m20 or m50 turbo?

[zimm_zimmer]

Hi all im looking to get my e30 touring in december time and will be saving and buying accordingly for a turbo conversion, now im not looking for much above 300hp but if i get bored in the future i want enough room to upgrade without huge amounts of engine work, so out of either the m20b25 or the m50 2.5 which one would you go for and why, i know which one would be cheaper the m20 but would it put up with 300hp without me uprating internals, i understand i would need a decompression head gasket and some arp head bolts but anything else? Many thanks for any help on this

[ross_jsy]

I would go for the m50 (which is what I'm doing).

With a MLS head gasket, ARP head bolts and standalone it will hold about to about 500hp, leaving you plenty of room to maneuverer.

[HairyScreech]

an m20 will hold 300hp turbo on a standard bottom end, been done plenty of times.

[Gunni]

What exactly does bored in the future mean?

Both will do 500hp on stock bottom end with MLS and ARP
Both will do 300hp on stock everything.

[HairyScreech]

and another thing to add to the above, the m20b25 low comp (late plastic bumper) version has 8.8:1 compression ratio which leaves you with a nice lump for boosting.

if you get bored later then a 2.8 turbo is only a crankshaft, rods and seal spacer away.

[Gunni]

A 2.8 won´t be able to make any more then a 2.5 as long as the pistons and rods are factory.

[oddisE30]

Well, Gunni - that depends on getting the map perfectly set up, doesnt it. Orginal parts you can never tell how long will last. Especially if we add E85 bioethanol the EGT drops quite a bit. Two friend of mine run 603whp and 530 whp with M20 2.7L's. The 603whp one had forged rods and stock preface pistons with down-machined and flatend out dome. And the other with same procedure on facelift piston and stock rods. The 603whp had 10:1 i comp ratio, and the other had 8,2:1.

They tend to crack in the ringland anyway when temprature gets to high. Eliminating the second weakest point, dome/squish is then regarded as smart. Then again, forged pistons have never been cheaper when you're building a serious 500whp+ machine. Even putting a expensive turbocharger on a stock engine i see as liability if the engines would start spitting out metal into the turbine.

[oddisE30]

BTW, to thread creator - I would also recomend you going for the M50. If there is no special passion for the M20 there is no good reason for working extra on welding the waterchambers in the head and always be taunted with the rocker arms issue.

A M50 MLS and ARP'ed with flat machined block and head surface should be a perfect setup and very durable and cheap! Non vanos is off couse preferable. There is lots of turbo chips on the marked, if not in the UK (?) i could point you in the direction of one who has done tons on OEM chips for M50 turbos up here, running for years and years with 330-360hp without flaws.

[Gunni]

Well, Gunni - that depends on getting the map perfectly set up, doesnt it. Orginal parts you can never tell how long will last. Especially if we add E85 bioethanol the EGT drops quite a bit. Two friend of mine run 603whp and 530 whp with M20 2.7L's. The 603whp one had forged rods and stock preface pistons with down-machined and flatend out dome. And the other with same procedure on facelift piston and stock rods. The 603whp had 10:1 i comp ratio, and the other had 8,2:1.

They tend to crack in the ringland anyway when temprature gets to high. Eliminating the second weakest point, dome/squish is then regarded as smart. Then again, forged pistons have never been cheaper when you're building a serious 500whp+ machine. Even putting a expensive turbocharger on a stock engine i see as liability if the engines would start spitting out metal into the turbine.

Everywhere but Sweden we must assume people use normal pump fuel,
this leads to much higher combustion temps , those temps can melt the stock pistons when E85 does not.


Therefor a 2.5/2.7/2.8 will all melt the pistons at about the same point when using normal fuel. Thats why I talk about them not being able to handle more then each othere.

[HairyScreech]

interesting and something i did not know,
i would have thought conventional wisdom would have seen a 2.8 produce more than the 2.5 due to the obvious advantages of a longer stroke, larger capacity motor when coupled with a turbo.

but if as you say they are piston limited then beyond a certain combustion temp/pressure there is nothing more to be had.

will a 2.8 cool the pistons more than a 2.5 due to the larger surface area of bore that the piston is exposed to?

my m20 specific turbo knowledge is not massive so genuine interest here.

[Gunni]

It´s not a larger surface area, they are both 84mm bore.

sure there are variances but these are rather slim as piston melts from temperature.

400hp for instance gives about the same peak cylinder pressure on a 84mm bore piston no matter the stroke. This is of course not absolute, so just take it with ease.

If a 325i can make 400hp then the 2.8 would only yield a little more really,

[HairyScreech]

but it is a larger swept area, the piston travels further down.
thus circumference of piston = 263.89mm
2.5 swept area = 263.89 * 75 = 19729mm^2
2.8 swept area = 263.89 * 84 = 22167mm^2

the heat of the piston is conducted from the skirt/ring lands to the bore wall, which then loses its heat to the coolant.

as heat transfer is proportional to the temperature differential, and the 2.8 spends longer time further down in the cooler lower half of the bore....

[Gunni]

Thats just 12% more area. Is there even coolant flowing past in that area in the block?

All I´m saying is that we aren´t talking about any big difference in power potential when you have stock pistons. There is probably some, but it´s nowhere near worth making a 2.8 for that power goal.

Rather a 2.8 with stock pistons would be used to get the boost on sooner to create a stronger low end.

[HairyScreech]

ok, i was just thinking, but i think you may be right that there may not be coolant at that part of the block anyway, also im not sure if it would make a lot of difference once thermal conductivity of the steel and the extra thermal load on the cooling system is taken into account.

all interesting info though.

the heat in the upper ring land is something im worried about with the shaved pistons for the 2.8 as theres no real way of knowing if they are too thin or not.