twin plugging question

[emcon5]

Originally posted by Grady Clay
Can someone post a photo of a 98 mm RSR piston, new and used? Photos of the combustion chambers (stock and RSR) would be useful also.

The second piston in my post is a new 98mm RSR. I don't have a photo of a used one.

82 SC combustion chambers here:




Tom

[Bill Verburg]

Here are some
my 993RSR Mahles


2.7RS Mahle


Stock 993


98mm Mahle & JE


MAx Morit 98mm


90mm JE RS

[rdane]

'83 SC heads with twin plugs And a Max Moritz 98 with the valve relief recut deeper (with a bit of damage).

New 98s Max Moritz with coatings

[rdane]

by Grady Clay:
Another significant difference between the two pistons is the one on the right is a “slipper skirt” racing or "S" type. Note the lack of piston below the wrist pin. I think this is a piston designed for CIS and certainly single plug as the combustion chamber is off-set to the right.

Grady, good eye and good commentary, thanks. The Max Moritz pistons (shown in both my pictures) were indeed designed for single plug and CIS. They are by all acounts a improvment in the combustion chamber over the typical CIS or Motronic piston.

Can anyone explain the differences and the advantages/disadvantages in the smaller skirts on the Max Moritz or "s" style piston?

Bobby sez: I had the motor in my F class race car with front mounted B&B and never had a problem, even running 1/2 & 1/2 race/pump gas at BIR when it was 95 degrees. If it was cool outside I had to tape off the cooler to get the engine to run at 190- if not it would never get to ideal operating temp, even in a race! So I think you will be absolutely fine with your setup.

Bobby thanks a bunch. You are one of the few who has actually added your actual experience and comments to the mix. First time hearing the oil cooler mod will keep the head temps down. But makes sense to me.

May I ask how expensive the P/Cs you have were...?

Sure, $3400 delivered as is:

"The new Max Moritz 98's with 23mm wrist pins, ceramic coating on the crown and dry film lube on the skirts, for the 3.4 on a 74.4 crank." They all seem to run the same price no matter what they are for in 95/98mm.

[rpiper]

rdane, glad I could be of assistance. The oil cooler just keeps the oil temp down though- the reduction in head temp is just a byproduct of the lower overall temp. I think Grady's advice re the fan/pulley ratio will lower head temps more specifically due to the increased airflow. On the race car airflow wasn't much of a problem because of the high speeds.

Last night I got busy with a small cutting wheel and modified a spare turn signal I have. I opened up the lens and housing so it is just the frame, and I'm planning on glassing in a stainless mesh following the curve of the lens. I need to locate a small dual element lamp, and I'll mount this behind the mesh, or else modify the side marker lamp housing to accept the turn signal bulb. This will allow a much greater airflow without having to resort to the goofy (but functional) oil cooler scoop. Yes I've got severe cabin fever.

I'm no expert, but my understanding is that the advantage of the smaller skirts is twofold. First of all, the pistons weigh less because of the lack of material. This helps with reciprocating mass. Second, the airflow inside the case is improved because the skirts are not protruding down inside the case when they are at BDC. This will work in concert with the boattailing of your case very well.

Yours should prove to be a great motor. I bet you'll get 225 at the wheels with the right exhaust. Makes me wonder how much I could sell my 95s for....

[Grady Clay]

Bill,
Can you please e-mail me the photos of:
The used 993 RSR Mahle.
The used stock 993.
The used 90 JR RS.
I’ll annotate them and tell what I see.
Send them as separate e-mails, my stupid system doesn’t like files larger than 500K or so and my in-box only holds 2M so spread them out over several days.

I trust that Max Morit 98mm piston never went into an engine. Hopefully that was shipping damage and wasn’t originally shipped like that.

Why is the crown to the used 90 JE RS piston so clean but the area by the rings obviously carboned up?

It would be useful to have some photos of the combustion chamber in the head (used) of some stock configurations and some carefully built high compression combinations (used).

Best,
Grady

[emcon5]

Grady, you should be able to just save the pictures Bill posted. Assuming you are using a windows based PC, just right-click on the picture you want, and select "save picture as" (or "save image as" if using Netscape\Mozilla)

The Max Moritz piston Bill posted was mine, I bought it as a used (~15K miles) P&C set, then resold the pistons and used the cylinders with the 98mm RSR pistons I pictured earlier. I don't know how that happened to the top, but the damage was really superficial. Based on the carbon build up in the dings, the engine ran for some time like that.

Tom

[ohecht]

Now to spoil the excuse to upgrade, but I was able to replace the seals on my 82's engine oil cooler without removing the engine (I think I just lowered it a few inches).

In case you just want to stop the oil cooler leak and try some of the less-invasive ideas like the 930 fan, upgraded front cooler, etc.

Olivier

[rpiper]

Good info Olivier! But the seals are almost new. I'm afraid the cooler is suspect. Also, I'm going to replace the thermostat in there to try and resolve the cooling issue. And maybe while I'm in there I'll do an RSR flywheel. Then I'll stop trying to upgrade. Really. I'm serious this time.

[ohecht]

FYI, the cooler itself can be replaced that way, too, but I understand the other needs to drop it out.

I just dropped mine all the way for the first time recently to replace a $50 TO bearing that just lost its circlip, and I have about $2000 into it so far for "while I'm in there" stuff, including replacing all the syncros in the transmission!

Olivier

[Grady Clay]

See, I’m trainable.

One of the critical issues in fitting pistons to heads is the clearances in various locations.
The goal is to have the piston as close as possible in certain areas but never under any circumstances actually touch the head. The arrow on the right is the “deck” clearance where the piston comes up flat close against the head. When I know everything is perfect, I have had this clearance as close as 0.022” without touching at 8800 RPM on 91x66 and basically this piston. That is VERY close and not for the feint of heart. No one should try this close with out creeping up on it over several maintenance rebuilds. This clearance is difficult to measure because of the “rock” of the piston in the cylinder.



The arrow to the left is “squish” (actually the deck area is also squish) and is potentially difficult to fit the piston to the head depending on the specific piston and head. I don’t think you can run quite as close clearance here.
In any case it should be close without ever touching. The space between the piston and head should be least at the perimeter and only slightly increase toward the center. There NEVER should be an included larger volume anywhere (like the transition from the deck) until you get to the combustion chamber proper.

Regarding this piston, the gun-metal overall coloration tells me that detonation is a problem and the lack of patterns in the squish area and not clean deck says the clearances are too great. Lots of clearance will lower the mechanical compression ratio but will increase the propensity for detonation.
It looks like the rings were sealing nicely from the coloration on the piston side.
What cams were run with this piston? IF RSR, we should be able to see a shadow outline of the valve in the coloration of the piston valve cut-out.
I can’t tell from this image if there is a problem with the piston in between the top compression ring and the 2nd ring. Are all the ring lands intact?

Bill’s Mahle (on left) in the “98mm Malhe & JE” image illustrates proper deck/squish. See how clean the piston is yet no sign of contact.

In the image of the “stock 993”, the piston on its side shows better (proper?) coloration in the squish area but not the other two pistons. Had that piston been cleaned?

Once we get some of these issues clear, we can properly address twin plug.

Best,
Grady

[Grady Clay]

Well, mabe not. The image didn't post.
Grady

[Grady Clay]

OK, back to the twin plug question.

Twin plugs are generally applicable to two-valve cylinders where the plug location is slightly compromised. The 901 engine was designed from the start to use twin plugs. The 4-valve DOHC heads have a better central placement of the single plug, however I have seen experimental 3-plug heads.

Help me fill in some history.
Of course all 4-cam Carreras (547, 692, 587) and the 4-cam eight cylinders (753, 771) were twin plug with two independent distributors.
The first twin plug six was the 901/20 with the two circuit single Marelli distributor with four sets of points. That basic configuration was maintained up thru 935s and 956s Bosch CDIs with no points and electronic ignitions. I think Motronic first showed up on 962s.
With the 993 the configuration became the single distributor with two independent caps and rotors to gain more physical separation between circuits.
Please correct my memory if needed.

When did the 6-coil, 12-plug aftermarket ignition systems first show up?
Who are the current manufacturers?
Which are integrated with EFI?

What is everyone’s experience?


Best,
Grady

[smdubovsky]

Emcon5 is correct in that the 964 dist rotates "backwards" compared to SC (and turbo?) engines. You need to install a 3.2 dist drive gear on the crank when doing a rebuild.

FWIW, I have the (weiner/andial/whoever) 964 dist converted to run in a SC. It still needs the 3.2 drive gear, but is modified w/ the CDI trigger wheel and mechanical advance. It could be for sale if someone is interested. (For considerably less than the ~$2800 rdane quoted from Weiner) Im currently using it in my SC but have since gone to full EFI and I don't really need the trigger/advance functionality of it anymore. Would anyone be interested at $1100? Includes dist, extra CDI & coil. I would have to find a plain "vanilla" 964/993 dist to replace it.

Tnx,
SMD

[emcon5]

I don't know where the ~$2800 number comes from, last I heard Steve charges ~$400 for the conversion.

In my case, the machine work for the second plug was ~$400, I found a 964 dist with wires for $475, the drive gear was ~$50, found a CDI and coil for ~$300 (which is a bit high). I made the harness myself, but I am probably into it $10 for wires and connectors. With the cost of 6 more plugs that still puts me south of $1650.

This number is of course above and beyond the cost of the rebuild, but I had to do that anyway.

Tom

[rdane]

$2800 is a complete conversion including the machine work on the heads and covers for twin plug, parts and labor.

"Using a 964 distributor (new) which is then converted
by Steve Weiner (Rennsport Systems) to use the advance
system of the SC. Twin MSD 6AL ignition packs and
tach adaptor (to drive the tach) Making up lower
igntion wire sets (existing upper ones remain)and
machining cylinder heads and lower valve covers.
All wiring parts, miscellaneous items, shipping and
installation which does include installing the 964
crank gear (the distributor turns the opposite way so
this would have to be done when the engine is apart)
would be approximately $2800 plus tax."

Steve Weiner's quote was "around" $2000 not including the machine work, which is typically $400.

[smdubovsky]

Emcon5,
You still need to "stuff" the SC distributor parts into the 964 dist. I don't know what is involved exactly, but you basicly have to sacrifice your old SC dist to get the the mech advance and trigger wheel. I suspect there is a bit of machining to get the parts to fit in the 964 dist (and also to get the mech advance to work spinning the other way). I think over 3 yrs ago Steve Weiner quoted me north of $300 labor to convert a 964 dist to run in an SC (I had to provide both distributors). I don't remember the exact $# but it wasn't cheap. I found someone w/ the whole system already converted and bought it. He went MoTec on his race car.

Since it works (and I dont really need it anymore), Im reluctant to pull it apart and see exactly what was done

SMD

[rs911t]

SMD, I pm'ed you regarding the dizzy.

[emcon5]

Originally posted by smdubovsky
Emcon5,
You still need to "stuff" the SC distributor parts into the 964 dist. I don't know what is involved exactly, but you basicly have to sacrifice your old SC dist to get the the mech advance and trigger wheel.

I know. Steve charged me ~$400 for mine, I provided a 964 and donor sc distributors. My cost for converting to twin plug was ~$1700, counting the cost to get the gear changed.



I think your price is fair, considering it comes with a CDI and coil.

rdane, that quote is from Waynes book? The "new" distributor, plus the cost of converting to MSD makes the number sound more realistic. I went with stock Bosch CDIs, so the ignition cost was a little lower. Plus I had a used distributor. If you take your time, and shop around you can do it for less.

Tom

[rdane]

Dane, that quote is from Waynes book? Tom [/B]

No. Prices I quoted are from one Seattle shop and Steve Weiner @ Rennsport directly. 2400/2800 seems to be the price (I have shopped 4 local wrenches) to have the twin plug conversion done professionally here.

You guys seem to be talking the parts alone. I am talking a turn key conversion to twin plugs.