15 replies to this topic

[RonH]

Hi all,
I posted a while ago about how tonreduce the compression ratio for a hot head I bought on ebay.
Main advice was to get some actual measurements and try again...

I now know I have a head space of 16.6 cc in the new head. My currrent head has 22.1cc head space providing a CR of 9.7. The new head will have a CR of 11.1.

As I don't want to rebuild the engine ( I just did that and it's not even run in yet!) I either need to find an extra 5ish cc or work out how to run the engine with a CR of 11.1 (or a combination of the 2?!).

Does anyone have any suggestions either way?

The car is not goig to be raced or commuted as it is a mid life crisis candy car - mini meets and fine weather running only (although there have been some intresting sggestions of runs to europe so it may need to be reasonably reliable???

Cooperman, if you're out there, you mentioned that you are running at 11.2 CR with a bespoke ignition advance. Is this something that needs developiing on a rolling road or cn it be tinkered with to correct in situ? Do you use a fuel additive and if so, what and how much does it work out at per litre of fuel used? (I'm fairly sure the head is not unleaded so I will be needing to use an sdditive anyway)

Any and all suggestions appreciated.

Cheers

Ron

[fwdracer]

Easy one this. Get the "Hot head" chamber opened up, respecting the gasket limits - reduce the combustion chamber beak size if the rest of the head chamber has been worked and increase the combustion chamber volume to 18/19cc. Next step is to fit the biggest capacity head gasket - they vary in CC capacity - some up near 4cc other as low as 2.5cc. It all helps.

 

The CR you mention is too high for a road car - it would mean a diet of super unleaded and something to stop valve seat recession (lead substitute) as a bare minimum. The highest CR ever offered by Austin Rover was 10:4 to 1 on the MG Metro - they can pink (pre ignite) which is a precursor to full detonation even in standard spec on crap fuel. Aim for a CR of 10:1 to 1 to account for the variable quality of fuel we get in the UK.

 

Hope that helps.

[Turbo Phil]

The head will have had a heavy skim to drop the chamber size that much. You're not going to get 5cc from opening the chambers, there just won't be the material there.
If you can boost the Octane rating of the fuel with a decent booster you can avoid detonation at your current compression ratio, though this will mean adding it every time you fill up, & you'll be in trouble if you forget.
Otherwise you could look at a thicker head gasket as mentioned above, Cometic do some of varying thicknesses.

Phil.

[Dusky]

Or swamp with mine, genuine Standard 998 head, no problems at all. lol!

 

You could try the thicker gasket combined with a bit of porting 

 

Good luck!

[Craig89]

Another option would be to fit pistons with a larger dish cc or a lower crown height. But obviously if you have just rebuilt then it's not going to be something you want to do in a hurry

[colinu]

Or (like me) use a thicker head gasket to drop the CR. Maybe not the best solution (some will argue it may fail or cause other issues) but seems a fairly common practice when lowerIng CR for turbo/supercharged applications. Only time will tell if it's a robust solution for my engine.
FYI - I chose a Cometic MLS gasket to drop what was a mid-11 CR (18cc head) down to a respectable 10-1 CR.

[Cooperman]

Yes, 11:1 is too high for a road engine, although around 10.5:1 would be just about acceptable.

To do this do as suggested above and open out each chamber by the necessary amount.

Then run an octane booster like Castrol Valvemaster Plus and 97 RON petrol. A different distributor advance would help and you could speak with Aldon Automotive who can re-curve yours for a moderate cost once you tell them the full engine spec. It may be that an Aldon 'Red' dizzy would work well.

I have had to remove 3.5 cc from each chamber for a previously gas-flowed head which was to be used on a turbocharged application. It was not too difficult, but a bit time-consuming.

[RonH]

Many thanks to all for the advice. I feel much more positive now.
I am thinking that a combination of the thicker Cometic gasket and removal of some more metal from the combustion chamber should get me down to the mid 10s, then a good octane booster to prent pinking should do.

Thanks for the offer Dusky, but I'm always up for a challenge so will have a play with mine.

Cooperman, do you have any picture of where you took the metal from to find your 3.5cc? Mine has been opened out to look like the top spec from Visard, then heavily skimmed. My main concern is that I may hit a waterway.

Many thanks
Ron

[Cooperman]

I didn't take any photos of the head I did for the turbo application.

You can certainly grind away the entire 'beak', but if your head is already done to a high standard you might be best off selling that one, buying another casting and working on it. There a lot of people who want really H.C. heads for true competition Minis, so long as there are no cracks, and they command a good price and as yours is a road car you will, presumably, not be looking for the very last bhp possible.

[Carlos W]

Tesco do a 99 RON I believe 

[nzmember]

I just bought a 92 thou thick Cometic MLS gasket from them direct (cheaper and can specify what thickness you need to reach your ideal CR).  Still expensive compared to a BK450 etc but much cheaper than machining work and avoids the 99 RON requirement.

The turbo forum's reckon it is a bodge but Mike from Ferriday Engineering is a big fan.  He makes decompression plates for the turbo/supercharged boys so a good recommendation.

[Turbo Phil]

This is one of my big chamber Turbo heads, & should give you an idea what you can safely remove. Though the shape is not optimal, it's a compromise when trying to achieve large volumes.
Phil. 

Edited by Turbo Phil, 16 January 2014 - 05:29 PM.

[RonH]

Late reply I know but again many thanks, esp to Turbo Phil (exactly the sort of pictures I was after) and nzmember (how on earth are you able to tell me about Ferriday Engineering from the other side of the world when I failed to find them from next door???!)

 

One way or another I now have 3 proper options:

1. Cut away more metal from the combustion chamber to make it look more like the turbo, (possible)
2. Fit a purpose made decompression plate or (prefered option)
3. Sell the head to someone who wants a hugely ported head and buy a more sensible one for road use. (any takers?)

 

Ron 

[Dusky]

Dynamic compression can be lowered by cam choice by the way

[gazza82]

This is realetd to my question. My 1275 engine is overbored to 1310cc. I need to get new pistons but wondering what sort of compression ratio I'd see with a standard 12G940 A+ head but that has had a 60 thou skim. Head already has unleaded valve seats.

 

Most pistons on market are 8.8:1 or 9.75:1 and I'm aiming for the 10:1 area ..

 

I guess I could to the maths (or use a CR website) ... but I'm pretty sure someone on her will already have something similar ...

 

TIA!

 

PS Not aiming for anything daft as it will be a road-car, albeit occasion use, but I would like power to be a bit more than the original 34bhp that the 948cc gave and up on the standard 1275 output to closer to 90-100bhp. HIF44 1 3/4 card is most likely add-on with larger bore LCB manifold.