bigger exhaust valves?


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By rexbd - 16 Years Ago
In reading the thread on the 292 heads a question popped into my mind.  I have some ECZ-G heads I sourced for my 55 tbird.  Does it make sense to put in bigger exhaust valves to help the breathing process or is that a waste of money for a street car? Or I guess the can't be enlarged?
By Ted - 16 Years Ago
Depends upon who you talk to regarding valve sizes.  My thoughts on exhaust valve sizes is to actually decrease them as the compression ratio goes up and an added perk to the reduced exhaust valve sizing is allowing more room for larger intake valves.  Unless exhaust scavenging is excellent, then a larger exhaust valve will typically not be of any benefit and in some instances can actually hurt street performance and fuel economy.  Most exhaust systems with any kind of mufflers or extended piping will promote less than ideal scavenging characteristics.  And if the corresponding port work is not performed in conjunction with any increases in valve sizes, then the valve sizes being increased is also a moot point.  All conjecture on my part as each engine combination is subtly different with no two liking the exact same variables.
By pegleg - 16 Years Ago
Unless you are talking a supercharged application. that throws a monkey wrench into the issue.
By Doug T - 16 Years Ago
Well Ted is one of the 3 or 4 foremost authorities on Y's these days and has a broad experience with other engines also so what he says has a lot of weight.  But when I was porting & flow testing ECZ-G heads I found that I could not achieve the 70-80% exhaust flow rate recommended in Vizzard's "Performance with Economy" without going to larger valves. This book was mostly for street applications and the example was scrub street heads. A lot of other info in that book proved to be correct so I have some confidence in that ratio. I believe I have seen that ratio in other literature also.

If you are going for hardened seats and new valves going to 1.6dia exhausts costs nothing extra and if the machinist doesn't sink the valve you wont loose compression on that.  To make use of the bigger valve and port you do need to cut away the some of the chamber wall and that costs compression.  You also need to open up the exhaust port quite a bit.

This is what makes hotrodding interesting,  all the choices to be made are different for each application and they should all be co ordinated.  For example the big exhaust valves do not make sense with a 2 bbl carb, little 55 ex manifolds and 1 3/4" stock tail pipes.

By rexbd - 16 Years Ago
Won't get to crazy as eventually would like to get the aluminum heads if they prove effective.  I intend to use FPA headers, a 2 inch exhaust and balance/crossover in the exhaust.  Won't get to crazy on muffler from a sound perspective.  Information is helpful.  I have learned a lot from this forum that I never knew so it makes it fun to explore these issues. 

Hopefully will start putting car back into paint this weekend.  Really ready to get this project done and enjoy the car.  Been two years of working on here and there and ready to get behind me.

By simplyconnected - 16 Years Ago
rexbd (5/27/2009)
... I have learned a lot from this forum that I never knew so it makes it fun to explore these issues...
  Amen to that, rexbd.

The only thing I can add to ted and doug's suggestions is one simple word, 'throughput'.  They summed it up nicely.  The only redeeming quality in a 302 is easy in and easy out.  The same potential holds true for the Y.