CIS: 3.2 or 3.4 Is it worth it?

[Michel J]

I can`t make up my mind on this one.

Here is what I have put together so far.

- 78 3.0 euro core motor with the larger ports heads
- heads have been rebuilt, with new valves, valve job, and with stronger HD springs from Dougherty Racing Cams
-heads are single plug and will be staying this way
-camshaft have been ground to 964 profile and rockers arms rebushed and re-profiled(Dougherty Racing Cams)
-new rocker shafts
-oil pressure chain tensioners
-longer stroke 74.4mm crankshaft with special oil groove modification machined in #5 journal (for better oiling)
-3.2 rods rebushed and balanced with ARP bolts
-set of 95mm (3.2L, 9.5 CR ) JE pistons and used (but good) honed 3.0 Nicasil cylinders
-SSI heat exchangers
-Magnaflow muffler

I plan on running CIS for a while and possibly going to EFI down the road in a year or two. We`ll see. This 3.2 seemed like a plan I could live with, but, now I am wondering if it would be worth my while and $ to sell the 95mm p and cs set and go for the 98mm setup. I am thinking this would set me back about $1200 after selling my 95mm set on the PP site of on ebay.

This engine will be running on 91 octane fuel and used mainly on the street with about 3 or 4 DE events a year. It will be installed in a 76 911S Targa. The original engine will be kept for smog checks. ( I rebuilt that one last year after missing a shift at SOW. Ouch!!$$ )

Given the above information, what would you do if you were me? Is is worth the extra $ and effort to sell the 95 for the 98s and gain 7% or so more power?

Thanks for your input.

Michel

[BReyes]

At this point yes it certainly does.

Regards,

[9elf]

I am extremely green on these matters but I am possibly heading down the same path. My one question if I may.

Will the 98mm pistons require twin plugging?

Some books say this is the cutoff size and some say the 100mm's are the cutoff for twin plugging.

[alphie]

Considering you have the 3.2 crank and 3.2 rods I would definitely go with 98 p&c.

[Steve@Rennsport]

Hi:

3.2 litre SC motors run VERY well and offer great drivability & performance while remaining smog-compliant.

The 98mm P/C's have too much compression for 91 octane fuels on single-ignition so anyone contemplating such a thing needs to budget for twin-ignition if they live in an area where 91 is tops.

[Michel J]

Steve,

Thanks for your response. The 95mm JE pistons I already have are 9.5 CR(that is what is written on the JE box). If I were to install these, I was thinking of setting(measuring) the compression to 9.5 exactly and hoping for the best running it without twin plugs.
Could the 98mm P/Cs be set to run at a 9.0 CR without twin pluging?
If this would work, is there a realisable power advange between the 95mm and 98mm setups here?
Another thought I have is the difference in weight of the pistons. Less weight with 95mm may mean better reving...I don't know.
Keeping in mind, I live in an area of 91 octane fuel.
Thanks,

Michel

[Steve@Rennsport]

Michel:

98mm P/C's could be made to run at 9.0 and obvaite the need for twin-ignition. I'd still machine the lower plug holes for later use as its quite worthwhile,...

98mm P/C's at 9:1 will be better than 95mm P/C's at 9.5:1.

The difference in how quickly each combination picks up revs depends more on flywheel/clutch and camshaft, than a few grams of piston weight, unless you are comparing Mahle's to JE's.

Maintain a conservative compression ratio for 91 and you'll be just fine.

[Michel J]

You're killing me Steve! It is amazing how one thing leads to another with these engine projects. If I go through the trouble and $ of getting the lower plug holes done, I will be thinking (I know myself), of completing the dual plug setup($2K) increasing the CR, 98mm P & Cs for sure($1.2k), which leads to 3.2 intake(1K), different ignition sytem, maybe a different muffler too, yada yada...

Still trying to make up my mind on the 98s running at 9:1(now).

thanks

Michel

[Michel J]

I have finally decided to stick with the 95mm JEs. After assembling all of the motor parts for the past year, it just seems to me that this engine configuration is meant to be.

Here are some pretty pics for your enjoyment.

[Michel J]

It's a shame these heads can't stay looking like this.



The first spark will be seeing this! Double shame!!

[Michel J]

The process has begun. Just finished putting the gears onto the end of the crank. I iced up the crank and heated the gears in hot canola oil. They went on like butteh. ya!

Notice the special oil channel machined in no. 4 main bearing.

Michel

[ChrisBennet]

Originally posted by Michel J
Steve,

Thanks for your response. The 95mm JE pistons I already have are 9.5 CR(that is what is written on the JE box). If I were to install these, I was thinking of setting(measuring) the compression to 9.5 exactly and hoping for the best running it without twin plugs.

You better measure them. The last two sets of 98mm "9.5:1" JE's (for an SC) I measured were both 10.5:1CR.
-Chris

[Michel J]

I will definitely measure all cylinders and report back. My goal is 9.5 CR...no more no less.

Thanks,

Michel

[JP911]

You can do twin plugging cheaper than $2K (not including the machine work) by going with a 964 distributor converted to mechanical advance. Steve (rennsport) does this mod. You'd have to decide now though, since you need to use a 964 distributor gear on the crank. This is the setup I'm using on my 3.5 CIS motor and it works great.
Parts list:
964 distrib.
donor 3.0 distrib.
2nd 6-pin CDI
Andial twin plug harness
2nd coil
3.6 distrib gear

[Michel J]

JP911,

That is a good idea for twin plugging. For me, single plug offers great ease of maintenance for the time being and one less thing to go wrong.

I may go for Clewett's XDI system down the road. However, I am not sure if this will help as I have heard that the spark energy is not enough to clear fowling plugs. I have also heard that the increased spark timing accuracy is a great improvement over a distributor setup.

Thanks,

Michel