prob stupid stroker question

[der affe]

just checking my facts: if i am going to build a 2.7 stroker i will need:
2.7 eta crank from 325e (cast) or 525e (forged)
2.7 con rods
custom pistons
2.7 oil pump? higher pressure/ volume???? why do i need?
i am going to put it into a 2.5 325i block, so just get a COMPLETE long block right?

i already have stand alone, itb's ford maf conv, headers, cat back, p&p head, schrick cam...althe top end bolt on stuff. is there anything else i am missing other than gaskets, machine work, stuff like that?
thanks,
greg

[der affe]

ok i just found out i was wrong on the rods i need to use the "i" rods with the eta crank. anything else?

[MrK]

Depending on your pistons and cam, you may need an adjustable cam gear. Otherwise it looks like you're set.

[der affe]

onw more stupid question...what is the difference between the s-50 and m-50 cranks? is the stroke the same? is it just a better forging for one over the other? just got a big raise and it has opened up my options to go bigger on the stroke.

[Madhatter]

essentially the same dimensions (bearing sizes, etc) when you compare US S50's, its just the stroke is different on some of the cranks (you can get different stroke cranks from M50/M52/M54's to make up different engine sizes compared to the 3L crank you will get from the S50). European S50 are different, they run larger bearings so they dont fit.

Depends on what you want the motor for, but if you go with I rods you need a reduced compression height piston (because i rods plus pistons result in the piston sitting way out the top of the bore), so you definately have to go custom pistons as nothing off the shelf really exists.

Otherwise, use the original e bottom end (crank, rods, etc) which will go straight into an I block, you just have to decide what you do about the pistons. Either use available bmw items and mix around compression heights or go custom to bump the compression ratio back up.

What way you go totally depends on what head you want to use and just how much money you want to spend.

[der affe]

thank you again madhatter! i am going to go with Pauter rods for the lighter design and the ability to have a custom rod legnth to fit the application, (i have used/worked with Don and Mario before on custom vw applications) and either a JE or MM piston. i would like to try to get a rodratio as close as possable to the stock "i" combo for longevity and i am using a late i engine to base it off of (including the head) that i have already picked up. i would like to also get the over all rotating weight as close as possable (or maybe lighter ) to the 2.5i i have now, with the new stroker, i think???? it might be possable to do using the lighter pauter rods, knife edgeing the crank, custom pistons (forged are usually lighter) and a lightened flywheel. i will P&P, unshroud the valves, match the cc's in the chambers myself. i already have ITB's, stand alone, mustang MAF, schrick cam, shorty headers,& cat back from my engine that is in the car already and will be reusing them on the new motor. i am going to keep it N/A and i am not looking to rev the hell out of the motor (7000/8000rpm) because of the rocker issues until someone comes out with a real solution, would like to be able to when that issue has a fix.
what do you think? is it possable?
greg

[Madhatter]

dont worry about custom lengths, just go with 130mm or 135mm already used and match the piston compression heights to fit in the block correctly. Pauter already produce rods in those lengths so it should be a little cheaper than having something made custom. If you are going with the 325i head i would go with 135mm rods and eta crank, with pistons having a compression height of 31 - 31.2mm (given the deck height of the motor is 206.2 - 206.7mm). That should bring the piston right back to the top of the bore, rather than sticking a good 3mm above it like it would do if you used I pistons with the same rod/crank combination. When you go with the custom pistons try and get a crown produced which is similar to the stock 325i pistons (send an original to them if they want to copy), its important to retain the basic design as it matches the combustion chamber shape.

If you want to rev to 7000/8000rpm i hate to tell you but that is reving the hell out of the motor, thats quite a lot of revs, especially for the M20. You will break rocker arms if that is your goal in mind. Once you have your head work done and you fit the ITB's, i think you will find that reving the motor a lot is quite useless as peak power falls off long before then. Have a look at what your engine puts out (something that will shift with the longer stroke of the motor and any cam changes), then go with a realistic rev limit. No point running 1000+ rpm above peak power, its better to reduce the revs (and reduce piston speeds) and gain some reliability.

[der affe]

thanks, i actually ment to say that 7000/8000 WAS reving the hell out of the motor. looking to stay below that for max rev's for a reliable combo. wouldn't you end up with a little short rod ratio with the 135 rods on a m52b28, m54b30, s50, or s52 crank? that's why i was looking into the custom rod legnths so i could end up with a rod ratio of around 170/175 with custom pistons. i would be really happy with 200 whp or close to it with out having to go with big compression (going to run on super, which in AZ is only 91 octain:( ) it also get friggin hot here in the summer so the high compression would cause issues too. as far as the reving is concerned i would just like to have it rev like my 2.5 does now, and with the extra weight and mass of the stroker crank that's where the light eng components would help out. i figure with the topend stuff i have now and a 3.0 stroker that hp goal is atainable and with the right combo also a reliable for daily driving. what do you think?

[Madhatter]

no.

If you want to talk short, S50 crank + 130mm rods = 1.51 :o From a reliability standpoint its not that big a deal if you dont plan to rev the motor a whole lot (im going to keep mine around the 6500rpm rev limit), you do encounter higher pistons speeds, but if you arent hammering the engine, with the new hardware (and balanced) you shouldnt have a problem with reliability. Just make sure you use a good set of rod bolts, big ends are a bit of a weak point on the M20 bottom ends, dont want the bolts to let go. From a wear issue, its going to produce some side loading of the piston (around the skirt), but it isnt a major issue when running the larger cranks, not to the point you should worry about it anyway. 135mm rods with the eta crank will result in a ratio of 1.66', you certainly arent going to have any issues running that.

Is there any reason why you want to use the M50 stuff? like compared to the eta crank i mean?

If you want 200hp you are going to have to go big, head work, manifold work, cams, extractors (headers), good exhaust, plus then possibly look at a larger crank to increase the capacity. Its all going to be small gains which push you there, check jordan's thread for a bit of an idea on what he has done (and he is still a little short). 200hp isnt as easy as people like to think it is.

Not a large amount of weight difference between the cranks, not like you seem to think there is anyway.

As for the 3L, you really dont have any alternative. The M50 motors are 2mm larger in deck height, thats why they can run larger cranks/rods, the block was built to accomodate it. Given the 206.7mm of the M20, you only have a few choices.

206.7mm - 42.9mm (crank throw) - 135mm (rod length) = Piston compression height of 28.8mm.

Compare it if you went with larger rods

206.7 - 42.9 - 140 = 23.8mm. That is seriously worrying, that leaves very little metal between the pin and the crown, probably so little that you wont even fit ring lands on it.

Realistically, 135mm is about the maximum rod length you can run with the S50 crank, i dont know a whole lot of manufacturers who will make you a piston with a compression height lower than that, most will probably be happier if you went with 130mm rods and 33.8mm pistons to keep some meat on them.

Have a chat to whoever you plan to get your pistons from, i think they will advise you away from the longer rods you wish to run.

[der affe]

good point on crown thickness. i hadn't concidered that. the reason i want to go with the m52/m54/s50/s52 cranks is for the longer stroke (more torque) and the larger displacement they will yield with the +1 pistons. i think jordons hp limitations could be overcome with the bigger combo, everything else seems to inline to get to the 200whp mark and with a little more tweeking and prob some ITB's he'll be there......and this is with a 2.8! 200 more cc's ought to make it easier to accomplish this don't you think? "there is no replacement for displacement" that (and my raise) are making me lean toward the 3.0 option insted of the ETA 2.7/2.8. i want to keep the car a daily driver (until i get my mcoupe) and reliable and when i build/ put together a motor for myself i tend to overbuild it as far a the quality of the parts for my needs ,but it gives you a nice cushion if you go and beat on it. i know that the cheaper way out is the eta but i would really like the 200 whp. as far as the wieght of the cranks, i wouild think that the 85/86/89 stroke would have a lot more rotating weight/mass over the 75mm of my current "i" 2.5's crank, that's a pretty big increase in stroke. i am not looking for some superlight combo but to get the rev responce out of a 500 cc bigger motor (or better revs/ responce) would be great. i am basing alot of questions off of my knowledge from building vw motors (both air and watercooled) and what stroking, rod ratio, compression VS available octain, can do to a motors powerband, cooling, reliability, etc. this will be my first bmw motor but it shouldn't be all that bad with the properly selected parts to start off with.
btw. not to kiss ass but it is nice to talk to someone with some knowledge not just what they have read from the forum and other people's opinions.

[nando]

if you want a longer stroke than the eta crank, but don't want to get wild with r/s ratios, I think the 84mm M52B28 crank is the way to go. you'd have 2.9l with an 85mm bore, and that'd still give you a pretty decent 1.6 r/s using a 135mm rod. if you use S50 rods or similar you'll also lose some rotational mass so your revs won't be hurt much (that's what I'm doing, although I still haven't decided on my crank yet).

[Madhatter]

indeed, i posted it on e30tech, you could run a stock bmw piston from a different motor if you went with the 2.8L crank too, good cost saving method.

Added plus for the longer stroke/shorter rod combination is going to more efficient as it passes TDC when drawing in the intake charge, simply because of the increased piston speed at TDC. So a good breathing head, plus ITB's and a large plenum should see a nice improvement in volumetric efficiency without sacrificing response (this is aside from the added response you will gain with ITB's) like would occur otherwise . Basically you make it too large, you dont end up with the compression effect where the air entering the plenum slows down and begins to compress itself increasing its volume (well, technically i surpose it is density). Its a combination you have to get right between slowing the air down enough so the plenum can actually do its job, and not having high air velocities where the air just flies right through the plenum and throttles, but doesnt have any time to build up (so you sacrifice low end torque).

It's good because the higher piston speeds actually develop more vacuum to suck in the charge, this simply equates to an efficient engine at lower engine speeds. Coupled with the long stroke of the motor anyway, it produces more low down torque which is what should see the motors pull really strong from low engine speeds. The other plus to this is that because of the increased speed away from TDC, it allows the cylinder to fill with the intake charge much quicker (so it's volume is increased faster), this is great for forced engines because it is simply about trying to cram as much air as you can into the cylinder in the period of time before the valve closes.

Only problem is, gains due to increase late intake valve timing isnt as significant, this is simply because the piston begins to slow (and is quite a difference between those with longer rods) as it moves towards and away from BDC. It takes away from of the scavenging effect during valve overlap, basically it just isnt as quick to compress the cylinders contents. Any failures in port design is going to show up here, mainly the exhaust port, you will find flow rates begin to trail off quite significantly, so you need to make the exhaust porting as efficient as possible.

You also can have issues with the piston actually moving faster than the flame front can travel which can result in a loss of cylinder pressure (and power due to it). So you may have to make adjustments with ignition timing to find the optimal point (so dyno tuning is going to be real handy).

As for weight, yeah but you have to remember, there is up to 15 years of difference between cranks, compositions would change, so would casting techniques, i dont think you will find the later model S50 crank to be a whole lot heavier than the eta item. Anyone got a set of scales to weigh an eta crank? ive got one here but it is still bolted in place so i cant check to see how much it weighs at the moment.

[der affe]

so what you are saying is that it may be a more efficient combo to go with the shorter stroke of the m52b28 rather than the s52 even with the 135 rods. the problem being that there may not be enough air reserve in the plenum (even with the ITB's..i am running the Dbilas itb's btw) for the cylinder to fill up because of the faster piston speed with the shorter rod ratio?

[Madhatter]

Mmm, i wouldnt say it would be more efficient, but if you are worried about the rod ratio the 2.8L crank from the M52 is a perfectly acceptible item to use for a stroker motor.

Its pretty rare to find a plenum that doesnt deliver enough air volume, when manufactured they are usually built with a percentage above what the engine actually requires. If it becomes a bottle neck its usually due to poor design. I dont think you will have a problem with any stroker motor running an ITB setup, not given the area the actual throttles take up compared to runners, the plenum would be quite significant in size (compare it to the M engines which run the setups and you will see just how much bigger they are to fit over the throttles).

Go back and read what i was saying, they were advantages to the faster piston speeds, basically you can suck more air into the engine which will equate to a rise in efficiency (which will increase horsepower/torque).

[der affe]

i understand what you are saying that you shouldn't discount the smaller strokeof the 84mm just because it is smaller, the faster piston speeds of the smaller combo will yield gains in addition to the stroke/boring by virtue of the faster piston speeds and filling of the cyl. quicker. do you or can you print the formual for finding the rod ratio vs crank stroke, i would like to have that. also what do you feel is an acceptable cr to run with the 91 ron gas i have to deal with. keeping in mind that i have the stand alone and the ITB's and i would like to run on the high side of the safe range.