Best 850 head option?

[mnicoop63]

I have an 850 that has always ran great and pulls pretty good that I am about to replace the timing chain and gears on because the chain is pretty stretched. Since the engine is out of the Mini I figure the time is now to replace it. I will be pulling the head off in order to find true TDC so I can make sure I get the cam and timing chain all correct with a degree wheel. While it's off I figure its a goodtime to put a bigger valve head on it and believe it will really benefit from having one. The engine breaths thru an inch and a half SU with K&N air filter and Howley intake manifold. It has an LCB with twin box RC40 exhaust. The distributor is an old Aldon 100ayv with vacuum advance. It has a VP7 mild street cam that has 270/276 Duration and .289/.292 lift. The power range is quoted at 1600-6500 rpm. It has a really small valve head on it ( i believe its a 12a1456) and really only gets to about
5000-5200rpms. I have several other small bore heads and I am wondering which would be the best to get it to breathe a little better. I a 12g202, 12g206 and a 12g295. The 295 head seams like it would be way too big and I am leaning towards either the 202 or 206 head. They are all stock from what I can tell but will have to have the chambers measured to make sure the compression is correct. Will I be running into any problems with either head if I have to have them skimmed? Like valves hitting the pistons or anything? Is it simply just a bolt on and go kind of thing? What have any of you done before? Just looking for a little better performance.

.

[850man]

202, 206. 295 will all work well with your engine. I would pick the 295, as it has the largest valves & duel springs, it will be excellent with your current engine specs.

[Dearg1275]

The 12G206 and 12G295 are essentially the same head, same chamber shape and size. The ports are very large too. They have the largest chamber size (28.3cc) of all the A series heads and you won’t get the compression ratio you need even if you skim the life out of them. You would also suffer a loss of port velocity with such a small engine capacity.

The 850 heads are 24.5 cc. You need to keep the combustion chamber volume down beneath this figure. If you can take enough meat off the 997/1100 12G202 head which is 26.1cc that is probably your best route. The ports are a better shape and not too large.

I fitted a 997 inlet valve in my standard 850 head and relieved the area around the valve slightly. It has worked very nicely.

D

[bwaminispeed]

Yep, modified standard head, or, 202 is best......295 valves, ports and, combustion chambers are all way too large for a stock block 850......

Standard head, you need to grind the beak back by about 3/16", The biggest restriction is the exhaust ports, for the first inch bellow the valve seat. Re work this area, mostly the short side radius, and, the guide boss. 1.070" valve. Intake ports can be opened up a bit, and, next size larger valve........Keep the intake ports as rough as possible, polish the combustion chamber, and, exhaust ports......

All the above relates to the 202 head, but, it doesn't have enough metal under the exhaust valve make a good radius into the port....Standard head, is still your best bet, although, slightly more work.....

I also use Rimflow style valves, and, 30 degree seats instead of 45......This helps the cam pull in earlier, and, gives more torque at the low end of the rev range........

All you need to know about making small bore engines go fast is right here..... http://russellengineering.com.au/projec ... gine-pt-1/

[850man]

Just use the 295.
Any slight compression loss will be made up for with the far superior flowing head.
Or you could simply skim the 295 down.
Its quite simple really, you would think no one has ever done it before considering the "expert" comments.

[bwaminispeed]

It's not a slight compression loss.....It's quite a bit (a lot actually).......compression is power, pure and simple......

And, lots of flow is no good if the port velocity is not kept up......

[mnicoop63]

I appreciate the info. I pulled out the heads and after looking at them I am thinking the 206 and 295 are just too big and I think the 295 has had oversized valves put in it at some point because it appears to have had the exhaust seats replaced and the valves are bigger than those in the 206 heads. It doesn't look like the ports have been worked at all, even around and near the valve seats or bosses. The 202 head looks more like a good candidate but I am almost thinking I am going to just put the stock head back on it and run it as is. I am currently building a 1275 that will eventually make its way into the mini. I would just love to see what a little bigger head would do for the 850. It was always the plan as everything else has been done already in order to get the benefit of a bigger head. Here is a picture of the 206 heads and 295 head which is at the bottom. It definitely appears to have larger seats than the 206 heads. I always thought it was a stock head. I have had it for probably 20 years and never used it. All these darn heads and several others I have, I have had for a long time and never used. It's kind of a shame really. I'm frustrated at the moment and might change my mind about putting a bigger head on the 850. Tonight I started to put on the new timing chain (its a minispares vernier setup) and the damn cam gear when installed and tightened hits the three bolts on the triangular thrust plate. Any suggestions as to what to do about that?

The 12G206 and 12G295 are essentially the same head, same chamber shape and size. The ports are very large too. They have the largest chamber size (28.3cc) of all the A series heads and you won’t get the compression ratio you need even if you skim the life out of them. You would also suffer a loss of port velocity with such a small engine capacity.

The 850 heads are 24.5 cc. You need to keep the combustion chamber volume down beneath this figure. If you can take enough meat off the 997/1100 12G202 head which is 26.1cc that is probably your best route. The ports are a better shape and not too large.

I fitted a 997 inlet valve in my standard 850 head and relieved the area around the valve slightly. It has worked very nicely.

D

[mnicoop63]

And I would like the damn thing to idle as well...

I replied to another user above and wanted to include you in that response but don't know if or how to reply to two users at the sametime so maybe you can read it and offer some advice. Cheers.

It's not a slight compression loss.....It's quite a bit (a lot actually).......compression is power, pure and simple......

And, lots of flow is no good if the port velocity is not kept up......

[Dearg1275]

Just use the 295.
Any slight compression loss will be made up for with the far superior flowing head.
Or you could simply skim the 295 down.
Its quite simple really, you would think no one has ever done it before considering the "expert" comments.

Since when did offering an opinion in response to a genuine request for advice make anyone an “expert”?

D

[850man]

Just use the 295.
Any slight compression loss will be made up for with the far superior flowing head.
Or you could simply skim the 295 down.
Its quite simple really, you would think no one has ever done it before considering the "expert" comments.

Since when did offering an opinion in response to a genuine request for advice make anyone an “expert”?

D

My point is that "experts" come along and start going on about port velocity and huge decreases in compression ratios lose sight of the original question.
Spend a thousand pound modifying a 202 head, or bolt on the factory made performance head you already have. Its a no brainer.
I highly doubt anyone will notice a difference in comp ratio because a head with a 3.5cc larger combustion chamber has been used. Combined with the current mods of exhaust, cam profile and bigger carb & better flowing manifold a 295 is an excellent way to increase the performance of any small bore engine, including this 850.
In fact, I just fitted a 295 on a 803cc engine in my A30 recently, along with extractors and twin carburettors, and guess what, it goes much better now. Funny that. Not all experience is gained from internet forums and websites to "gurus" in Australia that build hand grenades. Sometimes the best knowledge comes from bolting on parts and seeing what happens.

[CooperTune]

Way back in 1968 I rebuilt an 850 for my first mini. Having worked for CARMATIC ( an engine and transmission exchange ) for three years
I knew a little and had a few contacts. Had the block cleaned and line bored for the additional cam bearings. Bored for .060 Power Max pistons.
Had the coil spring flywheel surfaced and lightened along with the back plate adding double springs. Crank ground and balanced with a two part Cooper balancer.
Zero decked block installed Isky lifters, push rods, retainers and springs. After carefully measuring the location of the rocker oil passage
had the 295 surfaced .125 new guides 3 angle valve job. Using a Cooper S camshaft, simple header and twin 1 1/4" SUs. I would suggest the
295 it worked well for me. Steve (CTR)

[Dearg1275]

Way back in 1968 I rebuilt an 850 for my first mini. Having worked for CARMATIC ( an engine and transmission exchange ) for three years
I knew a little and had a few contacts. Had the block cleaned and line bored for the additional cam bearings. Bored for .060 Power Max pistons.
Had the coil spring flywheel surfaced and lightened along with the back plate adding double springs. Crank ground and balanced with a two part Cooper balancer.
Zero decked block installed Isky lifters, push rods, retainers and springs. After carefully measuring the location of the rocker oil passage
had the 295 surfaced .125 new guides 3 angle valve job. Using a Cooper S camshaft, simple header and twin 1 1/4" SUs. I would suggest the
295 it worked well for me. Steve (CTR)

This rather underlines the point I was highlighting about the 295 head. You can just bolt it on in standard form on a standard 850. If you do you would end up with a compression ration of 7.5 to 1. Standard 850 is 8.5 to 1. To get the best out of the 295 you have to remove a lot of metal, as you have done, from the block and head. You may get an improvement using it raw but it won’t exploit the potential of the head or the longer duration camshaft to the full.

D

[bwaminispeed]

Plus, the .060 overbore brings things back into the realms of normality compression wise.......just...

[mnicoop63]

I am not so sure the 295 I have would run very well on the 850. It is just plain too big and has the oversize valves from what I can tell. I think it would be a waste of a good head that is better suited for a 1000cc or bigger engine. I would be better off using one of the 206 heads and having it skimmed and see how it ran. I'd rather mess around with one of them since I have 3. The 202 head would probably be the best if I were going to just roll the dice and see how it runs. I also have a few 12a185 heads but am not sure of the combustion chamber size on them. No not the Cooper S big bore head but the early 997 Cooper version. One is a Downton modified head and like the 295 is too big in my opinion. I want something that is street worthy and will idle etc. The other two heads I believe are stock and I am wondering what the specs are on those heads. Anybody? All the information I ever see is for the S spec 185 head. Heck the first thing I need to do is figure out is why or what to do about the cam timing gear hitting the thrust plate bolts.

[Dearg1275]

The early 997 heads had the same valves, chambers and ports as the later 202s. So a combustion chamber volume of 26.1.

D

[mnicoop63]

The early 997 heads had the same valves, chambers and ports as the later 202s. So a combustion chamber volume of 26.1.

D

Thank you. I can never find any information on them. Everything always seems to be about the S head.

[ricardo]

I would try the Downton head which should have 22-23cc chambers and change the cam to something with less duration (cheap example: AEA630). The head is not too big... and the change of cam would help with the idle as well.