Recipe for disaster??

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lil_azza
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Recipe for disaster??

Post by lil_azza » Sun Jun 11, 2017 2:52 am

Basically been given a set of good 94mm stroker Pistons
Can I use these with a 69mm crank?
I guess I will have to use shims
With a w100? W110?
Got a set of stock 1600 heads
Got a machinist mate that can cut it all
For a heavy bus

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blue thunder
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Re: Recipe for disaster??

Post by blue thunder » Sun Jun 11, 2017 4:43 am

If they are stroker pistons, the wrist pin will be placed higher in the piston. You will then need to machine the cilinders shorter instead of shimming them. As the whole engine will be shorter, you will need to shorten the pushrods and pushrod tubes too. The combo with stock heads will result in alot of low down torque, but top end will be limited by the stock heads/valves/ports. A W110 will work, though you do need stronger (or double) valvesprings.
A W100 is a bit too tame IMHO.

Just my 2 cts....

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blue thunder
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Re: Recipe for disaster??

Post by blue thunder » Sun Jun 11, 2017 4:45 am

And if you have the budget, you could buy some longer conrods. Then machining the cilinders and shortening the pushrods/tubes is not needed...

lil_azza
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Re: Recipe for disaster??

Post by lil_azza » Sun Jun 11, 2017 6:24 am

Ok so torque is what im after im happy with a top speed of 60 just not 10mph up the steep hills
I have a set of single HD springs
What would be the better option mavhininh or conrods? Thanks guys

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blue thunder
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Re: Recipe for disaster??

Post by blue thunder » Sun Jun 11, 2017 12:11 pm

Longer conrods for sure... This will decrease side-load on the cilinders and will have a slightly better burn (longer piston 'dwell'-time).
If you choose the correct conrod length, the block will stay stockish width, no need for machining cilinders, no shortening pushrods and the exhaust and engine tins will fit with no hassle.

Cheers,

Robin

lil_azza
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Re: Recipe for disaster??

Post by lil_azza » Sun Jun 11, 2017 1:53 pm

Ohh ok that makes sence now u say it ok so il look into how far out they need to be and try find a set of longer ones 👍
Would u say stock rockers or ?

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blue thunder
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Re: Recipe for disaster??

Post by blue thunder » Tue Jun 13, 2017 12:46 am

You will need conrods that are approximately 6.5mm longer (82-69 = 13. 13/2 = 6.5mm) so you could choose between .2 (5mm) or .3 inch (7.5mm) longer rods. I would try to obtain a 1mm deckheight and unshroud the valves to get to the CR you desire.

lil_azza
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Re: Recipe for disaster??

Post by lil_azza » Tue Jun 13, 2017 10:41 am

Ok great thankyou just in the intrest of learning where did u get the 82-69=13.
From thankyou for your patience
Im off to learn about
Crs cheers

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Re: Recipe for disaster??

Post by Ol'fogasaurus » Tue Jun 13, 2017 11:36 am

lil_azza wrote:
Sun Jun 11, 2017 6:24 am
Ok so torque is what im after im happy with a top speed of 60 just not 10mph up the steep hills
I have a set of single HD springs
What would be the better option mavhininh or conrods? Thanks guys
I'm kind of confused: drag racing or hill climbing:

Torque for hill climbing (plus gearing) would be more likely using a longer rod and piston combined with maybe a higher lift shorter duration camshaft; what kind of RPMs are you expecting to work with?.

For drag racing I would expect off the line grunt (gearing/tire dia. is also part of the formula here) going through the hard pull I expect to be a different formula and higher RPMs.

Sorry about butting in here.

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Marc
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Re: Recipe for disaster??

Post by Marc » Tue Jun 13, 2017 12:49 pm

lil_azza wrote:
Tue Jun 13, 2017 10:41 am
Ok great thankyou just in the intrest of learning where did u get the 82-69=13...
He's considering that "B" pistons are made to be used on stroker engines and that 82mm is the most popular stroke, then assuming that the difference of the piston's compression height (distance from top to wristpin centerline) would therefore be half the difference between 82mm and the stock 69mm stroke.

Actually "B" pistons have the correct pin height for normal deck height with a somewhat shorter stroke, so this "shorthand" comparison is not completely accurate.

Pistons made for stock stroke have a compression height of 39.6mm. For reasons known only to them, AA Performance's are 39.8mm.

"B" pistons are usually 33.93 or 34mm. Again, AA is oddball at 35mm.

So the difference is ~5.6mm, not 6.5mm; and with AA pistons it's even less.

5.6mm = .220" (with AA "B" compared to stock, 4.6mm = .181")

A stock-length (5.394") rod on a 69mm stroke yields a rodlength-to-stroke ratio of 1.986:1 - higher than nearly any other production engine, that's been proven to be excellent for a wide range of applications of the VW engine.

Were you to go to a longer rod to enable the use of "B" pistons without shortening the cylinders or accepting big piston deck height, you might be able to use a 5.6" length IF the case has been decked some, since the rod's .206" greater length comes up short of the .220" compression height difference. But with AA "B" pistons the deck height would be .025" below nominal, so it should be simple to dial in a tight deck by using cylinder base shims.

A 5.6" rod on 69mm yields a rod ratio of 2.06:1 - now you're seriously outside of the common realm for passenger car engines, but still reasonably close to what the VW engineers had in mind. The effects tend to favor higher-RPM operation; for example manifold vacuum will be reduced but the slower descent of the piston from TDC also makes the maximum flowrate required from the heads to be less...the engine won't "run out of breath" at the high end, given the same port sizes, as quickly as one with a low rodlength-to-stroke ratio. Not exactly ideal for your needs but it shouldn't be a "disaster".

Were you to go to a 5.7" rod the ratio would be 2.098:1 with stock stroke. By this point the effects of such a high ratio become substantial...the piston dwell time near TDC is so long that you can actually rock the crank back and forth for 5-10° at TDC and see NO piston movement (bearing clearance determines exactly how much, but it's noticeable). At idle, the piston has time to flop back and forth at least twice before descending, making the engine sound a little like a diesel...but at high RPM more HP is produced because the cylinder pressure remains high for longer after the burn starts. Because of the slower piston motion near TDC the engine becomes more sensitive to small changes in camshaft timing and ignition timing, too. The higher peak cylinder pressure may increase the minimum octane requirement for the fuel as well. Great if the engine's going to be spending all its time above 4000 RPM or so, but I wouldn't recommend for normal street use, let alone for a "heavy bus"!

Increasing the rod ratio is the last thing you'd want to do for a heavy vehicle where low-end torque is more important than peak horsepower. Have the cylinders shortened and you'll need shorter head studs, pushrods, and overcylinder tin...and the intake manifold and exhaust system may not fit without some modification.

I would recommend that you simplify your life and find someone willing to trade the "B" set for some "A"s you can use. Ideal for a heavy Bus would be AA's "thickwall" 92s; next choice would be late 90.5s and the last would be 94s (due to the thinner cylinderwalls which don't take kindly to large thermal transients). Specifically the TW92s made to go into a case cut to 96mm, not 97.25mm - their P/N VW9200T1KS.

lil_azza
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Re: Recipe for disaster??

Post by lil_azza » Thu Jun 15, 2017 1:12 am

Thankyou very much for such a thorough reply I appreciate it
I think i will have to take your advice on the size
And rethink what would be the better option factoring usability reliability and cost

lil_azza
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Re: Recipe for disaster??

Post by lil_azza » Thu Jun 15, 2017 1:39 am

Thankyou very much for such a thorough reply I appreciate it
I think i will have to take your advice on the size
And rethink what would be the better option factoring usability reliability and cost
A friend has a set of brandnew unused in the box 82mm he thinks have got to be 10-15 years old
They are "normal" pistons i will check with him what they are etc and go from there

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Marc
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Re: Recipe for disaster??

Post by Marc » Thu Jun 15, 2017 6:47 pm

I assume that's a typo and you meant 92mm. The "classic" 92 size was based upon what was then considered the limit for opening up the case, 96mm (in the days of 10mm head studs). Current 94s and one version of the thickwall 92s from AA Performance require that the case be opened to 97.25. It's also possible to have 94s turned down to fit in 96mm holes, but obviously that leaves a very thin/fragile spigot like that of the slip-in 88s - once assembled it's been shown to not be a problem, though.

Classic 92s require a head register of 98mm, limiting the wall thickness where they meet the head to <3mm/.118". Compared to the stock 1600's 4.15mm/.163" that's scary-thin, and they're well known for problems because of it. NOT a wise choice for a heavy vehicle.

94s require a head register of ~101mm, allowing them to be 3.5mm/.138". Ironically, the same folks who say that's just fine are often the first to bad-mouth slip-in 87s which are a mere .004" thinner.

Late 90.5s were made to use the same O.D.s as classic 92s, allowing them to be used in the same case & heads and yielding 3.75mm/.148" ... that's about the minimum I'd recommend for a Bus motor.

AA's "thickwall" 92s use 101mm head register and come in two versions, one that fits a 96mm case and one that fits 97.25mm. Excellent choice for a Bus, they have a wall thickness at the head of 4.5mm/.177" (even greater than stock).

AA also makes a "slip-in thickwall" 88, that goes into an unmachined 90mm case (with the aforementioned super-thin spigots) but requires the heads to be opened up to the late 90.5/classic 92 register (98mm). Those too are a good choice in a heavy vehicle, with an even thicker wall of 5.0mm/.197". But 88mm pistons just aren't available (for a reasonable cost) except in "A" compression height, so you're pretty much limited to a stock stroke (perhaps 74mm) with them...even if you pushed that to 76mm, you won't get bigger than 1849cc.


I strongly recommend against the classic 92s for your application, but someone building an engine that won't be seeing a lot of miles, for a lightweight vehicle, could make good use of them. More trading material.

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