Ported Dart pro 1 pre platinum

[MadBill]

Here's a good example: A pal of mine has a number of BBC-based Batten four cam four valve engines, between 500 and 676 c.i. They are usually a 4.5" bore and use 1.875" intakes, thus a curtain area equal to a 3.75" valve. At higher lifts where valve area rules, they are equivalent to a 2.65" single valve.

[MadBill]

If 'too big' is a common valve issue, how do 4 valve engines with their much larger curtain areas prevail? If a bigger valve causes shrouding or the cam was optimized for overlap flow with a smaller one, etc. I can see it, but show me a 2 valve big block anything that can't use more valve (properly accommodated) than will fit.

I think Carnut showed the min. CSA is at the manifold gasket , which is already way less than the currently valve area. Do you think a bigger valve yet, could make even more power?

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For some engines (e.g. reportedly Pro Stock), despite the biggest valves possible the MCSA is the valve throat. For others it's somewhere upstream, and in such cases the throat is part of the pressure recovery zone, similar to the role of the top cut and surrounding portions of the chamber. Since the velocity drops big time at the seat in any case, a larger valve can provide a more uniform and energy-saving deceleration.

[Amilcar]

Ok Bill, I think that is two distinctive things, have the biggest valve you can fit and have the biggest valve the system is efficient.
Otherwise, CD wouldn`t mean a thing.
Lots of posts including Darin Morgan , who work exclusivelly with the highest output engines. Which on most time, has a throat as a MCSA.

[statsystems]

Ok Bill, I think that is two distinctive things, have the biggest valve you can fit and have the biggest valve the system is efficient.
Otherwise, CD wouldn`t mean a thing.
Lots of posts including Darin Morgan , who work exclusivelly with the highest output engines. Which on most time, has a throat as a MCSA.

I agree, and a lot of people never consider a larger valve requires a bigger port to use it. You see this in Chrysler heads that break over at .400-.450 lift. A bigger valve will make the break over occur at lower lifts. So you have to get the port to support the valve.

[KnightEngines]

Ignoring high end heads designed for racing use - everything is a compromise, most heads will not allow the MCSA to be in the throat, most will not allow a constant taper down to the MCSA, most require the air to turn a significant amount.
Applying knowledge gained from outright competition designed heads to heads available to the general public/sportman racers just does not work, most engines cannot utilise the valve train required to see the gains from a close to ideal intake port/valve/chamber design.
Trying to use high end techniques where a part of the overall package is compromised is a folly.

A 23 deg chev head is a big compromise.

Finding a valve size that allows the best cylinder fill for a given capacity & within the durability/budget limitations of the valve train is not as simple as it would appear.
Often with a limited valve train a larger valve has benefits - yes it makes sacrifices in CD & efficiency, but if it allows the mass air flow required to occur then that's the price you pay.

[Carnut1]

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Port #4 big valve experiment. No porting since last flow pull. Install 2.125" double backcut REV intake and 1.625 backcut Rev valve. I think running the clearance between the valves would be problematic. Thanks, Charlie

[Carnut1]

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Port #4 big valve experiment messy flowsheet. I have always been a biggest valve you can stuff in it type of guy. The intake beats or equals the 2.08 valve at all lifts. The larger exhaust beats the smaller at every lift. I need help understanding why the smaller valves will, make better power. Thanks, Charlie

[KnightEngines]

Charlie - if the bigger valve is picking up or equalling the smaller everywhere, but you haven't recut the seat to suit the bigger valve then what is that telling you about the valve job, short turn & throat size?

[KnightEngines]

PS - 2.15" is too big, maybe try a 2.1"

[GARY C]

Port #4 big valve experiment. No porting since last flow pull. Install 2.125" double backcut REV intake and 1.625 backcut Rev valve. I think running the clearance between the valves would be problematic. Thanks, Charlie

Is that in a 2.08 seat?

[Carnut1]

PS - 2.15" is too big, maybe try a 2.1"

I was thinking a 2.1" valve would be a winner. Thanks, Charlie

[Carnut1]

Port #4 big valve experiment. No porting since last flow pull. Install 2.125" double backcut REV intake and 1.625 backcut Rev valve. I think running the clearance between the valves would be problematic. Thanks, Charlie

Is that in a 2.08 seat?

Yes, no change to the seat. 2.08" seat on 2.125" valve. I would think the mismatch would cause a poor venturi effect off the seat causing a drop in efficiency and increased chamber shrouding lowering high lift flow. So I must have no idea at all at this point. Thanks, Charlie

[randy331]

If 'too big' is a common valve issue, how do 4 valve engines with their much larger curtain areas prevail? If a bigger valve causes shrouding or the cam was optimized for overlap flow with a smaller one, etc. I can see it, but show me a 2 valve big block anything that can't use more valve (properly accommodated) than will fit.

So a 4 valve head having a power advantage over a 2 valve head is proof Carnut's Dart pro 1s need the biggest valve he can fit in them?

Maybe I should do up a set of Dart pro 1s with a 2.05 valve and me and Charlie can meet at the dyno shop?

Randy

[user-9274568]

IMO, a 23 degree is about the only head you can over valve. Basically because it's a velocity limited design.

[GARY C]

Port #4 big valve experiment. No porting since last flow pull. Install 2.125" double backcut REV intake and 1.625 backcut Rev valve. I think running the clearance between the valves would be problematic. Thanks, Charlie

Is that in a 2.08 seat?

Yes, no change to the seat. 2.08" seat on 2.125" valve. I would think the mismatch would cause a poor venturi effect off the seat causing a drop in efficiency and increased chamber shrouding lowering high lift flow. So I must have no idea at all at this point. Thanks, Charlie

I was just curious because I had a similar situation a cpl of years ago when I accidentally put a 2.125 valve in a 2.08 seat on a test head I was flowing...I just didn't have the smarts to figure out what to do with the flawed data.