My friend and I have purchased a pair of k1100lt heads and just starting the twinky project. I am interested to hear anyones thoughts or recommendations on engine specs. We purchased the the heads with the lt cams so that we could get more usable street performance without having to rev the engines to high. I am running a mk2 s box with 10's and the engine will be based on an a+ 1275 block.
1. What engine size (1293 or straight to 1360)?
2. What CR ( mainly run 98 but it is not always available)?
3. What diff ratio? I am keen to get 3.1 but with mk2 s gears I am not sure.

Any advice or opinions would be greatly appreciated.

Thanks, Scott.

1. The bigger you go the less gap you have btw the bores, therefore harder to get a good head gasket seal. 1380 = 3mm btw bores

2. CR. Make sure when the welding is done there is not too much machined off the head. If combustion chamber CC is too small <25cc it makes it difficult to get the rest of the calculations right.

3. Don't know about the S box, but standard rod change 3:1 = 115kph @ 4000. If I changed I'd go 2:9 (lot of highway use and 90hp@w will pull it, unless you want to race btw traffic lights) But that's IMO

Good Luck

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Dalmeny

Thanks for the reply Dalmeny.
I read your build thread and your car and engine look awesome, I hope it is still going well.
If I remembered correctly you ended up going with a 11.4 cr are you happy with this and what fuel are you running?
Are you happy with the acceleration the 3.1 gives you and does the engine easily pull it?
What cams does your engine have?

Thanks, Scott.

I have a 1430cc big bore stroker, and have only ever blown one head gasket when I was running the 5 port.

Now that it's been refreshed for a 7 port I changed the diff from a 3.44 to a 3.67 for better acceleration. You don't reach top speed on most tracks, never mind the road.

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Too many cars, and too little time.

(1) any size that works for you,,, the bigger you go the more power you "should" make
73mm (with std stroke crank is 1360cc) is as big as i would suggest, as Steve says, bigger than that makes very thin cyl walls... still do-able but i like to reduce "Chances" of failure.

(2) 11.5:1 comp ratio is what we have run with Bimmer Twinkys (in N/A form) without dramas & with 30deg total ign,,, more than that comp ratio i won`t suggest for the sake of occasional "Chances" of detination,,, (one ping usually starts a series of pings)

(3) 3.2: F/D ratio is as high as i would go,,, but in saying that i rather like a slightly zippy-er ratio for around town & track day usage, 3.4:1 or 3.6:1,,, simple reason = quicker acceleration,,, & lets face it, if you`re building an engine for performance then what you`re doing is looking for quicker acceleration... if you wanted a burnt valved 850 with a worn-out automatic transmission that slips,,, then you`d have asked us how to slow your engine down,,, not ask how to make it faster

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

Scott, It's taken a while but the car is going really good.
Blew two head gaskets then put a Cometic MLS from MED UK, been good since and lowered the CR to 11:1 or just under.
Always run 98 fuel with no probs
I might be a little different to others as I live on the Hume Hwy and travel at 110 + which every way I go, and most drives involve 100kph country roads
As Matt points out, the lower the FD the zippier you'll go.
Also make sure you go for a good ECU and the jury is still out on the quad throttle bodies.
Cams are standard LT's

Steve

ps My Microtech ECU had to be heavily modified to speak to the Bosch TPS

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Dalmeny

Thanks for the replies.
Steve, it is good to see that the car is going well and that you managed to sort the head gasket issues. Do you know if other people have had similar issues with blown head gaskets? Your diff requirements for your mini are a lot different than mine, I will mainly be using the car around town with an occasional longer trip.

Matt, can the twinky engines handle a higher CR than a normal 5 port mini engine? Would 11.5 CR be suitable for 95 octane or only 98? Would you recommend the same CR for both the LT and RS cams?

Thanks, Scott.

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Dalmeny

I am not sure if anyone will see this thread, but after a long wait we are finally back to building a couple of twinkys so I have a couple of questions.

What +40 pistons would people recommend for the twin cam build? I will be aiming for 11.1-11.5.1 compression and need suitable piston crown thickness to handle being pocketed for the valves.

What head studs are people using for the build? I know specialist components sell an ARP kit, but they are quite expensive.

Thanks in advance for any help.

The standard BMW head studs are more than capable but ARP will no doubt be better. Better will always be the enemy of good though!

As for pistons I don't have too much experience in this space but anything with enough meat on it to still allow for the butterfly cuts would be my educated guess.

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-----Better is the enemy of good-----
-1976 Yellow Devil Leyland Clubman-
-1970 Morris Mini Van - Work in progress-

Thanks for the reply. Are the BMW head studs available new or would the second hand studs be fine?

The right answer here is - new ones.

The garage engine builder really says - yep, the ones that came with the head off the motorbike will be fine. If you were going to be going turbo you'd have a bigger argument as to going new.

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-----Better is the enemy of good-----
-1976 Yellow Devil Leyland Clubman-
-1970 Morris Mini Van - Work in progress-

The engine will be normally aspirated, but I may consider a turbo or super charger later on if I am after more power.

I am using the second hand head studs in my NA twin cam build too. I'm fairly sure dalmeny used the studs from the motorbike in his build too. I'm sure Matt can testify for heaps and heaps of his builds with re-used head studs so you will be fine Keen to see this one progress!!

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-----Better is the enemy of good-----
-1976 Yellow Devil Leyland Clubman-
-1970 Morris Mini Van - Work in progress-

It's good to know what other people are using so thanks for the info. I am also really keen to see my build progress.

What pistons are you or others using?