Eliminating Cylinder Head Valve Shrouding for Performance

[DesertSasquatchXploration]

Any gains with stock DP heads? I have my topend apart going back together as a 69 crank, 90.5, .040 deck height 49CC heads 9CR.

But why don't I bring the Chambers out to the bore size around the valves and have them cut back to 49CC I have a small step still so heads have some meat on them Is it worth it on a 5K rpm engine 100 cam 1.1 rockers.

Or will i end up with 75CC heads once i remove that material
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Key is to have downward travel Preload keep both wheels on the ground at all times once you lift a tire your DONE. Guys worry about clearance instead think of the opposite you want the suspension to drop that tire in the hole and keep you going. A spider for example they keep their body low but their legs can reach pretty far so they don't (bottom out)

[SGKent]

back cut at least the intake valves and that will help even with stock DP heads. Get rid of any excess flashing in the ports but do not make them shiny smooth. They need roughness to prevent laminar flow.
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"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin

[Alstrup]

[Brian_e]

I just finished up a pair of stock valve 040 stock valve heads for a local racer. Stock bore, and they are right at 125cfm at .500". They are my standard, quickie mild port job, with a minimal amount of material removed from the flange shape. 85.5mm bore. Mostly just bowl and seat work.

The same heads, near identical port with a 90.5 bore will flow about 134cfm at .500". That is with my 4 angle valve job, and a back cut valve on both. Only real difference is the larger bore diameter, and the slight bit more un-shrouding on the 90.5.

The un-shrouding is needed, but don't get carried away. Stick to the side walls, and the plug side of the chamber. I don't even touch the non-plug side. There is almost NO air moving there. Un-shrouding out to a 90.5 bore will gain you about 1-2cc is all.

So there are gains to be made there if you spend the time. Without a doubt, the biggest and best gains are made blending the bowls, and getting the seats narrowed and placed well on the valve face. Easy 15-18cfm gain just doing that.

Spending time trying to remove the casting lines in the port will do nothing for flow, and only add port volume in the wrong spots. Don't even worry about the casting lines. Also, you never back cut an exhaust. Only the intakes. Usually about a 3-4cfm gain just doing that.

The most important part of the whole intake port is the 1" before, and the 1" after the valve seat. Spend your time detailing that area, and you will see the most gains.

Brian
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So more or less the lazier and stupider you want to be, the nicer quality parts you need to buy.
-Modok


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Type E Engine Parts and Supplies
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[BFB]

Why do you say to never back cut exhaust valve ?
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"how am i supposed to torque the rear wheel nut to 250 ft lbs??? " - clonebug
An interesting thing happens in forums where everyone starts parroting the same thing and "common knowledge" takes over.
Most experts aren't.

[modok]

One mistake I made in the past, reading early David Vizard stuff with the idea that all flow is good, low lift and high, and thought it was a good idea to unshroud the valves all the way down by the seat get more low lift flow, to have a good path around the valve at even low lifts, like .200 lift, and....well..... that wasn't the right thing to do for every case.

Not a big deal to worry about, but turns out of you increase low lift flow, but hurt high lift flow, you end up making it run like it has a bigger cam, but in all the bad ways and none of the good ways. DOH

Increasing the gap around the valve at, say, .350 lift and up is a good idea, but I'd leave it alone below that, a lot of the time the stock heads is actually too far blown out down there already. The stock heads were cast a little big so a bit of core shift didn't matter. Check the gap around the valve you'll see it varies a lot one chamber to another, and making them all match the largest one isn't best, more like they had it right on average.

If you use a slightly larger valve and sink it down a bit, that helps improve the situation, as you can make each one match.

And the exhaust side is different but similar. Flow going around the valve on the cylinder-wall side is actually aiming the wrong way to go down the port, so you don't really want to increase that overkill, and maybe it flows more or maybe it doesn't, but flow likes to take the straighter path, so, focus on that the most.

So lay back to the bore size some, yeah, but don't go too far with it and hog out a big swimming pool around each valve. It DOES give more low lift flow, but not the way you want.
I think overall it just runs better being more like a funnel around the intake valve and less like a bowl. There is no one right way, but all the right ways have that aspect in common.

[DesertSasquatchXploration]

Great insight Thanks Guys! I'm going to have at it in the morning ill post a pic tomorrow.
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Key is to have downward travel Preload keep both wheels on the ground at all times once you lift a tire your DONE. Guys worry about clearance instead think of the opposite you want the suspension to drop that tire in the hole and keep you going. A spider for example they keep their body low but their legs can reach pretty far so they don't (bottom out)

[Brian_e]

I agree with Modok. Don’t dig a ditch around the valve. Ideally, the chamber walls should start at the 30deg top angle and extend out from there in somewhat of a straight line. More unshrouding as the lift gets higher.
Pictures are always hard to see it all, but this is one I just did. The plug side has a little too much bowl to it, but the material was already removed from there, and that is where 85% of the flow is happening.
Bigger valves blend better into the chamber walls than stock valves.

Focus on the valve bowls, and the valve job, then do the unshrouding.

Brian
_________________
So more or less the lazier and stupider you want to be, the nicer quality parts you need to buy.
-Modok


www.type-emotorsports.com

Type E Engine Parts and Supplies
https://type-emotorsports.com/collections/engine-parts

[Brian_e]

[modok]

LOOKS GOOD, and well said.

the stock heads have too much ditch aroud the seat, and that worked ok becase in stock form the valve only lifted .300 high.
If the heads were designed for .400+ lift they would have been designed different. You can only half correct it on old stock heads, but a lot of the NEW heads are getting better, clearly!

tho that one spot you didn't touch on the plug side.... and i agree with it

[Wreck]

Hopefully get a set of Doug's heads heading my way in a few weeks time.

[SGKent]

[modok]

[Brian_e]

[DesertSasquatchXploration]

Well you were right after I did a basic unshrouding they are at 51.5cc give or take. I'm going to drop them off at brothers in the morning. With the deck height @.040 and no rings when i rock the piston its .034. I had the jugs honed they are .0038 over the piston Dim. I think that looks ok.

Since im reusing the pistons and jugs im kinda tempted to do a .50 deck. What do you guys think should I do a .040 deck with used stuff?

I'm super happy i did a top end I had one exhaust valve with a sloppy valve keeper and it was pulling out was going to drop a valve at some point.
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Key is to have downward travel Preload keep both wheels on the ground at all times once you lift a tire your DONE. Guys worry about clearance instead think of the opposite you want the suspension to drop that tire in the hole and keep you going. A spider for example they keep their body low but their legs can reach pretty far so they don't (bottom out)

[Brian_e]

Not sure you are able to specify what kind of valve job you want at Brothers, but I would recommend against their standard valve job. You might have better luck with a performance V8 machine shop instead. The last few jobs I have seen from them were very wide seats placed very low on the valve face. Exactly opposite of what you want for good flow. If you can specify, I would ask for a .040” wide seat positioned about .015-.020” away from the margin of the valve. That is for intake only. Exhaust can be about .060 wide placed in the center of the valve face for better cooling.

It seems like a minor detail, but I have seen 8 to 10 CFM gain on a stock valve head just by moving the seat placement on the valve face.

Brian
_________________
So more or less the lazier and stupider you want to be, the nicer quality parts you need to buy.
-Modok


www.type-emotorsports.com

Type E Engine Parts and Supplies
https://type-emotorsports.com/collections/engine-parts