Am I smokin rocks? Or is the extra 30% not there?

[BLINGMW]

I picked up a '91 m42 318 a month ago. Wanted to play with one. My silver track car is an '85 w/ m10 for those who don't know. The m42 is known to have ~135HP, the m10, ~102HP. I wanted to see if the m42 was really much faster.

My test was a small stretch of road with a mild incline near my house. Super scientific. :lol: Start in 2nd at 35MPH at the speed limit sign, wind out 2nd and shift to 3rd, see how fast I'm going by the end of this white line at an entry to some business.

Both are due for an engine oil change, both have synth fluids in the trans/diff.

The m42 is stock stock with ~148k on it, the m10 has been rebuilt and has no cat. Might have a couple more HP than it should, no more than that I would guess. The m42 makes great compression and seems to be running very well.

The m42 actually gets an edge in the gearing dept (it has a 4.10 while the m10 has a 3.64) as it's turning a bit over 4k at 35mph, and the m10 is at 3.5k. And the gear spacing is a bit tighter going into 3rd too. I would think this would be a BIG edge.

The m10 has the edge in weight, though I had about 100lbs of tools in the trunk to help. Both had about a half tank of gas. With me in it, the m10 318 was prob 2700lbs. The m42 318 test weight should be around 2850-2900.

Both have stock size tires, so the speedos shouldn't be too far off.

Test was all done within an hour on the same day, so same air temp and all that crap.

The results? After two passes with each car (after warming up of course), the m10 is going 62. The m42? Maybe 63. Came out the same way with each run, so I didn't bother doing more. And I didn't really need the cops/neighbors to notice.

Was my test run too short? Too many variables? Does the weight difference matter more than I'm giving it credit for? I would have thought the m42 would be going significantly faster. Is 1 MPH significant for such a short run?

Statistical error, drink more beer?

[Sijray21]

what does the butt-dyno tell you :-P

haha, it's like MM Mythbusters on crack :lol:

perhaps different power ranges? would that make a difference? i think eliminating the shift would also make a more controlled experiment.

[BLINGMW]

what does the butt-dyno tell you

I don't really seem to have one, or it's just poorly calibrated due to too many indian and chinese buffets, that's why I did it. The m42 FEELS a little faster, but I think that's just because of the 4.10 so I move through the gears more quickly. I'm sure the difference is pretty large at 100MPH when HP is about all that matters, but I was just a bit disappointed. I mean, the only time the extra power's going to be any more fun (to me) is pulling out of a corner on track, likely 2nd or 3rd gear. Or an autocross. So if it doesn't pull much faster at those speeds, well... WTF?

I'm actually happy that the super old tech, old reliable could hang with the slightly less old but still out of date tech! :lol:

[WRXtranceformed]

Gearing makes a huge difference in acceleration.

[PGK]

Gearing makes a huge difference in acceleration.

astute.

[WRXtranceformed]

Gearing makes a huge difference in acceleration.

astute.

Indeed! And for most cars whose entire cost are less than 6 months of payments for a newer car, you may as well just buy a gear swap for the older car to make it faster =P

[PGK]

Gearing makes a huge difference in acceleration.

astute.

Indeed! And for most cars whose entire cost are less than 6 months of payments for a newer car, you may as well just buy a gear swap for the older car to make it faster =P

Brilliant!

[BLINGMW]

Gearing makes a huge difference in acceleration.

All the more reason why the m42 should have rocked my face off in this test, but it didn't. That was my point. With the same gearing, I wonder if it would have lost! hock:

[HAULN-SS]

find out for sure

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[Sijray21]

all plugs firing?

if yes, drink more beer.

[WRXtranceformed]

Or, you could just buy a new, fast car in the first place and don't fuck with old ass BMWs..... ; )

[Hunter]

Or, you could just buy a new, fast car in the first place and don't fuck with old ass BMWs..... ; )

You know people here don't speak Spanish... We can't understand you.

[JackoliciousLegs]

Or, you could just buy a new, fast car in the first place and don't fuck with old ass BMWs..... ; )

You know people here don't speak Spanish... We can't understand you.

hahahah... nice

[CaptainHenreh]

You know, talking about the gearing and the weight,

M10= 26.4 lbs/peak hp
m42=21.5 lbs/peak hp
M20=17.5 lbs/peak hp
S14=14.5 lbs/peak hp

By this scale, we should know which one is "faster", correct?

Well, we don't. A M20 325i manual will easily outrun a M3 in a straight line with a 4.10 final drive. "But this still doesn't explain it! The M42 has a lower final drive!" That's when we start looking at torque curves. Saying that "X has Y horsepower" doesn't mean shit.

Here's where it gets sticky. Horsepower is a function of torque over time. Assuming flat torque curve, the higher you rev an engine the more torque you have. So we need to take a serious look at each engine's torque:

M42: 127 lb-ft @ 4600 RPM
M10: 137 lb-ft @ 4000 RPM

The picture is a little more clear, here. The M42 has the lower gearing because it's peak torque is higher and its 16V head allows flow at higher RPM's. So to keep the RPM's there, the gearing must be lower. On the other hand, the 3.5 ratio for the M10 allows for lower engine speeds, to take advantage of the greater torque. My guess is that in a sprint to 60 (essentially what you're doing there) they should be about equal, which is what the data tells us. On a race track, however, I think the M42 would have the edge; my guess is that most of the M10's lead was made in the first couple of feet -- the higher torque, combined with the broader gears, got it ahead; leaving the M42 to play catch-up. Not to mention the extra time spent *not* accelerating by changing gears. My guess is also that uncorking the M10 a little gave it a pretty good advantage over stock, maybe enough to show the difference.

An objective, timed test would have to be done to determine it, but your results do not surprise me.

[BLINGMW]

Yay, someone's actually as bored as me to give this some thought! Thanks man.

Not to mention the extra time spent *not* accelerating by changing gears.

Just to be clear, I only did a 2nd to 3rd shift for both cars during my test. And that shift is even a touch quicker in the m42 since the gearing is closer.

BTW, where'd you find the tq figure for the m10? I've never seen anything that high, I thought it was supposed to be pretty close to the HP, like 105 or 110 or something. That's why I ignored it, I thought the torque figures for each engine were pretty close the HP. The G-tech meter I tested a while ago <!-- m --><a class="postlink" href="http://mmsports.org/forum/viewtopic.php?t=2974&highlight=gtech">http://mmsports.org/forum/viewtopic.php ... ight=gtech</a><!-- m --> told me I'm getting ~84ft/lbs to the wheels, so there's no way it's making 137 at the crank.

[CaptainHenreh]

I got it from here: <!-- m --><a class="postlink" href="http://en.wikipedia.org/wiki/BMW_M10">http://en.wikipedia.org/wiki/BMW_M10</a><!-- m -->

I mean, I realize that Wikipedia isn't infallible, but 130 lbs of torque seemed fairly reasonable, compared to the torque of the same-displacement M42.

*shrugs*

[BLINGMW]

dude, it says 103 in that article, not 130! And it's peak is 4500 just like the m42 :bootyshake:

I've seen this figure more places:

HP: 105 @ 5800
TQ: 103 @ 4500

<!-- m --><a class="postlink" href="http://www.e30tech.com/forum/viewtopic.php?t=1729">http://www.e30tech.com/forum/viewtopic.php?t=1729</a><!-- m -->

So the m10 had to start 1k off its peak while the m42 got to start just 500rpm shy.

[CaptainHenreh]

dude, it says 103 in that article, not 130! And it's peak is 4500 just like the m42 :bootyshake:

Sorry, I was looking at the 1.8 down at the bottom, which I guess is the euro engine. So, nevermind, forget what I said. Your M42 is broken. Although, if the engine did actually make 130 torque, my explanation would be sound.

[BLINGMW]

Although, if the engine did actually make 130 torque, my explanation would be sound.

true that

[CaptainHenreh]

I still think the weight might play a larger part than you're giving it credit for. Plus, having the M10 uncorked has gotta count for something.

I think this would be an ideal test application for that crazy microphone dyno thing.