7 replies to this topic

[ThermalEvent]

Have been calculating the compression ratio this afternoon and found it's at 10.46 to 1.

Having read some posts from AC the optimum compression ratio for a 266 cam is 9.75 to 1.

I've worked out I need to remove around 3cc's per cylinder to achieve the ideal compression ratio (bringing total cylinder head volume to 26.5cc).

Would any of you helpful chaps know where is the best place to remove material from a head?

I'm thinking mainly around the inlet valves to "deshroud" them but this is just an educated guess. It's also difficult to picture what "3cc's" would look like to remove?

Cheers,

[ACDodd]

Is it a standard head?

Ac

Edited by ACDodd, 22 August 2021 - 06:27 PM.

[ThermalEvent]

Is it a standard head?

Ac

Hi AC, It's an A series 11 stud head with 35/29 valves which I have done some work to myself by de-shrouding the valves. Averaged 23.5cc across all 4 chambers. For reference I have the below measurements which I hope I've got right:

1330cc (SV 332.5)
Averaged piston to deck height is 0.0035 = 0.35cc?
Piston dish 6.5cc
Average head volume 23.53cc
HG 4cc
Ring lands 0.75cc
UV 35.14cc
332.5 + 35.14 = 367.64 ÷ 35.14 = 10.46:1

Cheers,

[ACDodd]

Either the floor needs lowering and then the chamber reshaping to match and the valve seats re cutting or the pistons need dishing to a larger size.

Contact a specialist such as Phil Dawe at turbo minis to have this work done


Ac

Edited by ACDodd, 22 August 2021 - 06:58 PM.

[ThermalEvent]

Either the floor needs lowering and then the chamber reshaping to match and the valve seats re cutting or the pistons need dishing to a larger size.

Contact a specialist such as Phil Dawe at turbo minis to have this work done


Ac

Thanks AC, I did contact Phil a few months back as I was going to buy a modified head but he unfortunately wasn't taking on anymore work at the time. This is why I decided to give it a go and port my own. Is 3cc too much to remove for an amature with nothing but time and limited funds?

Cheers,

Edited by ThermalEvent, 22 August 2021 - 07:44 PM.

[ACDodd]

You have to have the valve seats recut. It’s best left to someone with all the kit. Ideal time to order a fully ported road head with the right spec chambers.

Ac

Edited by ACDodd, 22 August 2021 - 09:03 PM.

[maystro]

Have been calculating the compression ratio this afternoon and found it's at 10.46 to 1.

Having read some posts from AC the optimum compression ratio for a 266 cam is 9.75 to 1.

I've worked out I need to remove around 3cc's per cylinder to achieve the ideal compression ratio (bringing total cylinder head volume to 26.5cc).

Would any of you helpful chaps know where is the best place to remove material from a head?

I'm thinking mainly around the inlet valves to "deshroud" them but this is just an educated guess. It's also difficult to picture what "3cc's" would look like to remove?

Cheers,

Hi Thermal,

 

I just bought a 1293 from Minispares which has a compression ratio of 10.59 which had run on issues but since changing to 98 octane fuel and getting a proper dyno tune my new motor is running sweet with no run on.  It has 49 kw at the wheels and 134 NM of torque and  pulls like a teenager.  

 

Maybe you don't have to lower your compression ratio?

 

Brad

[Lplus]

Standard MG metro engine CR is 10.5:1 and the cam is pretty close to the 266.  I use 99 octane without problems.