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rebuild advice on 3ltr sc motor

I need to rebuild the topend of my 3ltr and was considering a change to 3.2ltr pistons and cylinders. Is this is pretty straightforward mod or are more complicated internal mods necessary? I plan to run the current cis until I can swap to carbs.
I think my question might make it obvious that i'm still green behind the ears!!!
Thanks in advance for your replies!!
Old 09-19-2005, 05:48 PM
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This conversion is known as a "3.2 short stroke" or "3.2SS". If you do a search on "3.2SS" you should find lots of info.

Here's the main points:
- It's pretty straightforward though not inexpensive.
- The conversion increases the bore from 95mm to 98mm.
- Factory 3.2L motors used the same 95mm bore but a longer stroke.
- You can get Mahle P&C for something like $4K.
- You can get later 3.2L cylinders, have them bored out and use them with 98mm JE pistons. (You can't use your 3.0 cylinders as cores). That will run you around $2.3K

Note: I have been unable to find 98mm 9.5:1 compression JE's. I've purchased 2 sets of "9.5:1" JE's that measured out to 10.5:1. Hopefully the situation has been corrected by now. Always CC your motor, never assume.

-Chris
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Old 09-20-2005, 08:25 AM
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Thanks Chris!

I talked to my mechanic and he basicly talked me out of of doing the mod.

Last edited by al74911s; 09-20-2005 at 03:29 PM..
Old 09-20-2005, 09:13 AM
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Quote:
Originally posted by al74911s
Thanks Chris!

I talked to my mechanic and he basicly talked me out of of doing the mod.
What were the reasons?

Wayne
Old 09-20-2005, 04:15 PM
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I currently run a 3.2 SS based on an SC.
Admittedly there are a lot of other mods done to the motor, but I am possibly one of the happiest Porsche owners arround.
I started with the Max Moritz 98mm conversion that i purchessed second hand. The power increase was about 8%, as expected with the capacity increase. Definately worth it.
One point to remember is that if you intend to go to carbs, is that the SC type piston desigen is not the best for carbs. It was designed for CIS.
If you wanted to use carbs in the future I would use a set of 98mm JE's. The JE piston desigen will work well with carbs and CIS with no ill effects.
Just my 0.02 worth.
Old 09-20-2005, 09:17 PM
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The initial reason I managed to get talked out of it was because I think I really am not sure of what i want. I dont plan to do any racing, possibly some a/x. I have been doing some reading of other posts and I am leaning toward a milder mod on the motor.

As far as why my mechanic convinced me not to do the 3.2 mod......it was due to total expence for power and how much of that power I would actually use.

could someone give me some feedback on a 964 cam regrind and larger exhaust porting?

Thanks
Old 09-20-2005, 11:11 PM
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The 964 cam will help a bit as will back dating the exhaust to the earlier SSI setup.
The exhaust porting is as large as you would like to go. The intake ports are actually too small. If you wanted to open up any ports you should open up the intake ports.
The only problem with this is a loss in bottom end performance due to the reduction in the intake air velocity with the larger ports. There would be an improvement in the top end performance, especially with the 964 cams.
Old 09-21-2005, 01:41 AM
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Quote:
Originally posted by 1meansc
One point to remember is that if you intend to go to carbs, is that the SC type piston desigen is not the best for carbs. It was designed for CIS.
If you wanted to use carbs in the future I would use a set of 98mm JE's. The JE piston desigen will work well with carbs and CIS with no ill effects.
Just my 0.02 worth.
Mahle Motorsport offers the 98mm P/C conversion kits for 3.0L applications in 2 versions: one for CIS cars like you stated and also a carb/mfi/efi version where there are larger valve relief's in the piston dome to allow use of more appropriate camshaft profiles with the "hotter" injection. These sets also typically have a higher advertised compression ratio than the CIS sets.

The price is more or less the same between the CIS & Carb/MFI/EFI sets, which is to say expensive. If you are doing this on a budget, the JE's will be a cheaper alternative.

Ralph
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Old 09-21-2005, 01:33 PM
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Quote:
Originally posted by Carrera3.5L
Mahle Motorsport offers the 98mm P/C conversion kits for 3.0L applications in 2 versions: one for CIS cars like you stated and also a carb/mfi/efi version where there are larger valve relief's in the piston dome to allow use of more appropriate camshaft profiles with the "hotter" injection. These sets also typically have a higher advertised compression ratio than the CIS sets.

The price is more or less the same between the CIS & Carb/MFI/EFI sets, which is to say expensive. If you are doing this on a budget, the JE's will be a cheaper alternative.

Ralph
Correct. In the original post, it was mentioned to run 3.2 P&Cs until a carb conversion was done. Well, you can't simply slap carbs on a motor and get power - you need to swap out the pistons and cams as well. Read Chapter 4 of the Engine Rebuild Book - it clearly explains this...

-Wayne
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Old 09-22-2005, 12:41 AM
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Quote:
Originally posted by Wayne at Pelican Parts
Correct. In the original post, it was mentioned to run 3.2 P&Cs until a carb conversion was done. Well, you can't simply slap carbs on a motor and get power - you need to swap out the pistons and cams as well. Read Chapter 4 of the Engine Rebuild Book - it clearly explains this...

-Wayne
Thanks Wayne,

I have been going through your book and I have found it to be very helpful. I read it during my down time at work and eves at home. I would like to have your opinion on whether going to the 964 cam is a good option considering I am, like many others, on a somewhat limited budget. I still would like to ad carbs later. but for now limit the work being done to higher rate springs and the cam, mentioned previously. If it helps, all I primarily am looking for quicker acceleration for the twisty roads and ax.

Thanks a million for your input!!

Rookie 911 owner
Alonso S
Old 09-22-2005, 10:22 AM
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Alonso

Putting a slightly "hotter" cam like the 964 in a 3.0 CIS motor should give you some more horsepower. Reading posts, it seems the concensus is that this works and doesn't have nasty drawbacks like terrible idle.

But why do you want stiffer valve springs? You only need stiffer springs if you are using a really radical cam which slams the valves closed or has other tricky characteristics. The 964 is not such a cam. Or if you are going to rev the engine way up. Opinions on what way up is varies. Some get worried above 7,000. I think I got along just fine in a race motor with stock springs at 8,000 rpm.

Now reving the motor up another thousand RPM is a classic method of gaining horsepower. That's why Formula 1 engines can make so much horsepower from such tiny engines. What do they get - something like 800 hp from a 2.0L engine at 19,000 rpm? Something like that - I don't follow those things exactly because I am never going to run an F1 motor. So if you rev your motor quite high for what it was originally designed to do, you certainly need stronger springs to control those valves (I'm simplifying this a bit here).

But if you take a stock 3.0 and rev it up beyond maybe 5,500 rpm you start losing horsepower (on mine the optimum shift points are about 6,200 depending on which shift is involved). Why? Because it is starting to have real trouble breathing. It breaths best down around or a bit below 4,000 rpm, where the torque peak is (the torque peak is always at the point of peak volumetric efficiency because that is where the most air gets pushed - or sucked, if you like - into the engine). Above that the beneficial effects on horsepower of more RPM keep the horsepower climbing even as the breathing and thus the torque falls off, but that doesn't last forever, and at the HP peak the falling breathing takes over and HP goes down above that point (for best acceleration you shift above the HP peak, but that's another topic).

So if you put a 964 cam in your 3.0 engine, we can assume you will raise your torque curve because you will get better breathing. It should move up and to the right on a standard graph. Or perhaps just move to the right. But will it raise the curve such that optimum shift points are now 7,200 rather than 6,200 or so? I'm dubious (did Porsche use stiffer springs on the 964s?), but that is the kind of fact you really want to know before spending money on stiffer springs.

Stiffer springs cost a little bit of power (it takes extra power to compress them compared with stock) and increase valve train wear some. More to the point, they cost money. There might be something else you could do with that money to cut time off your autocross runs, or give you a bit more acceleration for spirited highway driving.

Frankly, if you drive your SC to its limits often on public roads you ought to be put in the looney bin because those limits are so high already they will exceed your ability to figure out when some guy turning left on a country road isn't going to expect that dot in the distance to hit him before he completes his turn, or will think he can see far enough around a curve that he can make his turn or pull out onto the road. Suspension, for instance, will probably help you more in autocross than power. Practice will do more than either suspension or power until you get pretty good at it. But we all do like more power. Power is good (at least as long as you can afford the fuel).

Once you know the crude version of the theory (pretty much all I know - kind of a qualitative understanding), you can do a smarter job of putting the pieces together by taking actual data from others and seeing what their results were. Maybe somebody who has put a 964 cam in a 3.0 engine (and kept the CIS and didn't port the heads or bump the compression and so on) would give you his dyno sheet. That way you could calculate what the shift point RPM is (another subject). If it is above 7,000, maybe the slightly stiffer springs would be a benefit worth paying for.

Walt Fricke
Old 09-22-2005, 01:44 PM
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When I do a 3.2 or 3.0 motor with factory pistons and 964 cams I follow Bruce Anderson's advice and use a stiffer valve spring as insurance. Not because of throttle induced high revs but because these motors are invariably connected to 915 transmissions.

It's easy enough to kiss the pistons with the stock cams when you miss a shift. When you switch to 964 cams (w/their longer duration and slightly higher lift) the valves are lingering longer in the way of the approaching piston. In short, the stiffer springs are insurace against missed shifts.

-Chris
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Last edited by ChrisBennet; 09-22-2005 at 09:50 PM..
Old 09-22-2005, 02:51 PM
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I get a lot of calls from people that just bent a valve running an auto-x or D.E. Most are 3.0 and 3.2 engines that have missed a shift.
If you move the shift point up to 6500, the performance springs are good insurance for the reasons Chris pointed out.
The 964 cams are a good upgrade, but I doubt porting the cylinder heads will be worth the $$.
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Old 09-22-2005, 09:41 PM
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For a CIS and/or Carb discussion, check out this thread. Look in particular for Tyson Schmidt's comment on the conversion. I'm offering this only as a point/counterpoint argument.

Secondly, did you say what year your 3.0 is? There were, for the five-year-run of 3.0s, two U.S. variants of the engine, provided you have a U.S. 3.0. Both do vary in their personalities.

Surely stronger valve springs are insurance, but if you're not racing the car in a way where you can miss a shift, I'm not certain the value is worth the cost. If you are, or find yourself driving aggressively, then I'd change the valve springs out, particularly if you're putting in a more radical cam - not necessarily a 964 cam, but for a cam like a 20/21, which is slightly more radical than a 964 cam.

As is, the stock 3.0 valvetrain is supposed to be good up to 7,300 RPM.
Old 09-22-2005, 10:41 PM
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Nah, I would save your pennies and do the full conversion to higher compression pistons and 'S' cams. I think that putting the 964 cams in is a good option if you still need to smog the car, but it's really only a mild improvement over your other options. Not worth the $$$ if you're going to remove them later on...

-Wayne
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Old 09-22-2005, 11:21 PM
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Quote:
Originally posted by dd74
For a CIS and/or Carb discussion, check out this thread. Look in particular for Tyson Schmidt's comment on the conversion. I'm offering this only as a point/counterpoint argument.
Whoops, you forgot to post the link?

-Wayne
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Old 09-22-2005, 11:21 PM
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Quote:
Originally posted by Wayne at Pelican Parts
Whoops, you forgot to post the link?

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Sorry...
carburetors vs CIS
Old 09-23-2005, 09:31 AM
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Quote:
Originally posted by Wayne at Pelican Parts
Nah, I would save your pennies and do the full conversion to higher compression pistons and 'S' cams. I think that putting the 964 cams in is a good option if you still need to smog the car, but it's really only a mild improvement over your other options. Not worth the $$$ if you're going to remove them later on...

-Wayne
Ok. This is getting really interesting. The motor is from a 78sc made for the US. What kind of changes would I need to perform, other than pistons and new rings, if I do decide to go with your recommendation, as I'm sure will be the case. Is buying a set of used pistons a resonable way to go? Could the SC cam be regrounded to "S" profile?

I plan to run CIS until My wife hopefully gives carbs for my next B-DAY. By the way I am also doing a backdate to the very simple look of a 1972 911s with sc flares in the rear as well removing weight. front and rear f/g bumpers from renspeed will be used to go with the ducktail already installed. The car will be painted gulf blue and have an absolutely spartan interior with modified 914 seats by added bolstering and perlon carpet kit as well as aluminum floor boards/footrests. I look forward to posting pics of my car in its current state and updates as they happen.

Thanks a million to everyone for all your feed back and advice.

Alonso

Last edited by al74911s; 09-23-2005 at 06:47 PM..
Old 09-23-2005, 06:09 PM
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Alonso,
The SC cams cannot be used to make S cams. Its better to start off with a billet. The SC cams can be ground to 964 specs easily
until you get the carbs on your birthday..
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Old 09-23-2005, 10:42 PM
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Quote:
Originally posted by camgrinder
Alonso,
The SC cams cannot be used to make S cams. Its better to start off with a billet. The SC cams can be ground to 964 specs easily
until you get the carbs on your birthday..
John,

How will high comp pistons and the "S" cams run on CIS? I see the CIS as a temporary situation. I just wonder if I might have to deal with detonation or other dangerous hazzards?

By the way.......I assume you make these cams. right?
If so how much????

Alonso S
Old 09-24-2005, 12:28 AM
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