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Old 06-09-2003, 07:49 PM   #1
Neutrino
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Question Pushrod vs DOHC

Ok from what i know a DOHC setup is complettely superior to a pushrod one....it alowes for faster reving less moving parts and you can tune the exaust and intake timing and lift independently......



so why do american big V8 still use pushrods...i mean american companies obviously have the technology for using DOHC but they keep insisting on pushrods....even high tech modern V8's as the LS6 or the new 5.7 hemi still use them.....i guess only the quadcam in the new mach 1 and the cobra uses a DOHC setup


is there some advantage of using pushrods that i'm not aware of.....are they trying to keep the design more simple or keep the prices down....or are they afraid of making those engines too powerfull
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Old 06-10-2003, 05:57 AM   #2
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The LS6 and others use Pushrods for several reasons:

1, On a low revving engine large capacity with a flat torque curve and lots of grunt there is little advantage to be gained from useing an over head cam set up.

2, its simple. Two heads and 8 cylinders = 4 cams, AND lots of moving parts, which means lots of extra expense to manufactor, especialy when you are having lots of differnt versions of the same engine.
(note Lotus designed Twin Cam heads for the LS1, which was used in the Elise GT1, I think a special version of the Corvette, and as expensive extras from Chevy.

3, Americans are old fashioned, and speaking generaly somewhat naive when it comes to how and engine works, and what it means to be American. As a general market enough ppl assicaite lots of technology like Twin Cams with non American products, i.e. Cars from Japan.
Also a lot of American V8 owners like to tune thier engines, with things like new Cams etc.
By useing a single Cam pushrod engine Chevy keeps it "American" and makes it cheap and simple to add a new aftermarket cam. (think what 4 new cams would cost!)
(NOTE: This comes from a GM Marketing survey, and comments made by a GM engine designer to the Australian Motor Magazine)



4, Space. Twin cam heads simply take up a lot of space, especialy when you have two of them on an already large engine. Its ok in the frount f a truck, but when you want to squeeze it into a Van, or a Corvette?


Quite simply Chevy, stuck with greater displacement as and simple head and valve train design to produce more than acceptable amounts of power.
Quite simply its how thier enginers and company philosophy likes to build engines. It has its disadvantages, but it also has it advantages, but most importantly it works, and it works very well.
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Old 06-10-2003, 06:18 AM   #3
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Re: Pushrod vs DOHC

Quote:
Originally posted by Neutrino
Ok from what i know a DOHC setup is complettely superior to a pushrod one....it alowes for faster reving less moving parts and you can tune the exaust and intake timing and lift independently......
nope, more moving parts in a DOHC setup, MANY more, compare the LT5 to the LS6, 3 more cams, twice as many valves, springs and rockers (I believe it uses rockers anyway, if not then lifters) The comparable friction is completely lopsided, the DOHC setup will lose MUCH more power through friction, and that slows down the speed at which the engine revs, in this case DOHC is the worst setup available, in terms of friction, engine acceleration and power loss.



Quote:
do american big V8 still use pushrods...i mean american companies obviously have the technology for using DOHC but they keep insisting on pushrods....even high tech modern V8's as the LS6 or the new 5.7 hemi still use them.....i guess only the quadcam in the new mach 1 and the cobra uses a DOHC setup
because believe it or not, pushrod setups are JUST as good within their operating range. They may not be able to spin as high, but look at the size of the engines, there's no need to rev over 6500 RPM and if they wanted to, they could anyway. Pushrod engines are lighter, are less complicated, have less valvetrain friction and less moving parts. The engines are shorter in height as well being the single cam is housed in block, this makes for a lower hoodline and lower center of gravity. They cost less to make, and cost less to maintain and fix, did i mention they're lighter? by a good 40-60 lbs, all from the nose of the car.

Look at it this way, the Z06 can lap and run with the lighter 360 modena and heavier 911 turbo, get better gas mileage then both and has less aerodynamic drag, why? its engine is smaller in exterior size, it's likely lighter then the 360 modena V8 although i don't have its specs because of the fact that pushrod engines have less components, less heavy ones at that. GM has one of the BEST DOHC V8s around, the northstar, while you havent heard anything about it it's been in numerous SAE articles for its capability, now if they have one of the best DOHC motors around, why wouldn't they put it in the corvette? Dave Hill will do ANYTHING to lap faster and run better, tradition be damned, if the DOHC engine would work (and there isnt any other DOHC engine that exists thats actually better then what GM has available) better why wouldn't he use it?

As for valve timing, well thy can use single cam variable valve timing to help keep the powerband flat but its much less flexible then the DOHC versions, this is one place where DOHC has advantages. GM has a dual in-block cam motor prototype that runs, produces 450 HP & TQ, passes all emissions tetsing if bodied in a Fbody or corvette type car (weight and aeerodynamics mostly) and has variable valve timing. The motor uses a greater then 2V per cylinder arrangement though how many noone knows, likely 3V. Maintains all of the advantages of a pushrod setup and the second biggest advantage of DOHC, the first being its high RPM stability of course, beauty is in a 6.4L motor you dont need high RPMs.

Both engines have their place and i'm not knocking DOHC, but its far from better in every way, in alot of ways it's worse.

BTW moppie, the Lotus/GM designed engine was the LT5, found in the ZR-1 corvettes from 90-95, great engine as the car held the 24 hr speed record for like 14 years until the W12 supercar beat it, but it was terrible for a daily motor, rebuilds cost 10K+, the valvecovers from one engine cant be swapped onto another for some reason, i forget what, but it has to do with the valvetrain falling apart if it isnt the same. Also was very finnicky, required alot of chain tensioners and guides, the first 2 or 3 years were known for needing an overhaul after 40-50K because the chains would loosen and rattle around. Nice design overall but you'll notice its the only time GM ever let lotus design a motor with them
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Old 06-10-2003, 09:37 AM   #4
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Assuming you're comparing similar engine setups (2-valve per cylinder), a pushrod engine would have fewer camshafts, but add in the additional hardware that moves the valves and you'll have more parts in the OHV engine. Of course a 4-valve/cylinder engine is going to have twice as many valves...but compare apples to apples.

An OHV engine is typically smaller. In the late 1980s, Lotus was developing a DOHC setup for the Corvette engine but it proved too wide to fit in the engine bay of the car (Corvette's get their engines from the bottom and the engine wouldn't fit between the frame rails). Lotus needed to develop the DOHC LT5 engine from the ground up instead. And the reason why the LT5 costs so much to rebuild is that it's a low-volume engine...not necessarily because it's a DOHC engine.

As for OHC setups not being well suited to vans (I've already answered the Corvette problem), Ford uses SOHC engines in their Econoline/Club Wagon vans. GM does not (yet) use any OHC engines in their larger trucks and Dodge's van is going out of production, so there's no need to put the newer OHC engines in it. There would be no problem placing a SOHC or DOHC engine in a van...no more of a problem than a big OHV V8, which have been available.

Ford, Cadillac, Oldsmobile (until recently), and a number of imports use DOHC V8 engines. Ford and Dodge trucks use SOHC V8 engines (Dodge also has OHV V8s).

That survey quoted by GM, as with most all marketing materials, is to support the idea of GM's continued use of OHV engine in the face of global competition moving toward OHC designs. Using the small-block design saves GM money because they haven't had to design an all new engine in fifty years. If I were in business and the world asked me why I did a certain thing that seemed counter to the market, I would provide some sort of research backing my decision as well.
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Old 06-10-2003, 05:21 PM   #5
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Quote:
Originally posted by Hudson
Assuming you're comparing similar engine setups (2-valve per cylinder), a pushrod engine would have fewer camshafts, but add in the additional hardware that moves the valves and you'll have more parts in the OHV engine. Of course a 4-valve/cylinder engine is going to have twice as many valves...but compare apples to apples.
but its also got 3 more cam gears, springs, retainers and rockers on top of the valves, which in any case the valves are unnecessary unless you're going to use higher RPMs, the current LS6 heads flow more power then they can use up until 6500 RPM, people are making 30-50+ HP on just a cam swap as thats how GM limited it. the OHV does have alot of moving parts, lifters, rockers, pushrods and springs, retainers etc. but theres less friction inherent.
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Old 06-10-2003, 05:37 PM   #6
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Quote:
Originally posted by FYRHWK1


but its also got 3 more cam gears, springs, retainers and rockers on top of the valves, which in any case the valves are unnecessary unless you're going to use higher RPMs, the current LS6 heads flow more power then they can use up until 6500 RPM, people are making 30-50+ HP on just a cam swap as thats how GM limited it. the OHV does have alot of moving parts, lifters, rockers, pushrods and springs, retainers etc. but theres less friction inherent.

yeah but doesn't the pushrod itself care a lot of inertia...i mean its quite long and pretty solid too.....
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Old 06-10-2003, 05:58 PM   #7
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OHC engines are much larger than OHV motors the Ford SOHC 4.6L V8 used in the current mustangs is actually physically larger than a 428ci big block u couldn't fit a 4.6L V8 into the old mustangs that had big blocks in them my dad and I were giong to buy a 4.6 from a mercury marquis to put into my sister's 67 mustang but the engine compartment was much too small for the motor and back then u could order a 428 for those mustangs.
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Old 06-10-2003, 08:14 PM   #8
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Quote:
Originally posted by Hudson


As for OHC setups not being well suited to vans (I've already answered the Corvette problem), ....................
Didn't say they dont fit, just that since they are smaller on top it's easier for them to fit. IT also allows more interior room inside the van, something I would consider more important than how many cams and valves the engine has.


Quote:
Originally posted by Hudson

That survey quoted by GM, as with most all marketing materials, is to support the idea of GM's continued use of OHV engine in the face of global competition moving toward OHC designs.
Yes, good point. Anyone that has done even a basic study of statistics should know that with the right manipulation you can have a survey represent anything you want.

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Old 06-10-2003, 08:15 PM   #9
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Quote:
Originally posted by Neutrino



yeah but doesn't the pushrod itself care a lot of inertia...i mean its quite long and pretty solid too.....
Not so much inertia, but engineers do have to take into account how much the pushrods will flex (another reason OHC became popular)
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Old 06-10-2003, 09:22 PM   #10
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Quote:
Originally posted by 76_cobra
OHC engines are much larger than OHV motors the Ford SOHC 4.6L V8 used in the current mustangs is actually physically larger than a 428ci big block u couldn't fit a 4.6L V8 into the old mustangs that had big blocks in them my dad and I were giong to buy a 4.6 from a mercury marquis to put into my sister's 67 mustang but the engine compartment was much too small for the motor and back then u could order a 428 for those mustangs.
Name an OHC engine that does not have OHV.
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Old 06-10-2003, 09:31 PM   #11
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One of the more important reasons is because the cost of R&D is simply too high, especially since GM's current line of engines doesn't really have anything to address. However, GM is developing a new line of engines with contemporary technologies like variable valve timing, DOHC, etc.

IIRC, it's not so much the reciprocating mass that keeps the RPMs so low, but the fact that 2-valve heads can't breathe at higher RPMs. The higher stroke (compared to displacement) compensates.

FYRHWK1 - inline 4s use only 2 cam gears, and the same amount of springs per valve. And I do believe you're using OHV and in-block camshaft interchangeably.
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Old 06-10-2003, 09:37 PM   #12
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Quote:
Originally posted by 454Casull

Name an OHC engine that does not have OHV.


I actualy know of one.
There was a company here in NZ that developed a totaly modular all alloy engine that used double over head cams and Side Valves.
Unforunalty it was mostly a front to steal grant money from the Government, and while the engine worked, it didnt exatly work as well as claimed, and of course the whole project was shut down, and the directors prosecuted.
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Old 06-10-2003, 09:47 PM   #13
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Quote:
Originally posted by 454Casull
One of the more important reasons is because the cost of R&D is simply too high, especially since GM's current line of engines doesn't really have anything to address.

Remember that GM is currently the worlds largest company, and has under its wing some of the worlds leading car manufactors, and access to a lot of technology.

Infact outside of the US the only GM car I can think of that dosnt use an Over head cam engine is the Holden, and they use engine designs sourced from the US anyway.

Lotus already makes an excellent and compant DOHC V8 engine that still has plenty of life left in it, and plenty of room for development, they also have the resouces and knowledge to design a larger V8 for GM, at very little cost, since they are some what desperate for the work.
SAAB is also world leader in engine deisgns, and while they have little experiance with V8's Im sure they could be of great help.
And of course GM currently has a deal going with Honda where GMs desiel technology (Izuzu) is being exchanged for Hondas Hybrid and lean buring engine designs.
They also have a good relationship with Toyota, and have done a lot of Platform sharing over the last 15 years, they have even invested a few $$ here and there in Toyota projects.


I think there must be more to it than R&D costs.
I think keeping it simple, and control and prediction of the market are far better reasons.
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Old 06-11-2003, 12:00 AM   #14
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Quote:
Originally posted by FYRHWK1
the OHV does have alot of moving parts, lifters, rockers, pushrods and springs, retainers etc. but theres less friction inherent.
Ahhh, but a pushrod engine requires a much higher valve spring force to achieve a particular operating speed... so you end up with a whole lot more friction for each valve. On a per-valve basis, a pushrod system also has way more moving parts, especially when compared to a direct-attack valvetrain.

Quote:
Originally posted by 454Casull However, GM is developing a new line of engines with contemporary technologies like variable valve timing, DOHC, etc.
I'm curious to know where you heard about this. Was it in a magazine somewhere? I can say (on a tip from a very reliable source) that you're correct, but I wasn't aware that the knowledge was so widespread.

Quote:
Originally posted by 454Casull yeah but doesn't the pushrod itself care a lot of inertia...i mean its quite long and pretty solid too.....
The pushrod does have some amount of inertia, but it's usually pretty small compared to some of the other components. It's also on the "light" side of the rocker arm (rocker ratio means that the pr moves less than the valve does, so its mass is relatively less significant). In order of significance to dynamic performance due to inertia, I'd rank the valvetrain components for a pushrod valvetrain as follows:

1) rocker arm (mass ~160gm, inertia ~130 kg*mm^2)
2) valve + retainer + top of spring (mass ~90gm)
3) hydraulic lifter (mass ~90gm)
4) pushrod (mass ~60gm)

In order of significance due to flexibility, I'd rank them as follows:
1) rocker arm or hydraulic lifter depending on system
2) the other one (of the two above)
3) pushrod
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Old 06-11-2003, 02:04 AM   #15
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Quote:
Originally posted by 454Casull

Name an OHC engine that does not have OHV.
all of them, the OHC monicker stands for overhead cam, meaning the cam is over the valves, the OHV setup obviously has the valves over the cam.

Quote:
yeah but doesn't the pushrod itself care a lot of inertia...i mean its quite long and pretty solid too.....
actually they're hollow, but it does become a problem at high RPM, not only the springing action (flex like 2 stroke said) but keeping it in contact with the rocker and lifter.

Quote:
Ahhh, but a pushrod engine requires a much higher valve spring force to achieve a particular operating speed... so you end up with a whole lot more friction for each valve. On a per-valve basis, a pushrod system also has way more moving parts, especially when compared to a direct-attack valvetrain.
That happens when you're causing a force to change direction nearly 180 degrees, it's the biggest downfall of the pushrod system i'll admit, and on a per valve basis it does have more friction, but overall power loss is still less, it's like the HP/liter argument, brings up some good points but in the end the overall is what matters.

Quote:
I'm curious to know where you heard about this. Was it in a magazine somewhere? I can say (on a tip from a very reliable source) that you're correct, but I wasn't aware that the knowledge was so widespread.
The new 3,6L V6, the XV8, the XV12 are all new engines that come with all of that stock, the XV8 & 12 come with cylinder deactivation (DOD) and are meant for large passenger sedans, if those are the engines he's talking about. Rgiht now what I want to know is how far GM and BMW are along in the solenoid valvetrain issue.

In terms of powertrain GM is probably the #1 company for cost effective powerplants and transmissions, even BMW buys automatics for their 5 series from them, they have acess to DOHC motors that are very potent and make one thats world class in its abilities, noen of them outperform the LS6 as of now and the newly designed LS2 will be even better in design, for their application pushrods are perfect.
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