OHV vs OHC: Which is better?

[inferno]

Much like the other topic, I want to start up a tech discussion to ease my boredom. Which is better and why?

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"I know you don't really wanna, unleash the dragon..." Sisqo

[Unit 5302]

It depends on your application.

OHV generally makes more low end power. The reason being? Simple, since it's very hard to make a multivalve OHV engine, many times the head designs are more apt to focus on low end/midrange performance, say up to maybe, 5500rpms on a factory car. That usually means very good low end power, pulling force and solid midrange 3000-5000rpm performance. Going beyond 7000rpm on an OHV engine can also cause serious drivetrain problems. Valve float, bent pushrods and slop can cause serious engine damage. More stuff to go wrong, and it usually shows quickly when the engine isn't built to extreme tolerences. OHV engines are definately easier to work on, and they usually have a larger aftermarket catering to them, simply because they've been around since the 50's.

SOHC motors are much like OHV motors, except the improved drivetrain configuration eliminates some friction and makes them more rev happy. Due to the lack of multivalve heads they cannot really produce significantly more power than a comparable OHV motor, they are just better, as a genral rule, at running between 5000-7000rpm due to less chance of drivetrain failure. Manufacturer's usually take advantage of that property and design the heads a little more aggressive for higher rpm flow, making up for lost low end due to displacement losses. Usually due to increased external engine size associated in this configuration vs OHV the displacement drops somewhat versus previous OHV motors in the same chassis. The SOHC motors can be made to perform better than the OHV motor using their strength's which include, better drivetrain configuration, less moving componenets, reduced friction, and better head design to increase the output to a level exceeding or matching the previous OHV engine, but they generally sacrifice low end power due to smaller engine displacement.

DOHC motors are the ultimate in high performance applications. They allow for multi-valve setups which make huge flow rates easy to come by. The advantages to haveing more of the cylinder head availible to flow air is obvious, greater flow and huge increases in power. The DOHC motors are large, and they are generally heavy, many times this cuts down on their size, but they are extremely efficient especially when variable valvetrain technology is used, say like that in a DOHC 4.6L 32v V-8 found in the Mustang Cobra. The DOHC motor almost never makes solid low end power simply because they are designed for high rpm very high hp applications. I don't think you could find a DOHC motor made that doesn't run over 6000rpms, even the forced induction motors. They are very hard to work on compared to the OHV cousins, generally poor low rpm performers, very expensive, large externally, and heavy. Dispite this the power produced per cubic inch or liter can be completely out of proportion with the fallbacks.

[inferno]

Very well put Unit. I personally feel that DOHC is the best out of the three, especially with variable timing for a few reasons. Any guesses as to why?

[PGkelly]

i wouldn't say that one is better than the other. it's all in personal preference and the type of application you are using it for. DOHC/SOHC motors are better for road racing, because of the higher rpm power. OHV motors would be more suited to drag racing, or starting from a standstill, because they make a lot of power down low. OHV motors are also more often then not large displacement engines, giving them an advantage because they can make a lot more torque. that also means that their power will decrease substantially at higher RPM's (the kind of RPM's you would see in road racing) because they have to fill that big cylinder with air. That's the reason Ferrari makes a 3.5L DOHC 5V V8, because at the higher revs the smaller cylinders have an airflow advantage. DOHC/SOHC motors can be made to have low RPM power, but they aren't because they don't need to.

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If it ain't broke, make it go faster

[PGkelly]

btw, anyone who can say what company makes the only DOHC motor that uses rocker arms gets a million points.

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If it ain't broke, make it go faster

[Unit 5302]

An OHV 2v engine has no advantage over a DOHC multivalve engine in any racing application.

The powerband of the engine can be completely offset by the gearing.

For proof, please run a stock 5.0 with any gear of choice vs a DOHC Cobra with 4.10's.

The 5.0 has substantially more low end than the Cobra, it helps from 0-20 maybe, if you can get traction, but the overwhelming hp advantage of the Cobra will make short work of the OHV engine even by the time the 1/8 mile streaks by.

The answer isn't Ducati is it?

[MiracleMax]

That would be Ford right! part of the Cobra motor's advatage over other DOHC engines?

[97snakedriver]

The DOHC and SOHC Mustang engines use roller finger followers, which are a bit different then rockers.

Hondas use rocker arms.

[PGkelly]

yes, it is Ducati. their desmodromic valvetrain uses a regular cam to tilt a rocker arm that raises the valve, then is has a second cam, tilting a second rocker arm to close the valve a hold it closed. it uses springs but only to close the last 0.005". both cams are on the same shaft. Mercedes-Benz pioneered this system in the forties, when engine speeds in racing were getting too high for the spring material. Desmedromic timing effectively solved the problem, but the Mercedes system was very cumbersome. and it was made useless when the japanese discovered stiffer spring material. But Ducati chose to use the desmo system anyways, using the theory that it would eliminate valve float and give a higher rev ceiling. the current ducati redlines at 11,000 something, while most japanese bikes redline at 13,000 plus. but the Duc has much more torque at a much lower rpm, and it handles a lot better.

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If it ain't broke, make it go faster

[dinomite]

Best handling valves I've ever seen.

[Unit 5302]

The reason the Ducati's redline lower is due to them being large displacement V-twins instead of I-4's that are most often used by the Japanese sport bikes.

It's not a valvetrain issue, it's a piston size issue. Yes, they do have very strong torque curves, plus their light weight allows them to win many races against much more powerful bikes.

This is a good thread.

Take a look at my DOHC Cobra for comparison sake. The motor is stock from Throttle Body to Oilpan and has only bolt-ons for more power.

For mods list, my Cobra is listed in the Users Rides section under 94-98 Cobras in 12.0-12.9 ET range.

Later, Jason


BTW, you are correct. This thing got NO BALLS until about 4500 rpms. I use 4.56 gears to stay above this point the entire length of the 1/4mile.

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N/A 1997 Cobra

1/8 mile - 7.83 @ 90.08 MPH
1/4 mile - 12.26 @ 114.02 MPH
1.64 60 ft.

[Unit 5302]

jmeiers, I'm assuming the 4.6 Cobra's come on all of the sudden pretty strong due to the second set of valves kickin in around 4500rpms or so, correct?

Well, actually, the IMRC's (Intake Manifold Runner Control) open the secondaries at 3250 rpms. You can actually hear this function everytime you cross 3250 rpms from the kickpanel where CPU is located. The exhaust gets a deeper tone also and it pulls harder. You can also see this jump on any 96-98 Cobra dyno graph.

I just keep it above 4500 rpms because my only purpose in life is to run 11's on that stock motor and I am just below max torque and right in the heart of the HP powerband pull back to 7000 rpms where I make my next shift. The 4.56's get me up there quickly and keep me up thru-out the run.

FYI, the 99/01 Cobras do not use IMRC. They have a different head design (tumbleport) which promotes more low-end torque, enough where Ford decided not to use them on these.

There are also IMRC delete kits sold by FRPP to take them off of the 96-98 Cobras, but unless it is supercharged or some straight-up N/A race application, low-end torque will really suffer. There is also finally a new aftermarket intake coming out for the 96-98 Cobra which bolts directly to the heads and does not use the IMRC. But, this intake kills torque but raises HP significantly. 5.13+ gears minumum will be needed to take full advantage of this new intake. Stay Tuned!!!

Later, Jason

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N/A 1997 Cobra

1/8 mile - 7.83 @ 90.08 MPH
1/4 mile - 12.26 @ 114.02 MPH
1.64 60 ft.

[PGkelly]

Actually, the Yamaha R1 and the Honda RC51 are really giving Ducati a run for it's money. More often then not they will pass the Duc's in the straights, forcing the ducati to rely on it's exceptional handling to make up for it. the L twin in the ducati makes it much narrower, so it can lean a lot farther than the japanese I-4's, but Ducati has been losing more often. quite a bummer, for such exceptional bikes.

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If it ain't broke, make it go faster

[inferno]

Originally posted by PGkelly:
Actually, the Yamaha R1 and the Honda RC51 are really giving Ducati a run for it's money. More often then not they will pass the Duc's in the straights, forcing the ducati to rely on it's exceptional handling to make up for it. the L twin in the ducati makes it much narrower, so it can lean a lot farther than the japanese I-4's, but Ducati has been losing more often. quite a bummer, for such exceptional bikes.

I would like to get a RC51...I need to fix my bike and trade it in on a new one...more than likely though, I will get a 600F4i.

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"I know you don't really wanna, unleash the dragon..." Sisqo