The BMW head can come with four different cams. I don’t Know exactly which ones come with which heads but i do know which ones i want. I also seem to remember that the first head i did the correct cams were in it already and it was a 93 k1100 LT head. These cams have 254 duration and thats on the cam not accounting for lash.they have .313 lift they are timed at 106/106
The reason i chose these cams is the computer showed these cams to deliver the torque peak at 4500 rpm. The peak power is supposed to be 110 to 115 thats about 90 hp at the wheels. Just to be clear. The car this engine was going into is racing for two days at a time and has to withstand accelerating at full throttle for hours on end. The rpm limit we put on it is 6500 RPM One thing i know from racing other cars is high RPM’s is an engine killer. The mini is especially suseptable to bearing wear at high RPM’s because of its three bearing design. So we are working our way up in power to preserve the engine and the rest of the running gear. Theres more power at the top end if we want it.
Next up in the cams is the 284 RS cams these are timed at 102/106 .343 lift the intake is advanced 4 degrees this would boost top end power but pulls a lot of power out of the lower end 1000-4000 rpm. I may do this on the race car one day but the car is already fast.
The race mini when it had the five port head would come into the pits just blistering hot. Heat would just be pouring of of it. You couldn’t touch anything it was just super hot. The twincam is the opposite it is cool. So cool in fact that you can touch the valve cover right off the track. I am changing the thermostat to a 180 degree one just because the fuel injection won’t fully get out of cold start mode till its over 160 degrees.
These cams idle well but we are still tuning it and have not spent anytime on the lower rpms. Without any tuning it idles at 1000 rpm with a little lope. The engine pulls nicely straight from idle but it does stumble a bit at 3000 more tuning should fix that.
There are other cams other than the LT and RS and there will be more to come on those because i am having them plotted soon.
The engine block is the heart of the beast and careful measuring and drilling/boring can pay in easier assembly and replacement of parts. I start with a truing up of the mill head so all the cuts and boring are straight. Also i always index off the same point. For the Twincam this will be the #2 cylinder bore. To get it on the mill i leave one main cap dowel in on each end of the engine and place the block on the mill with the water pump side on the right and the dowels in the table t nut groove. i will pull the engine down with a stud in the #3 bore and one on each side to get the block stable. Here is where i kind of assume some things. First i assume the crank bores are straight and i assume that #2 is in the correct spot because if its not then at least all of the cylinders will be out the same amount. When you are measuring things on the engine you will notice that things will be out of spec and the bores and studs will be out one way or another .005. These old british engines are just not that accurate. Some things will be more like .015
Now we are ready to bore. I start by indexing #2 then moving the Bridgeport digital read out (DRO) over 3″ to start on #1 and take it out with increasingly more .010 cuts. I will end up at 2.845 on all four bores. Now i know this is not 76mm. Why because i know the bore walls are about .180 to .220 thick and if you add up the 73mm overbore of .97 and the fact that i moved the bore .028 this is .097/2+.028 this is about .076 thinner than stock bore wall and a .200 wall – .076 = .124 if i went directly the full 76mm i could end up under .100 wall thickness something i don’t want. Besides the reason for moving the bores is to make sure the cylinder is not shrouding the valve as much as possible but since we are at 73mm bore this is 2.5mm larger than the BMW stock bore of 70.5. All of this juggling has us clear of the valves and with minimal bore offset. The cylinder bore is now only 2.845 the rest of it will be done at the engine machine shop to get the final plateau finish at 2.874.
Here is a video of the offset boring http://s1346.photobucket.com/user/Hydrolastic/media/Twincam/IMG_2191_2_zpsynrs0hzz.mp4.html?sort=3&o=3
and one of the final to 2.845
Next post will be about moving the studs
Building an engine can be fun and for me its even better when you race it cause you really give it the business on the racetrack. The stock mini engine is pretty peppy and fun for the mini but its no race engine. it runs hot on the track and it overheats even in street trim.
I used a computer engine design program and found out pretty quickly that the five port head would make at least ten percent more power if it just had a regular eight port layout. Its a pretty poor performer right from the start. That doesn’t mean that it is useless it can be massaged to make some impressive power but its a lot of work! and i have spent a lot of time doing various kinds of mods. and i have changed countless head gaskets, usually at the track and once in a rainstorm. Something i have done enough and I was hoping the twin cam would cure the head issue with the race cars. yes the mini and now the 1100.
Measuring the bore centers it came out to 76mm (2.992) for the head and 3′ for the center two cylinders and 3.030 for the outer cylinders. I have found on the mini that the smallest piston to do the job is the most reliable way to go and the 73.5 is the biggest piston i will use. The 74mm just is a bit too big for the head gasket in my opinion. i really prefer the 73mm. The pistons i use in everything is the Hypatec 73mm available from minisport Australia. Now before you see the price and exclaim they can’t be good enough let me just say the team and i have put these pistons through the wringer and in the racing mini they have gone through many thousands of miles of full throttle use with a dozen different drivers and we have never had a piston failure. One key item to note is my machinist (who used to race mini’s himself) sets the bore clearance at .003 no more and no less. This is more than the recommendation on the piston box but after break in the engine will not use oil and the pistons will not be scuffed between rebuilds. I have used the chrome and the cast rings neither seem to have an advantage to me. Just to be clear the race mini also has a five port (used to be the main engine but is now the spare) with these pistons and with the main bearings replaced evey three races and the syncros at the same time. There are a few other things i now do at this time. Primary gear bushes and a inspection of the transmission and idler gear bearings under a magnifying glass. But i digress and here is one main point. OIL is the lifeblood of your engine and after trying seemingly every oil. I now break in the engine with castrol 20/50 with zddp additive and from then on MOBIL ONE 15/50 You will get three times the life out of your bearings. don’t bother with anything else. Thats it for now next post will be how to move the bores and get the engine block ready.
Have you ever wondered what your mini would be like with a modern engine or at least more horsepower?
I have built many engines from stock to wild and a few years ago i started on a twin cam project. I made some mistakes and found out a few things and thought i would share them.
This is based on Gregg Temkins masterpiece engine he made in 1989 and i originally contacted him and went over to his house to look at it about 2008 or 2009 I was really intrigued and started getting the parts together.
First item was to find a head. I got my first one from a BMW specialist. It is also the head that is on the only running twin cam i have finished but there are now a few in process. Anyway this head is the K1100 lt version and i am so glad i chose this one as it has now given me a starting place to go from.
Next thing to do was to take a good look and figure out just exactly it was that i was looking at and to get some idea of the measurements. physically it fits on the mini 1275 block perfectly and like a lot of people i thought ” it looks like it is made for it! ” However it’s not that easy.
I get most of the items needed from specialist components and they have done an excellent job of making the parts needed for the conversion. They will also help you out if you give them a call.
However not all of the components are the absolute best for my builds and i have sourced several items from MED and DIY auto mostly for the electronics the rest of the items i get from ebay or sites on the internet.
Here is a picture of the engine