Valve Lift/Diameter Ratio

I read a recent article about this, Valve Lift/Diameter Ratio. It said that for HP Street looks for about .25 and hard Core Perf. .28-.30 and pure race engines .35.

So what I can find on our Jeeps 4.0 with 1.6 rockers-
the Intake lift is .408" which is .255" cam lift on 1.91" valve
the Exhaust lift is .414" which is .259" cam lift on 1.5"valve

So stock Intake L/D ratio is .408/1.91= .2136 - I guess pretty poor
And the Exhaust L/D ratio is .414/1.5= .276 -seems pretty good

With 1.7 ratio rockers-
Intake L/D ratio is .434/1.91 = .227 better than stock
Exhaust L/D ratio is .440/1.5= .293 even better, but stock was good before

What does this all mean? ? Just something else to consider when comparing and building an engine.

For the 4.0 it looks like the stock intake valve size is fine, it just needs more lift. And the exhaust could need a bit bigger valve size. Everyone seems to run to big valve heads with emphasis on bigger intake valves. Just something to think about. I'm going to back test this on some known engines and engine builders.

"Everybody" does not run big valve heads on the 4.0l/strokers. Too much valve shrouding. Hesco doesn't even sell their aluminum head w/ large valves unless asked for.

That valve lift/diameter ratio doesn't mean a thing w/o figuring the head and its flow characteristics into the equation.

And I have seen many ask about larger intake valves 1.94" and 2.0".

CobraMarty said:

And I have seen many ask about larger intake valves 1.94" and 2.0".

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Yes a lot of people ask about them, because they don't know any better. They think if the hole is bigger, airflow we be better. Do they actually do it? I doubt it. Most people on here that ask about strokers tend to go the cheaper route once that get down to it.

NorCalChris said:

Yes a lot of people ask about them, because they don't know any better. They think if the hole is bigger, airflow we be better. Do they actually do it? I doubt it. Most people on here that ask about strokers tend to go the cheaper route once that get down to it.

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In the end it is probably better for them that they stay stock.

I don't know if there is some "Magical" number. If there is, I have not seen it. Generally speaking...

Increasing the valve size decreases the velocity of the intake charge. This is bad at low RPM. Only at higher RPMs will you see a positive from increasing valve size.

Increasing the lift is essentially leaving the valve open longer and allowing more charge in. That is why roller cams are the hot ticket, they allow the valve to be open longer and open an closed more rapidly than a tappet cam, thus not requiring a huge lift number to achieve the same performance without messing with idle.

So, if I was trying to get the most out of my 4.0L, I would not change my valve size. I would port the heads, and gasket match them to increase my fill rates without sacrificing velocity. I would probably upgrade to a mild can and spend the time indexing it for more bottom end torque.

You don't want to gasket match. Gaskets tend to be inconsistent as well as the exhaust port hole in the gasket is huge. If you don't have a flow bench what you want to do is just a clean up, polish the exhaust port, smooth out the intake port and polish the combustion chamber. Then match the intake manifold to the head better, but do not make it larger. If you have a flow bench and know what you are doing then go ahead and modify the ports then match the intake manifold to the port again. Going through all this trouble I would assume you would go with a header that has ports already larger than the head ports need to be.

"Gasket Match" is just a term, an you don't take it on face value.

Perhaps "Port Match" would have been a better term. You don't use the gasket, but India blue one side, like the manifold, then bolt it to the head.

Remove it and examine the surfaces. The smaller of the two ports gets enlarged and smoothed to match the larger one.

Another area to look at is the area under the valve. But you need to be careful there, you are generally close to the water jacket.

Also by "Smooth", also don't mean it literally. A rough surface is generally better than a polished one. All you want to do is to take out the obstructions. A bit of turbulence is not a bad thing.

As you can tell by my 1st post, the idea is to get performance by making the intake and exhaust paths better, not by making expensive and backwards performance modifications like increasing valve size over stock.

By smooth I mean by finishing it rough with 120 grit at most, not smooth like the exhaust. Boundary layer is not that important in an port injected engine as fuel does not travel with the air charge for a great distance.

I have seen people take "gasket match" literally. One guy took the exhaust port ALL the way out to the gasket.

This is not my theory, I am just sharing the information. Please don't shoot the messanger.

Here is an engine by Shafiroff Racing Engines. Very impressive to say the least. Not jeep, but it illistrates a high end engine build.

598CID, 93 octane, 10.95:1, 2.35" Intake valve, 1.85" Exhaust valve, 276*cam, 0.739"I/0.741"E, 114* separation, CNC ported Brodix heads, I= 434-457cfm at .7-.8", Ex=304-306cfm at .7-.8", 920HP at 6,300, 809TQ at 5.500.

So what does this mean?
920HP/598CID = 1.54hp/cid very impressive
Intake L/D Ratio = 0.739/2.35" = 0.314
Exhaust L/D Ratio = 0.741/1.85 = 0.400

This engine fits the theory very well in exacution and outcome.

By No means are we "shooting the messenger".

Think about this magic ratio a minute.
Does this figure by itself tell you anything about where the engine is designed to "perform"?

Does this figure have the same result on a NA engine and a blown one?

I am sure Shafiroff didn't even think of using that ratio. It basically says everything else is constant.

WB9YZU said:

By No means are we "shooting the messenger".

Think about this magic ratio a minute.
Does this figure by itself tell you anything about where the engine is designed to "perform"?

Does this figure have the same result on a NA engine and a blown one?

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All good questions, I don't know about 'magic' ratio but I have seen it used/discussed in other publications. It is most likely just a good guideline and who knows how much more.

Interesting that if you take the L/D ratio and multiply that by the head flow number and see how that correlates.

I= 0.314 x 445(average) = 139.7
E= 0.400 x 305(average) = 122.0
Intake is about 14.5% more, which makes sense.
Or exhaust is 87% of the intake.

On a stock 4.0L - I need to look up the stock head flow numbers. Intake ?190 and Ex ?145. Does anyone have easily available? Mine are at home.

Talyn said:

I am sure Shafiroff didn't even think of using that ratio. It basically says everything else is constant.

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That is a vague statement. There is more science and mathematical formulas in engine building today then ever. Engines every year are improving their performance in all catagories without just going bigger and bigger.

The whole lift/diameter ratio is a vague statement.

The driving factor behind all this sizing, porting, etc. is that you want to have the most flow that you can while still having the E/I flow ratio in the ideal range. My cobwebby memory says that for a NA engine that should be in the 0.6x:1 to 0.7x:1 range but don't quote me on that. Everything that you do to a head/valvetrain/intake/exhaust ought to be leading toward that goal. The thing is that to achieve that you either have to build a "formula" engine (e.g. copy someone else's work) or else have access to a flowbench and/or extensive prior knowledge of the exact casting that you're working on.

Also, port-matching is OK, but keep in mind that when you go to big ol' cams, reversion becomes a problem, and mismatched ports can actually help in that respect. The idea is though if you have a port mismatch the ports should always go smaller to bigger in the direction of flow, never the opposite direction. The step created actually helps stop reversion by creating a flow restriction in one direction only. I've actually seen designs for anti-reversion headers that took this one step farther and undercut the step slightly to make a little pocket. Don't know if that works or not, but apparently someone at least thought it made sense. Finally hogging out the ports to match the gasket does nothing unless the x-sectional area of the port is maintained all the way from the gasket to the back of the valve.