Valve Size

[GREG K]

Using the formula for the required intake valve diameter needed for peak Hp to occur..

Intake Valve Diameter = ((RPM*CID)/(Cylinders*306.7*282.7433388))^.5

My question is..Has anyone ever tried to use a smaller valve in a 2v or 4v cleveland head..intended to peak Hp at 7000 rpm..

Do you think,(taking into consideration the combo is to suit),that with the size valve,say for the 2v would be around a 1.950-2.000",would the car accelerate harder than with the 2.04" valve..It certainly does for the chevs..I realise that the port design differs,as does the port velocity gradient...I was just wondering if anyone has ever tried this...Because with the cleveland,if you intend to only rev to 7000-7500rpm,then the valve could really be a little smaller?It would certainly help to get the clevelands port velocity gradient a little more even,and drop the volume vof the port a little, perhaps to be more where it should be for this 7000 rpm peak?

[Ron E]

While it's mostly high lift stuff I do, I tend to use a valve size that relates to the CAS of the port. Even if the formula calls for a larger valve, I get better results with the reduced shrouding of a smaller valve if the CSA is already set by the port. Your question is the reverse of that. If you can maintain the flow, and back the port CSA down as you back the valve size down, i think you'll get the results you're looking for. But, just reducing the valve diameter with the big port is likely to just make the port more lazy due to reduced low-mid lift. That is unless these suffer from excessive valve shrouding.

[GREG K]

yes i would certainly reduce the ports csa in the right areas,to get/maintain even port velocity gradient..
I reckon their valve sizes are just to large,especially for a 7000 -7500 rpm application..I guess it depends what peak HP you are looking for,but i just thought that this would really help to get the velocity up,past the valve..It could go down to a 2.00" intake dia and still not be at a choke condition even if the port was flowing 350 cfm..discharge coefficient would also be up,as long as transition was okay..And thats with a 90% throat csa..Just makes you wonder why the factory made them with such large valves..Guess it was for high rpm circuit work..

[speedy]

Ive always thought the valves are too big for 7000 and under, but ive been told the clevo heads respond different due to being canted valve by a so called head specialist who casts his own heads here in oz by the name of AFD, INTERESTING INDEED

[MadBill]

Since the valve is the biggest impediment to intake flow up to around 0.500" -0.600" lift and is infinitely restrictive when closed, I don't see how it can be "too big", absent excessive shrouding. You can always use less lift if you want less flow.

Now a 'too big' port is a different story...

[Ron E]

Since the valve is the biggest impediment to intake flow up to around 0.500" -0.600" lift and is infinitely restrictive when closed, I don't see how it can be "too big", absent excessive shrouding. You can always use less lift if you want less flow.

Now a 'too big' port is a different story...

My reasoning is, and you hit on one, is shrouding. in high lift deals, keeping good flow where the valve is shrouded against the bore is important. The smaller valve can aid in that. All this hinges on a port with the CSA being the limiting factor. If the combination wants more, and the port will grow with it, I'll use a bigger valve. Now, in low to medium lift ranges, your point is well taken, as those lifts will usually benefit from a larger valve reguardless of the CSA.

[GREG K]

You might gain a litttle cfm with the larger valve in the lower lift range,but you would loose the velocity past the valve?
You would also need to have larger throat with the larger valve,and in doing this you would need to open the pushrod area up so that the min csa was not in that area?

[speedy]

agree^ gregk

[speedy]

just got off the phone to my mate he just made 100 extra hp and 100 extra ft lbs tq by going down on valve size and port cross section with a new set of CHIs compared to AFDs on 351 clevo .could be just head design i dont know, rest of combo is the same, so bigger aint better in this case hes stoked

[SStrokerAce]

Read some of Darrin and Larry's posts here on discharge coef. It's a odd deal, sometimes venturi sizes and valve sizes don't make "sense" but what the motor likes it gets.

Bret

[Erland Cox]

Not all heads have their smallest area in the throat. A 4-valve engine necks down to a point before the valve stem and then opens up towards the seat. I can not see any reason why it would work differently on a 2-valve engine? I would guess valve shrouding to be the biggest problem. Another problem with putting the choke before the short side radius in a 2-valve head is that they have ports that bend a lot more than a modern motor bike 4-valve. This forces you to lower the velocity past the bend. In a 4-valve head with really straigth ports you can run 390 fps in the smallest area at max horsepower revs. Fps at 28 inches. If you know the volumetric efficiency at max hp you can calculate your CA from that depending on how much you want to run past max hp. I think it is much better to design the CA from the revs and VE than from the flow. From the CA you need to diffuse the flow into the chamber. It is alot easier to start the diffusion in the port instead of having all of it in the combustion chamber. If you just dump all the flow into a big ugly chamber with flat floors you will loose a lot of flow in the process. This is one reason for the gains with 55 degree seats. Erland

[Ape]

It is alot easier to start the diffusion in the port instead of having all of it in the combustion chamber. If you just dump all the flow into a big ugly chamber with flat floors you will loose a lot of flow in the process. This is one reason for the gains with 55 degree seats. Erland

thats one of the reasons i guess why harry weslake made the min. csa midway in the port on some designs.

cheers

[SWB]

The min cross sectional area on a 2v head is right over the short turn where you would want it the least. I've seen guys take the guide protrusion into the port and grind it away completely as well as work the port to death in that area to try to get the flow up. That works (the head responds greatly) because you're slowing the air down as it's trying to turn (which is what you want). Unfortunately once the valve open area is greater than this minimum cross section, the short side turn is still the highest FPS in the port because the castings won't allow it to be made big enough for the "huge" valve downstream.

You're looking at poor transient response (throttle response) and greater reversion problems as a result, regardless of RPM.

I've got a set of 2v heads here I'm going to shrink down to a 2.02" inlet instead of the standard 2.06 size they came with. They're for me, so it will be awhile, but when I get them done I'll post results and see what you guys think.

SWB

[rooster]

If you must run a stock cleveland head, the "australian" 2v heads have the best CSA for your 7000 rpm application. You can literally start by raising the port floor by a half-inch or so at the intake flange, then maintain that CSA as close to the short turn as possible. Sinking that big intake valve a bit is a cheap and easy substitute for new smaller seats and valves and helps your response while maintaining high lift flow.

[Erland Cox]

Here is a mold of a Suzuki 2-valve Prostock intake port.



The CA is before the short turn and the port opens up all the way to the seat. Notice how big the port gets at the intake flange. Only a small part of the port is the high speed zone to minimize frictional losses. 280+ hp from 1500 something cc:s. This engine has a powerband between 10000 and 13000 rpm. It has a much better port to valve angle than most V8 engines. When you increase cylinder volume the size of the cylinder increases with the cube but the valve area only increases with the square. This forces you to move the CA to just before the valve seat as the cylinder volume gets bigger. ( If not only the bore increases.) A 500ci Prostock engine is actually undervalved. To increase the revs you must shorten the stroke and make the bore bigger to put in bigger valves. I would like to try porting a set of BBC oval port heads with the CA before the short turn and use the biggest valves the bore will allow. ( 0.53D). I have ported heads for Chinese mini bikes with 125 cc:s like the Suzuki port was made and they have worked fine. 15+ rear wheel horsepower from 8500 to 11500 and 15.76 hp at 11000. The available valve lift is the limiting factor so far. Engines are complicated and different designs require different approaches. Erland