One for the valve train experts

[zachmac]

So, I have a 2 ltr head off the car I inherited with twin down draft 42 webers and a header and as I began taking it apart I discover:
1) some PO has reworked the runners, very smooth, almost polished!
2) intake valves are oversized BUT 42.5 mm, not the current available 43mm from most suppliers, hmm.
3) exhaust valves seem to be stock, ~36.45mm

So, if I am going to 4mm dome pistons and 40/80 cams, do I,
1) clean it, up check the guides and replace if needed, new seals and run the way it is,
2)run the existing 42.5mm intakes but change to the 37mm exhausts,
3)install the new 43mm intake and 37mm exhaust valves?

How does one estimate the effect valve size has on flow differences and how does one match intake and exhaust sizes? I realize I may get a few tomes in response, but what are the basics?
Is total valve surface area even pertinent or does flow vary as some non-linear function with perhaps circumference? The later seems to make more sense as in all cases the stem occupies the center and the flow is through an annular area between the valve and the seat. That area is a function of lift and circumference.

For example a 43mm intake only represents about a 4% increase in valve face area, and only a 2% change in circumference versus a stock 42mm unit, but, depending on lift as well, might make a significant change in the flow area through that annulus? (I'm to tired to do that basic geometry at the moment) Wisdom please?

[zachmac]

Also, in general doesn't bigger intake valve size go along with higher RPM for power band? Maybe I should stay with the existing setup and go a little less aggressive on cams if I am mostly wanting a low end torque street car?

[RoyBatty]

Makes no sense to me to increase capability of the induction side of the engine and do nothing to the exhaust.
Normally this would create a bottleneck on the exhaust side of things and limit the engines ability due to the imbalance.
Is the fiat twin cam the exception to the rule?

[vandor]

FWIW polished runners are not good for flow.

Big valves don't cost so much that one should ponder - if you want more performance, head is ported, you'll have higher performance cams - makes sense to me to go with bigger valves.

[fiatfactory]

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[zachmac]

42 down drafts?? They must be DCNF type rather than IDF type as Weber never made 42IDF's...

SteveC

They are indeed 42DCNFs. Thanks for all the info.

[zachmac]

As far as the head goes, the seat throat size is what determines the actual flow... not the valve head size...optimum flow with a 43.5 valve is a seat throat too large for the stock seat... so new valve seats are the go... throated appropriately, without the throat enlargement the bigger valves will achieve very little.

Brief rundown on seat throat sizes...

1800/2000 37.0 - 37.2mm intake - 32.2 - 32.4 exhaust (these use a 42.4mm intake valve)

Unless you enlarge the seat throat, fitting a larger valve is a waste of time. SteveC

From a respected engine builder's site and expert on heads: “To get maximum velocity and airflow, the ratio needs to be a little smaller. On many heads, a throat dimension-to-valve size percentage of 86-1/2 to 89 flows best."

A quick check of the stock seat to valve ratios says were already running basically right in that range. Sounds like maybe good advise to run the stock setup UNLESS I want to enlarge the throats. EXCEPT, a 43mm intake (NOTE: NOT 43.5) puts you right at the bottom of that ratio and actually moves you from the high end to the middle for a 37mm exhaust, so it looks like a 43mm intake and 37mm exhaust stay in the "optimal" seat throat to valve size ratio without enlarging the throats.

Vandor,

it's not the cost of the valves I am looking at, it's bang for the buck. If I am building a street car then how much would I ever notice the difference.

In the end I guess it will come down to where I want to spend not unlimited dollars and how long I want to take to get it back together and running. "Like a dog with two bones, he runs to one, he runs to the other, he drop dead!" (DEVO I believe, most likely poorly paraphrased):D

[fiatfactory]

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[zachmac]

Thanks for the quick update and response, particularly the DEVO correction! I think rather than drop dead I may just have my existing setup freshened up with new seals (and guides if indicated) bolt it up with a fresh higher comp lower end and cams and see how it goes. I can always pull it and go to the bigger valves later if I feel the need.

Perhaps the DEVO reference was a subtle reminder that "freedom of choice is what I have, freedom from choice is what I want"?

[fiatfactory]

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