Back Pressure

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Great thread! So, guys, what are the risks of breaking in my newly rebuilt 351C with open headers and no mufflers? Should I worry about burning the exhaust valves?
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No, you should not worry about the valves. Chuck

 
Great thread! So, guys, what are the risks of breaking in my newly rebuilt 351C with open headers and no mufflers? Should I worry about burning the exhaust valves?
Click to expand...
No, you should not worry about the valves. Chuck
Click to expand...
Thanks, Chuck and James. That's how I see it, too. :)

 
My mechanic tells me the Cleveland was designed by Ford to use

back pressure from the exhaust. Installing headers would not

really be a good idea.

mike
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Time to find a new "mechanic" This guy knows nothing about engines. I'd even think twice about letting him do a oil change.

 
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It sounds like he is a good mechanic to use for stock rebuild, same way, same parts, same vendors, same results every time, a grocery getter engine that will last. I just wouldn't use him for something not stock.

 
Ok, Let me present this question about back pressure.

The pistons pull in as much volume of material as they

expell, different form. Why are the exhaust valves

typically ~15% smaller than the intake valves. Would

that not create "back pressure"?

mike

 
Mike, There is a lot more to your question than you may think. The exact valve size relationship depends on many things. However, the basic answer to your question is that the pressure available on the intake side is limited to atmospheric pressure (I'm assuming a naturally aspirated engine). The burn of the mixture raises the temp radically resulting in increased pressure in the cylinder (about 5 to 6 times as much as was there on the intake cycle). The instant the exhaust valve opens the high stored pressure "blows" most of the exhaust gases out very rapidly. The remaining minority of the gases are expelled by the upward motion of the piston while the exhaust valve is still open. Therefore, the exhaust valve does not need to be as large as the intake valve. Bore size limits available space for both valves so a lot of effort goes into getting the valve size ratios right for a given application. Chuck

 
Mike, There is a lot more to your question than you may think. The exact valve size relationship depends on many things. However, the basic answer to your question is that the pressure available on the intake side is limited to atmospheric pressure (I'm assuming a naturally aspirated engine). The burn of the mixture raises the temp radically resulting in increased pressure in the cylinder (about 5 to 6 times as much as was there on the intake cycle). The instant the exhaust valve opens the high stored pressure "blows" most of the exhaust gases out very rapidly. The remaining minority of the gases are expelled by the upward motion of the piston while the exhaust valve is still open. Therefore, the exhaust valve does not need to be as large as the intake valve. Bore size limits available space for both valves so a lot of effort goes into getting the valve size ratios right for a given application. Chuck
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I knew it was a loaded question and I expected a loaded answer

which is what I got . Thank You.

mike

 
Glad it was helpful. I sometimes get a little long winded in an attempt to be accurate and complete. I suppose it is a left over from writing tech material a long time ago. Chuck

 
Glad it was helpful. I sometimes get a little long winded in an attempt to be accurate and complete. I suppose it is a left over from writing tech material a long time ago. Chuck
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I read tech journals just to relax :)

Would buy a Kindle if I could get journals on one.

mike

 
My mechanic tells me the Cleveland was designed by Ford to use

back pressure from the exhaust. Installing headers would not

really be a good idea.

mike
Click to expand...
Time to find a new "mechanic" This guy knows nothing about engines. I'd even think twice about letting him do a oil change.
Click to expand...
Actually he knows a lot about engines.

He tells me the Cleveland was designed by a NASCAR engineer

to compete against 429's and the like; story for another day.

Your daily driver/commute car would do fine with no back pressure,

but, try running the engine at 6000 rpm around a 500 mile NASCAR

circuit all day. Without back pressure you will fry the valves.

The Cleveland was not designed to be used in a passenger car.

mike

 
Actually he knows a lot about engines.

He tells me the Cleveland was designed by a NASCAR engineer

to compete against 429's and the like; story for another day.

Your daily driver/commute car would do fine with no back pressure,

but, try running the engine at 6000 rpm around a 500 mile NASCAR

circuit all day. Without back pressure you will fry the valves.

The Cleveland was not designed to be used in a passenger car.

mike
Click to expand...
I have to dissagree a bit there...but all opinions...Cause the 351 cleveland has the biggest equast ports of any motor made from ford ..Bigger than even the 429's .Nascar does not use mufflers...but From what i know you only need headers too make sure the valves dont cool too fast from fresh cold air...Any back pressure is a loss of preformance and gas milage...And wont do the valves any better...Will make them carb up

Mufflers keep your car quiet by bouncing air waves around multiple times within different chambers and tubes, canceling out engine noise and keeping drivers from getting annoyed looks from pedestrians and other drivers. Each time an airwave bounces around, however, it creates backpressure. The level of backpressure a muffler produces depends on the specific design of that muffler. However, for a race car, any amount of backpressure is counterproductive: It produces pressure on the engine and reduces power, and without utmost power a race car might as well be a bicycle. For this reason, NASCAR race cars don't use mufflers. This undoubtedly helps the drivers, but it also offers a nice side benefit for NASCAR fans, in that they get to hear the full-throated roar of all those race cars flying past them in the blink of an eye....

I really believe backpressure is only to make the car quite in town..lol

Below is a good thread that was posted before about the 351 cleveland.. It was made to go 7,000 rpms...not just 6,000....And alot of racers took them much farther.

The Amazing 351C 4V

The following are excerpts from a thread on the forum that ran from May 13-29th 2009 on the 351Cleveland.net forum.

I excerpted George Pence's posts because I believed they were well thought out and brought an insight as to the design of the 351C engine that had not really been explained anywhere before.

Good work George.

-blizz

Deliberate Design

text by forum member George May 26th 2009

I have found the best way for me to understand the Cleveland is to first respect the knowlege & experience of the engineers who designed it. I am convinced every aspect of the design of the 351C 4V was deliberate. If some design aspect seems fudged to me I have learned its because I don't understand WHY they designed it that way. In other words, I'm the one who is ignorant, not the engineers who designed the 351C 4V. Over the years certain aspects of the motor have become more clear to me, while others still leave me scratching my head.

Measured in the conventional manner on a flow bench, the 351C 4V exhaust port is not as stellar as the intake port. Randy has mentioned the exhaust gas "bounce" design incorporated into the heads. What does it take to get the exhaust gases to bounce like that? Everybody, including the race teams, have always approached designing headers & exhaust systems for the Cleveland like any other motor's exhaust system, perhaps that's the problem. I doubt a conventional exhaust system is what the engineers had in mind.

Consider the 351C 2V head for a moment. The 351C 2V is never mentioned in the early documentation along with the 351C 4V. The lack of historical documentation regarding the 351C 2V makes it appear it was a last minute after thought. This is not as far fetched as it may sound because the summer of 1969 when the 351C went into production coincides with the time when racing & performance cars lost their desirability within corporate Ford & Henry Ford II slashed the 1970 racing budget by 75%. It would appear a decision was made to make the performance motor into a mom & pop motor too, so a 2V head was quickly designed. The exhaust port of the 2V head does not have any exhaust gas bounce trickery designed into it, its just a basic no frills exhaust port. On the flow bench the 2V exhaust port works better than the 4V exhaust port! Yet the heads were desinged by the same team of engineers! This indicates to me the engineers knew indeed how to design a good flowing conventional exhaust port, even for high shock towers, but there were tricks incorporated into the 4V head's exhaust port that the folks outside of Bill Gay's engineering group just never understood. Or perhaps those tricks were just too difficult for the race teams to execute, so the high exhaust port plate modification was plan #2 that the engineers had intended to fall back on if the exhaust gas bounce idea didn't work out in the real world.

If all development of the 351C 4V hadn't ceased in February 1973, I am certain there would have been a phase II 351C 4V head introduced that would have either featured something similar to the 2V exhaust port, or the head would have looked like a cast iron version of the A3 SVO head.

-George

Galileo Galilei

text by forum member George May 26th 2009

The engineers who designed the 351C 4V were heavy hitters in the world of race engine design. These guys weren't novices. Bill Gay's group represented an amazing depth of experience in the design of state-of-the-art race motors.

With all due respect to anyone out there that disagrees with me, I believe anyone among us who critiques the engineering of the 351C 4V is like one of the Catholic clerics who opposed the heliocentric science of Galileo. None of us have the knowledge & experience those guys were privileged to have. We are not in the same laegue. When it comes to engine design we are fumbling in the dark compared to the 351C 4V engineers.

There are secrets hidden in that design waiting for us to discover. But the first step in learning is putting ego aside and admitting to ourselves we are not well informed on the subject.

-George

The Amazing 351C 4V

text by forum member George May 27th 2009

Ford's literature says the 351C 4V's reason for being was (1) racing (2) a performance option for production cars (3) cost.

Its a racing motor. It was designed for 6 liter NASCAR racing, banging fenders with cars powered by 426 Hemis & Boss 429s. Let that soak in for a moment.

It was designed by the same guys who designed the 427 FE. The 427 FE featured a steel crank, thick bulk heads above the main bearings, cross-bolted main bearing caps, a main priority oil passage running down the side of the block. These engineers found those features necessary for the 427 FE to survive NASCAR racing.

These same engineers designed the 351C for the same type of racing yet they included none of those features.

Had they forgotten everything they learned? Had they gone daft? Were they idiots? You can't straddle the fence on this issue. They were either idiots and decided their new motor didn't need those features, or they were up to something new with the 351C 4V. Something very deliberate. The engineers knew they would have to contend with the same forces which required cross bolted mains, thick bulkheads, steel cranks and side oiling when they had previously designed the 427 FE.

The 351C 4V benefited from the new ways of doing things the engineers had learned while designing racing motors such as the Indy racing motors of 1963 - 1965. Design of the Cosworth DFV Formula One motor was also wrapping up in England in 1966. The 427 powered Ford GT40 was dominating LeMans and the World Endurance Racing series in 1966. When design of the 351C 4V began in 1966 Bill Gays engine group was literally immersed in applying high technology in the design of racing engines. Ford literature from the era described the Cleveland as "an engine that reflects the racing heritage of Ford products on the worlds toughest race courses". To achieve the goal of building a potent and durable racing motor that could be mass produced as inexpensively as possible the team relied upon the use of engineered solutions, intelligent design and finesse rather than the expensive, heavy, brute force solutions that had been applied in the past when building racing motors such as Fords 427 FE of the mid 1960s.

One example of solving problems with finesse rather than brute force is the extra wide footprint of the 351C main bearing caps (mentioned earlier by Joe). The footprint gives them great stability without resorting to using cross-bolts. And it makes the 351C 4V less expensive to build along the way.

When the 351C 4V entered the scene in 1970, NASCAR was dominated by 7 liter endurance racing motors than cruised around the ovals at about 7000 rpm making about 500 bhp. Endurance camshafts of the day had about 0.600 inch lift.

It was no accident that when equipped with a 0.600 inch lift endurance racing camshaft the 351C 4V makes about 500 bhp at about 7000 rpm. From 5.75 liters! 7 liter hemi motor torque and horsepower from 5.75 liters at the same rpm.

Let that very deliberate fact soak in for a moment.

The engineers hit their mark dead on. No mistakes. No getting lucky. It was all very deliberate. The Cleveland is a very damn amazing racing motor. It just lacks the curb appeal of the hemi motors with their big aluminum heads & centrally located spark plugs.

There's a reason why even today its hard for modern alloy heads to improve upon the intake port flow numbers at 0.600 inch lift achievable with the iron 351C 4V head. That intake port was deliberately optimized for 0.600 inch valve lift. Some people have ignorantly referred to the intake ports of the 351C 4V as nothing more than big pipes for gulping air, inferring there was no intelligent engineering in the design. This is, of course, far from the truth. Ford literature from that era described the ports as carefully sized. The cross sectional area of the 4V intake port was optimized to supply a 350 cubic inch motor at engine speeds up to about 7200 RPM and the port was engineered to be more uniform in cross sectional area than previous designs.

As the 351C 4V powered Fords thundered around the banked ovals at 7200 rpm for 500 miles, they did so with complete reliability. They were reliable in spite of their nodular iron cranks instead of steel cranks, in spite of their thin wall block instead of thick bulkheads, in spite of their lack of cross bolting AND in spite of their lack of side oiling. The engineers achieved the 351C 4V's reliability with all those short cuts because they weren't short cuts. Like the wide main bearing caps, the engineers deliberately chose engineered solutions instead of brute force to make the motor reliable.The 351C did not have a reputation for problems in the early years. Through 1973 all the press the 351C 4V received was stellar.

1974 was the first year the 351C received bad press in the magazines. It was the year of the first oil embargo. It was the year everyone started buying intake manifolds with tiny runners, economy cams and headers with tiny primaries trying to improve the fuel economy of their V8s. It was the year that the sbc with its small ports began to dominate the aftermarket parts industry, helped along by a bunch of guys like Vizard & Yunick with vested interests in the little sbc motor. It was also the year Hank The Crank introduced a new crankshaft for the 351C. A magazine writer (numskull) named CJ Baker was a significant source for the body of mis-information that grew up around the 351C 4V.

As is usual in motor sports, racers & teams kept pushing the limits. They were no longer satisfied racing at 7200 rpm and they began pushing the motors to higher and higher engine speeds. By 1977 they were cruising the ovals at 8500 rpm making about 600 horsepower. Quite a bit above the original design parameters of the 351C 4V. The problem was, US Ford abandoned racing in February 1973, and no further official development of the motor occurred after that.

Hank The Crank introduced a forged steel 351C 4V crankshaft in 1974. The crank was internally balanced, had extra counter weights AND ... very significantly ... relocated oil passages that would allow a greater range of strokes than the iron crank. Hank The Crank also introduced an external main priority lubrication manifold for the center 3 main bearings in the same year. It appears the relocated oil passages in the crankshaft screwed up the design of the 351C lubrication system. Everybody running the HTC steel crank needed the external lubrication header too. But the guys who retained the iron crank didn't need it. The steel crank was very popular with the NASCAR teams. But over in the Pro Stock world Jack Roush showed a preference for the iron crank, and he swore up & down the only lubrication modifications he used was fitting the standard pump for 0.002" to 0.003" rotor clearance, a high pressure relief spring and his little cam bearing oil restrictor kit. Pro Stock motors turned 9500 rpm. Jack Roush's partner, Wayne Gapp, was one of the 351C 4V engineers. I"m sure the insider information he had influenced his decision to retain the iron crank.

Shortly after the introduction of the HTC steel crank the engineers at Ford, working somewhat secretively, authorized the manufacture of a thick bulk head block in Australia. By the 1975 NASCAR season the Ford teams had steel cranks, main priority oiling and blocks with thick main bearing bulkheads, ala the 427 FE. It's not unreasonable to assume the 8500 rpm engine speeds required these changes ... but iron has dampening properties that steel does not. There's equal evidence the steel crank necessitated the heavy duty block just like it necessitated the external main priority lubrication manifold.

The HTC crankshaft was an aftermarket part. How many of us have had problems with our motors caused by aftermarket parts? The steel crank negated the engineering that went into the lubrication system and the block. The nodular iron crank, the oil passages within the crank and the thin wall block were engineered to work together as a very durable system.

So one person observes the design of the 351C and concludes the short block is nothing special, the castings are thin and flimsy and the head design is compromised. When I observe the 351C I conclude it is an AMAZING racing motor designed by a group of brilliant men. When the original castings are carefully assembled it is capable of making 500 bhp naturally aspirated at engine speeds up to 7200 rpm under NASCAR racing conditions (some of the toughest racing in the world) for a very long time.

-George

Australian Production

text by forum member George May 28th 2009

The earliest Cleveland engines installed in Australian vehicles were sourced from the US. Those vehicles were XW Falcons beginning about February 1970. In other words the 351C arrived in Australia only a few short months after it was introcuced in North America.

Even the Boss 351 was shipped to Australia topped with a 780 Holley instead of the Motorcraft 4300D and equipped with factory headers. It was installed in the 1971 Falcon GT-HO Phase III (based on the Falcon XY GT). The GT-HO Phase III is possibly Australia's most famous muscle car.

Australian manufactured Clevelands were introduced with the XA Falcon, which I believe was in March 1972. Some folks report finding Australian manufactured Clevelands in the engine bays of XY Falcons assembled in December 1971. Obviously the tooling arrived earlier & engine production had to be under way a few months before they showed up installed in vehicles.

Also keep in mind several parts of the Australian 302C are different (crank, heads, rods) and had to be designed, tooling made, molds created, test runs made all before production could begin.

Why ... I've never seen a Ford document explaining why. I think its because the Aussies recognize a great engine when they see one.

-George

The Cleveland Lives On...

text by forum member George May 27th 2009

True, Ford dropped the 351C 4V like a hot potato. 351C fans shouldn't take it personally because Ford dropped all racing and performance cars, performance parts, etc in February 1973. But don't let that diminish what the Cleveland is in your mind. It fulfilled its purpose.

People, including journalists, who bad-mouth the 351C do so out of ignorance.

It was, and always shall be, a 5.75 liter motor that in 1971 was capable of competing on equal ground with hemi headed 7 liter motors. With the same 0.6" lift endurance cams, at the same rpm, with the same carburetion, the 351 cubic inch Ford made equal horsepower as the FE427, the Boss 429, the 426 Hemi and the 421 Super Duty. It could power a Torino around the super ovals like Taladega and Daytona at equal speed, and with equal reliability as the big blocks. It could cruise around a super oval at 7200 rpm all day long without breaking. It did this with a thin cast block, no side oiling, no steel crank. The 351C 4V was assembled on an assembly line at 20% of the cost of a Boss 429 or the 427 FE.

It represented the culmination of everything the engine designers at Ford had learned throughout the Total Performance Era while designing NASCAR motors, Formula One motors, Indy motors, Le Mans motors, Trans Am motors and more. It was the most advanced, most precision built mass produced motor in its day. It was capable of making more horsepower per cubic inch than any mass produced motor that came before it. There is technology locked up in the design of the 351C 4V that people still don't want to talk about to this day.

When you stop and realize that the canted valve cylinder head was originally designed as Plymouth's cheap version of the Chrysler hemi, it just makes the 351C 4V that much more bittersweet. The technology locked up in the 351C ports and narrow valve angles far exceeds anything Chevrolet or Ford had accomplished with previous designs (Chevrolet's W motor, Mk II and Mk IV and Ford's 385 series). It's cylinder head is the ultimate canted valve cylinder head. It's basic architecture continues to influence the racing heads designed by Ford and Chevrolet to this day. If the design was faulty, this wouldn't be so.

-George


Btw...By no means am i saying you mechanic is a dummy...I talk to my cousin alot about such things...He has a bachelor's degree in mechanical engineering and enjoys talking about old motors with me...Even he agrees with me on this one...But alot of people believe in back pressure thou...For making your car quite It works great..But not for a race car or preformance....Nor do i believe it will last longer by plugging it up...But thats just my opinion..With some facts to back it up if i need..lol

 
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With all this talk about back pressure I thought everyone was concerned about me and those chili dogs I had last night!

 

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