Valve Sizing

[Ace VenTRa]

While it's fresh in my mind I thought I'd throw it out there in case anyone was considering new bigger valves. Having upsized my intake valve to 39 and considering a 40, I took a step back to look closely at the heads for a while. Based on the size of the combustion chamber and available room, I think that a 39 is the max you would want to go and even at that I think I can achieve better results with a completely unshrouded 38. Based on all that I have read and experimented with there is no more flow available after approximately .25D lift and at that point you would benefit from .20D minimum radial unshrouding. I'll post a photo later today but with a 38 you can definitely make that happen, with a 39 you're cutting it close. A 40 would be damn near impossible. Now, earlier I stated that I would rather have a smaller exhaust valve, here's why. The standard guide from just about everywhere for a street/strip motor is that the the exhaust should flow about 80% of intake. 85% is about the max though 90% can be used for mileage improvements in certain circumstances. To be clear I am referring to flow and not valve size. If you need a bigger valve to make the flow happen that's fine but it's more important to look at port cross section along with valve curtain. Either way, if you take the intake valve and multiply it by 85% (38x.80= 30.4) you get a 30.4 mm valve size. That's just a starting point but you get what I'm saying that at 80% the ex valve is way too big assuming that flow will follow suit. Later when I finish the flow numbers for the heads I'll compare them to a stock head but I will say that the stock ports, cross sections and valve sizes are pretty danged close (mathematically) and the most gain to be had anywhere are in the bowl, seat transition and 3 angle cuts.

[Ace VenTRa]

Ok, so here are some photos. The discoloration you see is just the dye I use to help visualize how the air wants to flow.

Notice the shrouding on the 40 as compared to the 38

Notice how the 34 ex valve fits just about perfect, a little smaller would have helped.

If you stand back for a moment you'll see that the valves take up all available combustion chamber area within reason. Considering that, and weighing costs of modification, we should focus on shrouding, blending, and getting a better designed cam as well as better quality valves, higher compression & lighter valve train.

AV

[mik_str]

Interesting stuff!

[Ace VenTRa]

I should mention that this is an unshrouded head! You should see the stock head with a 40. I'll post that later. For now heres a link to a new vid:




AV

[mik_str]

sounds healthy!

[Roger Ditchfield]

The Revolution/Moriwaki race bikes used o/size valves into suitably ported and gasflowed heads. Inlets were +1mm and Exhausts were +0.5mm over OEM. The heads were also skimmed by 28 thou/in.

[Ace VenTRa]

He speaketh!

You might agree that having an unshrouded set of 34/38 mm valves with a good 3 angle job would out perform just about anything else one could come up with for a street motor?

Maybe you could chime in on the cam timing events from back in the day that were experimented with? I've found the regrind that Web did for me to be stellar up top though I suspect that a set of adjustable gears on stock cams would perform admirably...how about you?

AV

[Roger Ditchfield]

Accurate or altered valve timing by the use of a crankshaft timing disc, dial gauges with slotted cam wheels even with OEM cams can be beneficial over the OEM stamped timing marks. Obviously, re-profiled cams through to high performance cams all come with their own timing figures which can only be installed by this method

[Ace VenTRa]

Not necessarily obvious to everyone, I think you should tell us how much of a variance you found when you were working on factory parts. For example, the RT/FT marks and their relationship to TDC might be a good topic to cover, as well as how to combat their differences.

AV

[Ace VenTRa]

Anyhoo, here's some more food for thought. I turned the old block upside down with a head on it so you can see down the bore and more fully understand the issues with bigger valves and shrouding should you choose to go down this path as well. Below is a pic looking straight down the bore at what would be the combustion chamber. If you look at the outer edges of the valve you will see that they are almost right against the cylinder wall and are on the razors edge of the gasket. So, if you are going to go to bigger valves, especially on the exhaust, consider how much room you have for radial clearance not just for the valves to piston reliefs, but also to the cylinder walls. As I understand it way back in the day chevrolet had this issue on 396's and maybe 454's. They ended up machining in a relief into the cylinder wall to help combat the issue. Now in this case you are looking at a 98mm standard bore, I'll have a 99mm more on the next engine and I have to say that if you are going with bigger valves, a bigger bore would really help the issue. The next issue is the diameter of the gasket, the stock gasket is not exactly perfect in that regard and I think that a cometic would be the way to go.

AV