sross

iTrader: (1)

Okay, multi-angle valve jobs are supposed to increase flow as are undercut valves. Does anyone have any experience on a flow bench to see just what kind of flow can be gained? Also, what about larger valves (assuming that there's no shrouding)? What's the potential here? Is there really that much to gain with a 5 angle, back cut 2.02 valve over a stock with a 3 angle valve job?

SStrokerAce

iTrader: (2)

Well I all I have on that is some tests that Dave Vizzard has in his book, "How to Build and Modify Chevrolet Small-Block V-8 Cylinder Heads" on valve seats and the flow he gets from them. These are not LS1 heads so the numbers are smaller, and most GM valves are cut to the right angles now stocks.

Vizzard did a presentation at the Superflow Engine Confrence in Colo Springs in 1998 about this, so he has really tested it to see what works. Basicly he talk centerd on valve seats, skirt area of the valves and flow. It basicly said this: the 30,45,60 3 angle is the BEST for flow and HP, The reason we have more valves is to get more valve skirt area therefore improving low lift flow and HP, (it's simple math. find the circumfrence of one valve = skirt area, vs. the circumfrence of two valves with a diameter half as big you get more flow)

These numbers are from a 186 Casting Chevy Head, for a small block.

He does this in a number of steps.

This is intake flow stock

Lift CFM @ 28.0 H2O
.025 14.8
.050 29.0
.097 54.4
.194 101.0
.291 148.5
.388 178.0
.485 179.7

After cutting the seat at a 30,45,60 degree cuts, reprofiling the valves and blending the back of the valve from the seat to the stem smoothly, cleaning up the throat of the Port the flow turned out to be this.

.025 17.3
.050 31.5
.097 58.9
.194 113.8
.291 161.4
.388 191.5
.485 194.5

So there is a good improvement. 14.8 CFM at .485" lift.

Then doing the same thing for 2.02 valves (up from 1.94)with the chamber deshrouded

.025 16.9
.050 31.0
.097 59.0
.194 116.5
.291 164.3
.388 193.2
.485 206.5

This shows that the larger valve will get gains for you above .100 lift. 26.8 cfm more at .485" lift.

Doing most of these same mods except for the larger valve on the exh side he went from this to this.

Lift Stock Mod Gain
.025 12.7 13.6 +0.9
.050 22.9 24.5 +1.6
.075 32.5 38.2 +5.7
.150 58.9 72.2 +13.3
.225 81.0 101.5 +20.5
.300 104.4 126.8 +22.4
.375 115.2 143.1 +27.9
.450 121.0 155.1 +34.1

Basicly this is easy to do relative to porting and give good gains with little or no modification to the port size, so port velocity increases.

If you want to know more this book is one of the best on porting heads. Go pick one up I hope it helps.

Bret

SStrokerAce

iTrader: (2)

One more post on head flow.

It's amazing what we can get since the 186 casting's were done. They were basicly the old LT1 heads on the 70 Camaro & Vette, the LS6 of it's day. Now here are three of the best street heads from GM, all under $1200 a pair!

xxx L31 Vortec Fastburn LS1
Lift int exh int exh int exh
.050 40 25 32 27 46 31
.100 70 49 62 55 83 62
.200 139 105 136 104 134 104
.300 190 137 187 133 189 136
.400 227 151 232 158 222 165
.500 239 160 266 180 240 179
.600 229 162 275 193 252 185

As you can see the new port shape gives the LS1 awesome low lift flow, the best street heads GM has ever made for the old small block are right here and the first head on the new small block is right up there. I would imagine the LS6 heads are a good mix of the LS1 and Fastburn heads. The mid lift numbers for all three are very similar, from .200 to .400 they are pretty much the same!

Just some interesting facts

Bret

One Monkey

Sross there's actruly alot too gain. The thing is one's head porter valve jobs usraly complement his ports. but there are lots of secrets that improve flow. 20-30CFM but there are secrets so the chance of you ever seen the posted is slim too none.

LS1derfull

Vizard knows his s-it, theres no doubt about that, i just wanted to add: low lift flow importance is combination sensitive. I mean if you have small cam, stock heads than these #'s are extremely important to making power and extending power range. Hi lift and duration cam could be tamed somewhat with not so great flow at very low lift ranges. Less flow, less caminess at lower rpm. The bigger cam is the more the focus of flow moves to higher lift flow . Make sense?