Fuel enrichment on a twin plug 3.5 CIS motor?

[JP911]

I'm getting to the final stages of putting my recently built 3.5 twin plug CIS motor (naturally aspirated) on the road. The engine has 78-79 CIS currently. My question is: do I need to enrich the fuel further to prevent detonation? I know that this is usually done on turbos, but what about higher compression N/A CIS motors? Any input would be greatly appreciated.

-Jon

P.S. For those wondering, "Why did you put CIS on a motor like this?" The engine was going to go in my 76 targa which still has to pass smog in CA, and I thought I might be able to get it by. Now the engine is going in my recently acquired 74 coupe roller, so it's smog exempt. I may go EFI sometime down the road once the trauma of the "Porsche Engine Build Money Hemmhorage" wears off.

[Porschekid962]

Unless you have gone nuts with the CR and cams you should be able to adjust the fuel at the fuel distributor just fine.

[JP911]

I believe the CR is 10.5:1 and I'm using a 964 cam, so nothing too crazy.

[starlifter1]

which piston and cylinder set did you use?

[JP911]

Mahle

[Carrera3.5L]

Hey Jon,

I would really be surprised if Bert built your motor anywhere near 10.5:1 CR. The 100mm Mahle's generally spec somewhere in the 9.2 - 9.8 range at normal deck heights when all is said and done, both versions for the Motronic dome and the RSR style with deeper valve pockets.

I think you got the same 100mm RSR's at the same price that I paid, right? If so, Bert will have to tell me where he found the extra point of compression.

When are you planning on firing it up?

Ralph

[JP911]

Hey Ralph!
You're probably right about the CR. I thought Bert had mentioned higher compression, but I'm pretty sure that they are the same P/Cs that you used. Sadly I did not get the same price as you.

I fired the motor last Sunday and it started on the first crank with spark and fuel connected, but I couldn't drive it as I needed an axle. I took it for its virgin voyage on Wednesday afternoon after Tom Amon (from Mobile Works) spent the day cleaning up the wiring and other odds and ends. It is a blast to drive! But, I'm already thinking about other means for induction to get more out of it. Carbs? EFI?

So what do you think about the fuel enrichment thing? Bert shimmed the fuel distributor to richen things up, but I'm worried about damaging this new motor. Maybe I'm being overly paranoid...

[iamchappy]

If you want to keep what you have for CIS and want to rich things up on boost why not add a micro fueler.
I use a 911 sc cold start injector that fires from a Hobbs pressure switch in mine. It will only cost you about 150.00 in parts and it keeps my AR in check.
It's not state of the art or fancy but has work well for me and many others.

[JP911]

Iamchappy-
Can you explain further how this works? My car is naturally-aspirated, so what would trigger the switch?

[Carrera3.5L]

Hi Jon,

I think some minor tweaks to the CIS will be adequate for what you have, assuming you have an actual static compression ratio in the low to mid 9's. We didn't do any type of ancillary enrichment but that was a long time ago and things may have changed.

I think if you really have 10.5:1 with CIS running on 91, you may be in trouble, even with twin-plugging the motor. I think you'll be spending some serious money putting in or blending 100 octane every fill up.

Now that your plans changed and the motor is in a car that doesn't require smog testing, you may consider changing the CIS to PMO's with a suitable cam profile when your budget allows. That would REALLY wake the motor up and make it fun to drive!

If you used the same high-dome RSR style pistons that I used, you will have more than adequate piston-to-valve clearance for a more aggressive cam profile without having to modify the piston domes. You really wouldn't have to go into the motor internally other than swapping the cams.

When are you going to come down with it and show Bert & I the finished product?

Ralph

[iamchappy]

The pressure switch would have to work on pressure or vacuum or the injector could be fired from an rpm module.

[JP911]

Ralph-
I looked up the part numbers on my documentation from Bert and checked them against the Andial listing on their Web site. You were right, 9.8:1 CR. Phew! I've already done a little research into the PMOs, however, and I'm tempted. What do you think of carbs?

[beepbeep]

You don't need it. Frankly, 99% of turbos don't need it either, it's just that owners like to slap all kind of widgets on them to feel that that they are doing something to "help their baby". Even at 1 bar, 930 (at least with euro fuel heads) run too rich anyway. CIS never had problems with fuel delivery, it has problems with air delivery.

Do a run with wide-band lambda and check mixture to begin with.

[Henry Schmidt]

The only place you will encounter lean running conditions is a properly tuned CIS engine is on cruise.
The mixture is controlled by moving a flap/ plate (air flow meter) and as the air moves across the plate pressure pushes the plate (either up or down depending on the engine) and turbulent air moves the plate farther than laminar air (smooth air). On acceleration the air is turbulent and as the engine starts to cruise the air flow goes laminar and plate settles producing a lean running condition.
In most cases this is ok and actually improves gas mileage but under performance situations this can do damage to the engine.
As an example, a 1+ mile straight away at 6500 rpm.

[Carrera3.5L]

Originally posted by JP911
Ralph-
I looked up the part numbers on my documentation from Bert and checked them against the Andial listing on their Web site. You were right, 9.8:1 CR. Phew! I've already done a little research into the PMOs, however, and I'm tempted. What do you think of carbs?

I would trust the documentation you have from Bert and not what is on Andial's site. That is just a generalized number, there are alot of variables when calculating compression ratios.

Me, I'm not a carb guy but in your instance they would be very good, much better for performance than CIS when complemented with a nice set of camshafts. Your power/torque/throtte response will dramatically increase, your fuel mileage will radically decrease, and the sound of a 3.5L on PMO's with a nice, sport exhaust will bring chills to your spine. You will easily have over 300 horsepower if set-up properly

The ITB/EFI route looks more trick & modern but the price tag will set you back much more than a new set of PMO's. The carbs would look right at home on your '74 and would be closer to being "period correct" if that is at all important to you.

Since I have a Motronic motor, I myself would never "backdate" to carbs but rather "update" to ITBs/engine management but since I live in California with a 76> car it doesn't really matter, does it?

Ralph