Valve seats, widths, angles etc.

[eugene]

Hello
I have a set of 44 mm intake and 38 mm exhaust SS valves (a friend bought them for me in vw a shop in the USA), but the seats i got for them seams too big to me. The intake seats is 47.9mm OD and 40.6mm ID. The exaust measured 41.4mm on the OD and 34.6 on the ID. Is this ok for a 44 intake and 38mm exhaust valve or are those made to fit 46 and 40mm ones?
 Also how much should be the correct measure for different valves. I think something like 0.88- 0.89 ratio between valve head dia and seat ID for a street engine. So that would be 44mmX0.88=38.72mm, for example. For my valves i was thinking 38.75-39 mm ID for the 44mm intakes and 33.5-34.2mm for the 38 mm exhaust.

How much seat width do you guys run on hot street engines?
The seat width would be like 1.2-1.3 on intake and 1.5 on exhaust- 45° seat angle with a 3 angle valve job. Is this OK ?
The engine is a type 4 ,ported heads, 100X71, high CR, 307° .500 lift cam, 48mm TB, 1 3/4 merged header,.. I will be doing the head work myself with a help from a friend who works in a reconditioning shop, but they know nada about performance mods, only stock.

What about modern 4 or 5 angle seats or radius blends to the bowl on ac engines. Any experience with this?

[John Maher]

Rather than worry about changing seat ID to suit your existing valves, make life easier by fitting larger diameter valves to dial in the seat ID ratio. If the ports have been blended and sized to the seats, it's not practical to fit a smaller ID seat anyway. With larger valves you'll also pick up flow and make more power.

The intake seat has enough outside diameter for you to step up to 45mm or even 46mm. That brings valve diameter to seat ID ratio into the range you're looking for (90% and 88% respectively).
With a good quality valve job you'll have much improved flow at all lift increments.
With 100mm bore even 46mm valve has no significant shrouding issues.

Similar thing with the exhaust but to a lesser extent. Max valve size .. maybe 39mm .. but any performance gains are likely to be minimal.

Regarding seat widths and angles... going really narrow (1mm or less) doesn't appear to deliver any worthwhile gains.
1.2mm to 1.5mm on the inlets and at least 1.5mm on the exhaust for durability.

Full radius seats are reserved for exhaust seats only.
Distinct angles on the inlet may hurt cfm on the flowbench a little compared to a full radius but improve wetflow dynamics, keeping fuel better atomised and distributed within the intake charge (less flow on the bench but more power on the dyno).

Full radius works on the exhaust side OK because there's gas only. No fluid suspended in the airstream..

I generally stick with the tried and tested angles people have been using forever or at least a very close variation. Basic angles... 45° seats, with 60° and 70°-75° below the seat, 30° and 15° into the chamber.

I use carbide cutters with preformed angles rather than stones nowadays... as used on Serdi, Newen etc seat machines. All 3, 4 or 5 angles, radii etc are cut in one hit. Accuracy, repeatability and consistency of the carbide cutters is unbeatable compared to grinding with stones.

It's important the 45° contact face is placed correctly on each valve... right on the outer edge for the inlet valve, a little inboard for the exhaust so it can dissipate heat more effectively.

Backcutting the valves can also really help flow. Backcut angle for a 45°valve is usually in the 25° to 33° area. People have different opinions on this and may advise backcutting or not dependant on application, type of port, shape of valve head etc and may carry it out on inlets or exhausts only rather than all eight.

Any gains found from the above are dependant the rest of the port being non restrictive and also being no bigger than necessary. Plenty of large valve heads have 'funnel' ports i.e opened up to accommodate the new larger seat and then quickly shrinking in size to blend into the stock port shape as you get closer to the manifold flange. The port will only flow as good as its smallest cross sectional area and that should occur close to or at the valve seat ID.

[eugene]

Thanks for your reply Mr. Maher. lot of good info as allways.
The heads are still bere and the seats  are still to be instaled and the seat receses cuted, so no problem going with 44/38 valves or 46/40 ones. But was thinking the smaller are enought for this engine size and 7000rpm max( with the righ porting), so i ordered those and got mismached seats with them, apparently.
 With the smaller valves it will be also easier to do an apropriate port- little or no welding needed and the eassier to keep the chambers small enought for the CR im shooting for (10-10.5:1). This is a type 4 head and as i understand, you mast keep the skimming to minimal(1-1.5mm maybe) for reliability, so welding up the chambers is the plan. We are going to use Mira cuters and i was just looking trought the catalog an got some interesting ideas what to order/use .

 

 

[K-Roc]

Hi, this is a cut and paste of ideas I had from another forum on basically the same subject..  Thanks,, K-Roc


Thought about a good way for an example.

Let's use an 044 head with a 42mm Vs. 44mm  ( these numbers are only for example )

First the 42mm Valve say it flows 200 CFM at .600" Lift
42mm = 2.14 in/sq. valve...    200/2.14=93.45 CFM per Square inch of valve area

Then the 44mm valve is put in the same head with only seat modification and a little chamber work.. ( no opening of the port itself.. it now flows 210 CFM at .600" Lift  so your all happy you picked up flow....
44mm = 2.36 in/sq. valve...    210/2.36=88.98 CFM per Square inch of valve area.

So even though you gained CFM by putting in the larger valve you have in fact decreased the overall efficiency of the port.

The other thing that often happens when putting in the larger valve and getting an increase in flow is that if the port itself is not resized properly. Or if the original port may have already been a touch on the small side for the valve size,and desired RPM for the engine.....now you picked up some CFM with the bigger valve.. you may actually be putting the port into sonic choke at a lower RPM than you had before, so the bigger valve actually causes you to loose higher RPM power.

So if going to a larger valve.. you must measure the seat and make sure you can cut a seat with a 89-90% throat,  ( or re-seat the heads with the correct sea)...then you must unshroud the chamber properly...last but most important you must increase the intake ports cross section to match your engine size and RPM range that peak power is desired.