InchUp

iTrader: (2)

There has got to be a reason why GM put 1.89" intake valves in the 5.3L heads instead of 2.00" valves like most the other Gen III/IV motors. The 5.3L is GM's bread and butter engine found in over a dozen vehicles. I'm trying to wrap my head around how a smaller valve will make more low end torque with sacrifice at the top end.

My theory; the smaller valve doesn't block the airflow as much simply because it's not as big of a cork in the intake. The air coming in has less blocking it from getting into the chamber. I figure a larger valve has more "pull" (not sure on technical term) in the higher RPM's creating a stronger vacuum when opened, pulling air into the chambers through the intake in combination with the downward stroke of the piston. So basically, the smaller valve is less of an obstruction where a larger valve acts as a bigger plunger with more vacuum force.

Or was the simple answer economics? The same 5.3L heads are on the 4.8L and I don't see it needing large valves so did GM just cut costs and make one single "mediocre" head for two similar displacement motors (5.3 and 4.8)?

Any ideas would be great. This whole thing is stemmed from my iron block 5.7L build for my truck and if switching to 241's from 862's (5.3 heads) is a smart choice. Btw I am an engineering student so don't be afraid to talk dirty with numbers and Newtonian theories and such.

Old SStroker

Perhaps the 5.3L didn't need as much airflow as the 5.7L. Perhaps the smaller valve works well with the 5.3 port.

I have seen modified LS6 heads with a stock-size 2.00 valve outflow the same basic heads with larger valves and porting by a different person. Just increasing valve size does not always guarantee more good flow. It can often hurt flow in a running engine.

Why do you think that might be? Think about the system, not just the head.


Jon

beastolds455

iTrader: (1)

X2!! Chamber design and cylinder volume are a big factor..

Stang's Bane

iTrader: (22)

People rarely think of this. They want to give these engines all of this air, sometimes the motor really doesn't want it....

InchUp

iTrader: (2)

By want it then, how would one sit down and have a nice cup of Joe with said motor to talk about what size valves are needed? Is there a systems approach to finding out what the motor wants? I mean that as in the way we all figure header size for example...we know that certain diameter pipes will move the torque peak lower or higher up on the rev range to best suit our engine goals. Is there a way to figure say..."at blank RPM, X" valves will flow best with Y heads, Z combustion chamber ....A....B....C...." adding variables as needed to complete the equation? Or is it really just a pick and shoot game played using the parts we're all given by GM mostly?

Like I said earlier I'm just trying to figure out in the end if my head swap is a good idea or not because like mentioned by Stang's Bane, maybe my motor doesn't want 241's.

Stang's Bane

iTrader: (22)

I understand what you are saying. The first thing you need to do is come up with your end goal. By this I mean do you want low end power??Or is it peak power you want?? I would assume it would be low end power you are looking for. Then set a budget. Maybe even switch those 2.

It is always easier to hit a target when it stays still and your gun stays the same.


Honestly, the valve size, port shape,intake manifold and cam will all have an affect on where the power is developed and how much.

It is not as easy as throwing together a bunch of certain parts and it working as described. Picking the best system of parts is the key.

There is a formula that I have seen that tells how much air is required to produce HP at a given rpm for a given displacement, I just don't know where it is at........

Cascazilla

You don't necessarily want to give the engine as much air as it wants right when it wants it. Often it is better to starve it on the early part of the intake stroke so that when the piston speed gets very high it creates a much stronger pull that pulls on the intake port much harder when the valve is wide open, the air speed is up, and the pressure in the cylinder will be higher when the valve shuts so that cylcle makes more torque.

Old SStroker

I don't know who you are, Stranger, but them is powerful words which not too many folks have thought about. They should start from there and think about what it means. It could change one's whole outlook on engine airflow...or maybe not. Of course, maybe you just drink too much Absinthe.

InchUp

iTrader: (2)

Ask and you shall receive.

http://www.performancetrucks.net/for...d.php?t=415573

Stang's Bane

iTrader: (22)

For your stated goals it what you had listed there should do just fine, probably make even more than that. If you have the money and time I would send the heads and have a good valve job done. contact BlackZ on here or MNFbody and ask him where he is going to have his heads worked over.

FWIW the runner on a 243 is not appreciably larger than a 241. Also it is a better head al the way around other than the price, which from the deal you got, I believe you made the correct choice!! Good luck and have fun!!

SS MPSTR

iTrader: (1)

bwahaha! Stranger? Who are you kidding? Just can't keep sonny off a board he's been banned from, eh? Too damn funny.

Stang's Bane

iTrader: (22)

That is not his son.

And yes I know who his son is

Old SStroker

Too damn funny is right! Bret is a valvetrain guy, remember? This is a thread about head flow and valve size....oh, now I see what probably confused you.

Do you have any technical comments on what the Stranger said? It certainly got me thinking.

I imagined trying to exhale thru my mouth with my lips closed. When the pressure built up and my cheeks bulged out (more than they normally do!), I opened my lips and an audible puff of air popped out. I could imagine that helping start a flow being pressurized by by lungs. Strange may have something there.

I suppose you could get the same effect from the other end of the body, but maybe that's more like the exhaust valve.



Jon

SS MPSTR

iTrader: (1)

What, no literary quote? I'm disappointed.

Don't get your depends all knotted up because many of us see right through the charade...this and the purported 'expertise'.

BTW, as far as the subject of technical merit is concerned - I read the same information over on Speedtalk, although the source was actually someone credible.

Yep - and you're absolutely right, there is a huge discrepancy between talk of valvetrain and valve sizes, despite your lame attempt at juvenile humor, oldjoker Maybe Joe Urban can chime in?

The Dark Side of Wil

Nobody's mentioned the bore... The 4.8 and 5.3 have smaller bores than the 5.7 and 6.0. The valve sizes may be related to that...

In fact, giving up the necessity of accomodating the smaller bores allowed the larger valve spacing that the LS7 heads use

Old SStroker

I'm sorry, but I missed all of your posts on Speedtalk, SS MPSTR. I'm sure they are as informative as those you post here. Link us to a few, if you would.

BTW, who was the credible source? There are a few over there. Inquiring minds want to know.

Cascazilla

There are very few people on Speedtalk that have a clue what they are talking about, and even less on here

lxarlo

Air flow velocity. smaller valves and ports create more velocity and torque.

Old SStroker

Case in point below:

Cascazilla

Velocity does not "make torque". pressure differential beyond a certain point will actually starve a motor to death. Put an AFR 205 on a 450 inch drag engine and see how much "torque" it makes. The highest pressure-volume diagram area makes the most torque. That is not up for debate because it's fact, not internet logic.