There are 350 larger exhaust valves for sale but is 0.050 worth the machining expense? The intake valves are even less of a gain at .045. It seems like a little porting would be more effective.
For that price you can't go wrong even w/o doing the intake (which I also would). I may have jumped in on the last set though if everyone else takes them. I could be wrong but if I was only going to do either porting or big valves I'd go for the valves. My last 350 had both & I was very pleased. Changed cam & compression @ the same time so hard to say what just the heads were worth but in the 80's KB 350 build they were worth a decent amount (don't recall exactly).
if the vlves are cheap it will be worth it when some porting getrs done You know we always keep talking about velocity and what if the heads were ported with the stardard valves in it. Would it have a faster velocity going in with the smaller valve in there? No one knows since I have never seen anyone do this. Roll this around in your heads for a while Think 600 cfm dbl pumper hmmmm
I could be off base here but I've sort of developed the opinion that due to the amount of work involved, the only time porting is really appropriate is when racing and even then only when you need just that little extra something to get ahead of the competition. If you aren't racing you never really know what anyone else is doing and there is no way of comparing your car to anyone else's. Therefore the edge to be gained by porting is nothing but bragging rights. Best don the flame suit I reckon. Jim
Good point, just cruising the streets or highways, even cruising at 3400 rpm im sure theres not much to be gained that you'd notice. Foot thru the firewall in the quarter mile balls out to 7 grand or more, then I'd say yes theres going to be a difference.
That was why I thought port with the smaller valve to stay on the stock side and would the intake velocity be higher? add a Sp3 600 carb, just over stock roller cam.
Mine is just a cruiser and fun car. I think I'll pass on the valves. It doesn't seem worth it. I'll just deck the block and shave the heads for a little more compression and call it a day.
Greg, if nothing else, new valves = non-worn valve stem diameters. Just like guides, you need to tighten those clearances up too.
Heads need to be rebuilt no matter what is going on so might as well upgrade the valves with bigger. It will probably run better with this improvement even if the rest is stock, will more than likely gain another 25 hp. The 1500-4500rpm stock will pick up a lot. and do what Hugger said he knows his sh...
Bigger valves will also put the valve seats into unworn material on the head. This could be an option to installing new valve seats.... which may not be an option for some engines (Nailheads). Thoughts?
Something that make sense to me, but I don't hear/read about much (at all, actually) is unported head with larger intake valve. The way I see it, if you don't port and just install a larger valve/seat, you might realize better (read more efficient maybe?) fuel dispersion within the cylinder. Its like the diffuser on a fire sprinkler. It throws the water out farther, thus the same amount of water sees more of the room. If the air/fuel mixture skids off a larger diameter valve, wouldn't it then be flung across a farther area of the cylinder? The end result would seem to be more a/f being dispersed across a greater area of the cylinder, compared to using a smaller valve. This could also be seen as a more efficient dispersion of the a/f across the cylinder, thus creating a more efficient burn during the ignition phase of the cycle. Its just my theory, but what do you guys think?
A larger valve wont help flow unless the bowl area is opened up enough to allow the additional flow potential of the larger valve. The purpose of larger valves is to allow more air/fuel in, and exhaust out.
So you think the degree of dispersion of the a/f mixture would remain the same, regardless of valve size?
How about the opposite idea port the heads and take more material from the top and keep the small size valve and with the small bore and the bigger port area the small piston will create more velocity and pull the air in at a faster rate with the small valve. this will make a more atomized spray within the engine, make that cam duration Big and the lift on the marginal side 500 lift lol. We don't have coffee can pistons to make a big pull on the engine. just a thought.
Years ago “swirl polished” valves were thought to impart swirl to the incoming mixture, but that theory kinda got thrown out the window when they came to the realization that the backside of the intake doesn’t stay clean for long, the “swirl” get covered in carbon
I don't understand how the comparatively small bore size will speed up airflow. If anything, I would think the air flow rate would decrease due to the larger port's ability to move more air.
