What are the pros and cons of going with larger valves. Does it cause any idle/drivability issues? Decrease gas mileage? Is the performance gain significant? Improve or decreased throttle response? Is this a higher RPM upgrade or will there be improvement at low RPM also?
Adding larger valves alone will not give you much gains in my opinion. Combining larger valves with professional head porting, larger camshaft, higher compression etc will for sure give major gains. I have seen 35 hp gains from larger valves added to an already pretty serious combo. With the alum heads bring released soon it is going to become very hard to justify thousands of dollars of investment into iron heads. Unless limited by racing class rules to keep iron heads there will be no real reason to spend hard earned cash on them. Nothing wrong with rebuilding a set of iron heads, or doing a home done port job, but pro porting and or oversized valves is not likely to be commonly done.
My machinist suggested that unless this is going to be a dedicated race engine that will constantly see 4K+ rpm then don't bother. Do you agree?
Putting larger valves in without the appropriate valve job including throat angle will worsen every aspect BUT a worn seat causing too much valve stem protrusion. Basically it is a bigger door to block the air until the air can pass around it. Dropping a smaller valve onto any port not specifically idealized for that bigger valve shows up better on the flow bench, esp. at high lift, even though it seems counterintuitive. Basically the throat percentage becomes larger. Despite what's being said, a bigger valve does help low speed performance as well as low lift flow by virtue of more curtain area per the same amount of lift, but the advantage goes away once the valve is open past the curtain area exceeding the port's. A good working combo or relationship trumps size every time. Good working port/valve combos regardless of minor size differences run much better in every aspect you've mentioned. Cams on the wilder side tone down some, wake up sooner, deliver better mileage, better low speed response. On several very high mpg 7.5l engines I tested, going from a well ported head to a bone stock fresh one lost 4 mpg at the same valve size. I do not fully agree with what your machinist told you, except as general advice. There aren't many benefits without making sure the vj and port are going to work well with that valve, but if so...nearly every aspect of your engine improves.
All good info in the above, but one added good side to bigger valves is to not have to sink a stock size valve when doing a valve job. Sinking the valve will add CCs to the chamber a lower compression which could be good , or bad depending on your combo. Sinking a Intake valve can add low lift flow with the right valve job done, but will near always knock off some high lift flow above .450" . Sinking a Intake valve will always make for more wasted fuel in a motor with good tuned Headers due to fresh charge getting sucked right out the Exh. If I am dealing with a motor rebuild that will still pump out thru stock Exh Manfolds , and I have the choise to add larger valves I will opt for bigger Exh valves. This allows for big increase in low lift flow by means of using a multi angle valve job. Since the stock Exh set up is quite restrictive and can not achieve high flow rates with even valve lifts in the .470" range this means we need to use the port / Exh area we do have better, and that time is at low lift when the Exh valve fiesta cracks open and the Exh Manifold is not saturated with more flow than it can pass. On low to mid comp motors ( under 9.4 to 1) this will allow the useage less added Exh duration Cam wise which will help to make more cranking compression .
Alright....so my takeaway is that it's easy to screw up and I could make things worse all while spending more time & money. I'll keep the stock sizes! lol
It does depend on your goals also. As mentioned it also affects the rest of your combo of parts. Cam/ intake/ rpm range/exhaust. But so far your on the lighter side. You want to keep rpm lower . But the better flow will help mid and upper rpm. I believe Sean posted a dyno sheet of an 8.3 compression build with heads flowing close to 300 with a 212 cam making close to 400 hp. Obviously the rpm range is different with that kind of flow . Seems that 350 builders like to have flow around 245-255. Probably worth 10-20 hp on an otherwise stock engine without porting. Just a guess thou. Haven't heard anyone that has gone to bigger valves say anything bad or performance was lower. The ports are already a higher velocity port. Biggest issue is expense.
You'll never hear of anyone stating they installed bigger valves and it caused it to run worse but believe me...they can and do under many circumstances. They just blame the nasty cam and it's specs. It's a gift to take on a project that has heads freshened and ported where you can actually see it run beforehand as well as correlate your flowbench findings to the end result. Simply installing a bigger valve tends to dam up the port and cause areas in the port to stagnate, so saying a certain head is already a high velocity head is kind of misleading because you've now changed things. High swirl isn't always high velocity. It also depends on if you are talking about stock, modified, ported and to what extent. In an A to B comparison the good working head shows stronger manifold vacuum, more throttle response (any of the above mentioned criteria)...it makes them behave and run better in every circumstance if properly done. Not many attempt to tune street manners into wilder builds but it is quite achievable to see a 3.7-4.0" stroke 1.52-1.60 rod ratio engine with 270* @ .050" 105-107* LSA cam idle at 950-1000 with 12-13" vacuum on a 300+ cfm port with a single plane intake. (The achievement isn't miraculous, it's that many don't bother the extra effort to street tune something like that. A slightly larger CSA port with 'only' 10-15 CFM+ peak might idle 1350-1450 @ 9-10" and suffer far less part throttle crispness for the benefit of 20-30hp at peak)
The oversize valves for the 350 are only .040" and .050". They will not hurt anything but your wallet. If your trying to save the heads without resorting to inserts then use them. The power gain with stock ports would probably not even register on a dyno.
There's usually a power loss with stock ports unless the VJ was sized appropriately, which they generally aren't after a rebuilder type auto machine shop cuts them and absolutely would show up on a flow bench and dyno (same dyno, no other changes to the engine that would serve to increase power). Of course, few tests of this would exist in common publications and no shop doing the work would admit to it. Obviously, if the engine in question is very cam and/or breathing restricted and doesn't come close to using the capacity of the heads, then post #9 would hold true. [So yes, larger valves without the appropriate VJ worsen any driveability issues that may already be a concern when adding things like aftermarket camshafts.]
Lots of great info for me to chew on. I appreciate it. It really does (for me anyway) seem like a waste of time and $$$. I'm just going to port the head and stick with the stock valves. It ran great before and it took a lot of time and effort to get it to that point. I'll stick with the old adage that "If it ain't broke then don't fix it."
As sean mentioned IF you planing on porting the heads .. either professional (too expensive in my opinion) or Homeport (makes the most sense) THEN the bigger valves will help gain very decent power if you also have a good grind cam . Guess how i know.. my combo is Ported heads , bigger valves , TA stage 1 springs, Custom Cam + SP3 intake dialed in qudrajet and long tube headers... Runs real good. BUT i have more to come based on this setup regardless.
A quick easy way to check if going to a bigger Intake valve will help or hurt besides what s been posted is to do this. Set the head up with the chambers facing you. Slip a stock size Intake valve in the guide and then raise it up from the seat little by little. If at anytime during opening the valve the a chamber wall come closer to the edge of the valve then the amount of valve lift your Cam will provide, then you have chamber shrouding / flow restriction going on now, and a step up to a bigger valve without dealing with the shrouding will make for less power.
Actually, if your valve job seat angle's outer diameter is more than .015" smaller on the radius...it's hurting your flow somewhere in the lift curve. I see a LOT of vj's where the seat OD is .020" smaller than the valve OD or much more, kind of a shop protocol of sorts. Safe for not burning valves or seats but not the best for flow. Just shy of the edge is best but it doesn't leave much room for erosion, not that you should incorporate that into your build (pointless, really). The only gift I see there is that I could potentially gouge a seat with a die grinder and not even worry...it's going to get re-cut anyways. It's usually a 10-12 cfm gift, so 20-25hp if the flow can be used. [This is a tough one for the garage guy to really see due to margin of error from bench to bench (or dyno), however you can see an immediate difference on YOUR own equipment immediately and repeatedly. So it may not translate well due to individual flow benches and calibration errors and the way they relate info across the country with varying atmo conditions and corrections. Basically the correction gray area lets important stuff like this slip by unless you do some ABA testing for yourself]
