26 replies to this topic

[liirge]

Hiya,
I worked out my compression Ratio, and have ended up with about 11.78:1 which im guessing is too high, can someone check my figures for me to make sense of it all:
Right so ive got a 998 that's being overbored +120 tho
which makes 1096 so i divided that by 4
to get:274cc
so then i added 4cc for compressed gasket, and the 21.4cc for combustion chamber.
to get:299.4
i then divided this figure by
21.4cc + 4cc to get:

11.787

Have i done this correctly?
This is pretty high for a compression ratio am i right?

[1293sleeper]

thats about the ratio i have on my engine. you'll need a high torque starter motor for that i found this out the hard way when i tried to start my engine with a normal starter motor which then set alight

[CraigyBoy]

I just worked it out and got the same, does seem quite high.

[liirge]

well, this isnt so good!
Hmmm, is this going to be a problem, so my inertia Starter motor i got is effectivley useless now??
What about the rest of the engine, Cam and Valves, and bottom end, what implications is this CR going to have
does this mean i can only use premium aswell?

[1293sleeper]

i will only run my engine on premium as it wil work a lot better. i have tried 2 different starter motors on my engine and both didn't turn it over quick enough even when i was jumping the car off a 2.0l diesel. all it should really do to the engine is give it more torque which is always a good thing.

Edited by 1293sleeper, 02 July 2009 - 09:55 AM.

[1293sleeper]

my engine has been designed to be high comp as i have a 286 kent cam and need the extra torque to get me passed the problem of lack of power at low revs so it should pull cleanly all the way through the rev range.

[jayare]

You don't have any allowance for piston dish, ring land volume or volume above piston when at TDC (unless you have had the block machined to have zero deck height, the pistons will stop short of the top of the block). Also, the cylinder head volume - is that a measured volume or just the standard volume for a 1275 head? IIRC small bore heads have more volume than that. If you are putting a 1275 head on it, do you have the block pocketed? If so you need to add the volume of the pockets too as they are in the combustion chamber.

Once you have all the numbers, then the formula for calculating the CR is (combustion chamber volume+swept volume)/combustion chamber volume, which is what you have used above.

You only need an extra 5cc or thereabouts to bring that to 10:1, which you will probably find if you get all the numbers together,

JR

[liirge]

i've got a SW-10 which is based on the 286, aswell as lightened bottom end, and 940 35.6 29.2 Valves.

[1293sleeper]

sounds similar to my engine spec but my engine is a 1293. all i can say is give it a try its a very racey cam for a 1098 any way my friend a kent 276 in a mini and that was as lump as i would ever like to go.

[liirge]

Do you have to use Premium Fuel on your engine?

[1293sleeper]

i'll let you know when the new starter arrives i can't get it started till it arrives

[1293sleeper]

but i have been told i will have to by a few people, they say it will run on normal fuel but not very well

[liirge]

ahh, more expense!!
i dunno if this compression ratio is worth it!

[1293sleeper]

you will change your mind when you first fire it up it won't just go well but sound beefier to. do it you know you want to

[liirge]

i just dont want to fork out for a High Torque starter, anyone else confirm that the old style inertia Starter motor will NOT turn the engine over with this Compression ratio.
Just been looking at Tuning the A-Series, david V, suggests that the higher C\R will provide me with better economy, which would be usefull as the car is'nt going to be exaclty mpg friendly! anyone confirm this?