twin plugging question
 

Hello all- I have a 78SC with a bit of a hotrod 3.0L in it: 9.8:1 Euro P/Cs w/78 heads, 964 cams, SSIs. The engine oil cooler is leaking so I need to drop the engine and replace it. As usual I've been thinking of things to do while it is out. I was considering a couple of upgrades, the first being to twin plug the engine. When its hot I get some detonation despite always running premium and having installed a new thermostat and Carrera cooler with fan. It is certainly possible that the engine oil cooler has gunk on it and isn't doing its job, but it seems that twin plugging would solve the detonation issue as well as allowing me to bump up the compression a bit further. If I had the heads decked and set up for twin plugs, could I bump up the CR enough for it to be worth it? Or am I going to run out of deck height before any gains are realized? What does it cost to go this route? Am I better off waiting until tbitz has the CIS to EFI kit done and switching to hotter cams instead? I guess what I'm asking is, have I already done all the "easy"(under $2000) upgrades or are there any I have left on the table??

Bobby,

I assume you have CIS and no catalytic converter.
Exactly what are your pistons like? Are they the bump and dip or the symmetrical type? If symmetrical, then yes twin plug and slightly higher compression might be reasonable project.

What is your current measured piston-to-head clearance? Where?
What is your current measured compression ratio?
What ignition timing do you have?
In the overall scheme of things, where are the 964 cams?

CIS and other mild cams can stress the fuel (even pump premium) and have detonation (knock) in the 2000-4500 RPM range. Your first test is to run the fuel tank almost empty and then fill up with leaded 112 octane race fuel at $4/gal. See if the detonation disappears. Is there any change in performance?

Please report your findings.

Best,
Grady
gradyclay@hotmail.com

Hi Bobby,
I am in the middle of a rebuild and this has not yet been answered for me. My suggestion is first measure the actual compression you have with your P&Cs. If I actually have 9.4 or higher is I'll twin plug. If it is 9.4 or lower I will not.

I am using a Max Moritz piston designed to improve CIS at a stated 9.8:1 and is suppose to be a single plug set up. Adding them (twin plugs) should get you 5/10 hp with any high compression set up.

I would be curious to know how your rings have held up if you have detonation?

My twin plug set up quotes started at $1500 or so from electromotive, $2400 installed, $2800 or so for a 964 set up from Weiner and a good bit more for a RSR set up from Andial.

Good luck! Let us know what you find.
Dane

I have CIS pistons (factory 9.8:1 for 81-83 euroSC) so they are asymmetrical ("bump and dip") type. I'm not sure of the piston to head measurement, but the heads were "trued" when I had the engine built 3000 miles ago. Measured CR is 9.77:1 using volumetrics. Cams are at 1.25mm. Timing is about 30-32 deg advance (the distributor was recurved again last summer, which helped the pinging and idle speed issues). I ran several tanks of 112 race gas last summer (there's a station near me that sells it) and the motor ran perfectly even when it was 98 degrees. Not sure if there was a difference in power, the car is really fast either way. Sounds like I have reached the limits of this motor. I don't want to go to Webers, as they are just too finicky for the street (BTDT). I actually really like the CIS but I'm just looking for options.

Grady I'm not sure I answered your cam question: to my understanding, the 964 cams are between SC and S cams, and are about the hottest you can run with CIS. There's another cam similar called 20/21 that several other pelicans use (Noah?rdane?).

rdane, that's going to be a neat motor. If I hadn't built this one for PCA stock class I would've done the short stroke 3.2. I really like the way these engines spool up.

I think perhaps I'll put on an RSR flywheel, button it up and call it good.

Bobby,
Are you planning on just using race gas then? I too like the CIS and I bought wedge shaped 9.8:1 P&Cs hopeing to avoid the expense of twin plugs.

Can you get by without detonation with the European 95 P&Cs without damage by running race gat as the weatehr gets warm?

Looks like I better plan on twin plugs if the 3.0 is having detonation problems at 9.77:1. Thansk for the info. Every bit helps. Nice write up on your race charity BTW.
thanks
Dane

Thanks a lot. Courage Center is a great place and it's nice to support them.

I can get by one of two ways when it's hot: keep my foot out of it or run race gas. It is my daily driver (in summer) so I hate to pay $4/gal. It is a real hotrod when it's cool out, so I may have to come up with a way to get it to run cooler. Maybe a second cooler in the driver's side fender, or the elephant racing oil lines, or replace the turn signal with a screen with an LED behind it. The Carrera cooler with fan isn't always up to the task.

I had no idea a oil cooler would make the difference on detonation?!
Good info.

I have a front mounted B&B. Since it was installed I have a hard time just getting it up to temp to check the oil level.

I also had a ceramic coating added to my piston tops to help control temps. The mean temp here in Seatle has to be around 50. Hitting 90 is a big deal here. I could burn race gas for those rare days and still have change from a $20 I suspect.

Sounds like both mods will help keep my engine in one piece.

Dane is right, a twin plug set-up is worth 5-10 Hp in most situations. However, if detonation is already an issue the gain can be a LOT more due to better (proper) combustion. Of course the other (worse) side of detonation is that it can do BIG TIME DAMAGE the mechanical parts of your engine.

Bobby, yes you answered my cam question. It is a CIS cam, just more radical profile.
Go ask the builder what the piston-to-head clearance was. If he measured the volumetric CR, he certainly measured the deck clearance. Sounds like a good builder.
You say your detonation is temperature related. Try using the ~’76 930 fan and pulley ratio to pump more air across the heads. Cooler heads = less detonation propensity. Also cooler heads make for better volumetric efficiency and more horsepower.

Dane; It’s combustion chamber temperature not just oil temperature.

Another trick for the street is a little bit (5%-) of leaded race 112 fuel, left over from the previous weekend track event, can significantly raise the octane of 91-98 unleaded pump gas.

Best,
Grady

Who knows the answer to this or can share some light?

How effective are twin plugs with asymmetrical (bump and dip) pistons as compared to classic symmetrical pop-up pistons at the same compression ratio?
The criteria would be the limiting octane number before detonation occurs.
We are talking about a 3.0SC CIS with appropriate cams and ignition.

Please offer your Pelican 2c.


As I understand it, the bump&dip pistons produce superior swirl in the combustion chamber resulting in better, more even and complete combustion. This allows for better fuel economy, lower unburned hydrocarbons, and good power. There is a limit to how high the compression ratio can be and they are not conducive to high lift –long duration cams (S, C6), and neither is the CIS. On the other hand the pop-up pistons can, with large valve cut-outs, accommodate radical cam profiles and much higher compression ratios (perhaps 13:1+). This takes careful attention to the squish area and the piston-to-head and valve-to-piston clearance. Of course this configuration benefits a lot from twin plugs. The down-side of high compression pop-up pistons is less efficient combustion, particularly at idle and very low RPM, and higher octane requirements. However, there is a huge horsepower gain.

Best,
Grady

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

Grady, the builder was Aaron Hatz at FlatSix here in Bloomington. He does superb work, and appreciates a sleeper the same way you do. I'll find out today the deck clearance and post it. Thanks for all of your insights. I think the switch to the '76 930 fan/pulley ratio is a fantastic idea; one I had certainly not thought of. When the engine was built, we swapped in a smaller fan pulley for a little more hp, but did not change it back when the engine was swapped into the street car.

Hey Bobby, can you post a photo of your car? I don't think I've seen it.

Can someone post a pic of these type of pistons...I am curious as to differnce. thanks.

v/r
Wayne C.
83 SC

dd74, are you being facetious? I post pics on any thread that even remotely concerns SCs or 3.0L engines or chrome trim or Fuchs refinishing or... well you get the idea. But since you asked.....

http://forums.pelicanparts.com/uploa...1076606456.jpg

Now I'm trying to decide, since there is very little more hp to be had with my engine, whether to go on a lightening binge. It weighs 2555 with full interior, sound deadening, stereo etc. I could easily get a lot of weight out of it, but would be sacrificing streetability. It is a slippery slope once you get started....

Not a very good picture, but this is a stock CIS piston:

http://forums.pelicanparts.com/uploa...1069133180.jpg

The left-top area is the dome (bump), and there really isn't a valve pocket. Where the carbon build up is, piston center to lower right is the "dip" My understanding of the reasons for this are the same as Grady's.


This is a RSR piston with a traditional dome with deep valve pockets:

http://forums.pelicanparts.com/uploa...1069134036.jpg

Just a note on the 964 based twin plug solution on 3.0L engines: The SC distributor rotates the opposite direction as every other 911 engine. You will need to change the distributor drive gear on the crank to make a 964 distributor work in a 911SC. If you aren't planning on splitting the case, you need to come up with another solution.

Tom

Ah ha! Yes, now I recall your car. "Sleeper" is right.

I've been battling the lightening binge. Aside from the lazy stuff like stripping the interior, etc., it's getting too pricey for my tastes. Plus, when going fiberglass, one starts down a slippery slope.

You'll find out...

Search out the recent thread asking about aluminum bumpers. Ed Bighi's SC is in there. It's nothing but bare metal @ 2200 pounds, but is still all steel! :eek:

Ooops! Sorry to go off topic, guys...

CIS with a good carbon build up on the left and a new Mahle, Max Moritz CIS piston on the right. Pistons are set with the rods running in the same direction for comparison.

I suspect but can not verify or give you a source that the CIS and Mortronic pistons don't respond to twin plugs very well. If they did I would have thought that the Max Moritz design would have been closer in profile and works with either single (which was the original intent on the 3.0/3.2 up grade) or twin plugs.

Wouldn't a twin plugged CIS piston have the second spark at the back of the lopside dome?

http://www.standingwave.org/albums/p.../ahy.sized.jpg

Hey Dane: I'm waiting with baited breath for word of your engine. Any updates? What's the prognosis and timeline to completion?

David, the lower is going back together now. Measuring compression next week I would think. All the parts (tinware, valve covers, fan) are out to powder coat and replate so I am getting close. Couple more weeks with the transmission changes and topend rebuild I think.

Take a look here for the update:

http://forums.pelicanparts.com/porsche-911-technical-forum/148354-3-4-coming.html

Dane,

Good photo,

On the CIS bump&dip on the left, notice the great clean periphery to the left of the bump. That is a good squish area.

The periphery of the piston towards the camera has more coloration, possibly indicating less piston-to-head clearance. The other possibility for this coloration is the piston is hotter there sinse there is less metal under the crown of the piston in that area. Note the discolored oil on the skirt of the piston below the wrist pin. That is an indication of how hot the piston has been run.

See the shadow of the valve on the right of the piston crown.

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.

On the piston to the right, see the machined perimeter to the left. That is the very important squish area. It looks like it extends ¾ way around the piston crown. When you are fitting pistons to heads that is where to push the limits; close but never ever any contact.

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.

Please offer some response to my prior twin-plug question.

Best,
Grady

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:


http://forums.pelicanparts.com/uploa...1060144468.jpg

Tom

Here are some
my 993RSR Mahles
http://forums.pelicanparts.com/uploa...1076614390.jpg

2.7RS Mahle
http://forums.pelicanparts.com/uploa...1076614423.jpg

Stock 993
http://forums.pelicanparts.com/uploa...1076614479.jpg

98mm Mahle & JE
http://forums.pelicanparts.com/uploa...1076614511.jpg

MAx Morit 98mm
http://forums.pelicanparts.com/uploa...1076614619.jpg

90mm JE RS
http://forums.pelicanparts.com/uploa...1076614836.jpg

'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
http://www.standingwave.org/albums/p.../ahk.sized.jpg

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 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.

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.

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....

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

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

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

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.

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

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

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

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

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

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

$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.

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

SMD, I pm'ed you regarding the dizzy.

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.

http://forums.pelicanparts.com/uploa...1060054453.jpg

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

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.