M30B35 with DGEVs - What is limiting peak power?

[sreams]

Wondering what the experts thoughts are on this.

I have a 1991 M30B35 with dual Weber DGEV carbs on the old Zenith intake manifolds. I have port matched the manifolds to the intake ports on the head (B35 ports are larger). Carbs were jetted using a wideband O2 sensor and AFR Gauge. The motor pulls brilliantly.

I have noticed, and a dyno test confirms, that while the motor will go all the way to the redline, power stops increasing at about 5100-5200rpm. I have a 165rwhp peak at 5148rpm, and a 185lbft rw torque peak at about 3400rpm.

According to BMW specs, a stock, injected M30B35 has a horsepower peak at 5800rpm. If I extrapolate the hp curve from my dyno run out to 5800rpm, the rwhp peak should approach 180.

Obviously, my motor is different from a stock M30B35 since I have carbs on it.

So here is the question for the experts: What do you think is limiting power after 5100rpm?

Is it the DGAVs (which suggests Weber DGES 38s might be worth the investment)?

Or is it the old Zenith manifolds (which suggests Weber 38s would do nothing other than change throttle response)?



Edit: Forgot to mention I am running M30B35 exhaust manifolds and a custom 2.5" exhaust with free-flowing mufflers and obviously no cat.

 

[Mike Goble]

Did you use an a/f monitor for the runs?
Valve float will cause power to drop precipitously.

 

[sreams]

Did you use an a/f monitor for the runs?
Valve float will cause power to drop precipitously.

I drive around with one. The engine runs very smoothly all the way to the redline, but power starts dropping off at 5100-5200rpm. The AFM gauge shows 12-12.5:1 all the way to the redline.

-Scott

 

[jmackro]

Is it the DGAVs (which suggests Weber DGES 38s might be worth the investment)?

So are you saying that you are running 32/36's now? If so, then yea, I would guess that the engine is under carb'd. I speak from zero experience with 3.5 engines, but given that venturi area goes up with the square of the carburetor diameter, moving from 32 & 36 to 38 & 38 represents a big increase.

 

[sfdon]

the pump is fine
Intake and exhaust are restricting?
That curve looks too steep.

 

[Stevehose]

Your Webers have smaller venturis than Zeniths and having owned a coupe with DGAV's I definitely think they restict the engine.

You don't appear to be leaning out at higher revs so maybe you're running out of air flow.

Perhaps look into 38's for the stock manifolds (or triples) if all else checks out.

 

[sreams]

the pump is fine
Intake and exhaust are restricting?
That curve looks too steep.

Fuel pump is fine. 42gph electric. Replaced recently.

I don't believe exhaust is restricting. M30B35 exhaust manifolds with 2.5" system after it. Mufflers are straight-through Magnaflows.

I do wonder if the old Zenith intake manifolds are insufficient for a 3.5...

The hp curve looks nearly identical to other M30 charts I've seen apart from the early fall-off after 5100rpm.

http://www.mye28.com/viewtopic.php?t=62090

If you scroll down a bit, naturally aspirated M30s seem to be at about half their peak horsepower at 2500rpm. That's what my chart is showing as well. The steepness could be attributed to the scale of the graph.

-Scott

 

[sfdon]

The pump is your engine
Exhaust sounds good.
Got a chart of afr and a matrix of hp/rpm?
Engine timing would be a guess- hot off the line and dead at top is a symptom of 10 degrees retard. One tooth maybe?

 

[Honolulu]

What does manifold vacuum do as you run through the rpm range? This could offer a clue about what's going on...

 

[sreams]

The pump is your engine
Exhaust sounds good.
Got a chart of afr and a matrix of hp/rpm?
Engine timing would be a guess- hot off the line and dead at top is a symptom of 10 degrees retard. One tooth maybe?

Good question... will check that.

 

[sreams]

I haven't had a chance to check cam timing yet. I should have a chance tomorrow morning.

Entertaining the idea of the 32/36 carb possibly being inadequate for an M30B35, I came across the attached chart, which is interesting. It suggests main venturi diameter for weber 2-barrel carbs based on individual cylinder displacement. The original 3.0L M30 would have had a single cylinder displacement of 498cc, and at 6000rpm, the graph suggests a main venturi of about 37mm in that case. The M30B35 has a single cylinder displacement of 571cc. The graph would suggest a 39mm main venturi for it.

The graph assumes synchronous carbs with matched-sized main venturis, so the 32/36 progressive DGV doesn't fit neatly into the graph. That said, the graph suggests that it is not enough for the B35's cylinders, and that a Weber 38 may be just enough.

 

[Mike Goble]

If you want a venturi size of 37mm you're going to have to have throttle diameter of 45mm or better. 38/38's aren't anywhere close to that.

I dyno tested a 331" SBC using a Holley 4412 2-bbl followed by a Holley 4779 4-bbl, ( essentially two 4412 2-bbl's in one body ) and the peak rear wheel power increased considerably, from 354 to 399, but the peak power rpm remained fairly close, moving up from 6700 rpm to 6900 rpm. The power does not drop off as precipitously as your graph shows, it simply flattens the curve and above the peak power it drops a little faster.

 

[jmackro]

I came across the attached chart, which is interesting. It suggests main venturi diameter for weber 2-barrel carbs based on individual cylinder displacement.

Yes, I applied a little of that same reasoning before replying yesterday.

The formula to calculate DCOE venturi diameter that I used is:

Venturi diameter = [single cyl displacement (cc) X max rpm / 2500 ] ^ 1/2 (this is probably the formula that generated the chart shown in post #11).

Plugging in your numbers gives: [ 575 cc X 5800 rpm / 2500 ] ^ 1/2 = 36.5 mm

Then the butterfly needs to be around 1-1/4 X the venturi diameter, or 45.6 mm in the above case. So if you are running DCOE's on a BMW 3.5, 45mm carbs with 36mm chokes should support a top engine speed of 5,800 rpm.

But with 38/38's you have four chokes per six cylinders; not the one choke per cylinder that the above formula assumes. And of course, you would need 6/4ths the choke area, not 6/4ths the diameter. Plus, it isn't clear that conversion would be as simple as multiplying the diameter by the square root of 1.5, since the 38/38 manifold shares two chokes across three cylinders, while the DCEO manifold is 1:1 between chokes and cylinders.

 

[sreams]

Yeah... it turns out the webpage I pulled that graph from had it wrong as to what it was indicating. It's clearly for choosing the right DCOE setup, where you have a single throat going to a single cylinder. I was also thinking carb throat size when the chart was referring to venturi size.

-Scott

 

[61porsche]

Progressive

carbs..... vs 38s. I suggest doing the sync conversion kit. That's the main difference and reason to change until recently when somebody figured it out. Cheap and you're going to notice it down low. But gas mileage is going down.

The vacuum secondary's open when they open, not optimum on a dyno run but great for gas mileage. Meaning there's actually a way to tune that based on what you're doing. Could be the reason you're all in a bit early. There's a trade off though on a driven street car. Even Weber doesn't say there's a big difference unless you're modified.

VE- no internal combustion engine is 100% efficient. If new or rebuilt right, maybe 75% on carb selection.

Driveline losses on dynos are only one part of the equation. There's not enough info on the F/A, set -up to get real specific.

Timing should be optimized.



Stock manifolds should be good for 230hp. Been proven by tuners back in the day on worse heads than a B35.

 

[sreams]

carbs..... vs 38s. I suggest doing the sync conversion kit. That's the main difference and reason to change until recently when somebody figured it out. Cheap and you're going to notice it down low. But gas mileage is going down.

The vacuum secondary's open when they open, not optimum on a dyno run but great for gas mileage.

Yeah... but the secondaries are mechanical on a DGV. Not Vacuum. They start opening when the pedal is pushed about 2/3rds of the way down. On a dyno run, there should be no difference between a progressive DGV and one modified to synchronous, because with the throttle pressed all the way down, primaries and secondaries are all the way open in both cases.

I actually like the throttle progression with the DGV setup. I think a synchronous setup might be too much too fast, and may take away from control at smaller openings.

I have a brand new 38 sitting here, and I've been considering moving parts over from one of my spare DGVs to convert it from synchronous to progressive, as backwards as that sounds. I think that would give the best of both worlds.

Stock manifolds should be good for 230hp. Been proven by tuners back in the day on worse heads than a B35.

That is good to know.

-Scott

 

[m5bb]

The pump is your engine
Exhaust sounds good.
Got a chart of afr and a matrix of hp/rpm?
Engine timing would be a guess- hot off the line and dead at top is a symptom of 10 degrees retard. One tooth maybe?

I'd go for a few more degrees of timing. M30 motors like up to 34+- degrees at high RPM.
May be your distributor is not moving the weights out to increase the timing to what it should be.
You can always advance the distributor at idle until the motor starts to run poorly and back off a little and then try that.

 

[sreams]

I'd go for a few more degrees of timing. M30 motors like up to 34+- degrees at high RPM.
May be your distributor is not moving the weights out to increase the timing to what it should be.
You can always advance the distributor at idle until the motor starts to run poorly and back off a little and then try that.

Good thought... but just checked and I'm at 35 degrees all in. Verified at the timing marks at the front and rear of the motor.

-S