Jan van Vurpa
Starting Member
USA
32 Posts |  Posted - 03 May 2008 : 23:06:58  
| quote:
Originally posted by Genty
This thread is about stroke/bore tuning options which was started in a recent thread.
Here is my 2p worth:
1500cc Engine
90mm Bore
58.86mm Stroke
45mm Compression Height
129.52mm Con Rod c-c 129.52mm (?) 130,5mm Note: Same
Rod in all Cologne 60 degree V motors
24mm Pin diameter
20.65mm Con Rod thickness
1700cc Engine
90mm Bore
66.8mm Stroke
40mm Compression Height
130.55mm Con Rod c-c 129.52mm (?)
24mm Pin diameter (?)
20.65mm Con Rod thickness
Where to start?
I suppose we start with that we CAN'T do anything about, and that is distance from crank centerline up to the top of the block: deck height
An uncut V4 or for that matter Ford "Cologne" V6 block has
deck height ..............205.33mm
Bore center...............120,9mm
All rods including V6.....130,5mm
When we're either searching for alternate pistons or trying to swap in say STRONGER RODS its simple addition and subtraction to figure where we'll end up
Always best to start with what we have and you guys in Angleterre always got 1,5 V4 so lets do one:
Block .............205,33
Rod...............-130,5
leaves............ 74,83
Now 1/2 stroke......-29,3 (cause stroke goes up and down)
Gives................45,53mm (which is comp height on 1,5 V4 pistons)
OK notice that comes out so the piston is right at the very top of the block?
That's the way it was designed, and that works best. In a 1,5 that gave a "nominal" 9,0:1 compression ratio, and a half decent 1500 runs well and pulls good with little fuss.
OK lets do the "Big 1,7"
Deck..............205,33
Rod..............-130,5
1/2 stroke.........33,4
Leaves.............41,43
Oh, dear......our North American 1970-> onwards 1700 V4s came with pistons 39.5mm from pn center to top---and a miserable "nominal" 8:1 compression and the bugger's were doggy and pinged.
Its cause the piston is about 2mm down the bore to lower compression by the easiest way possible: lopping the top off.
Now the boys who think they're smart over here always used to US Spec Ford Pinto piston at rebuild time to try and bump up compression and it was only OK.
US Spec Pintos were 90,8mm bore and had 40,5mm comp hgt so it made a nice 1st over (032") but it wasn't quite right, and they still pinged or you backed the ignition timing back.
Well there is a very convenient piston , indeed the most popular motor for decades in UK for modding: Ford Pinto.
Pinto? you say wrinkling your noses in distaste....
Yeah Europe only 2,0 OHC HC are 90,8mm bore and 41,5 comp hgt
Yep a smidge less than 004" taller than the deck.
Result? Over 10.5 compression, and it doesn't ping even with lots of advance.
So for 90% of people this has been my standard spec bottom end since about 1988 with some asking for overbore 2,0 Pinto pistons in the normal 0,5mm or 1,0 mm increments.
Now we should note that the US company TRW had forged, high compression Pinto pistons for years, may have still for all I know.
These were good Mahle slugs finished by TRW and could run at .0035" to .004" clearance rather than the common at the time .008-.010" for Arias, Venolia etc who used different forging alloys.
But! they were 40,5mm comp hgt.
And that was bad when paying big money for pistons.
quote:
In recent years there have been several successful approaches to increasing V4 capacity to as much as 2L. The usual options in increasing capacity involve over-boring the cylinders with larger pistons and the 1700 crank and con rods:
90mm = 1698 (standard in Sonett III & later US V4s)
91mm = 1740 (Works first overbore size)
!! No, 0,5mm was available 0,75mm 1,0mm 1mm being generally the maximum allowable overbore in Homologated classes in Rally.
quote:
92mm = 1776
93mm = 1815 (Works second overbore size)( No this was a sized used in Swedish "Nationella Klass" which came after the old "Standard B" rules were scrapped, and wasn't supposed to be used in International events--- but I've been told by people in the Competition Department they did use 1815cc motors "Nobody ever protested us, we weren't a big threat"
94mm = 1855
In order to increase capacity beyond this more severe tuning is needed; either by extreme boring or offset-griding the crankshaft. An offset regrind removes material from the inside of the con rod journal and moves the center of the journal (the modus of rotation) outwards by half that amount. e.g. a 6mm offset regrind moves the modus 3mm outwards and increases the stroke by 3mm. The issue with this is that increasing the stroke involves removing a significant amount of material from the crank and can jeopardize strength. That said offset regrinds have been used recently (and not so recently!) to achieve this with success.
Oh people have been offset grinding things for a century, and yes it MAY compromise strength. And in my first V4 offset grind it failed and broke but that was after 5 years and maybe 25 events averaging 120 stage miles and countless miles out in the woods just beating the hell out of it----rev limiter set at 7100 for 105mph on 62cm tall gravel tires. But in the similar time frame i knew of at least 2 other crank failures locally and like mine, they failed on steady state cruise at around 3000 rpm.
The roots of the failures is in the inevitable nitrogen bubbles which are always present when iron is in the molten state and if you're unlucky to have a chain of them line up thru someplace where there's cyclical stresses (like we always have in any crankshaft) then maybe at some point the begin to link and its like "tear on dotted line". These nitrogen bubbles are internal and microscopic and there's no way to crack check them. I once asked Bengt-Erik Ström about the "niter-härdad" or Nitrided cranks in the catalog and he said "Yeah they're harder so they should be longer wearing, but they break at the same rate the standard cranks break, cause its from the inside, so I wouldn't bother killing myself to get one or nitride a standard crank."
So crank breakage is something we just have to accept and try to mitigate by not stressing them with HEAVY rods and PISTONs.
quote:
SAAB ran a 2L in some of their rallycross cars. They used a 1.7 crank with 96mm pistons to give 1933cc. This is way beyond the accepted overbore limit (given by SAAB!) but they did do it. With cylinder liners you could theoretically go up to 97mm to give 1974cc.
Another method which I am aware of has been done much more recently in Sweden. Offset grinding the crank by 10mm will increase the stroke by 5mm (71.8mm) and reduce the con rod journal (from 53.99mm to 43.99mm) to give 1993cc but will reduce crank thickness dramatically. (SAAB produced a steel crank which was much stronger than the cast item but is incredibly rare and therefore very expensive!)
I've seen 2 of the forged steel cranks that Saab did a short run of.
One was owned by a guy near Sunne, and I saw it in Varmland in the shop of a friend who runs "Din Rallyboutique". That one was at standard stroke, and the boy needed pistons and as they used Saab 99 rods (at 134mm c-c) he was in a pickle cause those were custom pistons he had and they were beat to death.
I asked Bengt-Erik (I stayed at his place back in late 1989 for a month between job interviews with Ford Motorsports in Boreham, EsI am an idiot and did what I could during they day while he was out shooting moosies and in the afternoons we'd putter in the shop and then DRINK in the evenings and talk motors and history) about the steel cranks and he said "Oh we did a run of 20 "smidda" forged cranks and half were standard stroke and half were stoked to about 71mm". They were forged up in the North where everybody including SAAB and Volvo did their OEM cranks and rods, that is at Bofors Vapenfabrik. You know Bofors from their quick firing 40mm Anti Aircraft guns which i believe you Anglais called "Pom Pom" guns. Kinda appropriate for you guys, being Poms, eh?
Anyway, the steel cranks used 99 rods since they are pretty much unbreakable, and the stroke versions, with 94mm pistons, were intended for rallycross motors since the rules in rallycross were nearly wide open. Bengt-Erik did chuckle "well it did happen that some of the cranks ended up in VM rally motors and even a few time the 2,0 motors sometime found their way in, too. But we weren't winning overall very often so nobody protested us....(wink)."
Now when it comes to offsetting the center of a journal, if the machine operator is careful you can get a 1 to 1 change.
On my cranks we start at 54mm, reduce it to 50 and get 4mm more stroke.
Remember! The crank goes up AND down, so moving the center UP (or better said away from crank centerline) 2 mm at TDC gives you 2mm more everywhere and that means 2mm at BDC too.
Now when I found a set of new 134.25mm c-c Carrillo rods for a Datsun L16 motor in a friends shop, I wanted them just for the extra margin of safety they would give over the worryingly spindly stock V4 and V6 rods, but at the suggestion of the crank shop I figured "why not!!?"
So on that motor lets do the easy quick math:
Block...............205.33mm
Rods...............-134.25
leaves...............71.08
1/2 stroke...........35.4
Leave needed comp hgt..35.68
I chose forged Mahle VW pistons which were 34.6mm, and surfaced the block till the pistons were out the deck by 0,1mm or .004"
Now with more cylinder volume my compression would be nice and high, well over 11.5:1 so now I could finally do the combustion chamber profiling and valve unshrouding to get some advantage of those 42mm and 36mm valve we're all using, right?
This dropped comp to about 10.8 and this was a motor I could run on used Bass Ale (Oi! How is drinking Bass Ale like making lurv to yer sweetie while punting on the river cam? Fawkin near water!!!) Well I could run our 89 okt "Used beer" and not ping when using my local copies of Saab 7,6 or 8,3 cams.
Trying to find rods that we can use easily is hard. Most motors in the mid 80s "downsized" EVERYTHING so there's not a lot of things with journals bigger than 48mm since the late 80s. This lead me to the old Stand-by SBC and SBF and of course the Chevvy 2.0" journal for strokers and the Ford 2.125" for nonstrokers.
Now there's still work to be done opening up the big end ID, re-cutting the wee little groove for the tang on the bearing shell on the big end ID, narrowing the small end, rebushing small end for 22mm pin, finish honing the pin end, but at the cost of a set of say Eagle Rods, its not utterly absurd.
What else is there I'm forgetting?
Lots probably and I'm in the middle of an email to a boy in Sweden asking about rods and pistons etc for a rallycross so I'm getting scattered more than normal.
You boys can look around at www.jvab.f4.ca and peek at the photos there in the various folders, there some fun stuff like a piccie of my car when in 1995 I won a round of the US Rally Championship in our normal 2wd class, not for classics, normal open 2wd cars.
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ommony
V4 Beginner
70 Posts | Posted - 04 May 2008 : 16:45:57
| Now while I confess I don't know nearly enough about Saabs or the mechanical details of the car in particular, I can pass on the specs of my car, engine built by Mikael Klenfeldt and Ström in Sweden.
Bored to 93mm, 1815ccm
1700ccm crank
Special pistons to suit the bore
Modified to use copper gaskets
Blokk shaved/polished ~1mm
Heads shaved/polished ~0,5mm
Combustion chambers are ported/polished and are as near as possible identical in shape/volume and optimized for ideal combustion/flow.
Ported 44mm intake and 38mm exhaust
Exhaust ports out of the heads are ported as much as possible.
SAAB Sport and Rally korsinsugg
2x Weber 45DCOE carbs
7,2 cam
Lightened pushrods
Aluminium valve spring retainers
Stiffer valve springs
Pertronix ignition, hipo coil, magnecor custom lead set, distributor raised to make room for the crossflow intake manifold, etc.
Electric fuel pump
Compression is about 12:1 at the last check and 7000 RPMs is scary, but no problem (fingers crossed!)
Now, I'm still pretty new to Saabs, but I've had several of the old Saab guys in Norway drive the car and so far everybody who has driven it says it's the best setup V4 engine they've ever driven. :D
Not the cheapest tuning option, though, I'll admit. | |
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Topic 
V4 Stroke & Bore tuning thread
