little stroker recipe

[Crawdad]

My goal is to do a fast and easy upgrade on a stock 1600 DP that currently runs well. This recipe is dictated by a no-machining requirement. I am hoping to avoid case clearancing as well. Critiques and suggestions welcome.
By my calculations, this would give 1807 cc.
76 crank, stock rods
87 slip-ins
86A, 1.4 ratio
Merged 1 3/8 exhaust
Dual Kadrons with probably 30 or 32mm vents?
Question: Would Los Panchitos heads be too big (on valve size and port volume) for such a combo? This is for a daily driver, so I need a broad torque curve.

[petew]

Sounds pretty cool. May as well go 88s. They slip in. You just have your heads machined out a bit.

https://aapistons.com/products/88x69mm- ... -90-5-head

I would say that would be better than slip in 87s.

[Marc]

I agree, it'd be worth using heads cut to accept "90.5/92" O.D. cylinders so you could use the AA slip-in thickwall 88s (1849cc). Nice thick cylinderwalls (thicker than stock, actually) rather than the borderline-thin 87s.

There are no readily available stroker ("B") pistons in either 87 or 88mm, however - and that means that with the added 7mm stroke you'll need 3½mm (~.140") of barrel spacers to maintain normal deck height.
Also, consider that at BDC the piston will be coming 3½mm closer to the crank and opposing rod - the wristpin bosses may require clearancing. So, it can be done but don't expect it to go together without issues.

You can get 85.5mm pistons in a "B" version, however. Using them on a 76mm crank would result in excess piston deck and potentially even worse clearance problems at BDC, depending upon how long the skirts are - but the clearance to the wristpin boss is unchanged. By using a 5.5" rod rather than the stock 5.394" you move the wristpins out .106" which should eliminate any problems there. The really good news is that it only takes around .020" barrel shims to bring the piston deck back to normal with this combination. Displacement is down to 1745cc but the package ends up stock width.
http://vwparts.aircooled.net/STROKER-Pi ... 5x82cc.htm

Now, if you were to lower your sights to a 74mm stroke and stock-compression-height "A" pistons (1760cc w/87s or 1800cc w/88s) there'd be a much higher probability of avoiding any extra clearancing. The engine would still end up slightly wider than stock, but it'd take about .040" less in barrel spacer thickness.

Regardless of what approach you choose, it's almost certain that you'll end up needing pushrods that aren't stock length so be sure to allow for acquiring/building those.

I guess the bottom line is that even with a small/moderate stroke you often encounter just as many hassles as with an 82mm, probably why most folks just go all-in. The lone exception is that 76mm/5.5"/"B" piston combination, it avoids most (if not all) of the problems. How much case clearancing will be needed (for a given stroke) depends mostly upon the rod choice. I used some low-end 5.5" Chinese I-beams on my trike motor with a 76mm crank and still had to do quite a bit of whittlin'. Even with a 74mm stroke and those rods I don't think I could've avoided doing any clearancing.

[petew]

You know, I was watching a Hotrod magazine vid the other day on Stroker vs Stock.



I love the idea of a stroker, but I came away thinking... A 1916 is really the easiest option for ACVWs. Revs well, easy to build, etc, etc. Still like to see your baby stroker though.

[Marc]

For a given displacement, all other factors being equal, a larger bore will have more horsepower simply because it has a larger surface area for the pressure resulting from combustion to act upon.
When the stroke is altered, it also changes the ratio of the rod length to the stroke, which does remarkable things to the rod angularity (which acts as "leverage" against the crank arm and increases torque) as well as to instantaneous piston speed - a larger ratio makes the piston dwell near TDC longer so more peak cylinder pressure can be developed before the piston starts "running away" from the expanding gases. Strokers tend to have lower rod-to-stroke ratios, which increases the angularity but reduces the dwell time.
The rod ratio also changes the time spent near BDC, in the opposite manner (average speed remains identical, so if it's moving faster at TDC it must be slower elsewhere - and vice-versa).
Changing the piston location for a given crankshaft position also affects the ideal camshaft event timing - otherwise-identical motors with different rod ratios won't need the same cam to optimize their performance.
Optimum ignition timing would also be different.
Lower rod ratios cause the piston to descend from TDC more rapidly on the intake stroke, which increases manifold vacuum but causes the engine to "run out of breath" at a lower RPM with the same intake port - a stroker will need bigger ports than a short-stroke motor of the same displacement to make the same power at high RPM.

I could go on, but the point is that there are SO many other factors to consider. Dyno comparisons like this don't come near addressing all of these issues. In this one they did acknowledge that the heads are too small for the bigger engine, but little else. It's the curse of that "all other things being equal" proviso - they never are, since changing even one dimension has ramifications for some or all of the others. We’re told that the only parameter changed on those Mopar engines was the stroke, from 3.58” to 4”. If that were true, the pistons would be topping out .210” higher and the heads wouldn’t have even bolted down, so obviously they changed something else. Either the compression height of the pistons or the length of the rods (the latter change would have even a greater effect on the rod ratio than caused by the stroke alone). Either way, the compression ratio would be higher on the larger engine unless the piston dish was larger or the piston-to-head clearance was changed by a thicker head gasket.

Short story is, engines built for high RPM and peak horsepower tend to be large-bore/short stroke (and high rod ratio) while the opposite dimensions are better suited to an engine intended to have high torque at relatively low RPM and a broader power band. It's not cast in stone, things can be done to lessen the significance of those numbers, but that's swimming upstream compared to choosing the appropriate combination from the beginning. At a given displacement, the stroker should give a lower/broader torque curve which makes for a pleasant experience driving on the street - the question is, is it worth all the effort when you have a light vehicle where the lower torque of the big-bore/short-stroke combo is still adequate?

I've never liked 94s that much, many heads will hole through at the requisite bore size leaving a compromised seating surface and cutting the case out to 97.25mm weakens it unnecessarily. You can turn the spigots down to 96mm to address the latter problem. The cylinderwalls are still rather thin, only ~.006" thicker than 87s.
A "street" exhaust system and stock heater boxes is coming close to being max'ed out by ~1800cc, even with stock-valve heads, at higher RPM (say, by 4500-5000). To realize the potential of a 1915, one should include the cost of a more exotic exhaust system...1775cc (90.5x69) has long been considered a good stopping point when budget matters. Nowadays, thanks to AA's thickwall 92s I'd extend that to 1835cc provided the cam's not too wild.

[petew]

Cool, I can see all your points. And yes, you're right there's a lot more to it than just slapping a stroker crank in, but that was kinda my point. I guess I'm thinking, if you're pulling the case apart why not just machine it for a bigger bore like thickwall 92s. Cheaper and you're not messing with rod angles, clearancing or dwell.

Course I'd love to see him build a stroker anyway.

[Marc]

...why not just machine it for a bigger bore like thickwall 92s. Cheaper and you're not messing with rod angles, clearancing or dwell...

I agree, IMO the search for the ideal "machinework-free" engine is largely a fool's errand - even if you have no competent machine shop locally, it's easier and (possibly) cheaper to ship your case to & from one and have it opened up to a larger bore than it is to buy a stroker crank and deal with the hassles it brings. But impractical or not, it can be fun to do an odd-ball combination if only for a conversation piece.

If you're buying new heads anyway, you can usually order them already cut to the desired register (not so common with stock-valve heads unfortunately, but easy enough when ordering new bigger-valve heads) so the quest for a "no-machinework" build becomes more about finding slip-in cylinders that are worth using (i.e, no "classic" slip-in 88s). The AA slip-in/thickwall 88s will give you 1679cc and go together like a stocker. It's when you start trying to get all the displacement you can that things get challenging, even a 74mm stroke may still need some clearancing...but then, if you're putting in a counterweighted 69mm crank you still need to check, and often have to do a little carving - and the 74mm won't need much more. With the right rods, it could even be possible to squeeze in 76mm, but don't count on it...and as soon as you do need to clearance, you've lost at the "game" of building a no-machinework/no-clearancing motor.

Even if you're staying with a factory-stock crank (which implies not exceeding 5000 RPM) it's still advisable to have it 8-dowelled should you have intentions of flogging it - a healthy 1679 is capable of chucking a flywheel if abused - so if the goal is NO machinework that should be taken into consideration as well.

[farmer]

IMHO. STAY AWAY from the 88 slip ins. also the ones that require machining on the head side. The skirts are simply too thin and are prone to crack. The idea is good for budget displacement increase, but.... no.
There is almost no stroker kit that will drop in if you want to use stock rods etc. If you swop to slimmer rods there are a few that require no or very little clearance work with a Dremel.
The ministroker kit sold by Aircooled net is about the largest you can go with out machining. P

Panchito´s will be on the large side for what you want. Also, if you want to go large valve heads you will need to replace your header to a 1½" with a better flowing muffler.
That cam and Kadrons are not the best match either. Especially not if you want lower rpm torque. I´m thinking in the direction of this:
ACN ministroker kit. Your own cylinderheads rebuilt, ported and CR set to the combo, or Tims Super stock. FK41 cam or CB 2239 with 1,25 rockers. Keep the 1 3/8 header but find a better flowing muffler like a Dynomax or build your own.
Now, if you chose this way with the 87 mm pistons and 76 mm stroke, you can do a short cut to get a more durable engine. - get the heads done with a step inside so the deck clearance is in the heads instead of in the cylinder. This way the cylinders do not get heated quite so much during the combustion and it will make them live longer. Have them make the step 0,045 to 0,050" (But ONLY the with of the cylinder wall!!!!)and then zero deck the pistons.
Don´t be affraid of compression. Both these cam options need 8,5 + your altitude correction to work well on medium grade fuel.
30 mm venturies in the Kads would most likely be the ticket. Get the Kads tapped for vacum and use an SVDA distributor. adjust it for 10 idle, 30 WOT and about 38 on vacum.

T

[Marc]

...you can do a short cut to get a more durable engine. - get the heads done with a step inside so the deck clearance is in the heads instead of in the cylinder. This way the cylinders do not get heated quite so much during the combustion and it will make them live longer. Have them make the step 0,045 to 0,050" (But ONLY the with of the cylinder wall!!!!)and then zero deck the pistons...

I didn't propose that approach since the object of this exercise was to avoid needing to go to a machine shop. The AA thickwall slip-ins have a nominal wall thickness of nearly 5mm at the top (compared to 4+mm for 85.5 and less than 3½mm for 87) which'll hold up fine.

Slip-in 88s (including the AA thickwall ones) have a lower spigot that's less than 1mm thick - if you drop one on the bench it'll bend/break, and due to the inside bevel on some the edge is sharp enough to cut yourself on ...but once assembled, it's not an issue since this is not a highly-stressed region of the cylinder.
94s can be turned down on a lathe to fit into a case cut for 92s, again leaving a skirt that's <1mm thick, and those have a good record also.

Second-gen machine-in 88s (~97mm O.D. top, ~94½mm bottom) were great for durability but they're obsolescent now (just try finding cylinder spacers/shims that size - Bugpack used to list them but EMPI dropped them from the lineup when they took over). First-gen machine-in 88s (95.3/91.7mm) are even more rare, they haven't been produced in decades. AA once offered the second-gen barrels but no longer do, they feel that the limited market for 88s is adequately served by their 98/90mm product. These days there's only two sizes I'd ever cut a case to, 96 or 97.25mm (and actually I'd rather turn down 94 jugs than whack so much material from the case). My trike case is cut to 94½mm and rather than have it recut for 98/96mm 90.5s I had the cylinders turned down, leaving a skirt that's 2mm thick (same as classic 92s have)...I could've safely shoehorned even 92s in this way, just didn't need that much displacement.

[Chip Birks]

I had a 74x94 stroker, and after a season pulled that crank out and put in an 82. That 74 motor was harder to build and made way less power than the big one. Obviously its all about the total package, and mine completely changed when i went bigger. But still...

[farmer]

What was so problematic with the 74 stroke??? That would have been pretty much drop in.

[Chip Birks]

Mine fit okay, but then had to make the engine wide with shims, because of the wrist pin height. Its just kind of an odd size. The engine also fit worse in my engine compartment than my big one. If i had been a little more aware when i built it, i would have picked different rods and such. It wasn't hard, just a little annoying. I would way rather send the case off for machining and build a big one vs. avoiding the little bit of extra work. The cost in components is almost the same. Just add in the cost of the clearance work and move on with life.

[Marc]

...The cost in components is almost the same. Just add in the cost of the clearance work and move on with life.

That's what I was alluding to above, most "mid-size" stroker combinations present nearly as many challenges as a big motor, if you expect it to "go together like a stocker" prepare to be disappointed.

[Crawdad]

Thanks for all the replies, especially to Marc for detailing the hassles of even small increases of stroke. I think it's time to just build the squishy 2276 I've been meaning to do for years.

[Chip Birks]

Thanks for all the replies, especially to Marc for detailing the hassles of even small increases of stroke. I think it's time to just build the squishy 2276 I've been meaning to do for years.

Not a bad idea. I recommend using big chambered heads and throwing a lot of boost at it