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Old 7th September 2017, 08:57 AM   #1
Tom
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Cylinder head inlet valve sizes

Hi All,

I have a 331 stroker engine with modified pro sport cyl heads 1.9 in 1.6 exh.
Originally the engine was built with a 268XE cam from comp cams but this has now been changed to a 274XE cam from comp cams.
Question is, are the cylinder heads, especially the inlet valve size suitable for this camshaft? Or should I really change the cylinder heads for a bigger inlet valve size i.e. 2.01"
The engine is running 8 stack EFI on a Weber IDA inlet manifold.
Any comments on this cylinder head and camshaft combination would be gratefully received.
Thanks again
Tom
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Old 7th September 2017, 11:08 AM   #2
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Re: Cylinder head inlet valve sizes

Comparing the max available area of the opening at about 0.5" valve lift - when the valve is open the 1.9in valve has 2.98 square inches and the 2.02in valve has 3.17 square inches which is 6.4% bigger. The 268 cam has max power at around 5,000rpm.
I guess it would depend on how you are going to use the engine. You also say that the heads have been modified. If they have been well ported this will have a big effect on the way they flow and how well the engine responds to the high rpms cam timing.
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Old 7th September 2017, 04:29 PM   #3
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Re: Cylinder head inlet valve sizes

I am at a loss to identify your 1.9/1.6 valves and head configuration. I assume this is a 302 Winsor stroked to 331. I can't find "pro sport" cylinder heads when I Google.

With that being said the "bigger" cam has a bit more duration and lift than the first cam. With everything else being equal I would guess it also makes a bit more power and moves that power up the rev range a bit.

I also don't think that the two cams are radically different in their intended use (rev range) to cause a given "quality, ported head" make less power with the bigger cam because it can't flow enough air to take advantage of the higher lift and duration.

I do think that a 331 is big enough to utilize 2.02/1.64 valves (common valve package on Winsor high end heads). But 6000 revs isn't enough to really take advantage of the larger valve size. Given what I know about your motor (CR ratio, actual head performance info, valve spring package?) I would say, keep what you have unless you are unhappy with your motors performance, you really think your heads/ CR ratio need the increased air flow AND you can add 600-800 more revs to the rev range at a minimum. That means more cam IMHO...........but that is based on a minimum of information.

Last edited by Howard Jones; 7th September 2017 at 04:42 PM.
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Old 7th September 2017, 04:45 PM   #4
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Re: Cylinder head inlet valve sizes

The inlet valve throat dia & mods are more critical than the inlet head dia, in fact simply fitting large inlet valves on a 4" bore engine can result in degraded flow due to masking effect of valve head being closer to cyl wall & chamber in that area.
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Old 7th September 2017, 09:55 PM   #5
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Re: Cylinder head inlet valve sizes

Quote:
Originally Posted by jac mac View Post
The inlet valve throat dia & mods are more critical than the inlet head dia, in fact simply fitting large inlet valves on a 4" bore engine can result in degraded flow due to masking effect of valve head being closer to cyl wall & chamber in that area.
True and unshrouding the valve increases the combustion chamber volume and may also introduce cracks to the water jacket due to decreased material thickness.
My memory is playing games with me today Jac. What engine was that where the top of the cylinder bore was actually notched to just above where the top ring comes to rest?
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Old 7th September 2017, 11:21 PM   #6
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Re: Cylinder head inlet valve sizes

That was/is the 351 Cleveland, some of the later aussie 302c blocks which had 2.070" inlets did not have that notch and were handy to have when using longer rods and shorter ring stack height in pistons, the notch was only really reqd for larger 2.19" 351 4v heads.
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Old 8th September 2017, 04:30 AM   #7
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Re: Cylinder head inlet valve sizes

Thanks all, much appreciated and valuable advice as always. So it seems the general consensus is that the head and cam combination is fine for what I will be using the engine for. Does anyone have any feedback on the quality of pro-sport ali heads? I have heard horror stories of Chinese heads having rocker studs in the wrong place, valve seats falling out etc etc. Thanks again!
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Old 8th September 2017, 08:54 AM   #8
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Re: Cylinder head inlet valve sizes

Thanks Jac...

Re:Chinese heads
Might be perfectly fine or junk. Let others do your longterm evaluations on them the same way we all did Scat and Eagle...
Spend your money wisely and buy known quality components with a good track record.
Dart, AFR, Edelbrock, et al..
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