A 3.2 Short Stroke.. with room to grow?

IXjamesXI · 09-02-2013, 07:13 AM

Hello Pelican Denizens... I'm getting started with rebuilding my 3.0 sc engine due to broken head studs. I am thinking that I would like to improve it as part of this process.. hence increasing displacement to 3.2. I was fortunate to purchase a set of Max Moritz style Mahle P&C's from another member.. that will be the basis of my rebuild. I am hoping that I can build something that will still have "upgrade potential" once it is all buttoned up.

For instance, I would like to be able to switch to EFI at some point soon after the rebuild, however I am hesitant to do so immediately as I would hate to have fresh cams and piston rings in there not getting the break in that they need as I struggle to get the EFI running well enough to perform the basic initial break in and lubrication of those new parts. Or should this not be a concern? I would prefer to run the engine in on CIS, and then start working on the EFI once I have confidence that the new cams etc would not be damaged by what will probably be many attempted starts etc with a new EFI system.

Also, I'd like to consider twin plugging. With a Max Moritz style piston I have read that twin plugging is unnecessary, but I have also read that with cylinder diameters of 98mm and above that twin plugging is a sensible upgrade to avoid detonation etc. Is it worth even considering with MM style pistons? If I had the machine work done on my heads, I do not think it would be very expensive or difficult to implement it as part of the conversion to EFI.

Anyway, I am just starting to collect my thoughts about this process, if any one has any advice or suggestions for me, I of course would be grateful. Thanks. And I will try to update this space as my project progresses..

fred cook · fred cook's Garage

I am in the process of doing basically the same thing. I have the 100mm Mahle pistons (10.1:1) in a set of LN Engineering slip fit cylinders to give 3.3 liters. I am using a set of Carrera twin plugged heads along with an early SC CIS set of large bore runners. Cams are from a 964. Ignition is Electromotive XDi direct fire. I had this all together and running but had an oil pressure problem. Turned out to be a damaged oil pump seal requiring the block be split again. At this point, I am getting close to having the engine back together after replacing the bad seal. The little driving that I did showed that this configuration has good low end torque and runs/idles well. I only drove it a couple of miles before tearing it back down but was impressed with the combination. The big advantage of twin plugging is that you will run only about 24 degrees of advance instead of 35 degrees. Easier on the engine and should run cooler. If you do twin plug your heads, have the lower plug holes drilled and threaded for the 12mm plugs instead of the 14mm. Otherwise you will have to grind a 45 degree bevel on the cylinder head nuts next to the lower plug holes to allow room for the spark plug tool. Another alternative would be to use the plugs that have the smaller hex head than normal. Good luck with your rebuild. You will find that it will be quite an adventure!

KTL · 09-03-2013, 06:36 AM

Smaller 12mm plugs also have less effect on your compression ratio change. Smaller body, smaller drilling in head.

Max Moritz pistons were specifically designed for the CIS injection. So if you switch to EFI later, you'll be less than optimum for what the EFI system would like to see- typically domed, symmetric pistons.

toddu · 09-03-2013, 09:13 AM

I found it difficult to keep the CIS and nit some how limit future growth (whether it was cam choice or piston choice, etc.). I ended up just biting the bullet.

Todd

IXjamesXI · 09-04-2013, 06:23 AM

Fred! I have been watching your build intently.. I hope you get that seal sorted soon. Your car looks amazing.. it's my ideal look for an impact bumper era car...

Yes, I will be biting the bullet on fuel injection right away.. the more I think about it and the more people I talk to has convinced me that you need to come up with your "formula" and see it through in order to end up with an engine that makes sense.

I spoke to John Dougherty (camgrinder) yesterday and he had some good advice for my build.. Based on what he had to say and my goals for this as a street motor, I am going to keep my small port heads.. he thinks a 993SS cam might be a good choice given my Mahle wedge pistons, single plug, and ideally 9.8 CR, but of course it depends on what I decide to do in terms of induction. I think a proper ITB setup is a little out of the budget range at the moment, so I guess I need to decide how else to proceed.. possibly refitting the cis airbox and intake for EFI seems like the sensible route.. I will also have a set of SSI's for exhaust.

Anyone have experience with this 993SS cam in a similar motor..? I have searched and there seem to be a few folks out there that have shared their experience, and I saw one rather nice looking dyno that John D might have posted a few years ago. There doesn't seem to be as much documented about that cam as there is about 964 or 20/21's or GT2 grinds... Ideally I want to run the best cam I can for power from 2500-ish to 6500-ish RPM that a common plenum EFI system can handle and that my pistons have room for, although I think I can have my valve reliefs increased my 1 mm or so if needed.

KTL · 09-04-2013, 06:27 AM

993SS cam seems to be the most "radical" cam you can go with on a single plenum/single throttle body intake system. Seems that cams that are more aggressive have intake reversion that can cause havoc for the sensor plate (CIS) or airflow meter (Motronic)? I have no personal experience with this myself. However a few people I know, MotoSook and JeremyD, have chosen this cam for their hotrodded long stroke 3.2L to 3.4L conversions. Same would apply for the short stroke 3.0 to 3.2 conversion.

IXjamesXI · 09-10-2013, 08:26 AM

Hey pelicans.. thanks for your replies thus far. I have my new-to-me pistons and cylinders in hand, and to my untrained eye they look pretty good. I do have one question though.. I noticed one issue with one piston.. very slight pock marks on the side. Is it possible that these are by products of detonation? I have seen pics of detonation on piston tops before and I guess this looks similar. The edges of these little craters seem raised above the surface.. just by my feel. Ok to knock them down gently with a file?

Lastly, what's the current consensus on coatings? I was thinking just TBC on the top of the piston maybe dry lube on the ring lands and nowhere else.. although everyone seems to have their opinion ranging from coat everything to coat nothing. Any suggestions? Thanks.

KTL · 09-10-2013, 09:27 AM

Somebody drop the piston? Detonation pitting is a bit smaller. Like these

98mm Mahle Motorsport Pistons

No big deal to use a fine file to knock down the raised material. The skirt is the critical dimension, in terms of clearance, more than the area around the ring lands.

I'd coat the skirts with VERY thin dry film lube. Can't be a coating of considerable thickness or it'll close up your clearances by a large amount.

Crown coating is supposed to help. But to make it truly effective you really should coat the combustion chamber and exhaust port. Doing so contains the heat and forces it out of the engine via the exhaust. Leaving the head uncoated still allows the heat to soak into the head. Heat in the head is what we're most concerned with on these engines. Hot piston top not as much. But any little bit helps I suppose?

I got some new pistons and I plan to coat the crowns and dry lube the skirts. I don't want to take apart my heads again, since they're all cleaned up ready to be installed, so i'll pass on coating that for next time. Skirt lube is supposed to be beneficial at break-in and startup.

Those pistons I linked were coated on the skirts. I was shocked how little disturbance there was to the coating, which was quite fragile. The skirt film showed almost no removal due to engine operation. However when I mic'd the pistons, I disturbed the coating more than had occurred when the engine was run!

IXjamesXI · 09-12-2013, 07:54 AM

After doing some more reading I guess I'm not any smarter with respect to coatings.. I might just skip it altogether. If I coat the combustion chamber, with the intent of keeping the head cooler, does that help the exhaust valve in that when the valve seats itself the head is cooler and can absorb more heat from the valve, or does it place extra stresses on the ex valve because less heat is being sunk into the head and "all" the heat is going past the exhaust valve into the exhaust?

KTL · 09-12-2013, 02:30 PM

Well you can coat the valve faces too.

I would think you are correct. If the head is better insulated, then it has more ability to pull heat from the valve thru the guide.

jpnovak · 09-12-2013, 02:34 PM

I generally coat the following related to P/Cs and heads.

Pistons: TBC on piston crowns, DFL on skirts. DFL on wrist pins

Heads. TBC on chamber including valve faces, TBC on exhaust port including backside of valve to where it seats on guide. Make sure to clean valve seats with a quick lap. DFL on valve springs and a wipe on lubricant on valve stems.

Friction and heat are bad. Coatings do work but are a "system" just like planning your whole engine build.

Send me your parts. I can coat them for you. Coatings are applied after machine work.

PFM · 09-12-2013, 05:33 PM

James,

Is this for a street build or track? My experience with coatings is they help with cooling for track type events where you get to run it hard. We see little gain on short dyno pulls but good gains when we consider heat. With those pistons not sure I would go twin plug as it is an expensive plan on many levels. If you need max power then yes go for it.

cmcfaul · cmcfaul's Garage

A short stroke 3.2 that will spin close to 10k RPM's is the ultimate NA Porsche Engine, in my opinion.

Chris

73 911 E

PFM · 09-15-2013, 04:45 PM

Chris,

That 10,000 RPM motor sounds good if you have the budget to build and maintain it. The extra maintenance and service schedule is nothing to laugh at.

IXjamesXI · 09-16-2013, 06:34 AM

Yikes.. 10k rpm?

I don't think I will be looking for anything past 6500 with this motor.. I am adapting the stock plenum to EFI, and sticking with a 993SS cam.. No time, money, or inclination to split the case and go crazy making it a 9k RPM motor. Its only 66k miles and I feel like I can get away with this.

However, speaking of high rpm.. the rods WILL be coming off, as I need to have them offset bored and rebushed for the Carrera style pistons. Should I consider ARP rod bolts when I put the rods back on the crank? People say the 10mm stock SC rod bolt is plenty strong, but if the insurance is worth it... And if I do go with ARP... I heard that you need to do something to the big end of the rod, like have it trued up or something since the ARP bolts will cause it to deform differently than the stock bolts? Anyone know what that process consists of?

KTL · 09-16-2013, 06:58 AM

Quote:

Originally Posted by IXjamesXI

Should I consider ARP rod bolts when I put the rods back on the crank? People say the 10mm stock SC rod bolt is plenty strong, but if the insurance is worth it... And if I do go with ARP... I heard that you need to do something to the big end of the rod, like have it trued up or something since the ARP bolts will cause it to deform differently than the stock bolts? Anyone know what that process consists of?

Yes the ARP bolts are good insurance. Fairly cheap and very high quality. Comparo:

Factory Porsche SC rods bolt + nut is $11 ea. from Pelican. That's $132.

ARP bolts for SC rods are $230 at an aftermarket source I typically use when Pelican doesn't carry something. $100 is CHEAP insurance when building one of these engines.

The rods need to be resized on the big end when you use the ARP bolts because they are torqued to a higher spec than the factory rod bolts. Additional clamp load on the rod changes the shape of the big end. So its good practice to have the rods rebuilt/honed on the big end to proper size. Since you're having the pin end rebushed and offset bored, I would think it's best for the machine shop to also do the big end to make sure everything is in order- big end bore, small end bore and center-to-center length.

10K rpm from a 3.2 short stroke is asking a ton of that engine. You'd need a myriad of modifications thrown at it to survive. GT3R pump, Supercrank, Pankl Ti rods, super light pistons, Ti valvetrain, etc. etc. THat's big money.

IXjamesXI · 09-19-2013, 04:58 AM

Still thinking about these rods.. does anyone know if there is a relatively surefire way to calculate how much to offset bore the rod to ensure deck height within an acceptable range? I would rather not set the rods up once, measure, discover the deck height is wrong, and then re-bush it again.

I was thinking that since I am using Carrera style pistons that my SC rods should be lengthened by 1.2 mm. I got to that number by adding half the stroke to the rod length to the compression height.
SC Stroke: 70.4mm
SC Rod: 127.8mm
SC Comp Height: 34mm (not sure what this number is, or if it is accurate.. I assume it is the distance from the center of the wrist pin / small end to the start of the dome of the piston, or where you would measure deck height on the piston)

So the math I did was: (70.4/2) + 127.8 + 34 = 197mm

I assume that this tells me that the my "deck" would start 197mm above the center line of the crank, so I also assume that this is the figure I want to shoot for when adjusting my rods to my new (Carrera-style) pistons. I read somewhere on here that Carrera pistons have a combustion height of 32.8mm. I could not find this figure in Wayne's or BA's book, so I don't know if it is true, nor do I know if it applies to the Mahle "Moritz" style pistons I have in hand. But if that number is legit, this is the math I do: (70.4/2) + X + 32.8 = 197mm Where X equals the rod length I need.

Solving for X equals a rod length of 129mm. My rod currently is 127.8, therefore the difference is 1.2mm.

Now, if I lengthen the rod 1.2mm and my deck height is still too much, my only option would be to re-bush again, correct? Or maybe I could machine a little off the tops of the cylinders? If I want to avoid this, should I plan to maybe lengthen the rod another 0.5mm from the start? That way I assume I can give myself deck height back by using thicker cylinder base gaskets in case I end up with too little deck. Maybe this will also give me some flexibility with adjusting the compression ratio too, as I have heard that these Mahle pistons do not compress as much as advertised, and I would like to shoot for and hit the 9.8 spec squarely. This seems like a reasonable plan to me because base gaskets I think would be pretty cheap compared to re-bushing the rods a second time.

KTL · 09-19-2013, 09:48 AM

Yes your numbers for the compression height of SC and Carrera standard pistons are correct. 34mm for SC, 32.8mm for Carrera

Offset boring? Search is your friend............

Narrow engine, short chains.

Can I use 95mm 3.2 (84-89) cylinders on my 78 sc 3.0 motor

Upgrading 80 SC w/ 88 3.2 P&C's

Henry at Supertec has mentioned the offset bore a few times. He's done this successfully and likes the modification because it improves the rod length : stroke ratio. I think he said the offset is 0.030" or 0.040" Ollie's Machining lists it in their pricing schedule too, so you could ask them.

Home Page

I think the typical approach to adjusting deck with the cylinders is to machine the base, not the top?

IXjamesXI · 09-19-2013, 11:41 AM

Thank you, those are good threads.. the first one is new to me but I had already read the other two. I think what I am getting at, and what isn't really mentioned in any of the threads I have found is: Does it make sense to make the rod longer than I theoretically need, so that I can tweak deck ht and CR with base gaskets?

I am a little unsure if the CR of my pistons will be as advertised, so the strategy is to bore the rod such that if anything, I have too much CR and too tight a deck, which I can use base gaskets to ease off to get to the desired spec. What I don't want to have happen is to have too much deck, and not enough CR.. getting that back would require a second re bushing of the rods, which I would not want to do. Just trying to get some sense of if my strategy is foolish.

KTL · 09-19-2013, 01:29 PM

Some of the more common ways to raise compression:

Special pistons. Since you're set on using the MMoritz pistons as-is, you wouldn't want to buy different pistons to raise your compression. So that approach to compression increase is a no-go.

Head shaving on combustion chamber side. Cut the heads and use Henry's head-to-cam housing spacers to shift cam housing outward to maintain cam housing alignment. Doing this will avoid using an excessive amount of base gasket on the cylinders.

Offset boring the rods. This is a win-win situation in my opinion since you're postively affecting the engine in terms of mechanical operation. Increasing the rod length-stroke ratio improves the rev-ability of the engine. Just need to make sure you don't end up with too little deck height.

I think you should do two things to help direct your decision:

1. Measure the dome volume of your piston by "pouring" it in one of you cylinders. Also pour your cylinder head to know exactly what the volume of your combustion chamber. Then you can reliably calculate your compression ratio based on a desired deck height.

2. Do a partial assembly of the case with main bearings installed, one rod installed on the crank, piston & cylinder with rings. That'll allow you to actually measure your current deck height. Then you'll have a baseline to start with in terms of knowing where you're currently at with deck height and much adjustment you may or may not need to make.

Adjusting deck height can certainly be done with base gaskets. But its typically best to use a singular thickness. Stacking them isn't the best way to go. I know there are 2 typical thicknesses in 0.25mm (std) and 0.50mm (oversize) and i'm sure engine builders have a source for thicker ones.