Out of the ordinary?

[HapHazard]

Hi all,

I am exploring the theoretical possibilities of building up an engine from scratch that may on paper be out of the ordinary from what is normally discussed here.

I have a requirement for an engine that can ideally give me very good low down torque. Typically the engine needs to cruise at around 2500rpm, with a max of around 3000rpm. I am looking at either a 82 or 84mm stroke and a rather more unusual 88-90mm bore combo.

Rod length needs to be as short as possible.

So the question is, what is the shortest off the shelf rod to piston combo available to maintain piston compression height? (H beam rod)

Obviously shims are a way of maintaining piston compression heights and in this case, physical engine width is NOT an issue. Other things such as pushrods etc will be however.

I am aware that stroked engines may need clearancing etc and to prevent any off-piste debates about short verses long rod lengths, am aware of the cons of increased sidewall wear etc with short conrods.

Aim is not for out and out hp and would be quite happy for a low rpm sewing machine at around 40-50hp.

All comments positive and negative welcomed....

[madmike]

Luv my short rod motors
VW journal Porsch length rods is what I use

[risk]

I run a 2276 with 82 stroke and Porsche journal rods, 5.325" long.

On my old BPP crank (bugpack) I had to clearance the bottoms of the pistons where they would have rubbed the rod caps.

[Chip Birks]

Have you looked into Revmaster? They build right along the lines you are thinking.

[Marc]

No common (i.e. inexpensive) 88Bs available. Type I 90s have been obsolete for decades, but there're ample choices in 90.5s - you'd have no problem finding readily-available parts for a 2110 or 2161. The difference in compression height between an "A" (stock-stroke) and a "B" (most-common stroker piston) is ~5.65mm which would be ideal for maintaining nominal piston deck height with an 80.3mm stroke, if there was such a thing, so at 82 or 84 you'll be needing some cylinder shims even with "B" pistons & stockish-length rods, but the package width will be nearly stock - I wouldn't expect any special problems finding pushrods long enough for it, and sheetmetal/exhaust system fit should be no big deal either.

Conventional wisdom is that 82mm is the upper limit for a stock rod - the inertial forces from such a low rodlength-to-stroke ratio (1.67) put a lot of stress on the rod, even if the RPMs are kept low. At 84mm it'd be 1.63, serious Family Truckster engine territory for Detroit iron, let alone an ACVW that has 1.98+ in stock form.

A low rod ratio will enhance low-end torque, but at the expense of accelerated cylinderwall wear - I would recommend not going too extreme here. The effect isn't huge, you can still make good torque with a more conventional rod ratio - personally I'd go with aftermarket/stronger rods (I-beams should be adequate for a thumper, but H-beams may need less clearancing which could justify the extra cost for you) 5.325" to 5.4" with an 82 stroke and jump to 5.5" beyond that. 5.5" on 84mm gives a ratio of ~1.66, which is still quite low for a boxer engine.

Aircooled.net lists a set with 85.5B pistons...if 1883 or 1929cc will work for you. They're on sale for $200, too. http://vwparts.aircooled.net/STROKER-Pi ... 5x82cc.htm

Machine-in 88mm cylinders are 97mm O.D. at the head and 94.4mm at the case, and cylinder shims in that I.D. are almost nonexistent anymore...so even if you could find some 88Bs that were within budget you could get stymied when it comes time to set the deck height. AA (Chinese) does offer 88mm jugs that are the stock 90mm at the case and 98mm (normal for late 90.5s/classic 92s) at the head...no problem finding 90mm I.D. shims in nearly any thickness from .010" -.100". I'm not crazy about the idea of running cylinders that have <1mm wall thickness near the bottom with long-stroke/short-rod, but they'd probably live.

Berg still lists GB006 88s with a 33.93mm pin height (using the 97/94.4mm jugs) for $411 a set, but be aware that they're custom-made using blanks that were originally intended for stock 39.6mm pin height - they fudged the ring package as high as practical and shaved the crowns, so with short rods the wristpin bosses need clearancing even at a 78 stroke - by 82mm you've got to carve away a pretty scary amount of material (BTDT)...I wouldn't want to try fitting them with a short rod and 84 stroke. That same concern exists of course if you try to use "A" pistons on short rods with a big stroke - although you could use cylinder spacers to keep the piston from poking out at TDC, the wristpin bosses and possibly the skirts are not going to clear at BDC.

There's not much new under the sun, folks have been playing with outlandishly oversquare & undersquare VW engine combinations for a long time...if any of them actually worked well enough to warrant the trouble, everyone would have one by now. The difference between an odd-ball creation and something more conventional (with the appropriate cam, etc. for the intended mission) is all but indiscernible, not much point in it IMO except as a conversation piece. "One-off" combos can be a real P.I.T.A when it comes time to make a repair, too - unless you have your own machine shop it's wiser IMO to not get too creative. I've built more than my share of freakish combinations over the years, but as a rule I've done it using "expendable" parts so I wouldn't feel obligated to go to the trouble of refreshing them when they wore out...some even bore this label

[HapHazard]

Cheers Marc.
A lot to mull over here.
My target is LOW RPM as the primary governing factor with this engine, so its ability to produce its torque in these ranges will not be easy, hence the reason as to why I am looking at rod ratios and what the VW can comfortably handle. This is not for a car application, with 1000-1500 hours running between overhauls, wear is slightly less of an issue than a car…but is still a factor of course. Going too radical will of course mean radical fixes to get things to fit, especially with short rods at the bottom centre, let alone high sidewall friction which translates into more losses as you point out.
Current wisdom in some camps is that a n-ratio of 1.75 is about right for most normal applications, so I think that you are probably right when you say go for a 82mm with a stock rod for an n-1.67 (about the same as a small block Chevy 400 with a standard rod). An 84mm may require serious amounts of clearancing of the piston skirts with a stock rod? As for rods, would not consider anything but H-beams.
Pistons, will be practical on this as you have pointed out, with choice largely governed by what’s readily available anyway. As I said, at the mo its just a question, but if I can achieve a good workable combination that gives ample torque at 2000-3000 rpm, then great, But its asking a lot from an engine that was designed to produce power at much higher rev bands.
Thanks for the excellent and valuable feedback, especially as you have put so much time into your post.

[Marc]

I presume in your application cooling's not going to be an issue? In a car I wouldn't want to demand maximum power output at too low of an RPM since the cooling fan wouldn't be putting out enough (they can be overdriven of course to address this).
Cast pistons should be plenty adequate for the power level you're after (less than ½ HP per cubic inch - they're easily good for twice that and more). If you opt to have pistons made, hypereutectics w/higher silicon content are fine at 1½ HP per cubic inch at least - overkill indeed, but the skirts will wear longer. Forged would be a waste of money IMO.
Because of the short rods, you'll be limited as to skirt length - many conventional cast pistons will require a good bit of whittlin'....but starting with a long-skirt piston could allow you to retain more of the thrust surface area than exists in a typical "stroker" piston. Definitely do a mockup before you spend any time/money on the balance job, since odds are you'll be altering the piston weights regardless of which ones you use.

http://www.shoptalkforums.com/viewtopic.php?t=96850

P.S. I do have a set of wide/notched blanchard ground .160" spacers that're a little under 96mm I.D., made for old-school 90s - they'd work OK on machine-88s if you should end up going that route and need them.

[HapHazard]

Hi again Marc,

Cooling wont be a problem as they will be hanging out in the breeze (we throw away the factory fan). Also I will run a decent oil cooler, which will help the VW loads, as it isn't brilliant at dissipating heat, especially when you start pushing the limits of the engine (oil accounts for more cooling than some people give credit).

Again thanks for your valued replies Marc. You have given me loads to think about. If I can make a start on my project (house rebuild is in the way at the mo), I think I will l give a custom motor a go and play around to see if I can achieve the modest rpm bands I am after whilst trying to retain a healthy degree of torque.....if it doesn't work, the VW is wonderfully easy to convert back to a more conventional beast and move on.

[Marc]

....if it doesn't work, the VW is wonderfully easy to convert back to a more conventional beast and move on.

Yep, one of the reasons y'gotta luv'em.
I once built an 1879cc engine using some old racecar bits - a stock-stroke CW crank that had been offset-ground to 73.025mm (to achieve 1776 with a longer stroke/smaller bore than the usual 90.5x69 combo - but that's another story). Slapped 90.5s on it with a Bugpack 4063 cam and some home-ported 42x37.5 heads at ~10:1 w/48IDAs. Used 5.5" rods for a ratio of 1.91 (close to stock).
We stuck it in a bug belonging to one of my son's friends and took it to a "dyno day" where it took high-HP-of-the-day honors (besting a 2276 & a 2332 - closest thing to it was a G60 supercharged 1781cc waterpumper).
The punchline is that the torque curve was FLAT from 3000 to 6000. Short rods are overrated IMO, but for what you're after it's theoretically the right way to go...just don't bust a hump over it, it's not likely to be worth going to a great deal of trouble over.

[Piledriver]

IIRC, Larry Widmer once said that ~ the perfect rod ratio was that which puts the piston where you want it at TDC.

Even in a reasonably good simulator like EAPro short vs. long rod is lost in the noise for anything that you can actually build, and even taken to ridiculous extremes doesn't really help noticably.