85 octane CR?

tadd · 01-06-2006, 06:30 PM

Howdy all:
The background: I have a 912, no snickering...

... they are porsches too, even if it does only have about 80 hp at this point. Anyway, I was shocked (shocked, I say!) at the cost of a hot-rod 616 rebuild. I guess when everything has to be custom made, things get expensive quick.

I have access to a cheap, in boxes, 914-6 and would follow Mr. Andersons advice for the 2.0 911s and use 2.7 or 2.8 p&cs for a nice displacement boost. As for the CR each set provides, thats what the question is about...

First: Yes, I know this motor has a non-counter weighted crank and is therefore rpm crippled... I figure that the smaller valves (42/38) would run out of airflow about the time I run out of revs (5.8k rpm).

Since this will be a street motor, I was curious as to what would be the highest CR that could be run with 85 octane gas and a twin-plug ignition set up. The question is spured by a buddy that is an economics graduate student and his dissertation is on the oil/gas industry and he swears to me that gas will go back up - and peak about 3.60 a gallon for regular next summer. He says that the refining people are not ready to deal with the last phase of sulfur reduction laws passed many years ago, so there is going to be an artificial scarcity and therefore drive the price thru the roof. He thinks Katrina was a chance to test the waters... Hence, the 2.7 pistons being 'optimal' with only a 7:1 CR (Mr. Anderson again).

Keeping my wifes 2000 beetle turbo in premium last summer was as expensive as keeping my 76 international scout (@8 mpg) in the cheap stuff!

The 'other' choice is to go propane and with a 104 octane rating at 70*C, you can go compression crazy... I switched the scout a few weeks ago...

I'd like to stay gas for convience, but all options are on the table at this point.

Thanks in advance,
tadd

RoninLB · 01-06-2006, 08:08 PM

I use 8.5 RS/Euro pistons w/87 octane for the 1st time this winter.

I don't recommend doing this unless you can monitor EGTs and CHTs for detonation. Also the same engine specs may have different detonation parameters.

I'd say it's logical for you to maintain 91-93 octane on a streetable CR.

Carrera3.5L · Carrera3.5L's Garage

Where in the country are you that you have 85 octane gas?

Ralph

Jeff Alton · 01-06-2006, 11:00 PM

Apparantly you can get it in Utah? I am on my way to Salt lake City on Monday and tuesday, I will check it out...

Cheers

tadd · 01-07-2006, 09:47 AM

I am in new mexico.

Most modern commuter cars don't really even need 85-86 and will run quite happly on 82-83. It has been talked about for years to introduce a 'cheapie' grade of gas. The katrina mess has sparked the debate again. In the current 2.30 a gallon regular gas world the 82 would run around 1.50 or less.

The porsche is my daily driver and if I am going to build anyway, I wanted to know how low I needed to go to run the cheap stuff.

tadd

RoninLB · 01-07-2006, 11:32 AM

Oh.. So you want to build an engine around very low octane gas and still drive in desert heat.

I'd PM member camgrinder. There's more to combustion pressure than compression ratio. Then there's also ignition and combustion monitoring considerations.

Actually it may make some sense to spend $2,500 on the EGT & CHT monitoring, low compression 8.5, and T cams.

good luck

davidppp · 01-07-2006, 12:18 PM

Hello there.

This is a veery expensive way to get a 2.5-ish motor.

First, the 914-6 engine is worth a premium price..they are rare..so selling it will raise some useful cash.

Second, 2.7 motors are cheap and plentiful.

they come with piston squirters and a stronger case bigger valves much better combustion chambers for your crappy gas, as well as a better clutch and often the bigger oil pump.

not to mention the exhaust, correct oil lines and tank and tacho and 915 box often coming cheap with the motor.

A basic rebuild is then all you need..

I suggest you do some math and you will see you will be ahead and have a much better motor.

BA's unique book is now dated in many ways.. very few early mag cases now get machined out .....maybe only as cheaters for racing..!

kind regards
David

Kind regards

Michel J · 01-09-2006, 09:58 PM

I know that many 356 racers prefer non-counter weighted crankshafts. They are said to spin up faster, kind of like using a lighter flywheel. These engine spin up to 8000 rpm NO problem.
My 911S with 2.7 at a measure 8.5 CR runs perfectly on 87 octane with full advance, and passes CA smog too!
You may the better off selling the 914-6 engine and rebuilding the 912 at 8.5 CR with a stock cam after all. The 912 are actually a lot faster than most people think and they have better fuel economy than 911s.

kenikh · 01-10-2006, 08:27 AM

You don't have to be limited to low revs on an a non-C/W crank. If you do a balance of the ENTIRE rotating assembly after it is assembled (only to be done by the best Porsche machine shops) you can have your cake and eat it: you'll be bale to rev to 7500 reliably and spool up like lightning. The fatal caveat is that this kind of work is very expensive.

IMO, if your engine is truly an original 914-6 motor, sell it. They are indeed very rare and in demand. You'll be able to get a nice 2.7 for a lot less than you'll sell the 914-6 engine for.

74goldtarga · 01-10-2006, 08:21 PM

This is a fun theoretical question, and I have no expertise to offer. I don't understand your intent however? You have a 4 banger that runs, does it need a rebuild? If you want to go cheap just drive it as is - I'm sure the mileage is great. The cost of a rebuild of just about any kind can buy you gas for tens of thousands of miles. You should have your economist friend do break even analyses for the savings you would have to obtain with cheaper gas using 85 or less octane vs. the cost of a rebuild to 7.1 CR twin-plug. This is a lower CR than my tired 2.7L stock, I run 87 no problem, I bet it would run on 85. If you are going to rebuild the engine and twin plug it - rebuild it with a CR that will make it dance and pay the extra for regular (or premium gas). Someone correct me if I am wrong about the folly of spending money and time to develop a twin-plug rebuild to run low octane gas. I mean well Tad, good luck.

tadd · 01-11-2006, 02:59 PM

Thanks for the replies...
I guess a bit of clairfiation is in order. The 4-banger 616 can run 10-12k for a hot rebuild. Stock it was rated at 90HP, hence I would like a bit more. Alot of the 912 folks are switching to VW type 4 power since porsche did this anyway for the lone 1976 912 model year (and the 914-6 of course). Even T4 power is not 'cheap'. Well I crossed a motor-in-box that happens to be from a 914-6. It 'only' has 911T specs, so I figured I would add some bigger P&Cs, an E (or S) cam, some new bearings, and call it a day. I priced it to be much cheaper than 616 or T4 power since the porsche crank, rods, ect are quite up to the task.

It is also the lightest six (mag case, no couter weights) and would therefore be the closest in mass to the 4-banger. This is a good thing with the short wheel base.

Then I found the thread on twin-plugging 'at home'. Since I have access to a brigdeport mill at work, I figured on doing a two plug conversion (especially now since supertec has a distributor for the 2-2.2L engines)... Since it doesn't have carbs, I have been reading on MFI... even more weight lost.

So, who could have figured I would trapes across a motor that is even more rare than what I have??? At least you can still buy all the parts for the six unlike the 616.

So given all the pluses (lack of mass being the big one - and the fact that a bird in the hand...) am I really shooting myself in the foot using this motor? If I go ahead and build a well ballanced 2.2L twin plug with MFI will I have 'runied' it to the point that a 914-6 owner would pass it by or would I have increased it's appeal (value)?

tadd

edevinney · 01-11-2006, 03:16 PM

I think you really should review your overall goals, they seem over-constrained and at times at cross purposes. Is it more power? Lower operating costs? Inexpensive engine build?

My 2 cents: A 911 engine build will be mucho dinero no matter how you slice it. Remember, too, that Henry's dual-plug dizzy for 2-2.2L cases was posted with explicit mention of being Expensive. My understanding of MFI is that it will may be spendy to modify, too. In the end you might have a collectible engine case but probably not as interesting as a 914-6-in-a-box.

In the end, to spend all that coin for a motor hamstrung to run on 85 octane fuel seems like a waste to me. If you must have a 6 and want performance the LPG idea seems most promising. Otherwise build a nice street T4 and enjoy it.

IMO, of course.

tadd · 04:00 PM

Ed:
Guess I should have spelled out that I pretty much dumped the low compression ratio idea. I figured it would only be reasonable with much more displacement, e.g. 2.6L.

If I keep the case, crank, rods, heads (abet with an extra hole) and stick with 2L which it came with (or maybe 2.2) P&Cs (so no machining on the case like the 2.4 and bigger) I can't reason why it would have 'lost value' as compaired to a total stock 914-6... but hey, thats why I am asking the experts!

As for why six power, a bone stock 616 build is over 6k for that ~90 hp. The parts I have priced for the six is about half that if I stay with the stock 2.0 size and change to new JE pistons (~1.5k) and some used E or S cams. So that disty would have to darn expensive to put me over a stock rebuild. Clearly I must be missing something...

As for the MFI, I am really curious as to if it is the perfect answer for liquid propane injection... Peak octane all the time thanks to the latent heat of vaporization of the liquid propane.

tadd

74goldtarga · 01-13-2006, 07:12 PM

Tadd,

I just came back to see where your thread is going. You may want to repost as a new subject if you have let the 85 octane idea go - you will probably get more and much better (than mine anyway) replies if you post as a 914-6 MFI rebuild or whatever it is you really want to do. Good Luck.