Stroker question

[acolella76]

Well idk about the b25 outperforming the Seta with the b25 head.

My 'stroker' can and will outperform a b25, but honestly I would rather have put the money I spent on my stroker into building a b25.

[2mAn]

I have stacks of dyno sheets for 2.7i "strokers". I'd rather have an M20B25 with a cam.

can you post some of those please? what if you cam the "stroker" motor?

how is it possible that a car with more displacement cant make more power with the same head, cam, ecu, etc? is the loss in compression really that much? if thats the case, it can easily be remidied by shaving down the head to make up for the loss in compression due to the 2.7 bottom end. are people just lazy??

[Robotin]

Long story short = MR 325 has infinite knowledge, moreso than 'anyone on the forums' blah blah blah

i second that....

[nando]

can you post some of those please? what if you cam the "stroker" motor?

how is it possible that a car with more displacement cant make more power with the same head, cam, ecu, etc? is the loss in compression really that much? if thats the case, it can easily be remidied by shaving down the head to make up for the loss in compression due to the 2.7 bottom end. are people just lazy??

it doesn't have much to do with the CR drop. typically you've increased the displacement but the head flows the same as it did when it was on a B25. The head still fills the cylinder at the same rate, but the cylinder got larger - you haven't increased VE (the ability to completely fill a cylinder).

Torque and VE are related - your low end torque will go up because of the longer stroke, and the head flows more than enough air at low RPM. but your high rpm torque, where HP is developed, stays about the same, because the head flows the same as it did before. HP = torque*rpm / 5252. if you haven't increased VE, then you haven't increased torque output, and at a given RPM you aren't going to make more horsepower.

in fact, the engine is now *less* efficient now because you have the same headflow but more displacement - your VE has actually gone down.

increasing displacement doesn't magically change how the head flows. you can cam it, but if you've got 8.5:1 CR a wild cam is only going to hurt the low end torque that you got from the increase in stroke, while not really being able to take advantage of it's higher RPM capability.

the dyno charts reflect this. Here's three disappointing examples:

9:1 CR forged pistons, 885 head:

great torque, no power.. well, it has some power (stock M20B25 levels) but the redline of a diesel.

straight up 885 head swap:

pretty sad..

both with an alpina cam and schrick 288:
96kw = 128whp, 102kw = 136hp (that's WITH a cam!!)


meanwhile, a bone stock, 260,000+ mile M20B25 with a crappy dinan chip:


if you want more power, increasing displacement is a good start (more torque potential), but you have to increase the total airflow and cylinder filling or you don't really get anywhere. as a power upgrade for somebody with an eta, it's a good cheap upgrade, but you aren't going to magically make 40 more HP than a stock B25 by using the same head and supporting parts (intake, exhaust, tuning, etc).

[2mAn]

Is it a 20% drivetrain loss, from motor to wheels? and VE is Volumetric Efficiency right?

Mines an ETA so I would be swapping over the "i" parts, but only if I can get them at a good price. If I did the whole swap, I would also get a cam and shave the head down. Id also try to avoid going with a full head rebuild, because at that point i dont think it would be worth it to stay with the M20. If anything, I would force the air in (turbo or supercharger) before I started building a motor from the ground up. Lets be honest here, a 24v will always be better than a 12v so that would be my starting point if I was to build a motor.

My goal would be 200bhp with those mods, am I dreaming?

[lennon]

if "those mods" is including the turbo or supercharger you mentioned...

[2mAn]

if i were to add forced induction i would disappointed with anything less than 250 from the motor and i wouldnt even waste my time with it. thats when an S50/52 swap makes sense

[ForcedFirebird]

it doesn't have much to do with the CR drop. typically you've increased the displacement but the head flows the same as it did when it was on a B25. The head still fills the cylinder at the same rate, but the cylinder got larger - you haven't increased VE (the ability to completely fill a cylinder).

Torque and VE are related - your low end torque will go up because of the longer stroke, and the head flows more than enough air at low RPM. but your high rpm torque, where HP is developed, stays about the same, because the head flows the same as it did before. HP = torque*rpm / 5252. if you haven't increased VE, then you haven't increased torque output, and at a given RPM you aren't going to make more horsepower.

in fact, the engine is now *less* efficient now because you have the same headflow but more displacement - your VE has actually gone down.

increasing displacement doesn't magically change how the head flows. you can cam it, but if you've got 8.5:1 CR a wild cam is only going to hurt the low end torque that you got from the increase in stroke, while not really being able to take advantage of it's higher RPM capability.

the dyno charts reflect this. Here's three disappointing examples:

...

if you want more power, increasing displacement is a good start (more torque potential), but you have to increase the total airflow and cylinder filling or you don't really get anywhere. as a power upgrade for somebody with an eta, it's a good cheap upgrade, but you aren't going to magically make 40 more HP than a stock B25 by using the same head and supporting parts (intake, exhaust, tuning, etc).

That would be true IF the b25 was already using peak flow in the head. The head already flows more than what the engine needs.

For example the GM 2.8 has the same exact heads, cam and compression as the 3.1 yet there's an increase in power (130hp/160tq to 140/185) with only a difference in crank throw. It's the same with the late model 3100 and 3400.

It's definitely not because of the compression loss. The b25 is 8.8:1, seta is 8.5:1 and an un-shaved 885 head on an eta is 8.43:1.

People wouldn't have been putting 400 small block cranks in a 350 (using the same heads, again) if it didn't make more power.

This would be like saying the m54b25 would be better than an m54b30, they both use the same head and have increased stroke.

[MonkeyMan]

well this might explain my slightly disappointing dyno numbers... 134hp and 136 lb-ft from a mildly stroked m20b25 (323i crank: 76.8mm = 2554cc) on a stock (although rebuilt) 885 head with the 323i cam.
time for some tuning...

[nando]

That would be true IF the b25 was already using peak flow in the head. The head already flows more than what the engine needs.

For example the GM 2.8 has the same exact heads, cam and compression as the 3.1 yet there's an increase in power (130hp/160tq to 140/185) with only a difference in crank throw. It's the same with the late model 3100 and 3400.

It's definitely not because of the compression loss. The b25 is 8.8:1, seta is 8.5:1 and an un-shaved 885 head on an eta is 8.43:1.

People wouldn't have been putting 400 small block cranks in a 350 (using the same heads, again) if it didn't make more power.

This would be like saying the m54b25 would be better than an m54b30, they both use the same head and have increased stroke.

but isn't it? If the head flowed more, wouldn't the engine make more power?

the M54B30 uses different cams. BMW also really restricts the B25s compared to the B30. an example is the M52B25TU. BMW detuned that motor by an easy 30hp.

that's just it, I was talking about the system as a whole - intake, exhaust, etc. you have to spend a lot of money for quality headers that do anything, and the only viable alternatives to the stock intake manifold are extremely costly. Nobody doing a 2.7i is going to spend $1500 on exhaust and $3000 on a new intake manifold..

how many of those guys doing 4.0 strokers are changing nothing but the crank? I bet few of them.. they're doing other stuff too, to tap that extra displacement potential. why wouldn't you?

and look what happened to your GM example - HP only went up 10hp, but torque went way up. that is DIRECTLY related to the flow potential of the head. Power is mostly a product of RPM and headflow, if you bump up the displacement by 15% and power only increases 5%, but torque increases 10%, then you know where the bottleneck lies. You can keep on that tangent, only increasing the displacement, and your HP gains will continue to diminish even though your low RPM torque increases more linearly.

I think the dyno evidence spells it out pretty clearly. Show me a 2.7i that doesn't have a worked over head, custom intake manifold and high dollar exhaust that makes over 160whp. I don't believe one exists. even getting to 200whp is very difficult (mostly expensive). I do have dyno sheets from M20 strokers that make a lot more power, but all of them have something else done other than a stock 885 head swap.

people will do whatever makes them feel good, not because it's a good idea or because it has the best results. how many people here actually go to a dyno and *tune* their car? most of the people here, they're lucky if they aren't running the stock computer and take it for a couple baseline pulls.

[ForcedFirebird]

I believe tuning is a big part of it. Last trip to the dyno I was able to make 7 pulls and the power curve was getting broader and higher until the stock injectors were getting close to peak (15afr's).

As far as the torque being much higher than the power in the GM example I used, not so. There's a 9.6% increase in displacement, 7.7% increase in power and 9.3% increase in TQ.

...and yes there's more to the story. With the m20 a lot of people don't take rod length into consideration. BMW changed the rods to compensate for stroke, the GM example uses the same rods and move the wrist pin. This makes for a better rod/stroke ratio. The b27 has more stroke and less rod which in turn makes the piston sit at TDC for a shorter period and a worse rod/stroke ratio.

Also I feel the piston design has a huge role to play in the b25 vs. 327i. BMW put a lot of effort into the piston shape to promote swirl and edge out detonation. I finally found a SETA rotating assembly and am curious to see what happens when the parts are swapped from my current setup. The current rotating assembly is getting tired and starting to knock (believe it's because an oil cooler was never installed for high RPM duty) but will take one last trip to the dyno when the SETA is ready to drop in.

As far as the head flow to power comparison, while, yes, it is true the head porting will make power - but only if done properly. "Hogging" out the ports will make the head flow more, but if you pass the point of velocity/flow ratio then that extra flow does nothing or hurts power. Typically heads will make more power through porting because of the limits of casting procedures. Manufacturer's will sacrifice the end product if the process to making said parts isn't cost effective to be optimal.

I totally agree it's a combination of parts and procedures, and while we are discussing "budget stroking" and budget minded people they are going to just put stock parts together that aren't always the best.

[MonkeyMan]

most guys that buy a 20-25 year old e30 are gonna be on a budget (most) to start with.. as much as even half-decent headwork costs (a grand minimum), its not surprising that they're gonna do the easy stock-like head swaps and the budget (ie junkyard) strokers.. I think it just comes down to expectations - when you're just adding parts to your car, its not really a matter of adding all the expected gains together, but how they all work together.. however you look at it, you get what you pay for.. if I had an extra 2500 sitting around, I'd definitely put it into the head if I'm looking for more power..

edit: how this pertains to this thread.. it seems like a trend with these cars is to jump into the 2.7+ stroker right away (which, although not too expensive, that money can still go towards decent headwork) because of ease and popularity.. I'd say leave the displacement alone, work with what you got, and try to get as much out of it by improving flow.. these motors have a lot of potential, even with the stock displacement..

the PO of my car did a mild stroker (2.6), which is nice for potential, but I knew right away that I'd be spending some money on the head...
my .02

[digger]

the 885 flows plenty for most peoples needs but you need the correct cam to "release" the flow if you start pulling harder through the head (i.e. larger stroke and higher pistons speed). As already said above a proper tune and proper burn combustion chamber also shouldn't be underestimated

[totheredline]

Bringing this thread back from the dead. SO, long story short, I'm converting my car back to m20 power after selling my M30 setup. I just bought a complete B25 drivetrain and I was planning on regasketing it, slapping some eBay headers on it and calling it a day. Well, before I bought the B25 I had my eyes on a super super clean SETA out of my friends e28 for $350. SO, should I buy the super ETA too (can basically make my money back selling the oilpan and 885 head ) and combine the bottom end with the B25 setup I have? OR am I wasting my time? Am I better off with a stock b25? Is there a noticeable difference?

Sent from my XT1585 using Tapatalk

[digger]

sounds like a plan, should be a better DD due to more torque.....if you get the piston to head clearance to 1.0mm it will improve things a bit to