461 heads flow numbers - Duke et. al.

[Don C.]

I've finally got around to rebuilding the heads on my 65 350/327. Several months ago I used this board to get information on head porting and the flow numbers to expect with a good porting job. I've read everything I can find on the subject and now feel quite confident about doing it myself. The deal I struck with the machinist who does this work using a flowbench is this. He ports one cylinder and performs before and after flow tests on that cylinder, turns the heads back to me to copy his work then I bring the heads back for a flow bench test of my work. He has done the one cylinder and I am sharing the results here to get input on the numbers. First some technical data. Valves are Elgin Stainless steel 1.94/1.50 with the reduced diameter neck. No back cut on the intake valve nor radius on the chamber side of the exhaust valves yet(I plan to do that as part of my work). The exhaust seats were given a nascar cut(?)with a sunnen cutter (30/45/60 degree angles). The intakes were also supposed to be done with a cutter but previous work had opened the throats too much so they are ground 30/45/60 degrees. The outside and inside diameters of the intake seat on the one finished cylinder measure 1.925 and 1.840 respectively. The throat is around 1.7 and depends on the circumferential measurement points. The same measures for the exhaust are as follows 1.50/1.43 and 1.33. My observations suggest that the short side radius has been compomised somewhat on the intake and both throats are on the big side. This observation is based on my armchair knowledge and no practical experience. The flow numbers at 28 inches of vacuum for the intake are as follows.
Before is the second column and after the third column.

.050 29.6 23.9
.100 60.4 57.9
.150 89.3 89.6
.200 119.5 119.9
.250 144.9 148.3
.300 165.7 168.6
.350 182.7 186.6
.400 188.3 204.6
.450 206.4 217.9
.500 198.9 208.7

The exhaust flow for the same lift points:

.050 27.5 23.8
.100 52.6 48.4
.150 74.0 71.4
.200 91.7 93.1
.250 106.8 108.9
.300 122.1 127.8
.350 130.3 144.8
.400 135.8 157.0
.450 139.6 168.1
.500 144.0 176.3

I am interested in comments on these flow numbers based on your experience. I expected more of an increase on the intake. I am concerned about the big drop off in low lift flow and how it may affect performance in the dynamic operational state.

Regards
Don

[Michael H.]

Re: 461 heads flow numbers - Duke et. al.

Don,

It sounds like you've made some positive modifications on your heads. It also sounds like you may have made a few in a negative direction. Opening up the valve seat diameter to (nearly) match the OD of the valve, along with the proper approach angle and top cut, is a good thing. However, increasing the overall dimensions or size of the port itself may actually reduce the effectiveness of the port and produce less power. Bigger is seldom better. You may have increased the overall air flow amount with port size but by doing so, you would have reduced air velocity through the port which kills the positive effect of the late closing intake valve. When the piston reaches the bottom of it's travel and is no longer causing or creating a negative pressure in the cyl or intake port, the flow continues into the cylinder because of enertia. The column of air that was traveling at high speed through the port has enough positive pressure and enertia to continue adding to the cyl volume. When the intake port size is increased, the same (or greater) volume of air will flow but at a reduced velocity which tends to reduce the effect of "after the fact" pressure that continues to fill the cyl. So..the overal flow numbers on an intake port may look great on paper, but they tend to do it at a sacrifice of pressure and air velocity at the end of piston travel so the actual numbers in a running engine are less than they would have been before any work was done on the size of the port.

The secret is to try to eliminate any small areas in the port that restrict or disrupt flow, without actually increasing the cross section dimensions of the port. Some port configurations actually flow better as material is ADDED in certain areas. Small, but efficient, ports that maintain high air velocity are much better than large ports with low air velocity.

Major valve seat rework can increase air velocity numbers but at a price. As the valve seat is reworked, it sinks deeper into the chamber as it's ground, which increases total chamber volume. This lowers the C/R and also makes a deeper, less efficient chamber design.

I would try to stick with a modest valve seat rework, clean up and slightly match port the head and intake, and call it a day. All of the other magic hocus pocus mod's are only going to decrease your checking account but not increase horsepower. If you're talking about a competition engine that will see only open exhaust high RPM operation, that's a different deal but it's also a discussion for some other board. If this engine is for street operation, try to find some discussions on this subject by Duke in the archives. I'm sure he'll agree and have a few things to add.

[Michael H.]

Re: 461 heads flow numbers - Duke et. al.

Don,

It sounds like you've made some positive modifications on your heads. It also sounds like you may have made a few in a negative direction. Opening up the valve seat diameter to (nearly) match the OD of the valve, along with the proper approach angle and top cut, is a good thing. However, increasing the overall dimensions or size of the port itself may actually reduce the effectiveness of the port and produce less power. Bigger is seldom better. You may have increased the overall air flow amount with port size but by doing so, you would have reduced air velocity through the port which kills the positive effect of the late closing intake valve. When the piston reaches the bottom of it's travel and is no longer causing or creating a negative pressure in the cyl or intake port, the flow continues into the cylinder because of enertia. The column of air that was traveling at high speed through the port has enough positive pressure and enertia to continue adding to the cyl volume. When the intake port size is increased, the same (or greater) volume of air will flow but at a reduced velocity which tends to reduce the effect of "after the fact" pressure that continues to fill the cyl. So..the overal flow numbers on an intake port may look great on paper, but they tend to do it at a sacrifice of pressure and air velocity at the end of piston travel so the actual numbers in a running engine are less than they would have been before any work was done on the size of the port.

The secret is to try to eliminate any small areas in the port that restrict or disrupt flow, without actually increasing the cross section dimensions of the port. Some port configurations actually flow better as material is ADDED in certain areas. Small, but efficient, ports that maintain high air velocity are much better than large ports with low air velocity.

Major valve seat rework can increase air velocity numbers but at a price. As the valve seat is reworked, it sinks deeper into the chamber as it's ground, which increases total chamber volume. This lowers the C/R and also makes a deeper, less efficient chamber design.

I would try to stick with a modest valve seat rework, clean up and slightly match port the head and intake, and call it a day. All of the other magic hocus pocus mod's are only going to decrease your checking account but not increase horsepower. If you're talking about a competition engine that will see only open exhaust high RPM operation, that's a different deal but it's also a discussion for some other board. If this engine is for street operation, try to find some discussions on this subject by Duke in the archives. I'm sure he'll agree and have a few things to add.

[Duke W.]

Re: 461 heads flow numbers - Duke et. al.

If the engine started out as a L-79 the OE valve sizes would be 2.02/1.60".

What am I missing here? Are these original heads? Are they 461 or 461X?

I distinguish between "pocket porting" and "porting". The former just involves cleaning up the bowl area, particularly the annualar ridge above the valve seat where the initial conical seat forming tool intersects with the as-cast section of the port and opening up the manifold-head interface usually using a gasket as a template. The interior of the port should not be modified except to clean up any casing flash or excessive roughness.

"Porting" involves opening up the entire port, which is not necessary for a street engine.

Finally, attention should be paid to valve seating, including narrowing the seats at the top with a 30 deg. stone and the bottom with a 60 deg. stone - known as a "three angle valve job." Also, the top of the inlet valve should be cut at 30 deg. to eliminate excess material that's above the seating surface.

Pictures are worth a thousand words, and I recommend that anyone interested in cylinder head improvement buy "How to Hotrod your SB Chevy" and obtain a copy of the old Chevrolet Power Manual. The later has drawings and details on head modification, the basics of which are applicable to most production heads. The former includes information from the Power Manual and also has good details on seating recommendations.

The exhaust port will normally show a greater percentage increase in flow than the inlet side, which can usually be improved about 10 percent to somewhere around 220 CFM @ 28" at about 0.4" valve lift. Flow numbers above this level of lift are not meaningful with OE cams since total lift (taking into account actual rocker ratio behavior) is not much over this level, certainly no more than .450".

Duke

[Duke W.]

Re: 461 heads flow numbers - Duke et. al.

If the engine started out as a L-79 the OE valve sizes would be 2.02/1.60".

What am I missing here? Are these original heads? Are they 461 or 461X?

I distinguish between "pocket porting" and "porting". The former just involves cleaning up the bowl area, particularly the annualar ridge above the valve seat where the initial conical seat forming tool intersects with the as-cast section of the port and opening up the manifold-head interface usually using a gasket as a template. The interior of the port should not be modified except to clean up any casing flash or excessive roughness.

"Porting" involves opening up the entire port, which is not necessary for a street engine.

Finally, attention should be paid to valve seating, including narrowing the seats at the top with a 30 deg. stone and the bottom with a 60 deg. stone - known as a "three angle valve job." Also, the top of the inlet valve should be cut at 30 deg. to eliminate excess material that's above the seating surface.

Pictures are worth a thousand words, and I recommend that anyone interested in cylinder head improvement buy "How to Hotrod your SB Chevy" and obtain a copy of the old Chevrolet Power Manual. The later has drawings and details on head modification, the basics of which are applicable to most production heads. The former includes information from the Power Manual and also has good details on seating recommendations.

The exhaust port will normally show a greater percentage increase in flow than the inlet side, which can usually be improved about 10 percent to somewhere around 220 CFM @ 28" at about 0.4" valve lift. Flow numbers above this level of lift are not meaningful with OE cams since total lift (taking into account actual rocker ratio behavior) is not much over this level, certainly no more than .450".

Duke

[Clem Z.]

the ID of the intake port at the valve should be

.81% of the valve head OD.

[Clem Z.]

the ID of the intake port at the valve should be

.81% of the valve head OD.

[Don C.]

Re: the ID of the intake port at the valve should

Michael/Duke/Clem

Thanks for your assessment, advice and education. The heads are 461 but are not the originals which explains the 1.94/1.50 valves. Pocket porting as outlined in David Vizard's book How to Build and Modify Small Block Chevy Cylinder Heads along with the excellent pictorial's and descriptions in How to Hot Rod Small block Chevy's are my basic guides, along with the wealth of info you have provided in these archives. You have confirmed my opinion that opening the throat ports was not a good thing. I will be much less agressive on 'my cylinder', and we'll see what happens to the flow numbers.
Clem you suggested that the I.D. of the intake port at the valve - which I assume to be the I.D of the smallest cross sectional area in the throat just above the valve - should be 81% of valve O.D putting it at 1.57 inches. Is this correct? Does this same percentage apply to the exhaust valve.

Many Thanks
Don

[Don C.]

Re: the ID of the intake port at the valve should

Michael/Duke/Clem

Thanks for your assessment, advice and education. The heads are 461 but are not the originals which explains the 1.94/1.50 valves. Pocket porting as outlined in David Vizard's book How to Build and Modify Small Block Chevy Cylinder Heads along with the excellent pictorial's and descriptions in How to Hot Rod Small block Chevy's are my basic guides, along with the wealth of info you have provided in these archives. You have confirmed my opinion that opening the throat ports was not a good thing. I will be much less agressive on 'my cylinder', and we'll see what happens to the flow numbers.
Clem you suggested that the I.D. of the intake port at the valve - which I assume to be the I.D of the smallest cross sectional area in the throat just above the valve - should be 81% of valve O.D putting it at 1.57 inches. Is this correct? Does this same percentage apply to the exhaust valve.

Many Thanks
Don

[Clem Z.]

don send me your email addy

and i will send you a picture of what a professional job looks like. these wre done by katech for me

[Clem Z.]

don send me your email addy

and i will send you a picture of what a professional job looks like. these wre done by katech for me

[Clem Z.]

Re: the ID of the intake port at the valve should

you are correct about the intake because if the "throat" is too large you will lose mixture velocity and the exhaust should have a venturi shape just above the seat.

[Clem Z.]

Re: the ID of the intake port at the valve should

you are correct about the intake because if the "throat" is too large you will lose mixture velocity and the exhaust should have a venturi shape just above the seat.

[Bryan H.]

Re: the ID of the intake port at the valve should

I just got home this morning with a set of 1960 model 792 heads that have been reworked for vintage racing with a 287ci small block using a Bill Thomas modified '62 Injector and a '57 air box. These heads have 1.9 intakes and the flowmaster readings at 28" were about 10 percent above your readings up to .150 lift and then dropped off by about 5% below yours above that level in a fairly linear fashion. That would agree with the theory that velocity in a smaller port does really work. Bryan Hill

[Bryan H.]

Re: the ID of the intake port at the valve should

I just got home this morning with a set of 1960 model 792 heads that have been reworked for vintage racing with a 287ci small block using a Bill Thomas modified '62 Injector and a '57 air box. These heads have 1.9 intakes and the flowmaster readings at 28" were about 10 percent above your readings up to .150 lift and then dropped off by about 5% below yours above that level in a fairly linear fashion. That would agree with the theory that velocity in a smaller port does really work. Bryan Hill