Compression Ratio (How Boost Affects it)
Hi,
The question I’m posing is aimed more towards the tuners and engine builders who have experience fiddling with what makes a car run, and what suddenly earns you a window into your crankcase.
I’m planning on going the expensive, painful, and dumb route of wanting to take a BMW M54B30 (from various BMW vehicles, particularly in my E39 530i in this application) engine, and turning it into a fake S54B32 (E46 M3) engine by boring it out (stock stroke). Putting S54 diameter sleeves, pistons, forged rods, and S54 head in it up it from a 2,979cc engine to a ~3,194cc engine. (I do not have all parameters figured such as head volume, deck height, etc.)
Going from the 10.2:1 from the M54B30, would the increase take that up to 11.3:1?
M54 stroked/M54 stock, taking percentage x M54 stock CR (3,194/2,979= 107% x 10.2= 10.9)
I plan on using the engine like this until I can get an engineer friend of mine to help me fabricate a supercharger (twin screw) kit, at which I plan on running at MAXIMUM 10psi of boost. For daily driving, I’ll stick to 8psi, maybe keeping a party pulley for 10psi when I can get race fuel and a separate tune for higher octane fuel. I dare not take it further, as I’d sink so much money into it, I’d rather get an S54 or Alpina E5/1 iron block.
FINALLY! The Questions!
1: Will the block handle the stress when naturally aspirated after bored out?
2: Will the block handle the extra stress (I know the crankshaft will need replacing on top of pistons and rods) of 8-10psi of boost? If so, what compression ratio will I have to run on 91-93 octane gasoline?
I appreciate any feedback that you all have to give.
P.S.: I understand I’m crazy for taking a M54 and doing all of this rather than getting a S54/Alpina iron block E5/1 and sticking it into my car, then going from there. It’s something unique I want to experiment with as I don’t see a lot of extensive engine modification in the BMWs.
Comments
Question 1. It sure will.
Question 2. The engines are rated for 500+ HP. And we cant just do some magical math to know your exact HP out put with all your specs there. But im doubtful you going to go much over 500 HP. At about 700 HP is the point where most engines get upset about daily driving and life span is significantly reduced.
Question 2 part 2. All BMW engines right out of the factory have a high enough compression ratio to need to run on 91 to 93 octane. You only need to crack 9 to 1 about to even take advantage of the higher octane fuel. As for adding the boost into this mix i would say at 10 PSI you might be looking at race fuel instead of pump gas to run that. But i could be wrong. In my experience you find out on the dyno if you need higher than pump gas can provide.
10 psi of boost isnt that much really. For a supercharger its up there but turbos get much more into the 15 20 or even 30 psi.
So recap.
The block should handle it just fine. As long as we arnt looking at 700+ HP.
Thos rods and crank better be forged
You already should be running on 91 to 93 octane on a stock BMW engine. Luckily the engines computer can compensate at the cost of power to not damage its self. At a NA 11.3 to 1 ratio 93 should be fine. Add boost and id personally want to run race fuel witch is typically over 100 octane rateing.
On a side note here. E85 is really good with boost. You have to burn more of it but its still cheaper than race fuel. Look into an E85 conversation. Never messed with it personally but from what i have read and spoke with others about its cheaper and easier to get than race fuel with just as much octane rateing.
To answer the HP figure I’m looking at with rough calculations, ~550hp is the ballpark number I got with boost, ~325hp naturally aspirated.
I’ve done a little more digging since making the post, and found that adding boost to the equation would demand race gas or E85 to prevent killing the engine. Not sure if I would want to have two sets of rods or only get one and work around them for both NA and forced induction.
I live, as my name says, the midwest USA, so I can find E85 fairly commonly (yay farmland!).
After seeing the octane rating of E85 being about 112, I’m curious the compression ratios it can handle with boost. Not because I want to make it difficult on myself, but to know how to approach it safely. Seeing your crankshaft through a jagged hole would be an interesting sight for maybe 20 seconds.
Lastly, would methanol injection be a good idea on top of running E85? I’ve heard it reduces supercharger temperatures, but would it be beneficial enough in the grand scheme of it?
In my research, factory turbo cars usually have a lower compression ratio compared with factory N/A cars.
I think its something to do with the added stress of the extra air being forced in.