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2.4 Engine Build advice
Phoenix
Hello All,
It‘s been quite a while since I‘ve been here to the point where I lost the login info and needed to reregister. Life has gotten in the way.
Some of you may recall I have an Oxford/ Dalmatian Blue 73 911t. A lot has been done to get the body and core mechanicals to as perfect as I can afford :-). There have also been a few „upgrades“ and personalized modifications along the way. Bronze bushings, stiffer torsion bars, drilled rotors, side vent windows, and a few other bits and bobs. Nothing which can’t be reversed. Automobile Associates in Canton, CT have been terrific partners in getting Phoenix to where she is today.
The two areas I either do not trust myself to do or frankly do not have the patience for are engine rebuilding and wiring. AA will be doing both.
The engine build goal is to cost effectively increase reliability and low to mid range torque for spirited driving. I would like to switch to PMO EFI with Clewett management.
Beyond that AA is recommending boring the case spigots and going with 93 mm JE p&c’s and 10.3 –1 compression ratio. This will require twin plug, which I‘m told will help with low speed response. The heads will also need to be modified for the twin plug, and the port sizes would be 36 intake and 35 exhaust with GE30 cams. Front mounted cooler will also be needed. They estimate this engine would produce roughly 190ftlbs of torque.
Alternatively, I am considering something along the lines of a period correct (sans EFI) ST motor.
The current engine will be used as the base and is configured as:
- [ ] PMO carbs 40
- [ ] Bore: 74mm OEM Mahle 911s piston and cylinder
- [ ] Stroke: 70.4mm
- [ ] Compression Ratio: 8.8:1
- [ ] Valve sizes: 46mm intake / 49mm exhaust
- [ ] Port sized: 32mm intake / 32mm exhaust
- [ ] Camshaft specs:
- [ ] intake duration 270/ exhaust duration 254
- [ ] Exhaust opens at 36ABC/ closes at 20ATC
I‘m hoping this board can give me some advice on these options vs the goal of reliability, strong usable torque, fuel efficiency and cost to build.
What do you think?
Many thanks in advance!!
... Oliver
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Good to see you are back, Oliver...it was great to have you driving with us all those years ago. Hope you’ll be ready for a spring drive
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Thank you blucille.
I am hoping to get back into group runs. Phoenix has been sitting in a bubble for too long.
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Going to twin plug seems like a lot of expense for the HP gain. You should be able to get over 200 hp from a 2.7 S modification with carbs. Sure you can do twin plug and EFI, but how much HP are you trying to get? Anything over 180 should be a lot of fun to drive in that car.
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In 46, Ex 40. 84mm bore. In a low comp motor, twin plug will net u “0”.
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I think your goals of fuel economy and cost may pretty much limit you to what you already have. Just for grins check for full throttle, you'd be surprised how many cars don't have it.
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Thank you all.
I too believe twin plug may be overkill.
My goal is not necessarily an increase in HP rather a noticeable increase in low-mid range torque. I‘m not looking for top end speed which is limited to 75/85 mph speed limits anyway.
What are your thoughts on:
- PMO EFI w/ Clewette mgt.
- 93mm bore w/ JE p&c‘s
- keep 70.4 stroke
- GE 30 cams
- remain Single Plug
What torque would this motor generate in the 1500-4000/ 5000 rpm range?
Best... Oliver
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For torque you need displacement. I suggest you take a look at Bruce Andersons book, " Porsche 911 Performance Handbook", just about every combination of components is covered there. Or, just look at factory engine specs, pick the torque that would satisfy you, and build to that spec. You could also mothball your engine and plug a lump in.
Don't try and invent the wheel, copy the design.
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I remember the story of this car from the original fire a long time ago.
First off, I think you have an error in your valve sizes, I think you mean 46/40, not 46/49. Those are the valve sizes on the late T head, of course you currently have 32/32 nailhead valves, hence your desire to port to 36/35 2,7 RS spec (and not to 43/43 RSR!).
I assume the reason you are using 93s is because the largest factory Mahle was a 90 (From the 2,7) and you are using LN to get you to 93. The custom JE's don't cost any more, so might as well go for it. 93^2 * 70.4 *3.1416 / 4 * 6 = 2,869cc. No replacement for displacement.
Would I twin plug? Absolutely yes. The reformulated panther pee sold to unsuspecting motorists as gasoline is not getting better-- for the remaining few years we'll be able to BUY gasoline you want maximum insurance against detonation. Practically speaking that is accomplished by running less ignition advance, which has the added benefit of moving the PPP (peak pressure point) closer to TDC, hence more torque. I wouldn't hesitate to go to 10,3:1 static compression.
In terms of engine management the PMOs are the cheapest option, but you still need either an old-school twin plug distributor or a crank trigger. This is a no brainer in terms of the stability and tunability of the ignition signal-- crank trigger definitely the way to go. To run the plugs Clewett will sell you an Electromotive TEC, which can also run the EFI. The PMO EFI manifolds have the appearance of carbs for that sleeper look, although if I were doing it I would think about a TB that gave a straight shot into the outer cylinders for more balanced airflow, like the Jenvey kit.
Gordon's absolutely right-- the advantage of twin plug is nil if you don't bump the compression.
Why not preserve the original engine and start with an aluminum 3,0 and build a 3,2 short stroke or larger? The engine's physical dimensions are the same, you still need oil cooling, all the ancillaries cost you the same, you will spend less on porting, you already have a 915 box to handle the power (could be improved) and the extra 50 HP will make you smile every time you drive the car. :)
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I do like how Clewett gives you all the stuff in one package. This is not an advertisement.
http://www.clewett.com/bmz_cache/2/2...ge.687x550.jpg