46cc heads

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eashc
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46cc heads

Post by eashc » Fri Nov 18, 2016 8:11 pm

Doing a top end rebuild, and have to sets of heads to choose from. One pair is stock with the step and unknown cc’s. The other ones are stock valves with 46cc.

If I run the 46cc heads I end up with .060/.070 deck height I get 8.4/8.3 compression.

The question is: will that be too much for a 1641 with a stock cam and bottom end. Or do I need to split the case and put in a cam to match?

This is for a 66 bug daily driver with Kads.

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Marc
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Re: 46cc heads

Post by Marc » Fri Nov 18, 2016 8:54 pm

I wouldn't advise going over 7¾:1 with the stock cam, although you might get away with it using Premium fuel. The stepped heads are likely 52-54cc, which'd yield 7.5-7.6 at that piston deck - but they'd offer no better quench than the 46cc heads at .105-.115" deck (your other option, using .040"-.050" cylinder shims with the 46cc heads) and if you flycut the step out of them they'd also be ~46cc.

IMO the importance of keeping the piston deck tight is overrated with small valves/ports, where there's pretty good flow velocity to cause sufficient charge turbulence without it. Personally I'd just live with the big deck rather than change the cam on this one.

With an Engle W110-ish cam I still wouldn't expect to run Regular at 8.3, but mid-grade should do. You'd want some entry-level HD single springs. I wouldn't go much bigger since you don't want to buzz it over ~5500 RPM max with the stock crank, so you'd be giving up low-end power without being able to take advantage of more at the top.

eashc
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Re: 46cc heads

Post by eashc » Sat Nov 19, 2016 11:12 pm

Thanks for the tips.
How do you think a SCAT C35 cam would do with 46cc heads?

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Marc
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Re: 46cc heads

Post by Marc » Sun Nov 20, 2016 4:07 am

The C35 is essentially SCAT's analog of the Engle W-110 (a hair more advertised lift, same lift @.050", and .018" more valve lift). Should also have HD valve springs IMO - nothing too beefy, just a cheap $25-$30 set. Stock pushrods should be adequate, but with the Kads you may rev it high enough occasionally to cause the stock rockeram wavy-washer/springclip arrangement to fail so I'd also invest in a set of "solid" rocker shafts (~$40).

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Piledriver
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Re: 46cc heads

Post by Piledriver » Sun Nov 20, 2016 4:59 am

It has been said that deck be <70 thou or OVER ~.150ish (more like .200"), or you may find yourself in a particularly knock prone range.
I have always followed that advice, (down to tight decks that would be very unwise on a T1, apparently) so I haven't personally seen the effects, except from water coolers with extra thick gaskets to drop compression only to find it knocks much worse, despite lower compression.

The insane swirl/turbulence stock T1 ports impart may completely counter this.
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