I thought I would pose this question to you guy Underboost what are the effects of using a larger valves?

I am using this a 4.3 Chevy, but its basically a 3/4 350 some much of its the same. I am planning on using a Vortec Head that will be lightly ported by me.

I know what a lightly port head should do air flow wise (atleast for a NA engine). From what I have seen, it seems for light porting larger valve have show a minimal increase. This is NA engine though. Now adding boost, does the larger valves advantages become more pronounce because your shoving air through the valve or does it follow a relatively linear pattern with the port size before the valve creating the restriction.

BTW This going to a be 'mild' build, that is intend for street manners. I don't mind sacrificing a few top end horses for streetability. My RPM will will also be max 6000rpm. I expect my shift points to be in the 5500-6000 range.

 

Sounds like a Typhoon/Syclone build to me. I always go with the old adage "power is derived from the heads". The better the heads, the better the power NA or FI.

Aaron

 

Yes it a Syclone build up. I am doing what I want (my ideal) with the short block (ie no short cuts to save $), but it doesn't leave enough money for my ideal heads. I am going to make due with what I have; I can always pull the heads at a later time. Also I don't really have a transmission that will reliably hold up to the power that I would make with a max effort set of heads and high boost, so I am not just off by a couple of thousand. I am already planning on limiting my boost on this build while I save up for a 4L80E.

The advice I am looking for is I have a set of Vortecs, that will flow 240/210 with 1.94/1.50 valves. I am wondering if setup to 2.02/1.60 and the machine work for new valve seats are worth it on a boosted motor. On a flow bench under vacuum I don't think I will see very much of a gain with the bigger valves (at this point the port are probably holding everything back, but I don't have the porting experience or the $ to spend a professional on it right now).

However, I am unsure of if the air flow with bigger valves still will be neglible under boost. That is the question I am try to find th eanswer, does the port/valve size behave differently under boost and would larger valve be beneficial when a flow bench un vacuum says differently.

 

If the valves are not a bottleneck without boost, then they will not be a bottleneck with boost either.

 

Don't worry about the heads. I did some tuning on a Typhoon this year with those heads, and the heads weren't the bottle neck.

 

I built a 4.3L Scyclone engine a few years ago, and was surprised at a few things, when I got it apart.

1. How small the stock cam was.
2. The fact that the stock heads were infact the "High Swirl" intake runners and not the more conventional intake runner Vortec heads.

If you have a newer set of the 4.3L Vortec heads and intend to use them IMO you are already better off. If yo have mistakenly thought that the Scyclone heads were the Vortec style.. I hate to tell you but changing to bigger intake valves won't do anything to help that POS half cut off intake runner in the stock heads. Putting a 1.6 exhaust valve will help out some (20CFM across the board on my heads) provided you have a good valve job done and the valve bowl ported to match.

I don't know for sure at what point the stock heads would be a power restriction, but the one I built, I put 2.02 intakes and 1.6 exhaust with only a good valve job and bowl blending/short side radius work on the exhaust made it down in the 7.0's in the 1/8mile, so you can go pretty fast if you really want to... but the 700R4 tranny won't last long. His sure didn't.

Also by putting new valves in at least you get to upgrade to Stainless Steel which will stand up to the heat a little better. The one I built had 2 burned pistons and a burned exhaust valve when I got it, thanks to a stupid owner, pump gas and a boost controller. I tried to "learn" him a bit before he left my shop with his new engine. Nice to have that insurance thou.
Will

 

The larger intake valve won't be as important as the larger exhaust valve. When I had my heads ported I enlarged the exhaust to 1.60 and kept the intake at 1.94. With boost the intake valve is not nearly as much of a restriction as the exhaust valve. The most expensive part of the valve job is replacing the hardened seats (and it's a PITA). I would recommend a full set of nice new SS valves. Also, make sure the engine can breathe very well on the exhaust side; headers, larger system, high flow everything.

 

Make sure you have the vortec head as stated and rework the exhaust port with a 1.6 or larger valve and stainless or better material. I have a lot of aftermarket race 4.3 parts if needed 23 degree and 18 degree.