924Board.org

[Peter_in_AU]

Ahhh, Grasshopper* you've spotted the one flaw in my cunning plan.

Being that I design software for a "living" the simple answer is "I don't know". What I need to do is front up to a competent engine builder and find out what is best/cheapest, go for the 1G rods and shave 0.4mm off the rods' big-end or go for the 2G rods and somehow increase the BE width by 1.7mm or shim the crank.

How about I leave that up to you

The good ol' V8 boys have been doing this for a thousand years so the best solution will be known.


* If this means nothing, find someone really old and ask them about "Grasshopper"
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

[ESC944]

Peter,

yea their are lots of options to get them to work...

However I still ask my main question... can we get more of the rod journal another 2mm or something? and then we could try some other rods... their are other rods that are 150mm and have a 48mm size, at least on the domestic side I know of a few, have to check my import data.

I am still keen on a longer stroke... just 2mm in stroke would give us 2.2 liters.

I will post some stuff later... I want to use something forged for both, but that I can pull out of junker to try and then... get cheap new.

[ESC944]

Engine Series
Pin dia
B E bore
Length
B E width
Weight
Part Number

924
.945”
2.032”
5.670”
1.098”

Well if you wanted to go to bigger Rod Journals:

Pickup Truck
22R
.866"
2.205"
5.827"
1.090"



I will pull out my books and take a closer look at this approach. The only other thing I see is bearings.

Seems to me machining seems pretty easy, get right length and shave the rod... but what about the bearing.

I know their will be some rods that have slightly smaller width and the right length and the smaller than stock journal size... hemm but if they are slightly narrower... hemmm tolerances....

[sandgroper]

The machine shop ground the rod journals wider on my crank when he offset ground it,but you can also have both sides of the rod big end ground as well,costs more to do the rods,he did not charge me any extra when grinding the crank.

[ESC944]

Now that sounds like a solution... just grind the crank, while having it knife edged and all that... very nice... No issues with bearins either... simply match the width of the new rod.

Since we are talking about this, I figured I would revist some math and basic formula:

If a stroker crankshaft is not to be sued, the piston can be chosen first, and the exact stroke length selected by calculating backwards to make a common and inexpensive piston useful. <--PETER's Principal Idea

So to calculate stroke length from a known piston:
Stroke Length = 2 × (Deck Height - Rod Length - C-Dist. - Piston Deck Clearance)


Now with regard to a stroker crank... since we are talking grinding the rod journal size to fit a different rod... I assume Peter means that by not grinding all the way around, and instead ofset grinding, we are able to move the centerline, with regard to the rod big end and change stroke.

In other words, instead of adding material so that the rod journal center is moved to the out side edge and their by increasing stroke. The intent is to grind more on one end and the radius the journal to ensure it is round?


Stroker Crankshafts
“Stroking” simply means increasing the distance that the rod journal centerline (where the big end of the rod rides) is offset from the main journal centerline (what the crankshaft rotates on).

The stroke length is twice the offset: offset up = TDC, offset down = BDC.
Main journal size, rod journal size, rod length and deck height do not affect stroke.
Any stroked motor must be re-balanced, of course thats a given, but figured I would mention it. This should be easy to do by the shop doing the grinding.

It is generally possible to weld and “offset grind” the rod journal to increase the stroke. This is done by only adding metal to the outer (offset) side of the journal, then grinding the journal back to original size by removing more metal from the inside (original surface), which moves the centerline away, increasing stroke. <--Like I said before... if reducing size, it would be possible, but a competent machinist to move accomplish this without welding... just grinding down to the new journal size and making it wider to match the new rods.

So for example if we have a stock 50mm rod journal, you eclipse grind the journal, moving the centerline and increasing stroke... not something for the average DIY'er



Another method is to weld the rod journal all around to the larger diameter of a different rod, and re-grind to the new rod's journal size. The rod's other dimensions must be suitable: width of the bearing, length and pin diameter.

Once again Peter is going for the following, (not to mention reduced rotating mass... especially when you are talking lighter rods and pistons):

Three simultaneous advantages:
1. gives a slight stroke increase (equal to the difference between the rod journal sizes)
2. allows a more modern, or stronger, or different length rod to be used
3. does not reduce journal overlap (see comments to follow)

[ESC944]

Connecting Rods

All of the used rods considered for this process should be high quality forged steel parts, if building a boosted or high compression motor and probably need rebuilding. This is a straight-forward operation that can be done by any competent shop.

I insist on using ARP rod bolts. If you wish to polish the rod beams (not necessary, in my opinion), they should be shot-peened afterwards to restore the “skin” (more dense surface left by forging).

Now for a great little mod that can make a difference:
[b]Piston pin oiling can be slightly improved by drilling a 3/32” hole vertically through the very top of the rod eye. [/b

]Radius the entry carefully, remove any burrs inside the pin bushing, etc.

While you could do this on a stock rebuild of a 924 or 931, I suggest we explore with Peter all the possible alternate rod choices (Previous posts and Page) before making any investment in the old rods.

Any rod other than the original may not have the same piston pin diameter, which means either re-fitting the rod (where possible: either reaming to a slightly larger size, or bushing to a slightly smaller size) to the piston pin size of the piston you will use, use the piston intended for the new rod, or a custom piston.

[ESC944]

Clearance:

Any motor with a longer stroke, or longer rods, or both, will need additional clearance ground or machined in certain areas of the block.

Certain special rods (aluminum, H-beam, etc.) will also require more clearance even in std. length and stock stroke.

The most common areas of interference are on both sides of the lowest edges of the cylinders, on the stroke axis (on the piston’s thrust surface), 90° from the bank axis.

Metal must be removed from the block if, the rod beam passes does not pass with at least 1/16” clearance at any point.

[ESC944]

So now... if we are stroking our motor, we might want to visit the cam... just a thought... applying standard engine builder 101, back yard edition (in other words we gonna gestamate....)

To predict a useful cam for a “warm” street motor:
1. Take the size of the motor when completed. Divide by 10, and round off (131” becomes 13, 154” becomes 15, etc.).
2. Add 180. The result is the cam’s “duration @ .050” lift” (look for it in the cam manufacturer’s specs).

For a 131” motor in this case: 193° (131 ÷ 10 = 13; 13 + 180 = 193).

More power? Like that “racy” idle? Afford to lose a bit of low speed power? Add 10 to the .050” duration figure to get a hot motor (for a 131” motor in this case: 203°), best used with high compression (9-1 minimum), carbs, standard transmission or a loose torque converter, dual exhaust and high numerical gears (3.55, not 2.76).

Heavy vehicle? Stock torque converter? 7-1 compression? no carb, CIS, Subtract 5 from the .050” figure (for a 131” motor in this case: 188°).

3. Advertised (total) duration: use the lowest figure you can find for the .050” duration value predicted in Step 2 above (i.e., 290° advertised duration is preferred to 300°, if both have the same 220° @ .050” lift).

4. Lift: as much as you can get, limited by the choices in Steps 2 & 3.

5. When in doubt, select the smaller of the two choices. More is not better here.

Is this the “very best” cam? No, but unless you ask an expert who has successfully built motors very similar to yours, it’s better than guessing, and will provide satisfying results.

It will not be the “optimum” cam, which will require extensive testing and dynamometer work.

Don’t be too aggressive with motors under 8-1 compression; too much cam will make them hard to drive.

If the cam manufacturer suggests a specific valve spring, use it, don’t be creative. The motor will not last longer with weak springs, or develop more power with stronger springs.

[ESC944]

So here is my question, especially for Peter if you have the block apart... what is the amount of clearance between crank and pan, etc.. just how much has to be modified... if at all with the 48mm setup?

[HappyPuppy]

Man, all this talk about new pistons, offset grinding the crank, different conrods, etc., etc., blah blah blah, for a 12HP gain? This has already been said before, but you could get the equivalent of 12HP by just deleting some weight.

I've seen everyone go over this issue, trying to eke out that last hundred cc s. Why? As far as I can tell, it gives minimal gains and reduces reliability. I was once actually thinking about DESTROKING my 924 engine--the theory was that with shorter piston travel for the same explosion, the engine would have more torque across the powerband.

It's common knowledge that the biggest restriction in the 924 is getting through the cylinder head. That's what I'm going to focus on.
_________________
My frequent desire to perform violent acts on select other drivers is not road rage; it is simply preemptory self-defense.

[simsport]

Well I have heard a lot about these conversions and yet only seen one that works.....

Dave Dennett, UK speed champion for the Porsche Club here used a 2.4 crank, got his own rabbit rods, bored the block to 88mm and used JE Pistons, fitted Lumention management and a Piper cam , exhaust and got a very real 165bhp.

I drove it at the track, it was very quick.

That lot prob cost him around $4000.
Crank £500
Pistons £500
Cam £200
Bore £100
Management £500
Setup £250
Total £2150 x 1.9 = $4000

Not free, not the cheapest but less than a turbo conversion or my supercharger setup and proven to work well.
Also he reckons it could make a few more HP, perhaps as much as the 180bhp the factory cars made.

Cheers
Simon
ps but blown is still best!
_________________
Blown is always best!

[ESC944]

I want to know where he got the Crank and is it still available??

As far as stroking for minimal gains, well in the end it depends on where you are going with things. For me the appeal is the coolness of a stroker motor, made with minimal cash.

I can see paying for the crank or having it down, then using other parts from non-porsche engines and building a frankenstein and then adding boost... loads of fun. If you can do it cheaply... so Follow Peter's example, but I want more stroke!!

I also love boost> boost is easier than stroking, with regard to the huge gains.

Now if you are building a motor to handle boost or more boost, then why not stroke it, it adds up in the end. Just dont loose reliability. It isnt a drag car, so I want it to last a more than a season before a rebuild, more like years and years. It doesnt hurt to dream, but making it a reality might hurt the wallet.

[Jeffrey]

did you ever complete this stroker project? Is it possible?

[Andrew NZ]

Hi Peter, did you ever get any further with this idea?
_________________
Andrew
1977 RX924 race car
12a bridgeport supercharged
www.race4-dcup.co.nz

[Peter_in_AU]

No, I haven't done any further work since measuring everything up and deciding that it was possible. There always seems to be something else competing for my wallet at the moment.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you