OK so over the last few weeks I've been asking and reading up on the differences between all the heads that can be put on a 351/400 in some pretty heavy detail. I've learned about Australian quench heads (what a quench is) and why they're so desireable, learned what heads do fit on these motors, and learned fairly well what heads to use for what applications.

Something that has very much interested me is that it seems in many cases, especially with trucks, the 2v heads are actually better than the 4v. I've read that the 4v heads, with their much bigger ports and valves generate more torque at higher rpm's than the 2v's (I would have expected this), but what I don't really understand is that from what I've read, the 2v's actually make more torque at lower rpm's than the 4v's. I would be able to understand a 2v head being more suited to low end torque than a 4v (proportional to their TOTAL torque outputs), and a 4v being more suited to the higher end of the range, but how can a 2v ever produce more baseline power than a 4v? To be clear: If a theorhetical 2v 400 produces 300 peak lb/ft of torque, and a theorhetical 4v produces 400 peak lb/ft, then I could understand if the 2v hit it's peak at 3500rpm, and the 4v hit it's peak at 6000rpm, but I would think that a 4v engine would always have more, throughout the range. If this is really not the case, why not?

I'm also curious as to exactly how all of this stuff works in general, why would smaller ports and valves produce more of anything? Is it that with smaller air passages, and therefore reduced flow, when the 2v exhaust valve opens the pressure doesn't dissipate from the combustion chamber quite as fast, and so pushes down on the piston that much longer than in the more free flowing 4v heads?

If someone can explain to me how all this works, I would be very interested to hear. Don't be afraid to get technical, I'm not afraid to look stuff up, I've just been curious about this for a while now, and really feel I need to know exactly what's going on inside this thing I'm puting so much effort into.

Thanks, AleX

 

In a nutshell, either smaller valves, or longer intake passages, or both, can produce better fuel atomization (mixing with air) and velocity (which means the fuel/air mixture actually has some momentum traveling thru the intake system) than bigger (or more) valves when the engine is at lower speeds. In the old days of carburetors, a smaller carb would work better with a smaller engine and big engines running at lower speeds for the same reasons. You also get better emissions and fuel economy when you are moving the fuel/air mixture faster, because the fuel stays mixed more evenly with the air.

A number of engine designs like my 1991 BMW 318is (1.8 liter 4 cylinder) and the original 3 liter Ford Duratec have 2 sets of intake runners. One set is long and smaller in size and the engine runs on those most of the time. When the engine hits 4000 RPM (or something in that ballpark) the larger, shorter intake runners are opened up to "shotgun" more air and fuel into the cylinders.

Likewise, longer and smaller headers also work better at lower RPM's because the exhaust pulses get momentum and help "pull" the exhaust out of the cylinders. Exhaust systems also do better with some back pressure than no back pressure. It's all about the columns of air/exhaust moving in and out of the engine maintaining a smooth and constant velocity.

4 valve heads provide their advantage only at higher engine speeds and higher intake flow rates. Same as huge carburetors in the old days. My old 300 inch six (1978) had a ONE barrel carburetor, made its torque and horsepower peaks at low RPM's, and didn't do much of anything at high revs. There are some variable valve timing systems that change how the valves open. Subaru, at low RPM's, opens 1 of the 2 intake valves completely, the other only partially...this lets the fuel mixture "swirl" a bit more at lower RPM's into the cylinder, again staying more atomized.

This is pretty simplistic, but truck engines generally turn lower RPM's when they work, and don't need a ton of valves. The 5.4, though, has a very small bore (much smaller than the old 302 engine, for what it's worth), so putting in the 2nd intake valve (in the 3 valve head) was a bit of a help. An engine like the 5.4 with a smaller bore and longer stroke has more severe limits on valve size.

This may not be the best explanation, but I thought I'd start the ball rolling. I'm sure you could find a lot more stuff looking on Google.

George

 

O woops, just realized I posted this in the completely wrong forum. I have a 70's truck and thought I posted this in the 335 engine forum. Anyone can feel free to move it if you think that's appropriate.

Anyway though, the explanation may have been basic, but was perfect for me, I didn't know any of that, so thanks, very interesting stuff.

If anyone can add more, especially about 351/400 heads specifically (maybe better once/if this thread is moved), or about carbed engines in general, I would love to read it.

Thanks for this start though, AleX

 

To the best of my recollection, there were never 4v's on 351m / 400's. They were truck and big car motors and only had 2v's. (Aussie's may have had 4v's)
The 2v / 4v question was on 351c's although I believe the c heads will fit m's and 400's.

 

Right that's what I was asking about. 351m/400's did only come with 2v's (believe I've heard about an experimental factory 4v 400 from the early 70's, but that's not the point), but it is common practice in a performance 400 build to swap in 4v Cleveland heads, and something I have been considering as I plan out mine, so I've just gotten into the habit of thinking about these engines with both types of heads.
Good catch though, good info for everybody else.

 

And I kind of misunderstood your question as well...although my answer still had some relevance. The 2V and 4V in the old days meant 2 barrel and 4 barrel carburetors (2 or 4 venturi, I believe).

But the same thing applies to heads with smaller passages as it does to 2 valve or 4 valve heads, which are more of a current discussion topic.

Basically, an engine needs to have its intake and exhaust "right sized" for its application and desired power band. Another factor is camshaft profile...higher lifts and longer durations work better for higher revs, lower lifts and shorter durations for lower revs.

George

 

The 70-71 Cleveland 4v heads where closed chamber and had "massive" intake and exaust ports. Rare and worth cash, but not so good street heads for most guys. I've ran them and it would be my choice for a radical high hp engine. I've never played with a 351/400M, but on a 351 Cleveland you will be running high octane and roughly pushing between 11/11.5 to 1 with a flat top. The big difference between the 2v and 4v heads where the ports, if you want top end flow, you want to run the 4Vs.

 

George, I understand why you think you misunderstood, but you actually did understand, but just didn't understand that you understood, understand? Haha.

I waas saying 2v and 4v in the "2 or 4 venturi, matched with a 2 or 4 bbl carb" sense, not talking about 2 and 4 valve per cylinder heads. On the Cleveland engines, the heads that were put on the engines that got the 4 barrel carbs had the number 4 stamped on their corner, and were the ones with the big ports (but still 2 valves). Some of the normal sized port heads that came on 2 bbl carb Clevelands were stamped with 2's, some were not stamped at all.

I'm not sure if it is an official designation or not, but the way I've always heard it is that the heads with the huge ports, with 4's stamped onto them, that were originally paired with 4 bbl carbs on performance Clevelands, are 4v heads. I was confused the first time I heard 4v too, beacause I just assumed it meant 4 valve, and I'm not sure what it technically "means", but the way I've always heard it talked about is above.

Because of that though, your answer did make sense.

To continue the original discussion though, has anyone ever seen any data comparing 4v heads to 2v's, with all else (excluding intake and exhaust manifold channel size) being the same? That would be interesting I think.

 

In a nutshell, the 4V heads work well from about 3500rpm to 8000rpm. The 2V heads (Aussie heads are the best if you can find them) are much more streetable. The 4V heads have ports about the same size as the 429/460 heads. Even the 69/70 Boss 302 used a version of the 2v heads.

Oh, and I moved this to the 335 series forum for ya.

 

Found a chart for anyone interested and thought it might be helpful. Don't know how accurate it is. 351 Cleveland & Ford Head Specs

 

the Boss 302 was the first production engine to use the '4V' sized intake ports(there were no 2V ported Boss 302's) so the 351 Cleveland actually used a version of the Boss 302 head

there's no point comparing 2V's to 4V's using engine rpm as it relates to road speed. a proper 4V combo should be geared much higher than a 2V combo, and thus turn more rpm/mph. that's where the myth of the 'no low end' and '3500 to 8000 rpm power band' comes from, guys swapping 4V heads onto a 2V driveline combo and finding out the hard way(or not) that the 4V heads don't perform being bogged down by stock torque converters, highway gears and big tires

probably the most basic reason a 4V headed engine makes less torque at low rpm than a 2V headed engine is intake charge velocity. it gets real complex but low velocity in the intake generally makes less torque than higher intake velocity. a given amount of airflow through a 2V port makes a certain amount of airspeed and is able to carry a certain amount of fuel maintained in suspension. double the port volume and 1/2 the velocity(or less even?) and throw in poor atomization to boot. when the 4V engine's rpm & intake velocity comes up is when the party gets started, in a poorly matched combo that could be around 375 mph and 10,500 rpm

 

OOOK, I'm getting this more and more now. So a 4v head is meant to be on a high reving engine, a 4v headed engine likes to be spun up fast basically, and a smaller ported 2v setup can generate good air velocity at the lower speeds that are mandated by the low rpm setup that these old trucks have. So it isn't so much that a 4v isn't capable under 3,500 because Ford's engineers just somehow forgot to design for that range, they were just designing 4v systems to go in vehicles that were meant to go twice as fasrt on average than old trucks. If that's still wrong somebody say something, but I think I'm getting my head around stuff.
AleX

 

Yup, you're getting there. Think of an application like a fire hose. The pumper truck has a fixed pumping capacity so if you are pumping through say, a 3" hose, the water will squirt say, 100 feet based on its velocity. Now if you make that a one foot diameter hose, you will move the same amount of water but it will only squirt 7 feet. Not very effective for a fire.

But if the pump can squirt out of the big one foot hose for 100 feet, it will not be able to move as much water through the small hose...the size of the small hose will limit the amount of water.

Intakes and exhausts do best when they are "right sized" for an engine's size and the rpm where you want to focus the power.

The camshaft governs how long the valves stay open and how far they open. Different camshafts will work in different RPM ranges--the big cam with the high RPM power peak needs the big intake and exhaust systems as well.

George

 

Right right right, I'm getting it all together now. I was thinking of it too much as having just one variable, I understood the stuff about fuel mixing needing certain velocities and how air momentum can affect how an engine runs, but I was thinking of it too much in the sense of an engine just in and of itself, and I was also thinking that there would be much more conrete victors as far as 4v or 2v in each category. I see now that the differences involve many more factors than just how fast and hard an engine can spin up, and that the differences in all these systems are actually pretty subtle.
Thanks for the info, and keep it coming.
AleX

 

Good post I had an M code 70 Mach1 with the 351c factory 4spd and 411 gears. It was a beast, flat out hauled ***. Sold it a couple years ago to a guy over seas.