Decision to make - "4" or "6" Options

[Racer]

I am facing a suprising dilemma.

I have come across a pristine '76 2.0 with many of the track car trimmings I desire for my future DE/AX/Weekend car. It has accusump, safety devises cage, Chassis stiffening kit, front oilcooler and aux guages. Turbo Tie rods. 140lb rear springs. 19mm master cylinder, Larger (21mm?) Torsion bars, lowered, solid battery box and longitudinals. Stock seats. Red over black. Emissions equipment removed. Stock FI. Stock 4bolt wheels/brakes. Absolutely no rust. 2 owner car with 95K miles

The other one? a '74.. completely "rotissoried" and rebuilt with 80's SC 3.0 liter converted to webbers. 911S front brakes. 914 Six rear brakes. Stiffening kit. No Cage. No fAux oil Cooler. 6/7x16s with 205s. Beautiful black over black. No rust here either. Stock interior, Sideshifter conversion. Headers that are "heat" capable.

I know its not a bad dilemma to have (IMG:style_emoticons/default/wink.gif) For your consideration, my last 914 was a '70 1.7 that i added a cage, 5 bolt conversion (Carrera brakes), stiffer TBs and rear springs; 2056 motor, oil cooler and tracked and DEd for a number of years, only selling to get a 911, which has since been sold.

Any feedback as to which you might choose and why would be appreciated.

[Jake Raby]

OK so here is my question.....You say these type IV motors are as strong as the sixes Jake. What about (my idea of a failure point) the fact that there are only 3 main bearings in the 4 as opposed to the 7 in the six? I would think this to be the main failure point or area of 'twist' which leads to other 'misalignments'

I counter this with;

The reason the six needs more mains is because it is LONGER to support two extra cylinders, the longer span creates MORE flex over the span of the crank. This means that more support is needed to keep flex down.

We keep flex down by using stiffer cranks and also by using the largest rod bearing journals possible, this reduces "overlap" between the crank throws and makes for a stronger crank as well. This is why my rod journal of choice is .100 larger in diameter than the stock 2.0 journal size.

With our 4340 chromoly billet cranks flex is also greatly reduced, but not even the stock German cranks had flex issues, we turn these to 9,000 RPM with up to FOUR POUNDS removed from them, one of these has four seasons of peoduction racing under it's belt and still passes magnaflux every time.
Here is a picture of that particular crank


Here are the rod bearings out of that engine after 12 45 minute races, one season of use @ near 180HP from 1832cc


And here are the main bearings


I don't see any signs of wear, do you???? Want a few hundred more examples of reality? I have them because I keep logs on EVERYTHING, nothing like data and proof.

BTW- The TIV actually has 4 main bearings.

The 547 4 Cam Carrera engine also used "only 3" main bearings as illustrated in this pic I took while working on one last month(This is actually a 592 from a 904). The 547 engine could sustain 8,000 RPM at Le Man for 24 hours without failure and that was with metallurgy from 1955.
(IMG:http://rdtlabs.com/Pictures/4camcarrera/4%20cam%20Carrera%20pics%20305.jpg)

That being said I have seen TWO broken TIV cranks in my life, both came from 5,000 pound VW buses after being driven across mountains... I have never personally experienced one of these failures and lord knows I have broken at least one of everything else over the years.

The key is proper dynamic balance and combustuion balance, all 4 cylinders firing evenly with little variance in CR and mixture quality is key to eliminating failures.

This post has been edited by Jake Raby: Nov 28 2007, 06:19 PM