Building a 3.5L carbed Mod S engine CR query

[Bobboloo]

Hi guys,
I'm building a street engine for a customer and wanted to get some thoughts on the compression ratio. The engine is based off of a 3.0L SC long block. I was originally going to build him a 3.2L Mod S cam engine with SC 39mm intake, 35mm exhaust and 48mm PMO's. Single plug using the Mahle 3.0 to 3.2 P+C set. But after breaking the engine down I found he had a spun bearing. I suggested just repairing the crank and continuing with our original plan but the customer decided he would rather buy a replacement crank rather than repair. I informed him a good std/std 3.0 crank is very hard to find and expensive. Long story short he opted to get a 3.6L crank which are more plentiful and relatively inexpensive. So now I'm building a 3.46L Mod S (Dougherty DC40-106) So I needed a p+c set with a smaller dome volume to compensate for the additional stroke and bought the Mahle 3.2 to 3.4 set (motronic) which has a static ratio of 10:1 CR. The pistons have a Max Moritz style dome that pools toward the single plug. I'm setting the deck height at 1.25mm which should give me a static CR of 10:1.
My thoughts are that I'll be okay with the single plug at this CR ratio considering the piston design and the overlap cams. Car will be running on 91 octane (California premium) Mahle doesn't offer a lower CR for this application.
The car will run in Socal. Any thoughts on the 10:1 compression ratio? Do you think single plug is risky?

[stownsen914]

Are you sure it's still 10:1 with the longer stroke?

Even at that, I've heard that's pushing it with single plug and on 93 never mind 91. Let's see if some experts chime in.

[Jeff Alton]

Measure the actual CR first, dont rely on "should give me" XX CR. Camshaft choice definetely plays a role in dynamic compression.

Cheers

[Bobboloo]

I haven't cc'd the heads yet but if they are 90cc then the CR I calculated would be 10.04 to 1. Mahles claims 10:1 for a 1.25mm deck height.
Regarding the stroke. The pistons were designed for a 74.4 mm crank (PS 98-010). With the 76.4mm crank the extra stroke can be made up for with a 1mm shim. Besides I always measure my deck height which I've set at 1.25 mm.
I find it interesting that Mahle chose a static 10:1 CR at 1.25mm deck height for a piston designed for single plug head keeping in mind that it's supposed to be able to run a cam with no overlap. They don't offer a lower compression for 3.2 to 3.4 conversion.
I may just set the deck at 1.5mm which I calculate to give me a CR of around 9.8:1 if the heads measure 90cc. I just thought some feedback might give me the confidence to go with the 10:1 ratio.

[stownsen914]

I've heard that 3.2 heads can be up to 92 cc. Mine measured 93 when I disassembled my engine (mine are cut for a second plug). Extra cc would help you here, so probably worth checking.

[Henry Schmidt]

I would limit compression to a number closer to 9.5:1 unless you have a ignition management to can accurately control ignition at every low and rpm. The difference in performance can be recovered by additional ignition tuning.
California is a crazy place where "green ideology" overrides common sense and as soon as the "green" hysterics figure out that octane keeps old cars on the road, 85 octane premium won't be far behind.

[Bobboloo]

Thanks Henry. Better safe than sorry. The difference in performance between 9.5:1 and 10:1 can't be that much to make it worth while to risk it. BTW the ignition will be points and mechanical advance dizzy so not the most accurate spark setup.

[nospiners]

Originally Posted by Bobboloo

Thanks Henry. Better safe than sorry. The difference in performance between 9.5:1 and 10:1 can't be that much to make it worth while to risk it. BTW the ignition will be points and mechanical advance dizzy so not the most accurate spark setup.

What makes you decide on such an ignition set up to go with such good engine built ?
So many better alternatives...

[Dpmulvan]

Originally Posted by nospiners

What makes you decide on such an ignition set up to go with such good engine built ?
So many better alternatives...

I agree obviously money isn’t the issue.

[Henry Schmidt]

Originally Posted by Bobboloo

Thanks Henry. Better safe than sorry. The difference in performance between 9.5:1 and 10:1 can't be that much to make it worth while to risk it. BTW the ignition will be points and mechanical advance dizzy so not the most accurate spark setup.

What distributor are you using for a 3.0 based build that uses points? Am I reading something into your build that isn't there?

[Bobboloo]

I was thinking about using a 2.4/2.7 dizzy. I need to spin clockwise of course with the 76.4 crank.

[Henry Schmidt]

We have a way to covert the 3.2 Carrera distributor to mag pulse.
We are crazy behind on our distributor project but building the distributor you need is pretty straight forward.
If you do use the 964 distributor drive gear remember to replace the driven gear on the distributor. They are not the same. They turn the same direction but they are different diameters.
If the engine is still apart, the easiest way to deal with your issue is to put an SC drive gear on the crank and use a modified SC distributor.

[Bobboloo]

Originally Posted by nospiners

What makes you decide on such an ignition set up to go with such good engine built ?
So many better alternatives...

It's ultimately not my decision and money is always an issue. I'll try to steer him toward a programmable setup. I offer alternatives but the customer gets to decide on most things but there's of course some things I won't do that put me at risk.
I initially wanted to build him a 3.2 using the 98mm Mahle P and C's set using the Mod S cam because it's been done and the results are admirable. IMO it's plenty of engine for his car and his driving capabilities but someone else whispered in his ear about replacing the crank with a longer stroke instead of repairing the 3.0 crank. The 3.0 is a better crank than the unbalanced 964 crank IMO. One of my main gripes for me is not being able to use the 964 dampener with the 3.0 case. It won't fit in the 3.0 cradle. So not ideal IMO.

[Bobboloo]

Originally Posted by Henry Schmidt

We have a way to covert the 3.2 Carrera distributor to mag pulse.
We are crazy behind on our distributor project but building the distributor you need is pretty straight forward.
If you do use the 964 distributor drive gear remember to replace the driven gear on the distributor. They are not the same. They turn the same direction but they are different diameters.
If the engine is still apart, the easiest way to deal with your issue is to put an SC drive gear on the crank and use a modified SC distributor.

Thanks for the tip. I have a spare 3.2 dizzy I can rob the gear from if I can't convince to go to a different ignition.

[Mixed76]

What rods are you using, do you have to go from 130 to 127?

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[Henry Schmidt]

Originally Posted by Bobboloo

............edit........and 48mm PMO's...........

I think you might have meant 46mm not 48mm. PMO makes 40, 46 and 50 mm carburetors.
An engine with those specifications will greatly benefit from a proper exhaust. Porsche heater boxes or 1 1/2" headers will be too small.
We are making a new design 1 5/8" short primary header system that when mated to a sport muffler should be incredible with your long stroke small bore application.
These are bolted to a 3.4 liter with 46mm PMOs

[Henry Schmidt]

Originally Posted by Mixed76

What rods are you using, do you have to go from 130 to 127?

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The only Porsche 911 crankshafts that used 130mm rods were 2.0-2.2 and GT3 engines. 964/993 cranks use 127mm. The 3.0 used a 127.8mm rod.
For this application the stock rod would be 127.

[Bobboloo]

Originally Posted by Mixed76

What rods are you using, do you have to go from 130 to 127?

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964 crank with the 964 rods it came with.

[Bobboloo]

Originally Posted by Henry Schmidt

I think you might have meant 46mm not 48mm. PMO makes 40, 46 and 50 mm carburetors.
An engine with those specifications will greatly benefit from a proper exhaust. Porsche heater boxes or 1 1/2" headers will be too small.
We are making a new design 1 5/8" short primary header system that when mated to a sport muffler should be incredible with your long stroke small bore application.
These are bolted to a 3.4 liter with 46mm PMOs

I've informed the customer that his exhaust is his limiting factor for Max power output but he can change that if he wants to spend more. I also told him he should also consider that his 915 can only take so much without additional help.
I wish he would have stuck with the original engine I suggested but you know how it can go. You want people to be happy and not rain on their parade.
Carbs are 46mm. My bad.

[Bobboloo]

I really appreciate all the replies. I will be building a similar engine to this one in the future for my mid year car but I will do things differently where the build makes sense from the induction through to the exhaust. It'll use a GT3 crank, Mahle 98mm P+ C's ITB's, twin spark plugs with engine management and a nice 1 5/8" exhaust setup like Henry's. A reinforced 915 with oil cooling will also be a part of the equation.