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Location:
Melbourne
Registered:
November 2003
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Stupid DOHC question
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Mon, 24 November 2003 13:13
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I have been looking around and cant find a suitable answer to this question.
What does having a DOHC over a SOHC DO? And why is it better (obviously it is). Is it just to allow for more valves? Surly it's more than that!
Please excuse my ignorance.
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Location:
Brisbane
Registered:
July 2002
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Location:
Melbourne
Registered:
November 2003
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Re: Stupid DOHC question
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Mon, 24 November 2003 13:37
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Okay, what I'm getting so far (and this is what I have read before) is that there is no real difference between SOHC and DOHC. Just a difference between 2valves/cyl and 4valves/cyl... and that it's easier to put 4 valves/cyl on a car that has 2 cams.
I thought there would be more difference than this.
Does a second cam mean that the cams get to move at half the speed, resulting in less wear? Do the cams each control different valves (ie one cam controls the two exhaust valves and the other controls intake)? If so, how does this help?
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Location:
Brisbane
Registered:
July 2002
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Re: Stupid DOHC question
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Mon, 24 November 2003 13:43
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For starters you can do a hell of a lot more tuning wise with two independant inlet and exhaust cams.
Head designs are a hell of a lot better in the DOHC variety, they flow more. A lot of SOHC engines are non-crossflow as well, which is a killer for decent flow in comparision.
Also things like the spark plugs are in better position. You don't need rocker gear in a DOHC engine and can just use shims which directly open the valves.
Plus the later engines start using VVT and shit like that.
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Location:
Melbourne
Registered:
November 2003
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Re: Stupid DOHC question
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Mon, 24 November 2003 14:29
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Yes well obviously VVT-i, VTEC, MIVEC etc are very very very strong, ans much easier to do on DOHC. I was talking more about why it's so strong in 85-95 (obviously VTEC was around in 95, but that's not Toyota  ) model cars.
And I think you just answered my question. Thanks!
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Location:
Brisbane
Registered:
May 2002
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Re: Stupid DOHC question
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Mon, 24 November 2003 22:41
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The main advantage of DOHC is it allows more flexibility in port and combustion chamber design. If you took an existing SOHC head and added a second cam, there would be no real advantage. However if you re-designed the head around the DOHC valvetrain, you would be able to use different combustion chamber shapes, bigger ports, and more valve angle.
Note that DOHC/SOHC has nothing to do with the number of valves. There are quite a few 4-valve SOHC engines kicking around, as well as 2-valve DOHC engines.
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Location:
Brisbane, QLD, Australia
Registered:
May 2002
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Re: Stupid DOHC question
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Tue, 25 November 2003 01:28
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I remember posting somewhere once the main advantage of dohc as opposed to sohc with the same amount of valves.
Basically the main advantage of more power and higher revving, (dohc loves to rev) belongs to dohc.
The reason is due to less friction per 2 cycles (fuel in and exhaust out) in the 4stroke process.
I'll just explain this very simply:
If you have a sohc design, you have just the one cam pushing all intake and exhaust valves via a common rocker arm.
If you have a dohc setup, you have one cam to do the intake, and one cam to do the exhaust.
Less frictional energy lost on the dohc = more response and power. Plus you have the other advantes already mentioned with cam angles, etc.
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Location:
Sydney
Registered:
May 2002
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Re: Stupid DOHC question
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Tue, 25 November 2003 02:06
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Apollo - Aren't you forgetting that while the friction on each cam might be lower in a DOHC, there are twice as many cams, returning the overall effort requried to move the valves to the original value in a SOHC engine?
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Registered:
August 2002
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Re: Stupid DOHC question
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Tue, 25 November 2003 02:25
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Although a non Toyota example...
Take one Nissan RB30E (3L single cam straight 6) keep the bottom end but swap the DOHC head from an RB25DE onto it (2.5L twin cam straight 6) and the engine produces 30% more power in stock form.
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Location:
Brisbane
Registered:
May 2002
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Re: Stupid DOHC question
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Tue, 25 November 2003 02:56
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Not really a direct comparison of SOHC and DOHC there... most of the power increase comes from the 4 valves of the RB25DE head.
Perhaps a better example is the 18R-C (SOHC 2-valve) compared to the 18R-G (DOHC 2-valve). As we all know the former is less than inspiring (~100hp), while the latter is an absolute ripper (~140hp). Most of the difference can be attributed to the hemispherical combustion chamber design, larger ports, much larger valves, and of course the high-revving nature of the 18R-G. All of this is made possible by DOHC!
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I supported Toymods
Location:
Australia
Registered:
November 2003
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Re: Stupid DOHC question
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Tue, 25 November 2003 03:29
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ummm, are we forgetting 5m-e -> 5m-ge?
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Location:
Melbourne
Registered:
November 2003
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Re: Stupid DOHC question
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Tue, 25 November 2003 04:48
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Ahh yes. Thanks for clearning that up. I had a feeling it would be something to do with the shape of the intake/exaust. I just couldn't rap my head around why making the cams heavier would make the system better. But I get it now.
I was trying to explain why DOHC was so good to some mates, and then realised I didn't actualy know, hence this topic.
I think these guys are a lost cause though, as they where arguing that the VN pushrod system was just as good as a DOHC, and a DOHC was just cheaper. *sigh*
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Location:
Brisbane, QLD, Australia
Registered:
May 2002
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Re: Stupid DOHC question
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Tue, 25 November 2003 06:22
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| SXer wrote on Tue, 25 November 2003 12:06 |
Apollo - Aren't you forgetting that while the friction on each cam might be lower in a DOHC, there are twice as many cams, returning the overall effort requried to move the valves to the original value in a SOHC engine?
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Not really.
A DOHC typically has less moving parts than a SOHC (no rockers/fingers). Less than half the workload of a sohc is thrust upon each dohc per intake and exhaust phase of the 4stroke cycle.
I think I know what you are getting at though, but I've currently hit my limit on explainations.
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I supported Toymods
Location:
Ademelaide, SA
Registered:
July 2003
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Re: Stupid DOHC question
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Tue, 25 November 2003 10:43
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| Shraka wrote on Tue, 25 November 2003 00:59 |
Yes well obviously VVT-i, VTEC, MIVEC etc are very very very strong, ans much easier to do on DOHC. I was talking more about why it's so strong in 85-95 (obviously VTEC was around in 95, but that's not Toyota ) model cars.
And I think you just answered my question. Thanks!
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just felt like saying...
Toyota had vvti in 1994. it was released in the AE101 corolla in the form of the silver top 20 valve 4AGE.
Stupid Australia. well... Toyota Motor Corporation Australia. I love how they said on the ads for the ZZE-122 Corolla - "The all new 100kw vvti engine.." - it's like, big deal. the japs had 123kw in Corollas since the mid 90's....
still....i love Tojo's, and wouldn't own anything else.
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Location:
Melbourne
Registered:
November 2003
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Re: Stupid DOHC question
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Tue, 25 November 2003 11:32
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Heh heh. Okay.
Am I right in assuming that VTEC was the first variable vavle timing to come out of Japan though?
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Location:
Sydney/Wollongong
Registered:
October 2003
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