12 replies to this topic

[Java_Green]

I am looking for guidance for suitable compression ratio. Static CR figures are welcome but I find theoretical dynamic compression figures more reliable (perhaps as close to reality as it gets theoretically without a lot of assumptions).

My set up:
- A+ block, sleeved and bored to +020
- Omega diecast pistons, 10cc dish
- RS FI (int valve close 46 deg ABDC)
- Pierce 7 port Al head (36/29 valves)
- Supercharged (Eaton R410), ~10psi boost, no intercooler
- Plenum with 90mm full radii runners with an injector into each runner at head entrance
- Stand alone ECU controlling ignition (trigger wheel) and fuel injectors

Deck height have not been possible to measure but it is untouched since production so I assume >40 thou. At the moment, the combustion chamber volume is 20.2cc. I guess it is possible to open up tp some extent. As the head is made out of Al, I assume that I can stretch the CR a bit compared to a 5-port cast iron head.

The intension is that the engine is supposed to be an engine used in a road car and run on 95 RON petrol.

Edited by Java_Green, 19 February 2025 - 10:09 AM.

[DeadSquare]

Admittedly, my old Shorrock never produced 10lbs boost, but after over 50 years of sooped Minis, I have quite often needed to be pretty agile with anything over 120Bhp under foot, if the road conditions are unfavourable.

 

"In theory", at 10 psi boost, you are putting an extra 2/3rds of charge into your cylinders, so a factory 1275 S at 75 bhp with less than a 10:1 CR, would have 125 bhp and a 13.31 CR, which 95 RON won't like.

 

Bear in mind that a road car is very seldom 'flat out', so aim for torque (= acceleration) rather than a peak bhp and just build on the plus side of an MG Turbo Metro spec.

 

Don't forget that 2/3rds more charge, requires a greater heat dissipation..

[slidehammer]

I wouldn't run a standard Mini on 95 Ron let alone a boosted one, you will need Super Unleaded whatever boost you plan to use.

[Java_Green]

Admittedly, my old Shorrock never produced 10lbs boost, but after over 50 years of sooped Minis, I have quite often needed to be pretty agile with anything over 120Bhp under foot, if the road conditions are unfavourable.

"In theory", at 10 psi boost, you are putting an extra 2/3rds of charge into your cylinders, so a factory 1275 S at 75 bhp with less than a 10:1 CR, would have 125 bhp and a 13.31 CR, which 95 RON won't like.

Bear in mind that a road car is very seldom 'flat out', so aim for torque (= acceleration) rather than a peak bhp and just build on the plus side of an MG Turbo Metro spec.

Don't forget that 2/3rds more charge, requires a greater heat dissipation..

I am fully with you. The Metro Turbo had a staric CR of 9.4;1 and a dynamic CR of 8.1:1 (boost compensated 11.6;1). This with a 5-port cast iron head and distributer. An Al should be able to go a bit higher in CR.

What can actually 95 RON and 98 RON take in compression in an a series engine?

[timmy850]

If you’re going all out with 7 port, supercharger and fuel injection, have you got E85 available near you? That’d be my choice

You can also run water/methanol injection if you can’t get E85 and use a controller to inject under boost

[Java_Green]

If you’re going all out with 7 port, supercharger and fuel injection, have you got E85 available near you? That’d be my choice

You can also run water/methanol injection if you can’t get E85 and use a controller to inject under boost

I do have E85 available. However, E85 is a government product that very few use and that I assume can be removed on short notice the day somebody decides so. (There are several other drawbacks as well, but this the primary one.)

[Steve220]

You won't have time worry about static CR that much with an injector on each cylinder and modern ecu for ignition control. The alloy head will provide better cooling effect for the cylinders, also. Around 10:1 would be just fine.

5 ports usually run lower CR as the fuel distribution is terrible and more chance of knock.

[Java_Green]

You won't have time worry about static CR that much with an injector on each cylinder and modern ecu for ignition control. The alloy head will provide better cooling effect for the cylinders, also. Around 10:1 would be just fine.

5 ports usually run lower CR as the fuel distribution is terrible and more chance of knock.

Thank you for that answer. Do you mean 10:1 as static, dynamic CR? Boost corrected or not?

[Steve220]

Static.

[IronmanG]

Following this with interest. My turbo will be here shortly and though I will be running less boost approx 10:1 static is the aim

[Steve220]

Following this with interest. My turbo will be here shortly and though I will be running less boost approx 10:1 static is the aim

5 port?

[IronmanG]

7 port

[Java_Green]

7 port

Sounds interesting. Will it be port injected with a stand alone ECU?
What cam will you use and what boost level do you aim for?