How do i lower the compression on an M30 ?

[535ise]

Hi all,

As a sort of a follow on from an earlier thread i posted about putting a supercharger on my 535....

I was wondering if any one know's of any piston's or con rod's from anouther M30 that will reduce my comp' ratio ?

There's lot's of capacity variation's on the M30 over the year's(2.5,2.8,3.0,3.2,3.3,3.5) this was achieved through various stroke and bore permutations.

But which combination whould make the piston sit lower in the bore at TDC ?

If the piston sat say 3 mm lower in the bore at TDC than the standard set up then how much whould this lower the comp' by ?

I suppose the ideal set up whould be the piston's and con rod's from the Alpina twin turbo,as this whould lower the comp'(7.2:1 ? V's 9.0:1 as standard) and be tough enough to handle some serious power with out failing.But can you imagine how epensive they whould be ? $$$$$$$$££££££££ :(

The chap in the following link( http://www.cardomain.com/id/pphilpot08 )has what sound's like the Alpina set up with his turbo'd M30 that he's running 20 psi of boost that he's estimated to be producing 700 bhp !!!!

Thank's for your time and valued input guys ;)

Andrew

[Tiger]

Mmm... Normally you would not need to lower compression when using low pressure supercharger. I remember about this long time ago about those supercharged and turbocharged 535i... can't remember who did it... famous company.

To lower compression ratio, you would need shorter connecting rod or a different crankshaft. However, to do high pressure supercharging or turbocharging, you need stronger crankshaft, connecting rod, and piston. This gets into $$$$. JustinRed5 would have alot more experience in this matter.

[AllanS]

Have an engine shop redish your pistons, or have new ones custom made. To be honest, there's really no need to lower the comp ratio even if you plan on turbocharging your engine, unless you plan on really ridiculous amounts of pressure.

Whatever you do, don't use a thicker headgasket- this is the worst way to go about it.

[bahnstormer]

well my 91 535i has a 9.0:1 which is extremely low for a stock motor.
u can install a thicker head gasket...but for 10 psi of boost 9:1 should
be fine. so long as u don't live on teh sun =]

[George M]

a derivation I posted some time ago on how much compression is changed by milling the head of an M-30 engine and maintaining the same head gasket thickness. The following is based upon certain boundary conditions and therefore is theoretical but can be adjusted to back out compression difference based upon different piston height/stroke change etc as compression is simply a value X [how many atm or bar/14.7 psi.]
Since bore size wouldn't change for your calculation, and compression/pressure is proportional to stroke or piston height, it is pretty easy to back out the relationship if you care to. As a sidebar, you can crib a bit off of the 3800 V6 motor that GM offers in both NA and supercharged form. If you have driven a Pontiac GTP for example with roots style Eaton supercharger...they are quite fast at least to 120 mph or so. GM does opt to change the pistons to sag the compression for the supercharged v6 which max runs about 8 psi of boost....I believe a good practice for any blow motor if you want longevity. With a supercharger the tradeoff isn't as pronounced as with lowering the compression on a turbo motor because prior to spool with a turbo you are running on low compression. With a supercharger, boost occurs at a lower RPM hence more low end torque with a lower compression motor.

In any event the following provides a guideline if you want to take the dive and calculate the relationship between compression and stroke and/or piston height reducing combustion chamber cc's:

Solving for M-30 compression ratio using Boyle's Law:

You can back out the equation combustion chamber (cc) volume knowing the bore X stroke of the big six based upon known compression ratio as follows:
P1 x V1 = P2 x V2
P1 = std atmospheric pressure = 1 atm = 14.7 lbf/in2
V1 = Pi x B(Sq)/4 x S where Pi~3.1416 B=bore = 3.62 in
S=Stroke = 3.38 (the big six is wonderfully oversquare!)
P2 = 9:1 compression = 9 x 14.7 lbf/in2 = 132.3 lbf/in2
V2 = combustion chamber size
Solving for combustion chamber size V2 which is 9 x smaller than bore X stroke per Boyles law:
V2 = P1 x V1 /P2
plugging in quantities above:
V2 = V1 / 9 = 34.78 / 9 = 3.87 in3 (combustion chamber size)
Pi x B(Sq)/4 X H, (where H = combustion chamber Height) = V2
solving for combustion chamber height...assuming a cylindrical
combustion chamber shape..could also assume a wedge or hemispherical combustion chamber:
H= V2 / piston surface area:Pi x B(Sq)/4
we now know V2= combustion chamber volume and piston surface area so we can now solve for combustion chamber Height H:
H= .38 inches
we now know the height of the combustion chamber and what you want to calculate is the change or delta in combustion chamber height..will call it H'...due to milling the head .012" so:
H'=.38 - .012 = .368" (new combustion chamber height with milled
head)
OK...now you know everything to solve for "change in compression" due to milling the head .012". Employing Boyle's Law once again:
but first you have to solve for new combustion chamber volume due to milling the head..will call it V2'
V2'= .368/.38 X 3.87 in3 = 3.75 in3
therefore using Boyle's Law once again to solve for new compression:
P1 x V1 = P2' x V2'
where:
P1 = 14.7 lbf/in2
V1 = 34.78 in3
P2' = new compression
V2' = combustion chamber size with milled head = 3.75 in3
solving for P2':
P2' = 14.7 x 34.78 / 3.75
P2'= 136.3 lbf/in3
to convert to unitless compression ratio, divide by 1 atm = 14.7 psi:
P2' therefore = 9.27 : 1 compression ratio
In summary, if you mill the head on a big six by .012" you will increase the compression ratio approx. 0.27 of a point.

[Jeff N.]

I think the best think you can to do is find the block from a 745 turbo. M30 based but came with 8:1 forged pistons, oil sprayer in the crank, etc.

BTW, Pat P. sometimes drops by here - he's the guy with the serious turbo setups.

Jeff

[Paul in NZ]

BTW, Pat P. sometimes drops by here - he's the guy with the serious turbo setups
same dude as in the car domain link

[632 Regal]

take the plugs out for proper position and then drill a small hole through the piston, that should suffice.

[winfred]

or you could drill a hole through the head and cap it off with a valve, then you could fine tune the amount of compression, without all of the engineering expence of the varible compression ratio motor saab has been devloping for the last few years


take the plugs out for proper position and then drill a small hole through the piston, that should suffice.

[535ise]

Thank you every one for your reply's ! ;)

From your ideas i've got a few option's that need further investigation.

If i use a fairly light boost pressure of say 6-8 psi then i should be able to stay with the standard compression ratio(CR).But judgeing by the feed back i got from Rob (in the previous thread i started about super charger's(SC)).He had a SC M30 with a similar level of boost but he still had a head gasket go in a big way on his motor. So may be for piece of mind/longevity sake i should lower the CR from the start.Then later if i want to run a higher boost level i've less chance of it going bang !

I like the idea of having my piston's milled out as this will lower the CR with out breaking the bank.I think i'm going to get a 2nd hand short engine so i can have a machine shop look at the piston's to see if they can be milled out safely or not.I will also have the bottom end/piston's/bore's/bearing's etc reconditioned and balanced.

Thank's George M for your technical post on CR,it must have took a fair bit of time and effort to write.Though i must admit that certain part's of it are a bit beyond my math's ability to understand :( but i've printed it out and hope it will be more clear if i re-read it a couple of time's.

I'll have to find out some more info on 745's as i don't know any thing about them. :(

As for drilling either the piston's or the head......

I understand the principle's as to how it whould lower the CR but i don't think this is an easily realised solution to lower the CR and to get it to work properly.

So a few thing's for me to think about and some other stuf i need to reserch(745's) before i make any commitmant's.

Please keep the ideas and advice comming ;)

Thank's

Andrew

[George M]

pretty straight forward to calculating relationship between compression, bore, stroke and combustioon chamber size. Wanted to show those without an engine design background what the relationships are and how they are calculated. A simple fact is without changing piston diameter, compression is proportional to stroke and/or piston height which defines combustion chamber size. So without going back into the above equation, the following was determined: .012" reduction in combustion chamber height = .27 point of compression increase. Because the relationship is linear ..say you want to sag your compression down from 9:1 to 8:1, then the following ensues:
.012"/.27pt. = x"/1pt.
solving for x....x= 0.044"
In summary, if you want to reduce compression from 9:1 to 8:1 you will want to mill your piston tops by 44 thou which also satisfies intuition (it sounds about right)
If you want to go deep and really do this aside from just entertaining the idea which is fun in principle, I suggest you do a couple of things:
Compare the technical specs on a 745 turbo motor stock from the factory has 8:1 compression which I think would be a good baseline for turbo or supercharger. Specs compared should be piston height relative to wrist pin position and try to ascertain combustion chamber size as well as it is possible if not likely the head is a bit different for the M-30.
Two...I would have my head CC'ed ...very simple to do...you can yourself at home if you wish simply by performing a water test with intake and exhaust valves closed or have a speed shop do it. Then by knowing your bore and stroke you can empirically determine how much you should mill from your piston tops to reduce your compression by 1 point. Always good to have a second data point when taking this on. Keep in mind even if you reduce the compression of a NA M-30 it will still not have the structural integrity of a 745 long block motor...would be best to simply find one of those and build it on a stand and when ready with all the goodies just drop it in.
HTH and Good Luck....fun stuff...nothing runs like a blown car...something I have seriously considered as well...just easier to buy one installed at the factory instead of making one :-)
George

[George M]

http://members.roadfly.com/georgemann/M30PistonTopResize.jpg
Have a look at a pic of my M-30 when I had the head off for rebuild last year.
Notice the piston tops are topographical. Since the piston tops are not planar, change in piston height (delta H) will not achieve a linear reduction in CR. The best way to assess how reduction in piston height would affect CR change to combustion height because delta H is no longer linear with a topographical piston top, is with a computer CAD program...easy then...but would need the CAD algorithm for the piston shape...which only the BMW technical people have. Since this isn't feasible for the home mechanic or hot rodder or even speed shop, you will have to cc your piston top. This is an iterative process but you can get very close because "you know what volume of material (cc) you want to remove by calculation" to achieve the desired 1 pt CR reduction. You first start by CC'ing the piston top in its orginal form to see what the net displacement is...both in depth and highs. From there you know approx. where your first cut should be....safely underneath the desired result. Then reperform the same procedure. It goes fast. Once you define how much you take off a single piston...the same for the rest.
HTH,
George

[535ise]

Thank's a lot George ! :)

I'm going to look into the 745 set up as you say it's got 8.0:1 CR and they are strong componant's so ideal for super charging.I realy like those oil squirter's for the back of the piston's,they are supposed to fit in an M30 block OK.

The con rod's are supposed to use the same crank bearings.Only trouble is they didn't sell the 745 in the UK so getting hold of the part's 2nd hand is going to be a pain.May be i'll go and get them priced up at the dealers,won't be cheap but at least i'll know for sure that the part's are OK.

Heres a pic of the 745 piston's

BTW Why did they call it the 745 ?

http://members.roadfly.com/dukem535iot/SB2.JPG

[George M]

good stuff...thanks for the pic of the 745 motor...dished pistons...nothing new under the sun...a mainstay for low compression on both sides of the pond :-) A dished piston is a method of maintaining stroke and skirt height and still increasing combustion chamber volume to reduce compression. If you compare the two block pics, they look identical. You may be able to bolt those dished pistons and connecting rods into your NA M-30 engine. What I suggest is go hang out on the Roadfly E-23 board which is brimming with 745i blown motorheads. Most have modified wastegates and they go real fast :-) Ask them what your options are and what they suggest. They will know for sure. You maybe able to get pistons and rods shipped to your door from either the US or Europe...much better alternative than the dealer. Good Luck and let us know if do it. Turbo M-30's are much more prevalent than supercharged M-30's so easier to crib off BMW factory designs if you go turbo which would be my blower of choice.
George

[535ise]

Hi George,

I looked in to a turbo for my M30 but unforunatly they don't fit in a RHD car because the lower part of the steering colum get's in the way. :(

Cheer's

Andrew

[George M]

I see...RHD...must be interesting packaging around the steering shaft and exhaust manifold. Think you wrote you are in the UK. Believe it was Robbo who had a supercharger on an M-30...also in the UK and perhaps for that reason. You may want to check in with him. As I recall he didn't change his bottom end at all, just bolted on a low boost blower and that was it. Think he recently removed it. You may want to contact him and check the archives for info.
Good Luck,
George

[winfred]

they called it a 745 to compete with the 450sel mercedes, "it has the power of a 4.5 v8" i've read in a few places

[Bill R.]

the FIA equivalence factor for turbo engines...






they called it a 745 to compete with the 450sel mercedes, "it has the power of a 4.5 v8" i've read in a few places

[winfred]

it was some kind if crap i read in bimmer i think, it wouldn't be the 1st time a car company used creative badge engineering to sell cars

[Hector]

to figure out what the compression ratio for my m30 head would be which has been shaved by 4 mils. I get a P2' compression ratio of 9.274:1. This seems par for the course. Is there a nominal compression ratio range that one can safely be in without giving up too much performance? I think I read that if 12 mils are shaved from the head that one would have to buy a thicker head gasket to stay within a certain compression range. I purchased a regular gasket for m30 which I think I'm fine but I'm curious.


a derivation I posted some time ago on how much compression is changed by milling the head of an M-30 engine and maintaining the same head gasket thickness. The following is based upon certain boundary conditions and therefore is theoretical but can be adjusted to back out compression difference based upon different piston height/stroke change etc as compression is simply a value X [how many atm or bar/14.7 psi.]
Since bore size wouldn't change for your calculation, and compression/pressure is proportional to stroke or piston height, it is pretty easy to back out the relationship if you care to. As a sidebar, you can crib a bit off of the 3800 V6 motor that GM offers in both NA and supercharged form. If you have driven a Pontiac GTP for example with roots style Eaton supercharger...they are quite fast at least to 120 mph or so. GM does opt to change the pistons to sag the compression for the supercharged v6 which max runs about 8 psi of boost....I believe a good practice for any blow motor if you want longevity. With a supercharger the tradeoff isn't as pronounced as with lowering the compression on a turbo motor because prior to spool with a turbo you are running on low compression. With a supercharger, boost occurs at a lower RPM hence more low end torque with a lower compression motor.

In any event the following provides a guideline if you want to take the dive and calculate the relationship between compression and stroke and/or piston height reducing combustion chamber cc's:

Solving for M-30 compression ratio using Boyle's Law:

You can back out the equation combustion chamber (cc) volume knowing the bore X stroke of the big six based upon known compression ratio as follows:
P1 x V1 = P2 x V2
P1 = std atmospheric pressure = 1 atm = 14.7 lbf/in2
V1 = Pi x B(Sq)/4 x S where Pi~3.1416 B=bore = 3.62 in
S=Stroke = 3.38 (the big six is wonderfully oversquare!)
P2 = 9:1 compression = 9 x 14.7 lbf/in2 = 132.3 lbf/in2
V2 = combustion chamber size
Solving for combustion chamber size V2 which is 9 x smaller than bore X stroke per Boyles law:
V2 = P1 x V1 /P2
plugging in quantities above:
V2 = V1 / 9 = 34.78 / 9 = 3.87 in3 (combustion chamber size)
Pi x B(Sq)/4 X H, (where H = combustion chamber Height) = V2
solving for combustion chamber height...assuming a cylindrical
combustion chamber shape..could also assume a wedge or hemispherical combustion chamber:
H= V2 / piston surface area:Pi x B(Sq)/4
we now know V2= combustion chamber volume and piston surface area so we can now solve for combustion chamber Height H:
H= .38 inches
we now know the height of the combustion chamber and what you want to calculate is the change or delta in combustion chamber height..will call it H'...due to milling the head .012" so:
H'=.38 - .012 = .368" (new combustion chamber height with milled
head)
OK...now you know everything to solve for "change in compression" due to milling the head .012". Employing Boyle's Law once again:
but first you have to solve for new combustion chamber volume due to milling the head..will call it V2'
V2'= .368/.38 X 3.87 in3 = 3.75 in3
therefore using Boyle's Law once again to solve for new compression:
P1 x V1 = P2' x V2'
where:
P1 = 14.7 lbf/in2
V1 = 34.78 in3
P2' = new compression
V2' = combustion chamber size with milled head = 3.75 in3
solving for P2':
P2' = 14.7 x 34.78 / 3.75
P2'= 136.3 lbf/in3
to convert to unitless compression ratio, divide by 1 atm = 14.7 psi:
P2' therefore = 9.27 : 1 compression ratio
In summary, if you mill the head on a big six by .012" you will increase the compression ratio approx. 0.27 of a point.

[George M]

believe it is more a clearance issue Hector than a compression issue. I had .008" skimmed off my head and went with the stock gasket. Take note of the pic that shows piston recessing for intake and exhaust valve heads. My car runs fine on regular low octane gas with no chip. No detonation at all. Personally I wouldn't go to the thicker gasket unless you exceed .010" shave off the head....that is if the head has never been milled before :-) The calculation is theoretical based upon "nominal numbers". Keep in mind every dimension on your big six has a tolerance...+/- a certain amount. No two big sixes are the same from the factory or after a life cycle. Computer programs are written to calculate RMS (root mean square) tolerance stack up's to see what worse case compression will be based upon outliar tolerances. You should be fine with your modest head milling with the stock gasket.
George

[Hector]

Can't wait until I put the head back together. Rigth now the car is going through a major makeover.


believe it is more a clearance issue Hector than a compression issue. I had .008" skimmed off my head and went with the stock gasket. Take note of the pic that shows piston recessing for intake and exhaust valve heads. My car runs fine on regular low octane gas with no chip. No detonation at all. Personally I wouldn't go to the thicker gasket unless you exceed .010" shave off the head....that is if the head has never been milled before :-) The calculation is theoretical based upon "nominal numbers". Keep in mind every dimension on your big six has a tolerance...+/- a certain amount. No two big sixes are the same from the factory or after a life cycle. Computer programs are written to calculate RMS (root mean square) tolerance stack up's to see what worse case compression will be based upon outliar tolerances. You should be fine with your modest head milling with the stock gasket.
George

[AllanS]

There's a South African company that makes RHD turbo kits for BMWs.... I can't remember the name though, and I deleted the bookmark to their site.

If you can't mount the turbo(s) on the right hand side, you might be able to mount it on the left, using a custom manifold that wraps under the block. That's a little outlandish, though. The Porsche 944 Turbo used this method.

I would look into mounting it on the exhaust side, but far back and below the manifold. You may have to get creative in how you make the manifold.