23 replies to this topic

[Cooperman]

What would the next steps be based upon the CR found, say if it were too high what options would I have?
And what CR would be deemed 'too-high'?

You would need to grind metal out of the combustion chambers until the volume was large enough to give the CR you need, say 10.2:1 as a maximum. Each chamber must be the same volume, so it needs to be done accurately and in the correct manner to preserve or improve gas flow and 'burn'.
If you have a geometric Cr of 10.2 or lower already leave it as it is and get the ignition sorted.
What carb are you running and is it needled for the improved gas flowof the 295 head over the standard one? It could be running too lean which won't help.

[jakejakejake1]

I have a HS4 with AAU needle, although have ordered an AAA as I believed it was running lean as talked about in a different thread today.
I thought running lean would make things worse thus ordered the new needle, will an AAA be suitable or should I have gone for something richer again such as an AAM?

[miniman92]

Hi Jake. The head had 60 thou taken off so your compression ratio with flat tops should be around the 10:1 mark. I ran this head with no problems on my 998 with flat tops. You need to get the engine set up properly on a rolling road and with an AAU needle the fuelling will probably be way off.

[jakejakejake1]

Thanks for the info ash, I have order the richer AAA needle and will try that when it arrives.
And what recommendations do people have with regards to the distributor? Would I be able to have the current 59D re-curved suitably (who would be able to do this, and how much would it cost) or get a new dizzy such as an Aldon? Any input welcomed.

[liirge]

You can try and run the colder BP7ES plugs, I cured a constantly pinking mini this was a few years ago, the chap had also put on a head with an unknown compression ratio.

[Cooperman]

Aldon Automotive will re-curve the distributor if you send it to them. However, to do it correctly they will need to know what the C.R. is and what cam you have.
With regard to the needle, it's always a case of get as near as you can then fiddle to get the mixture right across thje rev range, or put it on a rolling road.
The problem with the 295 is that they are all old heads and whilst it's fine to say if a certain amount has been skimmed off it should give a certain CR, it depends on how much has been skimmed off since the head was cast back in the 60's.
That is why it is vital to measure and calculate. Also you need to know how far down the bores the piston tops are at TDC. This varies from engine to engine, again depending on whether the block has been skimmed previously and the part number of the pistons. Also if the engine has been re-bored the swept volume of each cylinder will be larger and this will increase the CR.
If you want it right, you have to build it right.

[klivins]

Just a thought, or more of a question - does there exist thicker head gaskets to get round the problem - as I think I have the similar one?

[Cooperman]

The good quality head gaskets are all about the same in compressed volume. For a 998 it's 2.7 cc.

[jakejakejake1]

Just to add to this topic, I changed the needle to an AAA, and throughout the rev range the car seems much happier, also when I changed the needle I re-gapped the spark plugs and points, set the mixture correctly etc.

Now i have no pinking, well nothing that I can notice, I took it easy for the last week or so, only driving on flat ground with just me in the car, but today after being happy things were ok took it for a drive up the local massive steep hill (bathwick hill if anyone is interested) with three people on board and all seems fine.

Thanks for all the help I think the car will be ok for the time being.