Based on the baseline dyno charts posted elsewhere in this forum, it's clear to see that the M112 engine has a beautifully linear torque curve which drops off at 4750rpm. What this means is that the engine basically puts out about 10hp in the last 1250 rpm of revs.

If I take a straight edge and make the torque curve flat to 6000rpm, the engine's projected horsepower should be about 40hp more than stock, putting the car at around 260hp instead of 219hp.

So my question is, why does the torque fall off at high rpms, given that the M112 has a fancy dual-length intake manifold? Is it the limitation of the SOHC, 18-valve design? Cam profiles?

I'm a turbo guy, not an all-motor guy. Anyone have any ideas?

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Correction, instead of 260hp, it should be 270hp. I forgot to take into account the 20% drivetrain loss.

 

My guess would be the target demographic of this car. They designed it to be short-shifted and driven in a more leisurely manner than a hard-core sports car. If you'll notice, you have 80% of full torque available at only 3,000 RPM. That puts the enegine right in the meat of it's powerband in the magic 40-80 MPH range (with correct gear selection, of course) of sporty driving.

 

Can you put a link to the charts, seems strange the drop occurs just when the traction control kick in.

The dual length intakes help power low down, at higher revs the are "switched" out, I thing HDDP had some photos showing the arrangemment. Two inlet and one exhaust valve might be hurting the VE at higher rev.

 

Thanks malcb. I took a look again and the original dyno sheet I quoted is from ELOW. I didn't know that TC cuts in at 4750rpm.

I took a look again in the archives and I found a new dyno sheet from x'd. His is similar to elow's, and according to his post, the shop helped him disable the TC. It leads me to believe that it's something to do with the cams.

https://www.crossfireforum.org/forum...ad.php?t=13786

 

There are several things choking this engine at the upper RPM range. But the major component that gives a graph it's shape is the cams. These are fairly small cams for good low end torque and to make to car feel faster than it is. The timing of the cams can be altered (advanced) to move the curve upwards a bit but really it's the small cam with very little overlap that is causing this. It's great for emissions and low end torque but hurt's high end HP.

The other two factors are the absolutely crappy exhaust manifolds coupled with the single exhaust valve, they are going to be restrictive at the upper rpm range where the engine is trying to push a large volume of air.

The next factor is going to be the intake manifold. A dual runner manifold like we have is a comprimize, the long runners aren't long enough for a true torque monster, and the short runners aren't optimized for high end HP.

Also tuning has a lot to do with it. The ignition advance is severly retarded (as in late during the combustion cycle) on this engine to move the flame front to the end of the combustion process to increase EGT's, and thereby light off the front cats sooner, to (again) lower emissions. I believe I read in MB tech specs on this engine that they retarded the ignition timing 10deg across the board to do this. They were able to retard the timing that much and still get complete burn by having the dual spark plug setup, with a single plug setup they said they couldn't have done this. (also, the next factor is going to limit the amount of advance you can put back into the tuning)

The last factor is the absolutely tiny fuel injectors on this car. They are speced very small (19lb), and the reason being is because this car is spec'd to run a very lean mixture (someing they can do, again, because of the dual spark system). The lean mixture helps to raise EGT's and again light off the cats faster...as you see another emissions concern.

Go to http://www.rceng.com/technical.aspx and you can put in 240hp (our guestimated crank HP), our BSFC (good guess of .45), 6 injectors, 56psi, and .80 max duty cycle and see it comes up with a 19.xxlbs injector. We really don't have the fuel available to go higher safer without larger injectors. If you increase the airflow to the engine and thereby the HP all we are doing is running our injector's past .80 duty cycle, and at that point most injectors spray pattern is getting wonky and the injector will wear out faster causing leaks and the possibility of one sticking open.

If we got more fuel we could run more ignition advance and gain a bit of power there, but the cams/intake/exhaust will be the real limiting factors in the upper RPM range. You can try www.crower.com ($920 for our cams + the cost of a set of factory fresh cams from MB) or www.webcamshafts.com (unknown cost, but they have a pn for MB of 42-020 for MB V6 cams) for a set of reasonably priced custom re-grinds (re-grinds are getting better and better and I wouldn't really be that concerned about them any more). Also, a REALLY good custom shop can build you a set of headers and an intake manifold (I got a quote of $1800-2500 for headers from a shop I had look at our manifolds). And lastly you would really want your exhaust ports fully worked to increase flow.

All that should bring this engine on par with an early vq35de (350z) engine and put you at around 280ish crank HP. But you're talking about lots of money to get there. And the limiting factor is going to be your tuning, these Bosch ECM's are tough to crack. A few companies claim to be able to tune them but I have yet to see definitive results...just claims.

So as you can see all the factors hurting the HP and torque are because of engineering trade-offs to lower the emissions. There's a lot of things coming together to rob this engine of power... But the damn thing sure does burn clean!!!!

 

Just my 2 cents... MALCB & DYEZAK hit one of the nails on the head... The flow is choked on the exhaust side...

PS: DYEZAK The NA injectors are not 19lb

NA ENGINE: 17 lbs/hr at 3.8 Bar, or 55 psi/ 14.5 ohms.
SC ENGINE: 51.6 lbs/hr @ 55.1 PSI / 14.5 Ohms based on 380 kPA = 3.8 bars = 55.1 PSI

 

HDDP,

I was trying to remember your reasearch off the top of my head.... The injectors are even smaller than I remember. At 17lbs we are running them well into 90% duty cycle stock then.......

EDIT: 92% duty cycle is what I just came up with

 

Ya gotta remember, the M112 was M-Benz first all-new modern V6. It was to be used in a wide range of models including sedans, wagons and SUVs weighing 1000-1500 Lbs. more than the lil 'ole Crossfire. It's a bread-n-butter all purpose motor designed to run long, clean and economically in a wide variety of everyday drivers. It never was supposed to be a high-winding sports car motor. It's not a finicky screamer, its designed for German luxury cars !

Slim

 

Thanks dyezak, hddp, malcb, ccslim. Great info

 

Based on my conversations with Stan Weiss, he determined that the specs of the NA engine HP etc. only require 14lbs. at 100% duty cycle. So your calcs seem to be correct...

EDIT. I posted the Bosch specs for both NA & SC injectors somewhere on the forum

 

just curious, but how many thousanths of an Inch of lift on the intake side of the cam? and what is the rocker ratio?

anychance we can get higher ratio rocker arms instead of cam swaps?

 

Thanks for the info dyezak. I did replace the tiny injector for new ones (19lb/hrs from TVT) added more air to engine bia bigger airbox (c-class) and retuned the ECU (LET Motorsports) to keep that AFR in check. I went to the dyno (speed industry, Troy, MI) to get results. The Stability control got on our way, so only AFR values were obtained up to 3500rpm. OEM claims 14.7 AFR and we were lingering around this value in our test. But it’s obviously partial.

Regardless of all of the factors here listed hurting HP it’s my personal experience that injectors, more air and ECU reflashed provided more HP. It’s over a month with the mod’s and a joyous car to drive.

“If you increase the airflow to the engine and thereby the HP all we are doing is running our injector's past .80 duty cycle,” I’m quite ignorant on mechanics, but isn’t the ECU supposed to prevent injectors to max-out past that 80% duty cycle hence the need, among other reasons, of a ECU retune when swapping injectors?


Respectfully.

 

you guys may also be suffering from internal turbulance inside the variable manifold, this could be the real cause of power drop off, there are some mods that can be done to the shorter runners to improve upper rpm flow.