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Old 01-11-2004, 04:14 PM   #1
beef_bourito
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Question Pushrod vs. SOHC vs. DOHC

What are the advantages and disadvantages, performance wise, of Pushrod engines vs SOHC and DOHC engines? Also what are easier to maintain?
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Old 01-12-2004, 10:21 AM   #2
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Re: Pushrod vs. SOHC vs. DOHC

Pushrod is best for power, OHC is good for economy.
That's why race engines are pushrod. I think there is something about losing power through the valvetrain when you deal with very powerful engines.
That is not to say OHC is BAD for power, just simply not AS good.
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Old 01-12-2004, 11:52 AM   #3
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Re: Pushrod vs. SOHC vs. DOHC

generally speaking, overhead cam engines allow higher rev operation, and thus are better for peak power output. They're found in many different exotic car and racing applications (Ferrari, IRL, etc) where high rpms are desirable.

Pushrods are generally cheaper to make for V engines than OHC, OHC is sometimes cheaper for inline engines.

Using two cams instead of one (DOHC vs SOHC) can give you a bit more flexibility when designing lift profiles, as you'll have less contact stress, and you may be able to achieve higher speeds. DOHC is a logical choice if you have a variable cam phasing mechanism, as you can put your intake and exhaust lobes on separate shafts.

Both OHC and pushrod valvetrains can use timing chains or timing belts, but timing chains are more common on pushrod engines, and timing belts are more common on OHC engines. The advantage of a chain is that it lasts longer; belts are quieter, easier to package, and offer better high-rev performance if the drive load requirements aren't too high. OHC and pushrod configurations can both use hydraulic lash adjustment, which almost eliminates the need for periodic manual lash adjustments.

Valvetrain friction can go either way. For reduced valvetrain friction, it is preferable to use a rolling follower, and it is preferable to use manual lash adjustment. Either of those can be done on a pushrod engine, but they seem to be more common on OHC engines. An OHC valvetrain will generally have less mass to control (hence the potential for higher operating speed), which translates to lower spring forces for a given operating speed, and lower friction.
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Old 01-12-2004, 04:03 PM   #4
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Re: Re: Pushrod vs. SOHC vs. DOHC

Quote:
Originally Posted by MustangRoadRacer
Pushrod is best for power, OHC is good for economy.
That's why race engines are pushrod. I think there is something about losing power through the valvetrain when you deal with very powerful engines.
That is not to say OHC is BAD for power, just simply not AS good.
What kinds of race engines use pushrods? I know there are those exotic sports cars that use domestic big blocks, but what others?
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Old 01-12-2004, 04:08 PM   #5
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Re: Pushrod vs. SOHC vs. DOHC

avg joe's american v8 hotrod. maybe nascar.
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Old 01-12-2004, 05:57 PM   #6
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pushrod engines are easily capable of keeping up with multivalve OHC engines mustang, but they're hardly "better". For smaller engines OHC is the way to go, you need to rev then to use the gearing necessary for fast acceleration. For larger engines it's not as much of a problem IMO since you're going to produce alot of torque anyway there's no need for high RPMs and short gearing.

If you aren't revving high then OHVs valvetrain issues won't come into play anyway, the LS6 can already rev to 7000 without any float, it doesn't need to go any higher and its head flows more air then it needs. If you are revving high then you use OHC, unlike ass-i mean NASCAR the general public doesn't know how to set the lash on their solid roller pushrod motors and hydraulic lifters haven't seen duty much above 7-7500 RPM to my knowledge.
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Old 01-12-2004, 06:00 PM   #7
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Re: Pushrod vs. SOHC vs. DOHC

4 valve DOHC heads have also showed that they can be used in high power engines without giving too bad exhaust emissions too.

For maximum power the best head configuration is a 4 valve pentroof type head with valves angled around 20 degrees. The combustion chamber should have the spark plug raised in the center and the smaller exhaust valves should be placed close to eachother, and there shall be a raise between the valve looking something like a + and the squish zone should be angled. The valves shall be actuated by twin cams (per cylinder bank) and fingerfollowers (preferbly).
This type of head can be found in F1, CART, touringcars and so on. Fingerfollowers and angled squishzones are however not always used since they are quite "new". If a cylinderhead is modified this also sets some limitations.
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Old 01-12-2004, 07:29 PM   #8
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here is a pic of a pentroof combustion chamber with a central spark plug...the valves angle at a very steep 48 degrees however



and i would like to agree that the DOHC setup is by far the better one even on high displacement engines....the 6.0 in enzo for example or the 5.7 in the carrera GT

another advantage of DOHC engines that IMO is crucial is the fact that you can adjust the lift and timing of the exaust and intake valves independetly from each other...this raises the tuning posibilities to a new level

the only advantages of pushrod that i can see are:
1. cheaper
2. smaller head
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Old 01-13-2004, 01:31 AM   #9
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Lower weight and less complexity as well, assuming you can meet your airflow needs with an OHV head (and since most pushrod designs dont rev high airflow need is decreased) and don't wish to rev over 7500 it's a fine choice. The lower and lighter engine has many handling benefits as well, a dry sump OHV engine is extremely short, supposedly the new corvette will have a high perf. model utilizing that.

There are concept 2 cam OHV engines, 1 for intake 1 for the exhaust, with that working an OHV engine with VVT is possible, you can run as many valves off of that as you like too. Pushrod weight and flex is always going to be a problem, but new metals have already fixed much of that, chromemoly rods are very light and stiff and for the enthusiast solid lifters will fix any valve float issues at high RPMs.
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Old 01-13-2004, 08:42 AM   #10
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Re: Pushrod vs. SOHC vs. DOHC

Pushrod race engines:
Top fuel dragsters 7-8000 hp
Nascar 700 hp
formula ford 170 hp (only 1600 cc w/ 2 barrel carb)
some LeMans cars (viper, vette)

DOHC is used in the IRL however. with 650 hp.
and toyota atlantic (cart) uses the 4age. DOHC
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Old 01-13-2004, 03:04 PM   #11
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Re: Re: Pushrod vs. SOHC vs. DOHC

Quote:
Originally Posted by MustangRoadRacer
Pushrod race engines:
Top fuel dragsters 7-8000 hp
Nascar 700 hp
formula ford 170 hp (only 1600 cc w/ 2 barrel carb)
some LeMans cars (viper, vette)

DOHC is used in the IRL however. with 650 hp.
and toyota atlantic (cart) uses the 4age. DOHC
Many of the engines you mentioned are limited to push rods, they don't use it because they are somehow better.

DOHC is used in almost all motorsports with a few exceptions. DOHC is for example used in F1, CART, IRL, most GT racing cars (Corvette, Panoz and Viper being the exceptions), WRC and touring cars (like BTCC). Some of the highest brake mean effective pressures of NA engines have been measured in touring cars like BTCC.
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Old 01-13-2004, 03:09 PM   #12
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Re: Pushrod vs. SOHC vs. DOHC

yes, but F1 and CART use pneumatic valves, which is kinda different.
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Old 01-13-2004, 03:47 PM   #13
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Re: Pushrod vs. SOHC vs. DOHC

OHV is certainly not superior, among other things already mentioned the pushrod intrusion through the cylinder head limits port design and shape. Of course more of the airflow differences between domestic and import cylinder head flow is due to the Big 3's reliance on displacement to make airflow rather than brilliant cylinder head design and RPM, but then either approach can work extremely well.
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Old 01-13-2004, 04:47 PM   #14
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Re: Re: Pushrod vs. SOHC vs. DOHC

Quote:
Originally Posted by MustangRoadRacer
yes, but F1 and CART use pneumatic valves, which is kinda different.
How so? They are not pneumatic "valves", merely pneumatic valve RETURN systems. AFAIK F1 and CART still use DOHC valve actuation systems.
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Old 01-13-2004, 05:50 PM   #15
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Re: Re: Pushrod vs. SOHC vs. DOHC

F1 use pneumatic springs instead of conventional springs. CART use conventional springs due to regulations.

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