12 replies to this topic

[Lund]

Hi there

I have just had a Cam 4810 (998) 'vanilla' head skimmed, as it looked to have been previously leaking on another mini.

Can someone let me know a little bit more about compression ratio's and if there is any performance increase with a skim?

What else can I do to get the best from my cylinder head? Can i sand the outlet ports with fine 1500 wet&dry? Or is it not that simple.

A little knowlegde would be greatful!

Thanks in advance

[Jordie]

Skimming the head removes material, which reduces the head chamber size, thus increasing compression.

Now we need to know more details then the head to work out the compression ratio. If you can get the compression ratio by looking up the engine number on the internet, guessworks guide etc....then it may be possible to work out how much the ratio as been increased. You will need to know how much as been skimmed off also.

To get the best you need the inlet ports machining and polishing for best flow, then the inlet manifold should be ported/flowed to match. Its all abit much on a standard engine really. The cylinder head unlocks the greatest performance but normally by increasing the valves aswell as the porting/gas flowing.

Make sure the valves are seating correctly, lapped in and not pitted. Give it a de-coke and a blast out with some air and re fit it if its just a standard engine.

Thats my opinion anyway

[Ethel]

If the head is for an older mini upping the CR a little will be useful.

It's only the 80's A+ HLE's etc that are near maximum for CR's. Getting tapered valve guides fitted is probably the single biggest thing you can do to improve breathing. It's all in the Vizard Book, get down the library 'n order it if you don't want to fork out for a copy. I bet you end up buying it once you've read a few pages.

[Lund]

Skimming the head removes material, which reduces the head chamber size, thus increasing compression.

Now we need to know more details then the head to work out the compression ratio. If you can get the compression ratio by looking up the engine number on the internet, guessworks guide etc....then it may be possible to work out how much the ratio as been increased. You will need to know how much as been skimmed off also.

To get the best you need the inlet ports machining and polishing for best flow, then the inlet manifold should be ported/flowed to match. Its all abit much on a standard engine really. The cylinder head unlocks the greatest performance but normally by increasing the valves aswell as the porting/gas flowing.

Make sure the valves are seating correctly, lapped in and not pitted. Give it a de-coke and a blast out with some air and re fit it if its just a standard engine.

Thats my opinion anyway

Ahh right I see, I dont actually know by how much the head has been skimmed. I guess they took off as little as possible and as much as necessary. If anyone has the link to Guesworks guide that would be helpful, as I cant find it.

I might sand the outlets till ther smooth and take my time, not going to machine it or use power tools, fingers and sandpaper for me! The head is unleaded with hardened valve seats.

If the head is for an older mini upping the CR a little will be useful.

It's only the 80's A+ HLE's etc that are near maximum for CR's. Getting tapered valve guides fitted is probably the single biggest thing you can do to improve breathing. It's all in the Vizard Book, get down the library 'n order it if you don't want to fork out for a copy. I bet you end up buying it once you've read a few pages.

The engine it is to be mated with is a 1979 BL Mini 850. (The last of the 850 production as it was discontinued in 1980.) The valves seem fine, might just fit it as it is and hope for the best! I want to improve gas flow though.

Does the 'Cam 4810' indicate a A + or A series head?

Edited by Lund, 18 May 2008 - 10:13 PM.

[Jordie]

Tried the FAQ's? im sure theres a engine no. guide somewhere.

[Lund]

Tried the FAQ's? im sure theres a engine no. guide somewhere.

Mines 85H - indicating 850, High compression. Thats all I could find out really. Im sure I have used Guessworks site to calculate engine preference, but canot for the life of me find the link! Even from his website...

Pre-Skim: (Its obvious that it has been blowing)

[ourkid]

Match the ports and maybe a 3 angle valve job but don't bother polishing.A slightly rough cast/shotblasted surface provides far better results as the turbulance increases flow velocity.Ultra polished ports create pockets of fuel is far less effective.I originally learnt this from tuning v-dub engines,but as mentioned above,it's all in Vizards book!

[Guess-Works.com]

on an 850, it's going to may squat difference... a head skim is only going to take a few tho' off which really is not going to make a lot of difference to the CR. it's only when you start taking 10's of thou' off is it worth looking into.

[ourkid]

Match the ports and maybe a 3 angle valve job but don't bother polishing.A slightly rough cast/shotblasted surface provides far better results as the turbulance increases flow velocity.Ultra polished ports create pockets of fuel is far less effective.I originally learnt this from tuning v-dub engines,but as mentioned above,it's all in Vizards book!

Sorry - just re-read,the turbulence helps ATOMISATION of the fuel,not velocity - anyway,polished is bad.

[Ethel]

998 & 850 heads are the same, I'm not aware of any differences between small bore A & A+ heads.

Guess Works is right that a skim to restore the gasket mating surface will make little difference and an 850 will put less demands on engine breathing so will gain less from improving it.

Edited by Ethel, 19 May 2008 - 09:08 AM.

[Lund]

998 & 850 heads are the same, I'm not aware of any differences between small bore A & A+ heads.

Guess Works is right that a skim to restore the gasket mating surface will make little difference and an 850 will put less demands on engine breathing so will gain less from improving it.

Okay thanks guys, I think I'll just paint it black (surely I'll lose a few BHP from this ) and bolt it on, mated with a maniflow freeflow and alloy inlet manifold.

[Guess-Works.com]

Apparently if you paint it RED it adds some too

[Lund]

Apparently if you paint it RED it adds some too

Its currently red, doesnt match my vermillion too well! So enamel gloss black it is!