Compression ratio for naturally aspirated engine
Turbo Mini Forums

I'm just wondering on say 98 or 99 octane fuel, what would be the maximum compression ratio you can get away with on a 5 port head before you start needing fuel additives etc.

I'm turning my attention to a 1380 I'm considering building, and with the current head, which has been skimmed a fair bit, using some 11cc pistons the compression would be about 11.5:1

The engine will be highely tuned, so will have someting like a 300 degree cam in it, mabie a touch more.

I want an engine that can really rev for the moment, therefore I'm going to do this engine which will cost about 1300 quid as I have most of the stuff already, then go down the 16v turbo route when funds allow.

Thanks
James

why dont you try the 11.5:1, take it to a decent rolling road and see what they say.

if theres a problem you can always bring it home, sell the head and replace it with another?

Got my calculations wrong on my (otherwise standard) MPi - ended up at about 11.3:1.
Use Super unleaded all the time (97-98 Octane) with no problems.

thats interesting cooper. 'some people say' that 10.5 is the maximum you should go with a mpi

i ran at 10.7 on my 1275 mpi

new engine is anywhere between 10.6 - 11:1 - though this isnt running properly yet

the piston dish i was quoted was 6-7cc, and didnt have time to measure.

used to run the old 1275i with cr of 10.7:1 on normal unleaded most of the time with no problems.

Edited by welshdan on 19th Nov, 2009.

with a 300 degree cam in it, mabie a touch more.

11.5:1 on good pump fuel super plus unleaded or optimax is no sweat

It does not only rely on the static compression. If you've got a shit head you might get away with it, if it's bloody good head, you might end up with pinking. What are you running?

You decide on the CR from the combination of parts, not the other way around.

Right, the spec is to be confirmed when I have a chat with my machinist, I'm just throwing ideas round really. He said he wants to have a look at the head then go from there. But as it stands Benross has given the head the full treatment anyway, and it can be tweeked to give the desired compression

the current known factors about the engine will be.

1380cc
fully worked bottom end
Probably omega pistons
Ultralite flywheel
1.5:1 roller tip rockers
Megajolt
I would like a power up to 7500 rpm

To be honest this thread is more out of curiosity for what the static compression of n/a mini engines on pump fuel can go up to rather then what I'm actually going to run, which as mentioed I will discuss with the machinest. There will obviously be a number of other factors which affect this but it can still give a general figure.

i run 11.5:1 on mine.. run 28deg max advance with bp ulimate with no pinking.. rr operator tried giving it more advance with no det, but it didnt make anymore power.

1380 n/a
morspeedfull race head
AGSP 300 dec cam etc etc

Running 11.8:1 on shell optimax, Runs super sweet

1330
Tom Bell Head
Fully worked bottom end
md296

my 1380 is 11.8 too, but will be coincedentally be using additive as it doesnt have unleaded seats, cvls not only is a lead replacement, but an octane booster too

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"

Mine will be 11.5:1.

No idea how it runs as its still in the garage

http://www.jolyonway.com