Maximum Compression Ratio
Turbo Mini Forums

i thought mine was strange but yours is even stranger. i've been told of jap engines runnig between 8.5 and 9:1 cr and running 20psi, dont know how.

my original head had the beaks removed, ported and 33mm valves, replacement head is standard MG metro.
is it a beak thing?
is it the porting?

Jap engines will have alloy heads with nicer shaped chambers for a start.

they also have different cams etc

Bud - How about a visual comparison of the two chambers (ie a picture of both) so we can compare any difference there.
I see you've already noted that the valves and ports are similar...

I do have one question though - I recall this head is from someone who has on many occasions 'accidentally' marked heads up with the wrong chamber volume? Any chance this has happened here - ie are you 100% sure it is what it says?

mine is the same old engine but with a change of head. Bud666 sounds like hes in the same boat.

is it all down to chamber shape?
or the different valve sizes?
or the porting?

i will take photos of both heads tonight

Edited by Turbo Shed on 13th Sep, 2007.

Every day is a school day ...........

How fast and how expensive ...... the same question...

Edited by stevieturbo on 13th Sep, 2007.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

dave, they're as near as dammit the same heads, same chamber shape etc. the only difference really is that mine had the inlet ports took out a lot more.

my original one was supposed to be 29cc when i bought it, didn't actually check it till xmas (2 years later, lol) and it came out at 26.6cc. and there was me thinking i was running around 8:1 all that time. wil measured his at 27cc so i'd trust his measurements.

here's a grotty pic of my old head a while ago.


and what happened to it :(


don't have any pics of the new one unless wil does?

Are we talking about reliabilty of head gaskets running boost on high c/r?

Remember Japs run stupid octane fuels, surely this must help?

Back once again like a renegade master

i've never blown a head gasket...

Oh, just pump the boost up then, kick the pedal and hold your goodies!!!

Back once again like a renegade master

ehh? thats something to do if you want a hole in a piston...

But think how good itd be for that 2 secs of boost. LOL
Im only messing.

Back once again like a renegade master

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Hi,
I too think that higher CR is a better way to go, it has to be more efficient which can only mean more power. Whichever way you go, high or low CR, accurate fuelling, timing and possibly boost control are essential.
Cheers,
Gavin :)

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My Mini build diary

Bah! Iv'e just searched the interweb for the last hour or so looking for this sort of info an not turned up anything usefull,

WHY did I not search TM first!!

It looks a bit out of date now though.

Enough people running 8.5:1 at 20 psi now.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Ah, but on what octane, not 95 that the graph was done for.

thread digging!
clearly im looking at CR and effects none of the link seam to work for me probaly to old lol any way..

so we know that the god old basic (static) CR ie spewed volume vs non sweped forgetting about losses and the fact of open valves ect has some efect on what is the actule ratio/cylender pressure but what really makes the difrence is valve timeing, stroke length, bore size, rod length, of cause boost and basic cr to get the most realistic CR..

so what am in trying to explain/ask? well basicly what should we consider to be the max dynamic cr for a motor? one thing to note is i will be running E85 fuel (85% ethanol) i have read that some people are running e85 up to 20:1 CR!? i dont know how true that is im just seeing what/if people have know to work or rather NOT WORK lol

my planed engine comes out as
bore size 2.6025
stroke size 3.2960
rod length 5.75
sweped vs non cr 9.5
intake closes abdc 69
boost 22psi

comes out at

Static compression ratio of 9.5:1.
Effective stroke is 2.45 inches.
Your dynamic compression ratio is 7.30:1 .
Your dynamic cranking pressure is 141.08 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing and boost of 22 PSI is 18.23 :1 true? CR on max boost

so considering that the effective cr is 7.3 with the late intake closing time should allow me to run 22psi with out to much trouble?

the intake closing time has a big affect on the cr from what ive seen looking around the www

what are your thoughts on this guys? agree about the cam late cam timeing reduced the cr there for alowing more boost? also max real cr? 15:1 18:1 ?

god bad otherwise let me have it

It has everything to do with temperature at the point of combustion.

Work that out and you are sorted.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

8.5:1 and over 30psi and with air charge temps of over 60deg's without issues

Team www.sheepspeed.com Racing

you can do anything if you set your mind to it...
i rather blow it up winning than keep it together losing.

finish date set for project 1... march 2018

i see so ill look at what temperature and pressure E85 will detonate at

try to work out a estimate on charge temps and ic efficiency

jim what fuel and cam are you using with that 8.5 and 30 psi

stumbled apron this an interesting and confusing read ill have to read it again tomorrow hopefully it will make more sense!
http://dspace.mit.edu/bitstream/handle/172...2/676953430.pdf

i spoke with the engine shop today about my build and regarding the E85 fuel
he said its a grate fuel if you get it from a proper souse ie not the local servo as the quality fluctuates far to much to tune the car properly