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>
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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>
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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>
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98 RON equates to about 93 pump in North America. With the CR at 11.3:1, this is a bit aggressive, but the largish cams help to reduce this. With total advance under 30 degrees, your probably safe, but at the same time, leaving power on the table without more octane or twin plug.

Judging by the general appearance of the graph, I would guess the mixture is way off. I'll guess lean (as in lean of stoich), but it could also be pig rich.

What do the plugs look like?

You really need to get a wideband O2 on there and confirm the ignition timing.

Matt

We will all say a little prayer to the gods of air/fuel ratio to get you through the next few days. From what I have heard of John G's work - the base engine will be perfect so that means its simply a tuning issue.

My (now departed) 70 911E would not run to redline at all when i first got it - one MFI tune up later and it was transformed.

Cheers - Ryan

ps: this probably goes without saying but i would minimize driving it until you know whats going on as either too lean or too rich can cause damage.

Matt, if I remember correctly you had the injectors tested and told they were "marginal" but not in what sense they were marginal? Is that correct?
Did John Gregory tell you anything about the engine other than it was on the very upper limit of single plugs? Perhaps there is a secret to be discovered there.

That really is too high for single plug on 93 (our equivelent) octane. But what I was worried about in my original post was that you may have been running more advance than necessary with twin-plug.

But since that isn't the case, then it becomes an issue of simply setting the timing where it won't detonate, and getting the fuel mixture set as close to 13/1 under load, and pay particular attention to what the mixture is at the top-end. You don't want to melt a piston from a high rpm lean condition.

I guess beep ... coming from the real world of turbo lag ... is blind to the lag in autofocus camera's shutters, EFI, and other state-of-the-art dsp application that are not quite instantaneous! The real-world response is a delay in EFI response is in the <b><i>HUNDREDS of milliseconds, not MICROSECONDS!!!</B></I>

That is only a difference of three orders of magnitude!

ps ... I assume the 'N' on the right axis means Newton-meters ... there does seem to be a missing decimal place, though.

Matt,

Well, when I last saw the plugs, I had a weird (gut) reaction to them. I didn't say anything because I assumed they had to be okay if my mechanic was putting them back in the engine.

It'll be interesting to see what they look like next week …


Ryan,

What's your opinion of the compression ratio in relation to the ignition system? Would you have questioned John? Or would you have also deferred to his experience?


Klaus,

I'll be having the injectors tested next week. I guess we'll soon know what Gus meant when he said "marginal".


Tyson,

If the car is developing the power it should be after next week's tune up, should I still be concerned about the compression ratio? What you would you if you were in my shoes? I currently lack the funds to have it twin-plugged or to take a compression point off the pistons. But if I did have the funds, I'd probably seek legal advice before shelling out the dough.


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>

162 kph What gear were you using? 3rd or 4th?

It is a bit hard to tell to much until you get the AFR.

I have had my car on three different Dyno within weeks of each other and the HP varied by about 60hp at the rear wheel. From 101 to 160, the real figure at the flywheel is 152, I guess it has a lot to do with the set-up of the dyno.

Does the car pull well when you compare it to a SC or a 3.2? Or is it off song?

Matt

most of the stuff i've read indicates that about 10.5:1 is the top comp ratio for single plug/high quality pump gas. but, to some extent, pinging can be controlled by ignition advance (but at the expense of some power).

did the dyno guy say it was detonating/pinging? - its pretty noticeable and they should be on the look out for it

high lift cams reduce the effective comp ratio somewhat too (i understand that, in terms of increasing lift, you have S, GE60, GE80, RSR) - so that is also on your side

if jg thought your thing would run well as is on good pump gas then, given his rep, i would be inclined to agree

i would get the thing re-tuned, check everything including the a/f ratio, then re dyno before you think of getting back into the engine

maybe consider having a fresh set of eyes look at it??

Ryan

ps: is there any chance you could have a tank full of crap gas?? the no name service stations in nsw sell some horrible mixtures...

pps: the 911E i mentioned above was 'tuned' by 2 well known nsw shops are never ran well - the 3rd guy took the time to get it right (purely settings no new parts) and has had all my work ever since

ps: just called my guy - no real diagnosis possible without the a/f curve

Krikey mate, stone the crows.

Was it running like that at Calder Park Sprints?????

Ryan,

Have you shown your mechanic the dyno graph?


Victor,

Look, it probably was -- I haven't done many k's since the Calder Sprints, and it feels the same to drive as it did then.

Given the car's (theoretical) power-to-weight ratio, I thought that a <b>1m 13s</b> at Calder was a little on the slow side.

Paul Stuart can lap Calder in <b>1m 08s</b> in his Carrera 2.7 -- at 8 or 9/10ths.


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
94kw at the wheels, using 1.31 which PCV uses, translates to 123kw at the flywheel.

I have a chart showing my 3.2 doing 107kw at the wheels, which is 140 at the wheels, handbook says 152kw.

So what are you looking for? Lucky its light.....

stuart

Nope - was on the phone to him on another matter (the high comp 7R MFI engine that I'm building actually...) and described it. a/f ratio first thing to check.

Stuart,

I was expecting around 160-170kW at the flywheel. With measly 8.5:1 pistons, these engines are good for 155kW at the flywheel.


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>