3.0 SC with MFI ????

[1972_911T]

Can you run an otherwise stock 3.0 SC engine on MFI without twin plugging and piston changes? Obviously the cam will need to be changed for an MFI drive one and a more suitable profile and the heads tapped for injectors, but other than this with the correctly setup pump will the STK SC pitsons do the job? If Sc run ok with PMO's I carnt see any reason why MFI should be any different.

Steve

[Grady Clay]

Steve,

Yes, the MFI has sufficient capacity. You will need a complete system, modify your 3,0 heads for the MFI nozzles, the appropriate cams, throttle bodies and MFI pump and appropriate pistons for the cams (CIS pistons won’t work). I would choose to start with a 2.4E or 2.4S system but any can be converted.

One reason many include twin plugs in this situation is the desirability to raise the CR, increase displacement, improve the combustion and slightly reduce the octane requirements.

This isn’t for the faint of heart (or pocketbook) but is very doable and great fun to drive. The larger displacement (compared to 2.7) will make the cams ‘act’ milder compared to an equivalent smaller displacement engine. I suspect the peak hp will be at 6800-7000 rpm with a 911S configuration. With some more expense it can be back up to 7300 rpm.

Best,
Grady

[1972_911T]

Hi Grady

Thanks for the input;

Is there no cam that would work with the sc pistons? I know for HP increases and to make full use of the MFI ideally the engine should have at least DC40 cams that will require valve pockets but is there not a milder MFI compatible cam that will work with the SC pistons.

If I was to upgrade to pistons with valve pockets it would be a case of I may as well get 10.5:1 cr which would then require twin plug and the $$ add up. At the minute I just want to work with what I have available without too much spending. I have a MFI pump freshly rebuilt by supertec for a 3.0 engine it was intended for a 10:5:1 twin plug motor but thats not going to happen at the minute. So I want to try and get the MFI up and running on the as close to STK SC engine as possible. It may be a crazy idea/pointless?? exercises but I want to examine the possibility.

If new pistons was the only possible route what would be a safe CR without twin plug?

P.S this is a street motor and I only want MFI for originality

Steve

[1972_911T]

What about a DC15 or DC19 cam ground with a more MFI suitable LSA 103 for example? Could this work?

Steve

[Henry Schmidt]

MFI on a 3.0 is doable and pleasurable.
The option the buying new pistons is to pocket your squish camber SC pistons.
We have been doing it for years. If your engine happens to have a 9.8:1 ROW piston the pockets will lower the compression to a very usable ration.
Low compression and lots of advance makes MFI sing like a canary on crack.

You will want to stay way from the more modern cams (I.E. : high lift less duration) in favor of old school Porsche in an attempt to create more piston to valve clearance.

There is no set spec for valve pocket cut, it's all one off stuff. My feeling is that a 73 S grind would be fine and although not as aggressive as you might choose on a blank page it will still offer thrills galore.

Remember that the pump will move plenty of fuel for 3.0 your next chalenge is air. Make sure the throttle housing are big enough to flow the appropriate air.

[1972_911T]

Hi Henry

How do you determine the depth of the valve recess? is it a case of trial and error, building the engine up then trying to work out how much extra clearence is needed by seeing how much extra tappet to valve distance is required to allow the engine to turn without contact? Or is there a better way?

Steve

[Jeff Alton]

What would be the minimum piston crown thickness one could machine the piston down to?

Cheers

[1972_911T]

Originally Posted by Jeff Alton

What would be the minimum piston crown thickness one could machine the piston down to?

Cheers

Good question Jeff, anyone know?

Anyone got any pics of a CIS piston machined for valve pockets?

Steve

[Henry Schmidt]

Originally Posted by 1972_911T

Hi Henry

How do you determine the depth of the valve recess? is it a case of trial and error, building the engine up then trying to work out how much extra clearance is needed by seeing how much extra tappet to valve distance is required to allow the engine to turn without contact? Or is there a better way?

Steve

Lift at overlap plus desired clearance minus the piston to head clearance.

Originally Posted by Jeff Alton

What would be the minimum piston crown thickness one could machine the piston down to?

Cheers

.280 " for turbo and .175" for NA with less than 10.5 compression. Higher compression's will require thicker domes.
This assumes a forged piston. A cast piston will require at least 25% thicker dome.

[Henry Schmidt]

Originally Posted by 1972_911T

Good question Jeff, anyone know?

Anyone got any pics of a CIS piston machined for valve pockets?

Steve

Ask and we'll do our best to fulfill even the obscure request.
Here is a 2.7 CIS piston cut for early 911 "S" cams.

[1972_911T]

Henry in the example posted above how much do you estimate the CR was reduced by the removal of the material?

Steve

[Henry Schmidt]

less than .2 points. 8.5:1 down to 8.3:1.
The gain in performance from lift and duration will far out weigh the loss of compression.

[safe]

Henry, is there any gain, other than the price, to cut a CIS piston instead of using a 2.7 RS piston or a JE?

[svandamme]

2.7 pistons are 90 mm
3.0 pistons are 95 mm

so the former isn't an option if you want a 3.0 MFI engine

[Henry Schmidt]

Originally Posted by safe

Henry, is there any gain, other than the price, to cut a CIS piston instead of using a 2.7 RS piston or a JE?

There are four benefits to reusing your low mileage pistons in this fashion.
1) You own them so they're free
2) You don't have to wait to continue your rebuild
3) Squish chamber dome functions better than other options in a CIS environment
4) Recycling your old useable parts is the "green thing to do" The production of aluminum products requires large amounts of energy and factories generate a large "carbon footprint"

[Fidalgo911S]

Originally Posted by Grady Clay

You will need a complete system...
This isn’t for the faint of heart (or pocketbook)...

I've got my '74 2.7 on the engine stand ready to tear down and rebuild. I planned on a stock rebuild but for years I've been hearing nothing but praise for MFI (when it runs right).

So that complete system....what kind of $$$ are we talking about? I'd like to know if it even falls in my budget. That way if it doesn't, I can quit thinking about it and get on with my life.

I'm modifying my nicely-optioned '74S into a '74 Carrera clone and how nice would it be to have a copy of a 911/83 motor in the back?

Thanks,
Nate

[svandamme]

pump will set you back at least 2 grand to find one and get it rebuilt to 2.7 or 3.0 spec
stacks and throttle bodies , another 2 grand to find em and bored out/rebuilt

mfi parts don't come cheap, there's a limited supply
the parts you might find cheaper, but there's no DIY about getting them in the right spec and good condition

4 grand is ballpark figure, you could off course be very lucky in finding very good components for very little price, but i wouldn't bet on it, when all is said and done, you might still overrun that 4 K budget

i'm not even including price for the belt, thermostat hoses, tuning tools , support bracket, engine tin with mfi tube, rear engine tin for mfi belt,oil feed and return tubes, fuel feed pump, injection lines, injectors, mfi fuel filter, airbox,relays,the little things

i think it still requires a dose of luck to find and get the stack/tb's and pump fully sorted out, properly for 4 K...

[Jeff Higgins]

Hmm... a '72 T with a 3.0 liter running on MFI? Why, what a grand idea. I'll have mine in my car and running this winter.

I think you will be leaving too much on the table if you do not change to much more aggressive cams, and possibly higher compression. The beauty of MFI (when compared to CIS) is that it allows you to do just that. Keeping the CIS pistons sounds like it will severely limit your cam choice; the clearance notches can only be cut so deep before they go right through. In light of this, I'm not sure I see any advantage to switching over.

I have not priced MFI parts lately, but it seems to me that complete systems come up (for T's and E's anyway, S's aren't showing up as much anymore) for less than half the cost of a set of PMO's. I scrounged all of mine (three complete systems in addition to the one running the car) back before they started going up. Anyway, assuming you can score a system for a reasonable cost, the actual cost of working on it and adapting it to the 3.0 shouldn't be all that daunting.

I had my set of plastic T stacks bored locally for less than 200 bucks. They are now 36mm S spec. I found a set of newly refurbished T throttle bodies that had been bored to 36mm for the same 200 bucks. I had the MFI port drilled into my 3.0 heads for, again, about 200 bucks. I have a freshly refurbished 015 ('72 T) pump that I picked up with a complete system; I plan on starting out with it as-is. If I find out I need to swap space cams in it down the road, I will do that then. (Henry, I'm not sure if you remember me, but I talked your ear off about this one last spring, and you gave me some great advice.)

I guess what I'm getting at is that the actual MFI system shouldn't have to set you back all that much. If that is all you want. Where the money should be spent is in the internal engine modifications that this system allows. On cams and P&C's. You just won't see the potential inherent in this system without these internal changes. Enough so to where I don't think it would be worth the effort without them.

I've spoken at length with John Dougherty (our own "camgrinder" here on Pelican) and he has come up with a custom set of cams that he feels will work well with my application. They have enough lift and duration that there is no way to make CIS pistons clear the valves, so I went with the JE 9.5:1's for their very deep valve pockets. The motor will have essentially stock internals other than these two items. They really don't add that much to the total cost of a full engine build, but they will wake it up in a way the CIS friendly cams and pistons never could. That is the real advantage of MFI - the ability to go a little wilder on this other stuff.

[shbop]

signing-up

[Fidalgo911S]

I see there is quite an expense involved. Maybe not this rebuild.

Thanks for the info and my apologies for the hijack.