How do you raise the compression ratio?

[Quaser]

In my old engine, it had almost 10.2:1 compression ratio from standard pisons.

How would this this be done, and what machining is required to do this?

Q

[Dan318-is]

iv been taught that skimming the head is a good sway to raise the ratio

are you sure they were stock pistons?

[Quaser]

Yeah - stock pistons, and the head wasnt skimmed apparently!

Q

[tourer-dan]

Are you comparing like for like?

The M42 engine has a higher compression than all the others IIRC.

The manufacturers set the compression of an engine at a specific level for a very good reason and by skimming the head to raise the compression, you are effectively reducing the cubic capacity of the engine.

[Dan318-is]

so skimming the head on my IS is a bead idea? how can it lower the ratio?

Q iv no idea then mate"

[tourer-dan]

The ratio is the voulume of the cylinder at BDC compared to the ratio at TDC. In simpe terms, if the volume at BDC was 500cc, then at TDC 50cc, the compression ratio would be 10:1.
By skimming the head you are bringing the valves etc closer to the block by a fraction which reduces these volumes.

Note that skimming the head RAISES the compression ratio, not lowers it.

If you need to have your head skimmed by a margin which is likely to affect the engine's performance, it is normal to fit a thicker HG to compensate.

[Jimbob]

off topic but the M42 head skim is pointless imo.

[Dan318-is]

how else can i extract power from it then

iv got BBTB viper 2 chip KnN panel filter and supersprint header and full decat system??

Any ideas as i was going to skim the head. how about polishing or flowing inlet manifold? or a new head gasket?

Back to the original question, how else do you increase compression ratio?

[Jimbob]

You've pretty much done all that's possible with the M42 (the head is weak!)

The only other thing I can suggest is getting 2 Schrick/Dbilas camshafts, possibly porting the inlet manifold if you can get it done for a worthwhile cost.

[Turbo-Brown]

Are you comparing like for like?

The M42 engine has a higher compression than all the others IIRC.

The manufacturers set the compression of an engine at a specific level for a very good reason and by skimming the head to raise the compression, you are effectively reducing the cubic capacity of the engine.

No you aren't

The cubic capacity of an engine is defined by the bore and stroke of the engine. A head skim doesn't alter either of these!

Also, the CR is calculated like this:

(Swept volume+chamber volume)/chamber volume.

So using your data:

(500+50)/50 = 11:1 not 10:1

If you really wanted to raise the CR without skimming the head, the only way I can think of is to use pistons with a greater than normal deck height (distance between the gudgeon pin and piston top).

[Dan318-is]

pistons cost a bomb tho!!

As for the head, is there any other heads that are stronger that would fit on? iv heard of cracking problems.

How hard/much is porting the inlet - iv got all the equipment at college and a year to do it in?

[Turbo-Brown]

Yes, pistons are expensive!

You can give the ports a quick tickle to remove imperfections etc, but major porting work should really be done in conjunction with a flow bench and a couple of scrap heads as unless you know exactly what you're doing for a particular head, it can be a bit trial and error

[Quaser]

So can you modify existing pistons to get a higher CR?

[tourer-dan]

If you really wanted to raise the CR without skimming the head, the only way I can think of is to use pistons with a greater than normal deck height (distance between the gudgeon pin and piston top).

If your information is correct (which I'm sure it is) then skimming the head will reduce the cylinder volume, and if the swept volume remains the same, then the CR will increase. Similarly, if the swept volume decreases (by fitting modified pistons) and the chamber volume remains the same, the CR increases. (I'm not suggesting that both would effect the same increases)

I think you've picked to shreds my original example which was meant to be simple enough that everyone can understand it. If you take what I quoted as 'cylinder volume at BDC' literally, then obviously, cylinder volume at TDC would be 0, and that doesn't make any sense.
It was meant to be nice and simple with round figures, so:

If swept and chamber voulme = 500cc (BDC)
and chamber volume = 50cc (TDC)
then compression ratio=10:1

sorry if you found my simple example ambiguous, it was just meant ot be easy to understand.

[tourer-dan]

Sorry, just read your reply again, and you're only saying about the cc's not changing.
My mistake. Sorry guys

[Turbo-Brown]

Dude, not picking holes, but with a 500cc swept volume and a 50cc chamber, you'll get am 11:1 CR as outlined in the equation above

[Widge]

Also if you skim the head it will change the chamber size which will affect the compression ratio.

[Dan318-is]

iv already got a thicker head gasket to go on, assuming i want to INCREASE compression ratio, is it really not even worth skimming it at all?

[Ant]

to make the M42 fly you need to alter the intake cam timing, give it 6 degrees more advance and she'll show some extra ponies for sure, DO NOT attempt to DIY this @ home though peeps, you need the revised pukka tool to time it correctly,or a decent cam protractor, no need for a vernier as the cam wheels are slotted to allow some tweaking. other than that a new rocker box gasket and shes done

I have done a few M42/44 cam timing alterations now, its defo worth the effort, idle is smoother too despite the extra overlap in timing

[Dan318-is]

advance 6 degrees on the intake? i know its an adjustable sprocket, my lecturer at college will know how and have the tool, thanks fo rthe tip pal?

anything else?

[slideways]

Are you comparing like for like?

If you really wanted to raise the CR without skimming the head, the only way I can think of is to use pistons with a greater than normal deck height (distance between the gudgeon pin and piston top).

sorry now if i'm wrong but if the deck height is increased with the same con rods, the piston will indeed go higher, but at bdc it will be higher also, so in effect the cc will not change, or am i talkin arse?

[Ant]

yeah, make sure itys no more than 6 degrees, turn her over by hand to double and triple check before you throw the key in the slot, other than that, enjoy.

Thats a gen BMW tweak BTW folks, not BS

Back to Q's original topic for a sec, if you mean your 2.7 ran that cr dude, then you need early higher comp 325 pistons, 14mm off the skirts to clear the extra throw on the crank, and I think if you add 8% to the capacity you should find it about right.


Maths was never my thang, I like oily bits

[Dan318-is]

the engien wont be runnign for a while pal, but that will be the last thing i check for sure

Any other tips for how could make my IS quicker, i thought of skimming the head but theres seems to be mixed opinions about it

got a big bore throttle body ciper chip and supersprint header and decat pipe in the works tho

any advice would be helpful pal cheers

[Quaser]

Cheers Ant

Could you please explain what you mean on the 8% part?

The reason i ask is cause i got some 9.4:1 pistons which i believe are the High comp ones - i know a little needed shaving off the bottom, but with all the explainations - i was confused as i thought that no matter what you take off the bottom to make it lighter, the volume itself will be the same wouldnt it?

Q

[Dan318-is]

sorry to semi hijack your thread quaser not much IS chat goes on round ere lol

[SwirlyE30]

Interesting thead.


318-is. Ive recently just stuck an IS lump in my 318 and im thinking of things to do to it. I dont want to spend a lot though thats the problem.Its done 150k and i cant warrent the money for a rebuild or anything.


Wouldnt mind trying to adjust the timming.But i dont know where to take it, BMW would charge a fortune!


Only things im thinking of doing which are cheap and give good enough gains and chip and TB, unless i have a fair bit of cash to do a top rebuild, new cams, ported etc then its gonna have to do!

[Dan318-is]

BBTB and viper chip are a must

luckily iv saved enough to install a supersprint header decat pipe and backbox, iv heard large gains can be had from decats

as for the head, iv heard the cams already quite a hihg lift for stock ones anyway.

Adjusting for me wont be a problem as my engien will be out for a year and the lecturers around me know what they are doing

You may be suprised by BMW as it seems like a harsh job but may be easy and quick, always worth askin pal. iw ould waste the money on new cams tho iv heard they create flat spots

have you had ne experience with FSE boost valves? iv heard mixed opinions

AS for heads and inlet manis, im probably porting and flowig my inlet mani, but im in the same predicament as you when it comes to skimming the head. all i nkow is that it doesnt cost that much for a specialist todo. i dont know about positives and negatives hto

be interesting to get some of the more experienced IS users in on thisone

[Simon13]

As u 318iS boys are poor and young like me i think everything has been mentioned that is possible with out spending stupid money.

Leave the exhaust standard sytems are very very good.

If i were u i would
viperchip
cam timing
BBTB

maybe K&N panel filter for the motorbike noise

Demlot has these mods and his iS flies compared to his standard one, i know i've had an iS and have driven both of his.

Cam timing is the best mod but gives it a bit of a patchy torque band at about 2.5 to 3.5 revs after that it pulls very hard for what they are. Even this is not very noticible

[Dan318-is]

il be living aboove 5 most the time ne way

iv heard standard manis r good but mines got a crack i think lol so i mite as well puta sprint one on - the dealer will probably charge the same as supersprint do for a standard one

[Simon13]

or weld your standard one, they all crack by the flanges either at the top or bottom. Is it blowing even?

BMW charge Ԛ£400 for a new manifold

[Dan318-is]

ye this supersprint one is 395 at trade price (teh lecturers at college can get em cheap)

is it surely not worth doin that?

[Simon13]

if it has proven gains off course, supersprint are a good firm!

u at guildford college?

[Turbo-Brown]

Think we're getting our terms mixed up here:

The swept volume of an engine is calculated by the area of the bore (pi x(bore/2)^2) multiplied by the stroke, all multiplied by the number of cylinders.

This can only be changed by changing the stroke to increase or decrease the volume, or by over-boring the cylinders and fitting bigger pistons.

The spept volume can not be affected by the cylinder head or piston deck shape etc.

The chamber volume (or clearance volume as I should have called it) is the volume that remains when the piston is at TDC. Is is the sum of the chamber volume in the cylinder head and any volume not occupied by the piston in the cylinder.

To calculate a theoretical compression ratio:

(Swept volume+clearance volume)/clearance volume

Skimming a cylinder head will reduce the chamber volume and as such will reduce the clearance volume. Typically, a skimmed head will be refitted in conjunction with a thicker head gasket. The thicker gasket creates a larger volume between the top of the block and the face of the head than a standard gasket would and so the compression ratio remains roughly the same with the skimmed head and thick gasket combo.

To increase the compression ratio with piston mods would require adding material to the tops of the pistons and is thus not really practical. Replacing the standard pistons with pistons which sit higher up in the bore will keep the swept volume the same as we haven't changed the bore or stroke, but the volume remaining when the piston is at TDC will be smaller, giving rise to a higher compression ratio.

Longer rods would also have the same effect as the piston would sit higher in the bore.

The most practical way to increase a compression ratio is to have the head skimmed to reduce the chamber volume!

[Dan318-is]

turbo brown and si your both quite local to me really! jsu realised that

na mate im at brooklands doing motorsport engineering, hard course but well worth it

gota do a project next year, im taking in an m42 engine hence why i wanted to know whether to skim head or not??

not sure about proven gains of teh S'sprint, might jsut polish n port the original one

[Ant]

the 8% comes from the capacity increase,



use the 9.7:1 pistons, clearance the skirts, transpose the 325 piston to deck height across(do this before stripping the 2.5 engine ! and make sure its @ TDC)) using a 2.5 piston on 320/323/eta rod, with the eta crank, in the block of choice, machine that block to the height as mentioned above, thats a 2.7 with stock valve clearances, thus your 9.7:1 CR become 9.7 + 8% gained from the extra 200cc = 10.476 CR

I've oversimplified as the increase is not quite 200cc but the math is sound.

Make sense peeps ?

Alex, interested to hear your comments, you can defo work it out better than me