DOHC vs SOHC 16 valve engines ?
Pramod John
I am curious as to what the difference is between a DOHC and an SOHC
engine (yes I understand physically what is different. the question is why
though.) It would seem that you could drive 16 valves using one camshaft
rather than two, so why bother with two ? Or i guess you could even drive
two camshafts off of one pulley connected to the timing belt/chain rather
than the two which seems to be the norm . Why ?
Thanks,
Pramod John
--
Pramod John Optical and Discharge Physics
email: pra...@uiuc.edu Electrical & Computer Engineering Dept.
Voice: (217)333-2483 University of Illinois Urbana-Champaign
"Blessed are the peacemakers"- Jesus Christ
Manly Matt Schulman
> I am curious as to what the difference is between a DOHC and an SOHC
>engine (yes I understand physically what is different. the question is why
>though.) It would seem that you could drive 16 valves using one camshaft
>rather than two, so why bother with two ? Or i guess you could even drive
>two camshafts off of one pulley connected to the timing belt/chain rather
>than the two which seems to be the norm . Why ?
It's mostly packaging and mechanical efficiency. In short, a
single stick for each head means mounting it between the two rows of
valves, and using rocker arms to actuate the valves. With two sticks per
head, the cam can be placed directly over the vlaves and actuate them
directly. At higher engine speeds (> 4000 RPM), the stiffness of
components like rocker arms starts to limit the performance of the valve
train, so the 2-cam setup performs better up there.
_____________________________________________________________________________
These words of wisdom from: _____
"Manly" Matt Schulman / \
[schu...@nova.gmi.edu] --------/--------/ \
( / )
( / )
\ /--------/--------
\ _____ /
Steve Won
> It's mostly packaging and mechanical efficiency. In short, a
>single stick for each head means mounting it between the two rows of
>valves, and using rocker arms to actuate the valves. With two sticks per
>head, the cam can be placed directly over the vlaves and actuate them
>directly. At higher engine speeds (> 4000 RPM), the stiffness of
>components like rocker arms starts to limit the performance of the valve
>train, so the 2-cam setup performs better up there.
Adding to the above list:
I recall reading a whole bunch of advantages to DOHC vs. SOHC but can't
remember many.
I think one of them to do with packaging was that a DOHC allows for a
narrower included-valve-angle, which leads to better breathing at high rpm
I believe.
Also, if the engine is equipped with a variable valve timing system such
as Nissan's (in 300ZX) which rotates the cam to alter timing at various
rpm, it works best if the intake and exhaust cams (DOHC) are separate so
that adjusting intake does not adversely affect exhaust.
Finally, I wonder if it's possible to fit hydraulic valve lifters on a
SOHC 4-valve head. I have a SOHC 16-valve inline4 and it needs frequent
valve clearance adjustments. This would not be necessary if I had
hydraulic lifters, such as on my friends' DOHC-16.
Is there any other reason why Honda used solid lifters as opposed to
hydraulic? Are there other pros & cons that would recommend solid lifters?
Steve
--
MountainBiking motto: "You know you're having fun when things begin
hurting/breaking."
Car-tinkering motto: "Less weight means more performance!!!"
arie o dezwart
: Finally, I wonder if it's possible to fit hydraulic valve lifters on a
: SOHC 4-valve head. I have a SOHC 16-valve inline4 and it needs frequent
: valve clearance adjustments. This would not be necessary if I had
: hydraulic lifters, such as on my friends' DOHC-16.
: Is there any other reason why Honda used solid lifters as opposed to
: hydraulic? Are there other pros & cons that would recommend solid lifters?
: Steve
No, you can't,
Solid cams/lifters give you faster revs & more horsepower due to cam
design...
ADZ
Bob Gilbert
->
->Finally, I wonder if it's possible to fit hydraulic valve lifters on a
->SOHC 4-valve head. I have a SOHC 16-valve inline4 and it needs frequent
->valve clearance adjustments. This would not be necessary if I had
->hydraulic lifters, such as on my friends' DOHC-16.
->Is there any other reason why Honda used solid lifters as opposed to
->hydraulic? Are there other pros & cons that would recommend solid lifters?
I believe that Nissan uses a hydralic lifter in their Maxima engine (and
others I'm sure).
-Bob
Bob Gilbert
->jpramod@cehpx3 (Pramod John) writes:
->
->> I am curious as to what the difference is between a DOHC and an SOHC
->>engine (yes I understand physically what is different. the question is why
->>though.) It would seem that you could drive 16 valves using one camshaft
->>rather than two, so why bother with two ? Or i guess you could even drive
->>two camshafts off of one pulley connected to the timing belt/chain rather
->>than the two which seems to be the norm . Why ?
->
-> It's mostly packaging and mechanical efficiency. In short, a
->single stick for each head means mounting it between the two rows of
->valves, and using rocker arms to actuate the valves. With two sticks per
->head, the cam can be placed directly over the vlaves and actuate them
->directly. At higher engine speeds (> 4000 RPM), the stiffness of
^^^^^^^^^
reciprocating mass
->components like rocker arms starts to limit the performance of the valve
->train, so the 2-cam setup performs better up there.
Otherwise, a very correct answer.
-Bob
Bob Gilbert
->Steve Won (bc...@FreeNet.Carleton.CA) wrote:
->
->: Finally, I wonder if it's possible to fit hydraulic valve lifters on a
->: SOHC 4-valve head. I have a SOHC 16-valve inline4 and it needs frequent
->: valve clearance adjustments. This would not be necessary if I had
->: hydraulic lifters, such as on my friends' DOHC-16.
->: Is there any other reason why Honda used solid lifters as opposed to
->: hydraulic? Are there other pros & cons that would recommend solid lifters?
->
->: Steve
->
->No, you can't,
->
->Solid cams/lifters give you faster revs & more horsepower due to cam
->design...
Yes, you can.
The problem is that it tends to counter the benifit of the reduced complexity
and mass of a solid lifter.
-Bob
Dan Cyr
I believe Honda uses solid lift in their engines to facilitate higher rpm
with the SOHC engines. For example the Vtec variant engine in the Civic
hatchback (USA Si model) revs past 7200rpm, which is pretty high for a
SOHC engine. I dont know if they coulda gotten away with hydraulic lift
in that application. I dunno about your accord however. Perhaps a way to
get more money from scehduled maintanence???
dan
ps. they dont get much from unscheduled????
Steve Won
In a previous article, c...@u.washington.edu (Dan Cyr) says:
>>In article 1...@freenet.carleton.ca, bc...@FreeNet.Carleton.CA (Steve Won) writes:
>>->
>>->Finally, I wonder if it's possible to fit hydraulic valve lifters on a
>>->SOHC 4-valve head. I have a SOHC 16-valve inline4 and it needs frequent
>>->valve clearance adjustments. This would not be necessary if I had
>>->hydraulic lifters, such as on my friends' DOHC-16.
>>->Is there any other reason why Honda used solid lifters as opposed to
>>->hydraulic? Are there other pros & cons that would recommend solid lifters?
>
>I believe Honda uses solid lift in their engines to facilitate higher rpm
>with the SOHC engines. For example the Vtec variant engine in the Civic
>hatchback (USA Si model) revs past 7200rpm, which is pretty high for a
>SOHC engine. I dont know if they coulda gotten away with hydraulic lift
>in that application. I dunno about your accord however. Perhaps a way to
>get more money from scehduled maintanence???
>ps. they dont get much from unscheduled????
You may be right about the higher rpms, but I wonder what kind of
lifters Honda stuck in the DOHC VTEC of the Del Sol. (8200rpm redline, 1.6L
w/160hp)
Concerning scheduled valve adjustments by Honda dealerships: No way!
Those idiots once gave back my friend's car with "adjusted" tire pressures
of 18, 18, 37, 43psi on the 4 tires! I'd never trust them to do a valve
clearance, especially since meticulous conditions, and a COLD engine are
required. I've done it myself once.
If anyone's interested, here are my opinions on what happens to engine
breathing and power if you neglect to adjust the valves. I figure the
valve seats recede due to normal wear, so lift
affectively is increased. This leads to better high rpm power, but lower
charge velocity=poorer low end torque. If you don't adjust your valves
frequently enough and the valve clearance gets depleted, then the valves
may no longer completely close, and you'll get loss of compression and
burnt exhaust valves. This will be accompanied by engine detonation due
to the burning exhaust valves pre-igniting the mixture.
Also, using synthetic oil (reduced wear) will not allow me to extend the valve
clearance intervals because the valve-seat recession takes place from
within the combustion chambers.
Steve
--
Mechanical Engineering, Queen's University. '94
Car tinkering: "Less weight means more performance!!!"
MTN Biking: "You know you're having fun when things begin to hurt/break."
James Carriere
the business about ease of breathing, it is all true, but in a
textbook sort of way. Nothing can replace an engine designed with
care, especially fancy dancy engineering. The engine in my car is a
2.1 litre carburetted, and tuned for torque. The figures, SAE net:
122 lbs. ft @ 2500 rpm
102 hp @5250 rpm
redline: 6500 rpm, but this much revs is only useable with the more
tuned version (more expensive) of my engine
The torque produced at best power is about 83% of the peak torque-not
bad for an engine tuned for low/mid range.
The engine was first available in 1975, it is a sohc 8 valve with
solid lifters that require adjustment about once every year. There
are no rocker arms, all eight valves lie in line right below the cam,
and you adjust by replacing shims on top of inverted bucket lifters.
This is the same design used on the first inline 4 Porsche engines of
around 1980.
My car? A plain as vanilla ice cream '82 Volvo.
---------
James Carriere SECOND! year mechanical engineering,
class of '97, Carleton University, Ottawa, Ontario, Canada.
"Two men claim they're Jesus. One of them must be wrong."
-Dire Straits
both of these work jcar...@chat.carleton.ca
am...@freenet.carleton.ca
James Carriere
Steve Won
In a previous article, jcar...@chat.carleton.ca (James Carriere) says:
>Surprisingly to me, I find that the clearance on the intake valves
>requires more adjustment than the exhaust do. Probably because the
>exhaust valves are built out of a way tougher material. I can only
>guess which type of valve would burn more easily if both were equally
>at negative adjustment (not closing).
The exhaust valves always run hotter, and in racing applications, usually only
the exhausts are sodium cooled. The intake valves are cooled by the fresh
intake charge.
>As for perceptible change in torque due to tinkering with valve
>adjustments, I didn't notice it on my car this year when I did my
>valves, and I reduced the clearance by .010", which is a whole lot. I
>tried tinkering with the valve clearance on my old car, and couldn't
>tell any difference, so I thought better of it and set them back to
>spec.
>All in fun, and I learned some things too :-)
Keep up the experimentation! And I'm sure we're all interested in
anything else anybody's tried.
As for reduced valve clearance, .01" is a lot. This reminds me about
what I've heard of valve jobs (porting). Apparently it's a trial&error
thing where results aren't always as expected (needs experience and
experimentation). So reducing the lift might have a different affect on a
different car than yours. Anyways, you were talking about the intake
clearance right? Cause doing it to the exhausts shouldn't help anything.
James Carriere
> If anyone's interested, here are my opinions on what happens to engine
> breathing and power if you neglect to adjust the valves. I figure the
> valve seats recede due to normal wear, so lift
> affectively is increased. This leads to better high rpm power, but lower
> charge velocity=poorer low end torque. If you don't adjust your valves
> frequently enough and the valve clearance gets depleted, then the valves
> may no longer completely close, and you'll get loss of compression and
> burnt exhaust valves. This will be accompanied by engine detonation due
> to the burning exhaust valves pre-igniting the mixture.
> Also, using synthetic oil (reduced wear) will not allow me to extend the valve
> clearance intervals because the valve-seat recession takes place from
> within the combustion chambers.
I agree with the above but have some of my own stuff to add. In my
old car the valve clearances would typically increase, I think due to
the adjusters being slowly vibrated out of adjustment faster than the
valve seats would wear down. In my current car, there are no rocker
arms or adjustment screws to turn, just shims to replace at the top of
the lifters. The adjusters don't go out of adjustment; there is a
bath of oil that stays on the lifters and cam after the engine is
turned off, and the valve train is very short, not like the pushrods
and stuff in my old car. The only wear is in the valve seats.
Surprisingly to me, I find that the clearance on the intake valves
requires more adjustment than the exhaust do. Probably because the
exhaust valves are built out of a way tougher material. I can only
guess which type of valve would burn more easily if both were equally
at negative adjustment (not closing).
As for perceptible change in torque due to tinkering with valve
adjustments, I didn't notice it on my car this year when I did my
valves, and I reduced the clearance by .010", which is a whole lot. I
tried tinkering with the valve clearance on my old car, and couldn't
tell any difference, so I thought better of it and set them back to
spec.
All in fun, and I learned some things too :-)
---------
A.Srinivasan
|> I remember reading that the new engine in the Maxima has solid lifters
|> instead of the previous hydraulic ones. Solid lifers that lie
|> directly under the camshafts are (usually) without rocker arms and are
|> adjusted by removing a shim from the lifter, measuring its thickness
|> with a micrometer, and replacing it with a more suitable shim to
|> achieve the desired clearance between the cam lobes and the lifter
|> when the valve is fully closed. These adjustments are required about
|> once a year. Other brands that have been using this setup in some of
|> their engines for years now include Porsche (inline 4 cyl) and Volvo
|> (inline 4 cyl) and probably many others.
|>
|> The way I understand hydraulic lifters to be constructed, and anyone
|> who really knows what they are talking about should correct me, is
|> like this: The inside of the tappet has a space that can increase or
|> decrease in volume. The less volume, the more valve clearance, and
|> vice-versa. The volume is kept at an equilibrium by the engine's oil
|> (hence hydraulic) oil pressure and some of the oil escaping every time
|> the valve is smacked open by the cam lobe whacking into the lifter.
|> The equlibrium point is designed to achieve a desired and regular
|> valve clearance.
|>
|> Engines usually have an oil pressure relief valve that opens up once
|> the oil pressure has reached its maximum design pressure. This
|> usually happens around 2000 engine rpm. Since the oil pressure
|> doesn't increase at all past this point, then there is an explanation
|> why hydraulic lifters do not perform well at high rpms-because the oil
|> loss in the lifter is not compensated fully by the oil pressure, so
|> that equilibrium clearance decreases and thus the valve does not open
|> fully anymore. I would guess that around 6000 rpm the average
|> hydraulic lifter ends up reducing clearance by an order of around
|> .050", based on my knowledge of cam profiles relating to torque
|> curves.
|>
|> This is a great discussion, let's get some more real world info going.
|>
|> ---------
|> James Carriere SECOND! year mechanical engineering,
|> class of '97, Carleton University, Ottawa, Ontario, Canada.
|> "Two men claim they're Jesus. One of them must be wrong."
|> -Dire Straits
|> both of these work jcar...@chat.carleton.ca
|> am...@freenet.carleton.ca
A good lifter is designed to achieve equilibrium at idle rpm.
So the lifter should charge up with sufficient oil to take up
any valve clearance, as long as the rpm is > idle.
The reason hydraulic lifters perform worse than solid at high
rpm is because they must get charged and discharged during each cycle
and the faster the rpm, the less the charging time.
Again, good lifter design should compensate for this by keeping
as much oil as possible IN the lifter at all times.
The only problem I have seen with hydraulic lifters is that
they can take a little time to charge up after a cold start
and if you rev your engine before properly warming it up, you
can cause excessive lifter wear.
Anyone care to second (or disapprove of) the last observation?
-- Arvind
Joe Hunter
: These adjustments are required about
: once a year. Other brands that have been using this setup in some of
: their engines for years now include Porsche (inline 4 cyl) and Volvo
: (inline 4 cyl) and probably many others.
Once a year sounds a little excessive. I happen to own a Porsche 944 and
a Volvo Turbo. The 924 does require shims for adjustment but the 944 does
not. I've never adjusted the valve clearance on my Volvo and it is still
running well and passes the smog tests (by a large margin) every year.
The Volvo has 136,000 km (85,000 mi) on it.
: the oil pressure has reached its maximum design pressure. This
: usually happens around 2000 engine rpm. Since the oil pressure
: doesn't increase at all past this point, then there is an explanation
: why hydraulic lifters do not perform well at high rpms....
: ..... I would guess that around 6000 rpm the average
: hydraulic lifter ends up reducing clearance by an order of around
: .050", based on my knowledge of cam profiles relating to torque
: curves.
Porsche has used hydraulic lifters in their race cars at high-rpms with
no problems. The recent models of 911 engines now have hydraulic lifters.
The new Ferrari F355 which produces 380 bhp uses hydraulic lifters to
operate the 5 valves per cylinder. The engine is rev-limited to 8500 rpm
but the entire valve-train is said to be safe up to 10,000 rpm!
I doubt the lifters on the Ferrari are of any special or new design. The
racing Porsches I know are not.
Joe Hunter
Joseph William Chan
>-> It's mostly packaging and mechanical efficiency. In short, a
>->single stick for each head means mounting it between the two rows of
>->valves, and using rocker arms to actuate the valves. With two sticks per
>->head, the cam can be placed directly over the vlaves and actuate them
>->directly. At higher engine speeds (> 4000 RPM), the stiffness of
> ^^^^^^^^^
> reciprocating mass
>->components like rocker arms starts to limit the performance of the valve
>->train, so the 2-cam setup performs better up there.
>Otherwise, a very correct answer.
>-Bob
Greets People,
Apart from the 'mechanical efficiency' aspect of the DOHC, I thought the
idea was to increase valve inclination, improving volumetric efficiency ?
Or so my propulsion lecturer claims ..?
Joe
--
Joe Chan,
s93...@minyos.xx.rmit.edu.au
James Carriere
> Porsche has used hydraulic lifters in their race cars at high-rpms with
> no problems. The recent models of 911 engines now have hydraulic lifters.
> The new Ferrari F355 which produces 380 bhp uses hydraulic lifters to
> operate the 5 valves per cylinder. The engine is rev-limited to 8500 rpm
> but the entire valve-train is said to be safe up to 10,000 rpm!
> I doubt the lifters on the Ferrari are of any special or new design. The
> racing Porsches I know are not.
> Joe Hunter
Impressive. I doubt that the hydraulic lifters in my mom's K-car
would stand up to 10,000 rpm, though. I agree that the lifters on
Ferraris and Porsches are not of any special design, but I think it is
safe to say that they are built to tighter tolerances than other more
mainstream brands.
As for yearly adjustments, the I thought I remember the maintenance
schedule saying that, but it seems that it is way overcautious. I
have only had my Volvo for less than two years, and I have put
40 000 km on it with one valve adjusment. However, when I bought it
it had 250 000 km already, and the records with it were not detailed
enough to mention the valve adjustments.
Good useful information, though. Fancy design can't really replace
design with attention to detail.
Bernd Felsche
s93...@minyos.xx.rmit.EDU.AU (Joseph William Chan) writes:
>>->
>>-> It's mostly packaging and mechanical efficiency. In short, a
>>->single stick for each head means mounting it between the two rows of
>>->valves, and using rocker arms to actuate the valves. With two sticks per
>>->head, the cam can be placed directly over the vlaves and actuate them
>>->directly. At higher engine speeds (> 4000 RPM), the stiffness of
>> ^^^^^^^^^
>> reciprocating mass
>>->components like rocker arms starts to limit the performance of the valve
>>->train, so the 2-cam setup performs better up there.
>>Otherwise, a very correct answer.
>Apart from the 'mechanical efficiency' aspect of the DOHC, I thought the
>idea was to increase valve inclination, improving volumetric efficiency ?
>Or so my propulsion lecturer claims ..?
Hmmm... seems like a typical academic over-simplification to me.
More valves => less lift required for same curtain area, higher engine
speeds are possible. The curtain area also increases more "rapidly"
with respect to crank angle, allowing more nett airflow.
A "cross-flow" setup allows for more generous air passages in the head
and the manifolds need not be as "crowded" and restrictive.
Inclination of the valves to allow airflow to "wash" down the cylinder
walls is of benefit (one of the reasons for 2-stroke intake efficiency
where the side-port air uses the piston crown as a guide). The result
_can_ be and increase in volumetric efficiency, especially at wide-open
throttle. [ A thought crossed my mind that engineers only ever test
engines at WOT during their studies. Is this to turn us into
motor-heads? :-) ]
Marked improvements can be achieved in volumetric efficiency any valve
inclination at all, such as in 1.8 and 2 litre 8-valve VW engines.
Reduced valve-train power losses more than make up for the slight
penalty to flow efficiency incurred by that valve configuration in
"average service". Most motor cars tend to be driven at part-throttle
most of the time. [ Notable exceptions being engineers' cars! :-) ]
I could go on (and will unless forcibly restrained :-)) with quite a
few other reasons. The above are by no means definitive answers.
Something to think about:
Given that in-cylinder swirl leads to better fuel-air mixing and hence
combustion, explain why typical multi-valve engines have inlet on the
same side of the head and not diagonally opposed, on both sides of the
head.
--
Bernd Felsche, MetaPro Systems Pty Ltd
328 Albany Highway, Victoria Park, Western Australia
Phone: +61 9 362 9355 Fax: +61 9 472 3337
Dave Williams
-> Given that in-cylinder swirl leads to better fuel-air mixing and
-> hence combustion, explain why typical multi-valve engines have inlet
-> on the same side of the head and not diagonally opposed, on both
-> sides of the head.
Cost, mainly. That'd require another intake and exhaust manifold and
associated plumbing.
It has been done - recently by some of the Lancia rally cars, long ago
by American hot rodders, viz some of the Frontenac and Winfield
conversion heads for the Model A and B Fords.
==dave.w...@chaos.lrk.ar.us=============================DoD #978==
== JOHNSON TOOLS - you'll always get your nut with a big Johnson! ==
========================================John De Armond Fan Club #978==
Neal Gaborno
: Given that in-cylinder swirl leads to better fuel-air mixing and hence
: combustion, explain why typical multi-valve engines have inlet on the
: same side of the head and not diagonally opposed, on both sides of the
: head.
I tried to visualize what the intake and exhaust manifolds would look like...
it would definitely present a packaging problem with an inline (intake plenums
and exhaust manifolds on each side, maybe?), and would be a nightmare on a
V-engine (exhaust manifolds on top of the engine, or perhaps draping over
the valve covers). The intake manifold(s) would probably soak more heat from the
exhaust manifolds, too. Just my humble, ameteur opinion.
Neal
Bernd Felsche
>Bernd Felsche (ber...@metapro.DIALix.oz.au) wrote:
BTW: I erroneously stated that a multi-valve cross-flow head allowed
for greater volumetric efficiency, especially at WOT. This is of course
nonsense because volumetric efficiency is only important at WOT - as a
fellow reader reminded me. I believe that I meant that it allowed for
greater volumetric efficiency, especially at _higher engine speeds_.
I promise to never ever post again until after my first cup of coffee
in the morning. :-(
Take a look at the VR6 engine from Volkswagen, only one inlet and one
exhaust manifold. It's a 6-cylinder Vee engine with 15 degrees between
banks, 2 camshafts and ONE head which has air tracts for exhaust and
inlet cast into it.
A wider-vee engine can therefore still be done with the same number of
external manifolds, usually 3, and an inline engine with 2 manifolds.
[ I'm not volunteering to design the resulting "bowl of spaghetti"
inside the heads! That is left to the "student" as an exercise. :-) ]