Maybe 6 More HP?

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volwing
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Maybe 6 More HP?

Post by volwing » Wed Jul 30, 2014 9:04 am

1973 SB with stock AK motor. Planning to go with 1641 piston/cylinders from AA, exhaust headers, and 1.25 rockers. Just wanting about 6 more horses. Am I too optimistic?
Not ready to go to dual carbs, splitting the case right now. Another idea is going with AA 88's slip ins but concerned about thin walls. I do have extra set of 90.5 heads. Thanks

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Marc
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Re: Maybe 6 More HP?

Post by Marc » Wed Jul 30, 2014 2:42 pm

Definitely do not use slip-in 88s, they have their place but it's not in a heavy, full-bodied car. Even the 87s are less-forgiving of being overheated than stock and IMO their use should be limited to lightweight rigs. Otherwise they're just not worth the liability for the relatively small (3½%) displacement gain.
AA also makes 88s which require that the heads (but NOT the case) be opened up to the late 90.5/classic 92 diameter (98mm), leaving you with thick cylinderwalls where it counts. The thin lower spigot is fragile during assembly but is not a liability in operation. I would heartily recommend those over the 87s; with a 6% displacement increase your goal of +6HP should be easily achieved. You'll be bumping the static C.R. up slightly even with identical chamber volume and piston deck height so it'll want better than Regular gas. I wouldn't exceed 8:1 with the stock cam - any idea of the chamber volumes on your 90.5 heads?
An 8-dowelled crank/flywheel would be wise, but if you don't go too stiff on the clutch and can manage some self-control when launching it should stay together without that...I'd still invest in a glandnut/washer upgrade (Berg GB202 should be adequate if you can find an N.O.S. one somewhere, or step up to the GB202F).
Consider 1.4 rockers if they're in the budget, but to make full use of them takes some HD single springs (cheap ones like Bugpack's would suffice)...
A pretty good bang-per-buck combo; a pair of 34ICTs would make it even sweeter, but it'll do OK with the stock carb (with slightly larger main and probably pilot jets). Even if you only go with 1.25s, you may as well fit the HD singles now so you'll be ready for more carburetion. New stock springs shimmed ~.060" would be the low-budget option. You won't know until assembly whether you can get away with stock-length pushrods, but the chances are better with 1.25s than with 1.4s...allow for custom-length ones in your budget & assembly schedule, and consider that if they're heavier they'll increase the demands made on the valve springs.


Before the AA thickwalls were available, to get 1679cc without splitting the case or using the tin-cans-with-fins slip-ins you had to take a set of "machine" 88s and have their lower spigots turned down to stock diameter, then open up the heads to 97mm...or, you could use heads cut for late 90.5 (98mm) if you fussed around making some sheetmetal sleeves to center the jugs as the heads were torqued down. With these babies, all you need is heads cut to 98mm and it'll go together as easily as a stocker. http://www.thesamba.com/vw/classifieds/ ... id=1341973

volwing
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Re: Maybe 6 More HP?

Post by volwing » Wed Jul 30, 2014 3:33 pm

OK so I already have heads opened up to accept the 88's. The AA brand that is tapered to slip into my stock case will work with the 90.5 heads? So heat distortion will not be an issue in my daily driving? I have an experienced engine builder who will be turning the wrench but feels the thin walls at the case end is a very bad idea due to heat. I guess I am looking for assurance that all else being in good order- sensible driving, normal stock cooling and proper assembly- the AA 88's will give reliable service.

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Re: Maybe 6 More HP?

Post by Marc » Wed Jul 30, 2014 5:35 pm

"Classic" slip-in 88s have given all 88s a bad reputation because they have cylinder walls which are only ~.114" thick where they meet the head - so thin that, in addition to having problems with distortion during heat cycling, they tend to actually cut into the head - resulting in a loss of headstud torque and eventual leakage. They are an abomination, justifiable only when you're after a cheap/sleazy way to have bragging rights to a "1700"; in a lightweight sand toy that probably won't see 20,000 miles use in a lifetime, they wouldn't be a total disaster (provided they heads were retorqued after a few thousand miles or so) but they're nothing one should use on a daily driver.

Long ago, 88s and 90.5s were made which had cylinder walls nearly as thick as stock 85.5s (requiring machine work to both the case & heads)...but you seldom find these around anymore. In more recent years, "machine-in" 88 cylinders went to using the same case & head register as those old 90.5s (94.4/97mm) and 90.5s were stepped up to the O.D.s of the "classic" 92s (96/98mm).
Presumably, your "90.5" heads are cut for the later cylinders - they should measure ~98mm (not 97mm). Stock is ~93.8mm.

The new AA thickwall 88s use the late 90.5 upper O.D. (98mm) so the cylinderwalls are a generous ~5mm (.197") thick where it matters. At the bottom, they still have the stock 90mm lower O.D., so the spigot is the same as that of the slip-ins. It's only ONE millimeter thick - there's no "taper" involved, but the inside is chamferred to aid in piston ring insertion and you can literally cut yourself on the edge of the spigot, and it's fragile enough that it'll bend or break if you bump it, so handle with care. This is not a stressed area, at least not with a stock stroke, so once the engine's assembled there's nothing to worry about. As I said above, back in the day we had to turn down the spigots of the then-available 94.4/97mm cylinders to pull this off and it proved to never be a problem. Today, many choose to do the same with the lower spigots of their 94 jugs - turning them down from 97.25 to 96mm so they'll fit into a case cut for 92s. That leaves them with the same ~1mm spigot wall thickness and even with the larger bore/higher power output it hasn't caused any trouble other than the aforementioned pre-assembly fragility. I haven't done this on any engine with a stroke longer than 74mm but others have, up to 82 and 84mm strokes, and claim that the thin lower spigot has held up just fine.

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