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Back Pressure
#1
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

[Image: 1_11_11_13_11_50_27.png]
  Reply
#2
goodnigh;63980 Wrote: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

Wow. Never heard that before, and I've never come across it in the 335 Forum. Huh I wonder what the purpose would be?

Doc

[Image: 6y14ea.jpg]

Project started 8-7-10
Completed: All new suspension, rebuilt 351C H Code bored .030 over with mild cam and intake, new 3.50 TracLok, custom exhaust system
Current "mini-project": interior upgrade Undecided
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#3
It was originally written in the engine builders Bible that BACK PRESSURE was necessary to help cool the exhaust valves and keep them from burning out.

Well, after thousands of miles with people driving headers instead of exhaust manifolds, the bible has been revised. No measurable valve wear can be attributed to headers.

Exhaust manifolds were also looked at as heat sinks. They were SO HEAVY they took a lot of energy to heat and kept the valvetrain cool. Again, this was looked on as necessary. However, there is no proof this is the case. Let's face it, castings were primarily used because they were cheap...heavy but cheap, reliable and lasted forever without maintenance.

I am still looking for THE Ford engineer(BY NAME) who designed the 351C 4v to hear what he/they were thinking. Until then, everything regarding their design is conjecture.

Follow my engine building rule: The louder the better.

[Image: 11jmcuc.png]
351C Bold Manners, Brash Attitude
Favorite Teams: Michigan Wolverines and Whoever Is Playing Ohio State.

When I drive past a herd of cows, the cows MOO at me
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#4
This off a test site


One thing that I get tired of hearing is the concept that engines need back-pressure. Simply, there is no properly tuned engine where increasing exhaust back-pressure causes an improvement - in power, torque or fuel economy. One of the reasons that this idea has gained support is because when people change their exhaust they seldom check the air/fuel ratio or re-map the ignition timing to once again give optimal performance. For example, some MAP sensed cars drop substantially in power with a large exhaust fitted because they are then running lean.

Atmospherically inducted cars that use a tuned length system to improve cylinder scavenging (via extractors, for example) are sensitive to exhaust diameters within the tuned length part of the system. This means that the maximum effect of exhaust pulsing may come from an exhaust system that is small enough that some exhaust back-pressure is developed. However, that is a quite different concept to saying that engines "need" exhaust back-pressure! Turbocharged engines require as big an exhaust as possible, with the same applying for naturally aspirated cars once the tuned length part of the exhaust is passed.

Few tests have been done that clearly show the affect of changing back-pressure. Most muffler and exhaust comparison tests change more than one parameter simultaneously, making the identification of exhaust back-pressure as a culprit difficult. However, Wollongong (Australia) mechanic Kevin Davis is one who has done very extensive testing of varying back-pressure on a number of performance engines. These range from turbocharged Subaru Liberty [Legacy] RS flat fours to full-house traditional pushrod V8's. In not one case has he found any improvement in any engine performance parameter by increasing exhaust back-pressure!

The tests came about because Kevin has developed a patented variable flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some back-pressure at low loads "to help torque". However, he soon changed his mind when any increase in back-pressure proved to decrease torque (and therefore power at those revs) on a properly tuned engine! What increasing the back-pressure does do is dramatically quieten the exhaust.

One of the engine dyno tests carried out by Kevin was on warm 351 4V Cleveland V8. Following the extractors, he fitted a huge exhaust that gave a measured zero back-pressure. Torque peaked at 423 ft-lb at 4700 rpm, with power a rousing 441hp at 6300 rpm. He then dialled-in 1.5 psi back-pressure. Note that very few exhausts are capable of delivering such a low back-pressure on a road car. Even with this small amount of back-pressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the butterfly position to give 2.5 psi back-pressure. Torque and power decreased again, both dropping by 7 per cent over having zero back-pressure!

And if you still believe that exhaust back-pressure improves performance, simply block off part of your exhaust outlet and see if your car goes any faster!
  Reply
#5
This waiting on my cast manifolds to wear out so I can put headers on. Shy

Jim

Jim

M code 71 Mach 1, 351 4V Cleveland, Ram Air (not factory), C6 Trans, 3.5 rear
  Reply
#6
hyena429;63990 Wrote:This off a test site..

You forgot he-he...and you thought no one noticed.

[Image: 11jmcuc.png]
351C Bold Manners, Brash Attitude
Favorite Teams: Michigan Wolverines and Whoever Is Playing Ohio State.

When I drive past a herd of cows, the cows MOO at me
  Reply
#7
Wolverine;63993 Wrote:
hyena429;63990 Wrote:This off a test site..

You forgot he-he...and you thought no one noticed.

Damn you got me...hehe...that better? lmao
  Reply
#8
goodnigh;63980 Wrote: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

Think I would find a new MechanicConfused
  Reply
#9
I have heard some good reasons not to install headers (i.e. ground clearance, leaks, Dad told me not to, fitment, etc.), but backpressure isn't one of them!

[Image: 4zw1hv.png]
Dave

1931 Ford Model A Station Wagon
1969 Mach 1 - 351C, TKO-600, 4WDB, R&P, A/C, Shaker, Fold Down, etc.
1972 Mach 1 - 351C, FMX, PDB, PS, A/C, Fold Down, Console
1996 Mustang Cobra Convertible - 10psi Procharger
  Reply
#10
Wolverine;63989 Wrote:It was originally written in the engine builders Bible that BACK PRESSURE was necessary to help cool the exhaust valves and keep them from burning out.

Well, after thousands of miles with people driving headers instead of exhaust manifolds, the bible has been revised. No measurable valve wear can be attributed to headers.

Exhaust manifolds were also looked at as heat sinks. They were SO HEAVY they took a lot of energy to heat and kept the valvetrain cool. Again, this was looked on as necessary. However, there is no proof this is the case. Let's face it, castings were primarily used because they were cheap...heavy but cheap, reliable and lasted forever without maintenance.

I am still looking for THE Ford engineer(BY NAME) who designed the 351C 4v to hear what he/they were thinking. Until then, everything regarding their design is conjecture.

Follow my engine building rule: The louder the better.

My mechanic is pretty adamant about the issue. He has been
rebuilding Clevelands for going on 42 years now. Maybe he knew
some of those engineers or read something in a trade journal.
His most recent encounter is pictured below 'before/after'.
I can at least trust him to change my spark plugs Tongue

mike


BEFORE

   

AFTER

   

[Image: 1_11_11_13_11_50_27.png]
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