Question about valve sizes?

[Jens-Ole]

So might be a bit of a newbie question to some of you but how big can you go on valve size and is there a drawback?
Engine specs so far (seems to be escalating though) 76x94, eagle 2242 cam, 1,4:1 rockers, 40mm idf's (but guess they will be 44's in near future) comp somewhere around 10 - 10,5
And the heads then? My engine builder guru, you know who you are  , convinced me from stock to bigger and bigger but can i go 44 intake and 37 exhaust without "problems" ?

This is all going in a street car.

[Zach Gomulka]

Bigger valves are heavier, and make it harder for the spring to control them. Bigger valves also go hand in hand with larger ports, if the port is too large for the rest of the engine, it will be sluggish in the lower revs.

As with anything engine related, bigger is not always better. It's the combination that counts.

[Bruce]

IMO, those valves are way too big for such tiny carburators.  A 44x37 head needs 48s.

Like Zach said, a smaller valve doesn't weigh as much, so it doesn't need super stiff springs and all the trouble associated with them.

[Jens-Ole]

So it's mainly low end torque and excess valve wear that is the problem?

[Zach Gomulka]

It's not so much valve wear, it's everything else in the valve train (cam, lifters, pushrods, rockers). You won't be gaining anything by having valves that are too large for the rest of your combination.

[DWL_Puavo]

One thing not yet mentioned is also that with bigger valves, head chamber size ("head cc's") also increase. Thus to get desired (static-, for nitpicks) compression ratio, if you're not going to increase stroke, you'll may need to machine down the head. That may then again reduce the amount of valve lift available.

With 76x94 this probably won't be such a big problem: with 1,5mm deck height you'll need head chamber size in 45-50cc's for 9.8-10.5 CR. That should not be a problem with 44x37 valves with your cam and 1.4 rockers. Disclaimer: please test the assembled combination with blu-tack or similar pressed to pistons.

I would go with bigger valves. Carbs are way easier and cheaper to change when (it's never "if") you'll get more hungry!

Puavo / DWL

[58vw]

42x37

[Jens-Ole]

So 42x37 is the thing?
Any recommendations or experience with CB 044 Magnum 42x37 (are they the same as the CB 044 Special except for single springs?)

Also, if I stay around 6-6500 rpm would it be enough with single high rev valve springs, og should I go for dual springs with the 1.4 rockers?

[Bruce]

..... 42x37 ....... 6-6500 rpm

It doesn't matter what cam or what rockers you use, you absolutely MUST use dual springs.

[Fiatdude]

Open the CB web site and choose a set of complete heads as big as your pocket book can afford -- then grow the rest of your engine to them

[Jens-Ole]

Open the CB web site and choose a set of complete heads as big as your pocket book can afford -- then grow the rest of your engine to them

 

[Fiatdude]

The Fiat ran the ultra wedge ports on a 2165cc engine --- added a turbo @ 20 Lbs boost - one of AJ's 500 cfm 2 barrel Holley carb's -- custom exhaust and intake to fit under the closed deck of the Fiat -- used 100LL aviation gas - 325 HP --- drove like a  stock engine -- until the boost came on that is LOL-- simple enough

[JS]

..... 42x37 ....... 6-6500 rpm

It doesn't matter what cam or what rockers you use, you absolutely MUST use dual springs.

So, choice of cam and rpm limit does not affect spring pressure requirements?

[TexasTom]

..... 42x37 ....... 6-6500 rpm

It doesn't matter what cam or what rockers you use, you absolutely MUST use dual springs.

So, choice of cam and rpm limit does not affect spring pressure requirements?

I think what Bruce was indicating is that either of those specifications, and most certainly the two together, will require the use of dual springs.