TheSamba.com :: View topic - Rod length and availability for a 74/76/78 x 90.5 bus engine

Tue Sep 30, 2014 3:22 pm

Building a upgraded engine for a early bus, based on a1974 Super Beetle engine. Current ideas / specs for review :
90.5 B series TW pistons for a longer rod and less clearance issues.
74 /76 /78 new forged CW crank w/ VW journals. Displacement open.
New H beam rods with "reversed" bolts, for clearance. Probably CB's.
New 40/ 35 as cast heads with single high rev springs.
Mild cam for mid range torque with stock 1 : 1.1 ratio rockers [new]
Thinking Dual single throat carbs. OR ? Engine balanced ,CC'd etc , etc
Questions :
1. Is there a rod length available so as to have near stock deck height and engine width with one of the above stroke cranks?
2. Is a new bubble case already machined for the cyls the way to go, and which stud size [8 vs 10] should be used and why? I assume the new case means minimum clearance work is needed.
Comments and recommendations [cam, carbs,rod length, sources] please.

Tue Sep 30, 2014 5:05 pm

Yes. The AA pistons are 35mm pin height, so 78 stroke is stock width with stock length rods.

A new case from CB seems like a good bet, tho all are a roll of the dice.

Clearance work?? mag cuts 2x easier than aluminum

Tue Sep 30, 2014 6:57 pm

go with 92mm thickwall pistons/cylinders..............

i did a 76x92b(2021), 5.5" rods, web 163 in my square and am very happy.

Tue Sep 30, 2014 7:15 pm

I believe the general consensus for rod lengths with each stroke to get close to stock length is:

74 crank - 5.3" if you can find it, or 5.325" with A height pistons

76 crank - 5.5" with B height pistons

78 crank - 5.395(Porsche lgth), 5.4", and sometimes 5.5". 5.4 might actually be a bit narrower. This is with B height pistons.

82 crank - 5.325, 5.359(Porsche lgth) with B pistons

Tue Sep 30, 2014 7:50 pm

Thanks for the posts with answers.
If a 76 stroke and B CH pistons is correct with a 5.5 rod, how can a 5.5 rod also work with a 78 stroke and a B piston? Maybe with shims?
Doesn't a B piston need the shorter 5.4 rod to cope with the longer throw of the 78 crank?
Another question :
If a 78 stroke and 90.5 B piston is selected, what combustion chamber size
[ assuming near zero deck and a .050" copper head gasket ] will give close to a 8.5 : 1 CR? I am also assuming the spigot height on the block is at stock dimension from the crank centerline.
Is there a online chart that has this info, to get me close?
Any cam ideas for mid range torque ?
Thanks Again,
BC

Tue Sep 30, 2014 9:39 pm

a 78 is only .040" higher than a 76 at TDC. I recommend 5.5" rods and B pistons for 76 and 78mm cranks. I recommend 5.4" rods and B pistons for 82mm cranks.

Tue Sep 30, 2014 10:08 pm

I don't know of a chart. Take people's recommendations, and learn from the experience after you assemble. You CAN measure where things will end up if you have all of the parts' dimensions, but that may be hard to come by.

As for mid-range cams, there are a ton. Figure out how many CCs you're going to go with, then buy a cam. The bigger the engine, the bigger the cam you can get and still feel the same mid-range pulling power.

Oh, and for the 78 crank, I didn't mean to add "Porsche length" VW stock rods are 5.394", which is basically a 5.4" rod. 5.359" is Porsche.

Wed Oct 01, 2014 3:12 pm

78 with 5.5 rods. or 82 with 5.4 or 5.5with .090 shim. or 76 with 5.5. but shim will depend on the assy.

Wed Oct 01, 2014 3:22 pm

I used 5.4 rods w/ a demello 78 crank. After mock up, choose to trim the cylinders so i had zero deck. Added .040 copper spacers for deck height. Engle 120 cam and Tims stage 1 heads. 40 dellortos. I had a little extra machine work but it is very fun to drive (in a ghia). I plan to build a 2110 for my heavier DC.

Wed Oct 01, 2014 6:07 pm

The answers vary because nobody measures anything. Mahle pistons are a half mm shorter but who cares if you use AA.

35mm pin height is all you need to know.

although if your AA cylinders are 94 length you may think it is wider but it isn't. the first AA TW 92 are the longer style cylinder, but you won't know if your machinist decked the case according to usual procedure for 94 cylinders.
Not sure about the ones with smaller bottoms, I asked.........but I think the guy I asked used a ruler to measure :lol: