76mm stroke -- short or long rods?

stan_tichomirov

I know 76mm stroke is a bastard size. It's one reason I want to try it.

I'd be tearing apart my running 1835 and putting a 76mm crank in there, reusing what I could. One way would be getting a set of "B" pistons and 5.5" rods, the other way would be keeping the "A" pistons and using a short 5.325" rods, possibly having to trim the piston skirts. Some say short rods will promote low-end torque, others swear by using longest rods possible. Which one would you do for a light Beetle and why?

Rest of variables would be:

Web 163 cam, around 9.4:1 static CR
40x35 heads, single HD springs, 1.1 rockers
1 1/2" merged header, A1 phatboy
KEP Stage 1, lightened stock flywheel
40IDFs

I know leaving the current engine alone and building a new one with 82mm crank is more proper, but I want to try this and improve what I already have, it's one of the ways I learn things.

Stan
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ravivos · Posted: Tue Jul 29, 2014 1:48 pm Post subject:

just finished building my 2021 T3 engine.

5.5" SCAT rods
76MM CW crank, VW journals
92mm Thick wall cylinders and B pistons.

on the block i used, and these rods, the deck height was 0.055" without any shims under the cylinders, so it was rather easy to set this part.

goind with the 5.5 rods, beside it been easier to build (dont need to modify the piston's skirts), it should make the engine's life easier with less wear and tear.

since i am not racing my car, i would prefer longevity over top-top-end performance.

Ravivos.

luckystiff · Posted: Tue Jul 29, 2014 1:58 pm Post subject:

2021 in my square.....

76mm cb crank
92B thickwall AA piston/cylnders
Revmaster 5.5" rods
Brothers 40x35.5 heads "cleaned up/worked" by my builder and myself
Brothers 1.25 rockers(actually measured out to 1.34)
Web 163
CB straightcuts
manton pushrods
dual 40 drla
berg full flow kit
dynamiclly ballanced with 1/2 gram
9.2:1cr

FUN!!!! and ended up stock width.....
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Quokka42 · Posted: Tue Jul 29, 2014 1:58 pm Post subject:

Go the long rods - the difference in rod ratio doesn't make enough difference to engine operation to be worth the effort of shortening it. Grab a piece of paper and a calculator, until you get below about 1.65 changing rod ratios make little difference to rod angle and piston speed in the real world.
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stan_tichomirov · Posted: Tue Jul 29, 2014 2:24 pm Post subject:

I'm doing some calculations based on my current deck height of .05" (1.27mm), "A" piston height (39.8mm) and rod length of stock VW rods I'm using (5.394"/137.008mm). I'm coming up with this for 76mm crank, assuming it would be adding 3.5mm to my deck measurements:

"A" piston and 5.394" rods: -0.0877953"/-2.23mm
"A" piston and 5.325" rods: -0.01877953"/-0.477mm
"B" piston and 5.5" rods: 0.0004330709"/0.122mm

I think I'm calculating correctly.. need to re-check. My case is decked. Looks like I'd need to shim either way, with "B" pistons and 5.5 rods putting me basically at 0 deck. Now I need to figure out how to calculate rod ratio.

Stan
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Jansen Enterprise
My 1835cc build
My budget 2276 build

vwracerdave · Posted: Tue Jul 29, 2014 3:32 pm Post subject:

Rod ratio is grossly over rated on street engines that cruise around 3500 RPM on the highway. Build whatever is easiest and cheapest and forget about it. Now if your building an 8500 RPM screamer, then it might make a difference.
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stan_tichomirov · Posted: Tue Jul 29, 2014 3:44 pm Post subject:

Not a screamer, 100% street engine. I like to drive twisties so throttle response is important. If it can wind up as high as it does now it will be fine. I'm hesitant to step up to dual springs, if I did I could run the 1.25 rockers I have. I'm also a little concerned with CR being close to 9.5:1 with this cam, but I think I can get a couple more CCs in the chambers if needed.

Stan
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Jansen Enterprise
My 1835cc build
My budget 2276 build

Danwvw · Posted: Tue Jul 29, 2014 3:52 pm Post subject:

Having used those 5.325" rods in a 74mm crank with standard pistons and watching them come out the end of the cylinder as far as they do, I can't imagine using them on a 76mm crank! Use the Stroker P&C's and the 5.5" rods or whatever it is the B's need to keep it stock geometry!
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stan_tichomirov · Posted: Tue Jul 29, 2014 4:04 pm Post subject:

Yeah, the short rods would probably be pushing it, there must be a reason it's almost never done.

Stan
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Jansen Enterprise
My 1835cc build
My budget 2276 build

modok · Posted: Tue Jul 29, 2014 4:35 pm Post subject:

short rods are good with 74-78 stroke and A pistons.

B pistons and 5.5 is good too.

Danwvw · Posted: Tue Jul 29, 2014 4:39 pm Post subject:

Has anyone ever measured those 5.325" Rods that ACN say's are Porsche Length? Cause Porsche 356/912 rods are 5.352" long. I have a set of the AA I Beams here but not sure I can get an accurate measurement, I will try!
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modok · Posted: Tue Jul 29, 2014 4:42 pm Post subject:

measure edge to edge, then add 1.570" should be accurate +/- .005

Danwvw · Posted: Tue Jul 29, 2014 4:58 pm Post subject:

Yeah, doing it and get 5.3285 one way and 5.3235 the other. Which are .005" apart. So I think it's safe to say they're 5.325" which is .027" shorter than a 356 Rod.
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[email protected] · Posted: Tue Jul 29, 2014 5:06 pm Post subject:

I've used 5.352" rods with 84mm stroke cranks, works fine.
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oldschool5er · Posted: Tue Jul 29, 2014 8:11 pm Post subject:

mark tucker · Posted: Tue Jul 29, 2014 8:17 pm Post subject:

I would go with 5.5 & b pistons.

stan_tichomirov · Posted: Fri Aug 01, 2014 10:21 am Post subject:

Well, I now have a CB 76mm crank in my possession, time for a plan. I do have a spare AS41 case with 8mm head studs, I could utilize it if it checks out okay. How would this sound?

76mm 4140 crank, VW journal
5.5 I-beam rods
AA 94mm P/C (could use TW92s)
AA 500-420 heads with 40x35 valves, minor p&p. Currently 51cc and single springs, would need to open chambers/cut for duals.
Web 163 cam (own and can re-use), or FK-8/something similar, 1.25 SCAT rockers. CR would be around 9.5:1.
1 1/2 Bugpack merged header, A1 Magnaflow phatboy
Berg 26mm oil pump and Full Flow kit
Kennedy Stage 1, lightened stock flywheel, stock clutch
40IDFs.

Good for 120hp or so?

Stan
_________________
Aircooled.net
Jansen Enterprise
My 1835cc build
My budget 2276 build

luckystiff · Posted: Fri Aug 01, 2014 10:41 am Post subject:

very close to my 2021 build i listed above...........

i wrecked my square with about 800 miles on the build and before i put it on the dyno but the butt dyno is guessing somewhere in the 125-130hp range plus or minus 10hp possibly as i was still taking it easy and don't know 100% as i can tell you the 163 with 1.25s(that measured out to 1.34) was still pulling hard when i let out of it before hitting 6k on the tach.

one options that you can also look at is running an 86B with 1.25s. that gives the very similar caracterisics as the 163 with 1.34 but is as easy on the valve train as the 163 with 1.1s.

if/when my case is opened again it will get a 86B in place of the 163. if you have no plan for higher that TRUE 1.25s the 163 is fine. i figure the 1.34s are probably gonna increase wear/shorten longevity by 10-15%.
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[email protected] · Posted: Fri Aug 01, 2014 12:47 pm Post subject:

The 163 is designed for 1.3:1 rockers. So using 1.35:1 is not going to hurt anything. It's NOT a cam designed for 1.1s that guys use with 1.25s.
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It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
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stan_tichomirov · Posted: Fri Aug 01, 2014 2:07 pm Post subject:

Case is at the machine shop. If it checks out, it'll get punched out for 94s. John, you think I can get away with 9.5:1 CR with Web 163 and 1.25s?

Stan
_________________
Aircooled.net
Jansen Enterprise
My 1835cc build
My budget 2276 build


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