents itself, How much better are the 998 cranks?
But ill do up some measurements, I don't like the sound of this off set head mounting.

Also MiniMan any more info on these cams would be helpful.
Author:  mini_mad_matt [ Wed Sep 29, 2010 9:13 pm ]
Post subject: 
I don't mean to be a negative nancy... but

If the sound of off-setting the head concerns you then maybe you should stick to something a bit simpler and cheaper. If your a student strapped for cash worried about the cost of a 1275 over a smallbore and your planning on supercharging a fuel injected bike head, well i'm sorry but i don't think you've looked into it very hard at all.

In my opinion doing the block mods to fit the bike head would be much much easier then fabricating components, finding somewhere to put the supercharger, setting up the fuel injection and whatever ignition you run etc etc.


If this is still what you choose to do then by all means go ahead and make one awesome motor and enjoy it, but it just doesn't sound to me like it's likely to get to the point of enjoying it.
Author:  kirby [ Wed Sep 29, 2010 9:23 pm ]
Post subject: 
I'm more in it for the challenge. I'm not flat broke and I'm not chasing every half horse power, so I guess my question should have been is the 1275 worth the etc cost over a small bore.

Now in regards to offset boring, how concerned should I be about the crank no longer lining up perfectly with the bores?
Author:  mini is my ride [ Wed Sep 29, 2010 9:32 pm ]
Post subject: 
hmmm
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