Dyno graph
 

Here's a scan of the dyno graph.

<img SRC="http://www.holscope.com/74Carrera2.7/dyno_readout.jpg" height=624 width=787>

The car will be in the hands of my mechanic next Monday, so by this time next week we'll know exactly why this engine is not developing the power it <i>should</i> be developing.


Matt Holcomb
MFIWDP
<b>1974 911 Carrera 2.7 (RoW)</b><a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank"></a>
<a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank">Dedicated Homepage</a>
<a href="http://www.holscope.com/74Carrera2.7/rebuild" TARGET="_blank">Engine Rebuild Homepage</a>
<a href="http://www.pelicanparts.com/cgi-bin/pmpre/pm.cgi?action=display&login=Matt_Holcomb" TARGET="_blank">Pelican Gallery Profile</a>

So is that a ~94kw peak?

Something ain't right, MFI 2.7 was 155kw stock, and yours isn't stock, is it?

Tom

Tom,

It's a bone stock 911/83 RS engine except for the 11.3:1 JE pistons and the GE-60 cams.

Something is very amiss.


Matt Holcomb
MFIWDP
<b>1974 911 Carrera 2.7 (RoW)</b><a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank"></a>
<a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank">Dedicated Homepage</a>
<a href="http://www.holscope.com/74Carrera2.7/rebuild" TARGET="_blank">Engine Rebuild Homepage</a>
<a href="http://www.pelicanparts.com/cgi-bin/pmpre/pm.cgi?action=display&login=Matt_Holcomb" TARGET="_blank">Pelican Gallery Profile</a>

That curve looks a little ragged.

I think a valve-adjustment will help.

Hmm...my gut response would be that's what you get when you are persistant in wanting to keep that MFI crapola. I know, i know...."but it's got better response, fuel pressure is higher than in EFI which helps atomize the fuel better and charge flux capacitor yadda yadda"....Junk that crap out of car, sell it to a purist and get yourself a programmable EFI and ignition.

It's pity you cannot see A/F ratio in your papers but i bet it looks really weird. We have too little info but it wouldn't surprize me if you have fuel delivery problems, as motor seems to run relativly well, besides drop in power. It's just not worth it fiddling with those space-cams ... expensive too.


Cheers!

Ahem,
BeepBeep, Matt *is* a purist...
GeorgeK

Goran,

I know for a fact that the injectors are marginal. That's what I'm putting my money on.

The pump was completely rebuilt -- with a new space-cam -- last year.

And yes, George, I am a purist!


Matt Holcomb
MFIWDP
<b>1974 911 Carrera 2.7 (RoW)</b><a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank"></a>
<a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank">Dedicated Homepage</a>
<a href="http://www.holscope.com/74Carrera2.7/rebuild" TARGET="_blank">Engine Rebuild Homepage</a>
<a href="http://www.pelicanparts.com/cgi-bin/pmpre/pm.cgi?action=display&login=Matt_Holcomb" TARGET="_blank">Pelican Gallery Profile</a>

Matt:

Most dyno shops have wideband probe that can be inserted into exhaust so you can get plotted A/F graph. I would look at that and ignition timing, as very retarded ignition will yield same sort of problems you have: low power on otherwise well-functioning engine. As far as i remember, you had problems with engine-knock which forced you to retard timing. Is it maybe retarded too much? Or are your high C/R pistons just too much, so you cannot run optimal ignition w/o knock?

I suggest looking at A/F ratio, fixing that, then fidling with ignition timing in order to find sweet spot.

I don't remember if you had dual plugs but with C/R that high, i bet it would help a lot.

Don't take my comments about MFI too hard, but it hurts to see people going to all sorts of trouble just to keep it...it's old, it's bad and it's obsolete.

Good luck!

Matt are you twin-plugged? What's your total timing advance? Those spikes in the graph can indicate either detonation, or tire-to-roller traction/vibration issues. If you are twin-plugged, try running 22-24 degrees total advance.

And definitely find out what the air/fuel ratio is on the dyno. Static readings are almost meaningless with MFI, since MFI can't sense load. The CO readings are quite different once you put it on a dyno.

Stupid question -- am I reading the chart wrong, or did the engine never rev higher than 3600 rpm?

I believe that's 3600 newtons, a value of torque. Rpm is always an x plane value.

I'm certainly no expert, and I'll admit up front that modern EFI systems are hard to beat. Now...having said that, I'll also bet that a properly tuned MFI system, with proper space cam, etc., would be JUST AS HARD TO BEAT, and impossible to beat in the area of throttle response.

11:1, using pump gas and single plug would probably perform rather poorly because you would need to retard timing so much that the cylinders would not be loaded with pressure right after TDC, like they're supposed to. Race gas/aviation fuel -and/or- twin plugging would be necessary for this engine, I would think.

But let me remind you of one important thing. I am not an expert.

The x-axis is speed on that particlular graph. So the run went to about 101mph.

You would loose that bet. EFI with Alpha-N input will give you instant response. No lag whatsoever...(couple of microseconds maybe)

Alpha-N: throttle position sensor...as soon as it starts moving, more fuel gets squirted in. Even better, EFI will measure rate of movement and squirt additional enrichment fuel for even better response when throttle moves fast, which MFI cannot do.

The too high compression issue is pretty easy to check I reckon. Just bung in some race gas and put the timing where it belongs. Tyson's concern about detonation would be a concern to me.

Is full pedal throttle opening the butterflies to full throttle (and the MFI pump)? It is just way too much power loss...

(Oh, and what Superman said applies to me too - I'm not an expert either).

even as a comp. sci major I'd think it would be hard to beat the mechanical linkage of the MFI.

But in the end I"m willing to bet that the extra fuel is squirted on the very next intake stroke for both MFI and for Alpha-N

man beep-beep isn't going to rest until he rids the world of MFI, CIS and carbs :P

Goran,

MFI is a great system; I'm going to persist with it. Sure, EFI has its advantages, but not enough of them to make me want to convert to EFI.


Tyson,

I only have one spark plug per cylinder. From memory, my total timing advance has been set to 27 or 28.

We'll be checking the a/f ratio next week.


Superman,

I'd have it twin-plugged right now if I had the money.


Cam,

The problem is, AVGAS has been banned in the Porsche Club of Victoria. This engine has to run -- and run well -- on 98 RON fuel.


Matt Holcomb
MFIWDP
<b>1974 911 Carrera 2.7 (RoW)</b><a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank"></a>
<a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank">Dedicated Homepage</a>
<a href="http://www.holscope.com/74Carrera2.7/rebuild" TARGET="_blank">Engine Rebuild Homepage</a>
<a href="http://www.pelicanparts.com/cgi-bin/pmpre/pm.cgi?action=display&login=Matt_Holcomb" TARGET="_blank">Pelican Gallery Profile</a>

Cam,

The problem is, AVGAS has been banned in the Porsche Club of Victoria. This engine has to run -- and run well -- on 98 RON fuel.

Matt - I mean as a once only thing, to try and figure out if the compression/advance issue is the limiting factor. I figure it costs only as much as a few litres of avgas and retiming the dizzy. It doesn't provide a long term solution - just a cheap troubleshoot.

Cam,

Gotcha. It's a good idea. Thanks for your interest in my latest car crisis!


Matt Holcomb
MFIWDP
<b>1974 911 Carrera 2.7 (RoW)</b><a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank"></a>
<a href="http://www.holscope.com/74Carrera2.7" TARGET="_blank">Dedicated Homepage</a>
<a href="http://www.holscope.com/74Carrera2.7/rebuild" TARGET="_blank">Engine Rebuild Homepage</a>
<a href="http://www.pelicanparts.com/cgi-bin/pmpre/pm.cgi?action=display&login=Matt_Holcomb" TARGET="_blank">Pelican Gallery Profile</a>