Van

Why do you want both valves open? Wouldn't you want just the intake valve open, and then run a separate test (reversed with your switch) for exhaust?

ehall

Wait, what? You thought I read any of your drivel?

67King

Yes and no. One of the problems with just looking at flowbench numbers is that they are not a good representation of what goes on in an engine. What was written is true for the INTAKE side. On the exhaust side, the valve seat area is MUCH more critical than the port area, because you have significantly higher pressure when you begin opening the valve. Furthermore, the amount of time a valve is actually open enough for the port to even matter can be very small. The attached shows some multivalve production cams - the flat area is where the engine is port limited. Unfortunately I only have this data on the intake side, but it demonstrates pretty well the point.

Again, this is just about the exhaust side. On the intake side, the pressure deltas are the opposite - lowest at IVO (that quickly changes, then quickly drops, and then starts to level off). This is one of hte reasons you see a lot of 30 degree seats on the exhuast side - it increases the valve seat area at a given amount of lift.

I'm curious - why no bellmouth on the intake manifold?

A.Wayne

Interesting if accurate as those are good flow numbers and much better than has been mentioned here in the past......


Bada bing .......

Of course that all changes again on high boosted engines and the exhaust becomes even more of a concern ...

Chris White

Ok Van, wake up! - no sleeping in the back on the class!

It’s a 16 valve head – there are two intake valves in each cylinder!

Duke

Hmm I just got a stellar idea - if the exhaust is so important, and the intake flows more... why not just use the intake ports for the exhaust and vice versa!

Chris White

OK, now I feel like I was sleeping during class! Quoted like that I would have to disagree with myself. I should have put it better!
Its easy to get too fixated on one aspect of a test result. Its also easy to come up with general rules about numbers that are a little too general….kind of like that quote!

The same thing happens with dyno charts – people can get fixated with the peak HP and miss the big picture. So what’s a better way to look at the results? I would start with the ‘area under the line’ approach. To take it a step further you need to know the cam specs to figure out the time function of the flow. Combine those two and you could do a quick and dirty total cubic feet per valve event calculation. Once you do that then some of the advantages of particular flow rates will make more sense. That works great for the intake side which is, in theory, under a constant pressure differential. The exhaust has a different dynamic – the pressure differential goes from a very high peak and eventually equalizes (or close to equal). That calculation can get pretty complex with a lot of assumptions.

Looking at the intake flow on the 16v head you can see that the flow is valve opening limited up to .300” and then starts to become port limited (warning – this is very generalized!). The port flow hasn’t become ‘choked’ at max cam lift so there is some benefit available by adding more lift to the cam – but not a huge amount. If you could make up a cam that simply opened the valves quicker but kept that same max lift and duration you would see a lot more total flow per valve event. Then you have to figure out the complex valve train dynamics….thats a whole ‘nother discussion!

The summary is that low lift numbers are important because of the time frame in the valve event that they occur (67’s exact point!)

No bellmount? – the intake runner is bellmouthed into the plenum…and I thought about putting one on the end of the manifold – but a previous comparison test showed that it made no difference in the flow bench readings so I don’t bother with it anymore.

Thanks for the catch ’67, after seeing it quoted like that I would not want folks to go away with the wrong idea

Chris White

Hey – I can quote the flow rates down to .01cfm and lift to .0001” – so it has to be accurate….right? .

It is a modified head – the measurements on a stock head are inline with known 16v head numbers (just under 300cfm). I have some flow tests results from the 80’s somewhere….
Yes, the turbo guys are a little more ’**** retentive’ about their exhausts… as they should be!

Chris White

Because sometimes bigger is not better…its just bigger!

You should be more careful on what you post Duke – now we have some real proof that you are a ‘backwards thinker’….don’t let Lart know!

Duke

It was a joke in case you missed it Chris
No backwards going on around here!

Chris White

and mine was a joke about your joke

But I may just quote you out of context when needed!

Duke

Damn it! I'm always one step behind (sorry there I went backwards again..)

Chris White

You have to try extra hard to be one step behind when you are already 6 hours ahead….!

Van

Doh! Totally missed that. I must have been asleep (or day dreaming about being at the track).

A.Wayne

At big boost numbers , the exhaust port size becomes very critical ....

LOL... a joke it maybe , but it has been done on many race engines ....