twin plugging question

[rpiper]

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??

[Grady Clay]

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

[rdane]

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

[rpiper]

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.

[rdane]

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

[rpiper]

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.

[rdane]

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.

[Grady Clay]

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

[Grady Clay]

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

[rpiper]

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.

[dd74]

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

[curlesw]

How effective are twin plugs with asymmetrical (bump and dip) pistons as compared to classic symmetrical pop-up pistons at the same compression ratio?

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

v/r
Wayne C.
83 SC

[rpiper]

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.....



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....

[emcon5]

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



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:



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

[dd74]

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!

[dd74]

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

[rdane]

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?

[dd74]

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

[rdane]

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:

a 3.4 is coming

[Grady Clay]

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