10.5 without twin plugging.

[dipso]

My 2.7 is in the shop. Here is what I currently have. Runs fine, just old.
2.7R case, 40 mm PMOs, MSD, Elephant Big Mouth oil cooler, JB Racing 6 plug adjustable advance distributor, LSD differential, fairly new tranny.
The engine runs very cool with the oil cooler. SSI's.

Here is what we are adding.
JE 10.5 pistons in existing Nikasil cylinders, new rod bearings and rods, reworking heads, valves, springs guides etc, #402 Webcams. I think this is it.Web Cam Inc. - Performance and Racing Camshafts / Porsche 911 SOHC 12v Timing Card
I am sure there are a few things more but that is all I can think of right now. He is a trusted mechanic.

I told my mechanic that 9.5 is what I have read one should do for single plug. He insists that 10.5 is fine with the PMOs. The adjustable advance distributor can be used to find the right spot for timing. The camshafts he is choosing will take care of any air flow problems. He thinks we might have to change the MSD to programmable MSD to tune things, if necessary.
I live in California with 91 pump premium being the highest octane available. The car is mostly a weekend toy and not a daily driver. There are no smog requirements for my car, where I live.

Does this configuration sound like it will work.
I am using 90mm cylinders and personally don't see the issue with flame travel across 90mm. I understand with larger bores twin plugging is necessary.

I don't want to spend the money if I don't have to for a twin plug setup.

[Flieger]

High compression pistons have a high dome and do more harm to flame travel than a big bore, flat top piston. I personally would not go higher than 9.5 with single plug fuel. By retarding the timing you are giving up a lot of power and stressing the components more. Go with the 9.5 compression and proper timing.

[304065]

It won't work.

10,5 to 1 pistons have a large enough dome that the combustion chamber is divided in half, inhibiting flame propogation. The factory figured this out in the '60s. The fact that you are using California 91 makes it even worse.

It does not make sense that someone would propose to use a programmable MSD box with a distributor. A programmable ignition box is only usable when you have an independent trigger for the box, like a crank trigger. The reason why is, the distributor is not locked: the rotor position changes with centrifugal advance. Since the rotor position changes, the ignition trigger point also changes. You cannot have the distributor doing one advance curve and then a box doing another one without a non-changing reference signal.

That particular cam has pretty high lift and duration for street use. The 101 LC will result in higher dynamic compression than the stock 2,7's 110, further compounding your detonation problem.

[Steve@Rennsport]

Flieger & Professor Cramer are both spot-on.

You could use 10.5:1 on 91, however the resultant safe timing curve results in a lethargic, hot running engine and this isn't anything I'd ever do to a customer.

Set it up at 9.5:1 so you can dial in reasonable timing for best throttle response and cooler running.

Your current MSD is fine; no need to spend any more money on a component that you do not need (or want) for your application.

[dipso]

Nothing is assembled yet, in fact I don't even think the heads are back yet.
I think they are due back in a few days.

My options are still fairly open. I am very nervous about the 10.5 and single plug.
But I am not totally opposed to adding twin plug either. I am imagining about an additional 2000.00 in costs? Maybe 2500.00.

I have 2 choices at this point.
A. 9.5 pistons
B.10.5 pistons and dual plug it.

Is a 10.5 twin plug a 'have to have' machine. Or should I just save a few bucks and go with the 9.5?

[Steve@Rennsport]

Originally Posted by dipso

Is a 10.5 twin plug a 'have to have' machine. Or should I just save a few bucks and go with the 9.5?

There is a big difference in throttle response, torque & HP when adding a point of CR & twin-ignition.

The issue is really about your budget,..............

[cstreit]

Originally Posted by Steve@Rennsport

The issue is really about your budget,..............

Ain't it always?

Dipso, I think the point is while you might even theoretically get away with it, there's just no advantage to doing it really. As Steve said, a lethargic engine as a result? So why?

You're better off twin plugging or lowering the compression...

[911pcars]

I agree with the others. In its present configuration, increasing CR isn't conducive with the combustion chamber design and piston crown.

Steve, Has anyone tried reshaping the chamber to create a wedge-shaped squish area using flat top pistons?

Sherwood

[dipso]

All right guys, thanks.
I'll look into costs for twin plugging or drop the CR to 9.5. The 9.5 and 10.5 are both stock sizes and JE is just around the corner, so I will figure out something.
I'll have to see what my total is so far for what he is planning and then look into the cost for twin plug.

Was my guess about right as far as cost goes?

[Walt Fricke]

If you twin plug, have it made for 12mm plugs. You'll want that for plug wrench clearance. I'd also use flange nuts instead of the stock barrel nuts on your head studs. You need clearance. Assuming you aren't springing for the Supertec studs.

Electromotive waste spark ignitions ought to be available fairly inexpensively used as guys go from this rather simple multiple coil ignition system to programmable EFI. A pair of them work fine for twin plugging.

[sjf911]

Originally Posted by dipso

All right guys, thanks.
I'll look into costs for twin plugging or drop the CR to 9.5. The 9.5 and 10.5 are both stock sizes and JE is just around the corner, so I will figure out something.
I'll have to see what my total is so far for what he is planning and then look into the cost for twin plug.

Was my guess about right as far as cost goes?

If you want to think outside the box, I'll happily donate (for shipping cost) all of my EDIS parts that I removed from my car when I went to MS3X with sequential (if I can find them, I may have tossed them). It includes an AC crank pulley (double by GSF) with 36-1 wheel designed to fit the 3.2L or later 930, Ford VR sensor, 2 EDIS-6 controllers, 2 EDIS-6 coils and, I believe, the related wiring connectors. You would only have to source your plug wires. You would then have electronic ignition with your carb's. The crank pulley likely wouldn't fit your engine mount without the AC pulley being machined off.

Modify heads: +/- $450
Microsquirt ECU $350 + external MAP sensor (450 for a MSII which has an internal MAP sensor)
Vacuum manifold to convert carb's/throttle bodies for MAP signal: ??
Twin EDIS-6 controllers: free
Twin EDIS-6 coils: free
36-1 wheel: free
Crank VR sensor bracket: $90 at Clewett, you might get it cheaper at GSF.
Crank VR sensor (Ford): free
Spark plug wires: ??

[dipso]

Thanks SJ, but I am not sure what I am going to do just yet. I put a call into the shop and I guess I will figure this thing out tomorrow.
Either way, I don't think I will need what you are offering.
Thanks for offering though, that is very cool.

[Steve@Rennsport]

Originally Posted by 911pcars

Steve, Has anyone tried reshaping the chamber to create a wedge-shaped squish area using flat top pistons?

Sherwood

Oh yes,....

Mahle did just that with the so-called Max Moritz 98mm P/C's that safely gives 9.7:1 on single ignition as long as one pays careful attention to deck height.

I've not seen anyone do this with 95mm and smaller pistons since most folks simply match CR with available fuel. I would have CP make custom sets for me if there was a demand.

The factory did similar things with (air-cooled) 962 heads since twin-ignition wasn't legal so they used a modified (closed) combustion chamber with a flame trench that worked very well. I've done similar things that have been successful on race engines using very high compression ratios.

[Henry Schmidt]

Originally Posted by 911pcars

I agree with the others. In its present configuration, increasing CR isn't conducive with the combustion chamber design and piston crown.

Steve, Has anyone tried reshaping the chamber to create a wedge-shaped squish area using flat top pistons?

Sherwood

We built this engine last year.
Peanut chamber heads were fabricated form standard 2.2 "T" heads.
The pistons were a set of 8.5:1 Mahle MFI 90mm. (RS)
With .050" deck we produced 11.3:1 compression.

[Jeff Jagusch]

Pm sent

[haycait911]

Originally Posted by 304065

It won't work.

10,5 to 1 pistons have a large enough dome that the combustion chamber is divided in half, inhibiting flame propogation. The factory figured this out in the '60s. The fact that you are using California 91 makes it even worse.

It does not make sense that someone would propose to use a programmable MSD box with a distributor. A programmable ignition box is only usable when you have an independent trigger for the box, like a crank trigger. The reason why is, the distributor is not locked: the rotor position changes with centrifugal advance. Since the rotor position changes, the ignition trigger point also changes. You cannot have the distributor doing one advance curve and then a box doing another one without a non-changing reference signal.

That particular cam has pretty high lift and duration for street use. The 101 LC will result in higher dynamic compression than the stock 2,7's 110, further compounding your detonation problem.

on my 3.4 I'll be using an MSD6AL-2 programmable. my thought was to lock the distributor and use the MSD to set the timing curve. it's strictly fortuitous that the MSD also has a boost/nitrous retard feature.

[304065]

Originally Posted by haycait911

on my 3.4 I'll be using an MSD6AL-2 programmable. my thought was to lock the distributor and use the MSD to set the timing curve. it's strictly fortuitous that the MSD also has a boost/nitrous retard feature.

Won't work, the factory included rotor phasing for a good reason. Would be better to do coil-on-plug.

[911pcars]

Originally Posted by 304065

Won't work, the factory included rotor phasing for a good reason. Would be better to do coil-on-plug.

"Rotor phasing" was discussed at length in another thread. Suggest that be reviewed.

Granted, coil-on-plug ignition systems would be most accurate. However, the rotor on legacy 911s (66-90's and maybe beyond) advances ahead of the "idle" location pretty much in synch with the advance curve in order to be in close proximity to the contact in the distributor cap.

One should remember that 30º of crank advance is but 15º of rotational difference compared to distributor rotation (15º isn't much). With the normal broad-tipped rotor and the factory centrifugal weights, this should be more than adequate to roughly replicate any variation in a custom timing advance. If there's any doubt, one can confirm by removing a section of the dist. cap, then with the advance curve as reference, observe the rotor location as the distributor shaft speed increases (perform this on a dist. machine or equivalent).

S

[haycait911]

Originally Posted by 304065

Won't work, the factory included rotor phasing for a good reason. Would be better to do coil-on-plug.

what's 'rotor phasing' ? my intention was to use an old 2.4 dist, open it up, lock the rotor to the shaft in method yet to be determined, and go to it. no?

couldn't I just do the above, then use a timing light to set the timing at zero. wouldn't I be 'in-phase' then?

[304065]

Rotor phasing (not sure why Sherwood suggests that I review that thread, I am one of its authors ) is the use of the centrifugal advance weights to move the rotor tip under the appropriate distributor cap terminal at the time of firing. In older cars where spark timing is established by the movement of the rotor, these things occur together. In newer cars where a crank trigger signals the DME and the DME fires the coil, even though the advance mechanism doesn't determine the timing, it's still present to position the rotor approximately in the right place. The factory felt this was sufficiently important to put it in the workshop manual.

Therefore, locking a 2,4 distributor won't work, because the spark will either jump to the wrong terminal, or have to jump a larger or smaller gap resulting in inconsistent ignition performance, just when you don't want it, at high RPM under load.

The only way you would be in phase with a locked distributor would be if you ran at a constant RPM (like one of those German Air Force stationary 356 engines! )