DOHC vs Push Rod

[svsgt1]

I have a question for you guys. To put it plainly, why are Overhead cams better then push rods. I'm sure they are better but exactly why is that the case. I mean everyone knocks the Chevy V8 for being old school technology because of the pushrods but then again how long ago were over head cams invented. But seriously, I coudl imagine that there would be less friction and movement with overhead cams as opposed to pushords, but then again it adds a good amount of mass to the top of the engine, which may not be good if you really want to try and make a race car and lower the center of gravity. The Saleen S7 and Viper both use pushrod engines and they seem to make good power and be good cars.

Then this discussion will go into the 1UZ vs the chevy engine. Do the overhead cam make it possible to do 4 valves per cylinder and is that why chevy's only have 2 or is that just because thats the way they make them? Then it will go down to the four bolt mains but I have seen after-market chevy blocks with 8 bolt mains, and some that are made out of all light weight aluminium and weigh less then the 1UZ. So I guess I'm also trying to figure out why everyonoe always knocks the engine.

I'm by no means an advocator of the chevy engine as my race car has a 1uz in it. I just feel that everyone knocks these cars and engines too much because they are american or for some other stupid reason when in fact they can be great motors. I would say that from the factory that the 1UZ is a "better built" engine and and probablly better designed but the chevy engine can be done up much better then what the 1UZ can be in my opinion. But that is probablly unfortunatley true as we don't have many engine upgrades or aftermarket parts for our 1UZ's. But before everyonoe goes anal about how much better the 1UZ is then any piece of crap american engine (sarcasim of course) I really would like to know overhead cams are better. I have seen an overhead cam conversion for the LS1 and supposedly it gives impressive horsepower gains. But I don't know much about it.

 

[Zuffen]

The benefits of overhead camshafts were put to use in the period 1910-1915. I can't remember the exact date or manufacturer but there were quite a few DOHC engines in production in the 1920's.

The principal benefits of DOHC engines are lower reciprocating mass in the valve train. As the cam doesn't have to lift the follower, push rod & rocker (and the valve spring have to push them back down) the engine can pick up revs faster and also rev harder without valve float. To overcome the problem of the mass of the valve train in a conventional engine they use very strong valve springs. These valve springs cause a loss in power output due to the power needed to turn the cam to open the valve.

A Lexus has valve spring pressures of about 1/3 of that used in a typical pushrod engine. It can use lower pressure springs as it doesn't have the mass of the valve train to overcome.

You will notice people lust after Titanium valve gear due to its lightness. This takes the concept to the next level.

One of the many downside of pushrod engines is the sronger the valve spring the more likely the pushrod is to bend. To overcome this problem they use thicker/stronger pushrods, which increases the forces involved. Quite often you will see performance pushrod engines with pushrod guides/retainers in an effort to stop the pushrod deflecting and failing.

Few pushrod engines will rev past 9,000rpm unless they are very small capacity whilst DOHC engines are known to rev to 19,000rpm. I know Fuel dragsters are an exception but their pushrods would be 1/2" in diameter

Another problem with pushrod engines is expansion of the pushrod when hot and closing up the valve lash. This means they can be noisy when cold.

You can have 4 valve pushrod engines but they tend to use on pushrod for the inlet that acts on a rocker that then acts on 2 valves. Same with the exhaust. Because of this complexity pushrod engines tend to be single valve. Because they are single valve the valves tend to be big and heavy meaning more load on the valvetrain.

Dollar for dollar it wouldn't cost much more to have DOHC than a pushrods in a new engine design.

All that and I didn't suggest the Chev is a bucket of bolts. It isn't it fills a need in the market and has done so for quite a while. I just won't own one thanks.

I don't think you will see another pushrod engine developed by any manufacturer other than Ford, Chrysler or GM. Americans like simple engines that do the job. Maybe the price of fuel will kill off the dinasours?

 

[Cobra]

Having a quiet day are we Rod?

 

[rarson]

Not to mention, you also get more flow area with less valve shrouding on a 4-valve head than a 2-valve head. The only real downside to DOHC (aside from double the valvetrain parts, in other words, $$$) is the timing belt/gear/chain setup, due to the fact that it has to be much larger.

 

[svsgt1]

wow, that was actually the best reply one could give. Know I see that over head cams are a better design then pushrods as I always knew they were. Although you have a point of the less mass in the OHC engines as opposed to the pushrods, I don't really always see the reality of the advatanges. Such as the higher revs, My 1UZFE will do about 7000rpm while the new 427 LS7 will do the same and it is a much larger engine with much larger valves. I guess that is why they use the titanium connecting rods and springs. I guess that further proves the point that no matter how good your equipment is, it really doesn't matter for street use. Now for race use and all out performance I fully agree about the overhead cams as the F1 engine are now doing over 20k rpms in their new V8 design. (Of course they have pneumatic valves)

Actually I think the correct response to American liking simple thiings is not really accurate. It is partially true it is just not an important thing to us whether the engine has DOHC or Pushrods. It really boils down to performance and in all honestly, plain horsepower. Unfortunately we don't concentrate on things like power to weight ratios, just horsepower. Different cultures I guess.

 

[Peewee]

Zuffen has pretty much summed it up there.
I'll say a few more points.

You mentioned the Viper
8.3L - 500hp.
Saleen S7
7.0L - 575hp
Other pushrod motors for comparison
LS7 - 7.3L, 500hp, 485ftlbs
LS2 - 6.0L, 400hp, 400ftlbs

Out of all of those, the most impressive is the Saleen @ 82hp/L (62kW/L)
(The website I found also quoted 81ftlbs/L (or 110Nm/L, but that has to be bullsh!t)

These motors are fairly to very worked (while being safely tuned), and they are far from 100hp/L

Now look at the STOCK 1UZ
250hp - 4.0L
62hp/L
This is COMPLETELY unworked.

Throw some cams of similar specs as the Saleen, some extractors, bigger throttle body, and some tuning, and that 62hp/L will increase to over 90hp/L without even struggling, wouldn't surprise me to go over 100hp/L.
I got up to 75hp/L on STOCK cams.
I'm aiming for as close to 100hp/L when I get my cams (hopefully later this year)


Some people will laugh when you start talking hp/L figures, but thats where I get impressed.

A 7.3L motor making 500hp (with all the fruit that the LS7 has, CNC heads, Titanium valves, 15mm lift on the cams (1UZ stock is 7.35mm) dry sump, etc etc), is no where near as impressive as a a 4.0L motor making 400+hp
Some places around the work have pulled more than 550hp from an NA 1UZ
Thats almost 140hp/L.


Pushrod motors are good at doing what they were designed for.
Making cheap, reliable power.

 

[Peewee]

Oh, another thing about the 6-bolt main.

It the 1UZ had only 2 bolts, it would probably self destruct at about 300hp
4 bolts, it may last 600hp.

Basically, being an big aluminum block, 6 bolts is NEEDED.

4 bolt mains on an iron block would be stronger then 6 bolt mains on an alloy block.

 

[svsgt1]

Peewee,

the hp/l figure is impressive as you state. Thats why I like the Subaru STi as it makes 300hp with only 2.2L 4 cylinder (I think that is the right size). Of course it is turboed but that is still very good for a production car. But I still wouldn't buy one even though it is very impressive.

I have cams, headers/extractors, intake, and slight head porting and I don't think I have 400hp. I'm hopiing that the 8 individual throttle bodies woudl give me about that. Unfortunatley what you say about the 1UZ is also true about the other pushrod engines. You can easily put on an exhaust and intake on a corvette and viper and easily make more horsepower. My brothers Viper GTS is 450hp stock. He put on new headers and exhaust and is making over 500hp now and still has cats on the car. But that is pretty much standard on any production car. And of course the bigger the engine the more HP you seem to get easily with exhaust and intake. But 500hp is still 500hp regardless of the size. And to get 550hp out of an NA 1UZ must have cost an insane amount of money. I wish I could do that. ha ha ha

 

[Peewee]

It depends on your cams profile, and your tuning.

If you are still running factory ecu, or a cheap piggyback, then that it where the problem is.

Cams with around the 9.3ish mm lift, along with tuning, should be getting close to the 400hp make. Easy over 350hp.



I chose the motors stated, because those were the top spec motors those companies build.
Pretty much the only increases would be in tuning, as manufacturers need to build a safety margain into tuning.
(I was going somewhere else with this, but completely forgot....)

And 500hp is not 500hp.
500hp from a hypo 2.0L turbo charged 4cyl, is nothing compared to 500hp from a blown LS1.
Overlay the 2 dyno graphs, and you'll see the difference instantly.

 

[svsgt1]

Well I have a LINK ECU in my car so the tuning should not be a problem. I am planning on getting it retuned here shortly so then I will have an exact idea of what type of power I'm making exactly. Here is a spec sheet of my cams. I don't know much about this topic so if you could tell me if they are good or not then that would be great. Or explain to me how to read these specs and what it all means.

 

[Peewee]

The 'remarks' sound about right.
Its just over 8.5mm lift, so it should be a very drivable cam. ("In Valve Lift" is the important one, along with Duration @ .050" - 206deg)

Its by no means a high performance cam, but you should not loose any drivability with it.

I'm going for 9.5mm lift, depending on if I want to convert to shim under bucket.
Anything over 9.5mm lift requires shim under bucket conversion, and then I may as well go 11.5mm lift.

 

[Lextreme II]

Peewee,

Would longer valve stem be able to compensate for the shims?

 

[Peewee]

Not sure Lex, I haven't looked at the exact setup of the 1UZ.
Best to ask a CamShop.

 

[Trvln Nalzmn]

The pushrod engines like the LS1 and Viper motors get their power and reliability from capacity because gas is cheap in the USA. If you doubled, or tripled the price of gas in the USA then the car makers would start to look for ways of lowering capacity and but still maintaining power.

 

[Zuffen]

Cobra,

Very quiet day!

Trvln,

You hit the nail on the head.

Whilst fuel remains cheap in the States (and by world standards it is!) they won't build fuel efficient engines.

I have never seen a thread grow so quickly and no one start arguing about the merits of the 1UZ over an LS1. Congratulations gents.

If pushrods really worked Shouey would have them.

 

[turboandrew]

About gas milage and pushrod V8's: some actually get great gas milage. A friend has a Z06 and he gets 19 mpg in the city and 28 on the highway (averaging 80 mph hwy), and this is not abnormal for that type of car. I really doubt an 1UZ equipped car is doing any better than that. Curse chevy all you want, one thing they have done right is making a easily packaged, high displacement, high torque, high HP, good milage engine. And I challenge anyone to take a look at the new LS7 and not call it a work of art.

Not that I don't love the 1/2/3UZ motors, but pushrods has about 1% to do with how these motors are going to fare on the street. The 1/2/3UZ motors -could- do this and -should- do that, but how many here are making 100hp/liter without turbos or superchargers? I bet not a single person on this forum. I personally don't care what the HP/liter is -the bottom line is how much power under the curve you are making -and you can do it on something that's steetable and reliable. I think both the 1UZ and the pushrod motors are equally capable.

 

[ChrisV]

"Pushrod motors are good at doing what they were designed for.
Making cheap, reliable power."


Funny thing is in ANY industry other than cars, and for any other product, this is enough. In most industries, and if you go out looking to buy products, you don't try to find the most complex, expensive way to deliver the end product, do you? It's rarely regarded as a great business move.

An engine's job is to move the car. How much power, where the power is made, and how much it costs to make and maintain, are all primary considerations. On the street, sky high redlines are simply not necessary for the majority of driving.

DOHC engines started being used in teh early 1910s, and by 1932, the US had a couple companies with DOHC, 4 valve per cyl, all alloy 8 cyl engines. In the '50s, advances in metallurgy due to the war had made thinwall pushrod V8s a possibility, and it was then easy to make a smaller, lighter, stronger engine that was less complex, less expensive, and more reliable than the overly complex OHC designs while making as much power.

Let's look at a coule of engines for comparison. The pushrod Chevy DZ302 in the '68-69 Camaro Z/28 and the K20 2 liter DOHC VTEC 4 cyl in the Honda S2000.

Both rev to 8k in stock form. But the larger Chevy made more power in total, and more poiwer under teh curve, as it had to move a car almost a thousand lbs heavier than the S2000. And it did so through a 4 speed transmission with 1:1 top gear ratio (limiting it's top end as much as teh aero did).

But, both accellerate to 60 and through the quarter at about the same rate, and both get about the same fuel mileage. The S200 would do better with more cubes, but the DZ would do better with less weight and more gears. Oh, and fuel injection instead of carbs..

So where has all that tech gotten us?

In street use, modern pushrod engines are low emitting, highly fuel efficient for their displacement, and full of almost as much electronic engine management tech (other than VVT, which is necessary on a small engine vehicle to get back the low end driving ability that high end cam lobes don't have. A problem larger displacement engines don't really expereince to the same degree). They are physically smaller for their displacement, less complex, and less expensive to build. If the problem is that they don't work as well for F1 racing, well, so what? I don't race there.

Sure, the OHC engine has lighter individual valvetrain weight, but with twice as many valves and twice as many cams, overall valvetrain mass is greater, and frictive surfaces (as in bearing and valve stem surfaces) is increased a BUNCH, as well as drive belt/chain friction. And physical engine size is increased to house the extra parts.

I'm using a 1UZ due to it being different than the Ford smallblock that would normally be used in a car like mine. It's the "coolness" factor of going where no one has gone. But not because of any sort of efficiency issues.

 

[ChrisV]

Of course, hp/liter is brought up. Hp/liter is important in racing classes where displacement is limited, or countries where displacement is taxed. Other than that, it's merely a sort of interesting engineering side note.

A small .049 r/c car/airplane gas engine makes 450-500 hp/liter. Do you want one powering your car?

hp/liter almost ALWAYS goes down as the displacement goes up. Just the nature of the beast, even in companies that are expert in coaxing hp from small displacement engines. Honda's 600 cc street bike engines make over 100 N/A hp. At that hp/liter rate, their 1800cc Type R engine should have made over 300 hp in stock, streetable, N/A form. The S2000, would have had nearly 400 hp. And that's comparing engines in sport models from the same company (one that is usually praised for high hp/liter numbers)

And interesting tidbit: the NEW S2000 gets worse hp/liter than the first S200s. They upped the displacement, but kept the same hp. What happened?

The answer is the other part of the reason hp/liter by itself is meaningless on the street. They wanted a better torque curve BELOW the peak figures. Increasing displacement may not make MORE power per liter, but it makes BETTER overall power and torque. A more useable torque curve means you don't get caught out of the powerband. It means flexibility in driving around town or out of low speed corners on the track. F1 cars get around that by having a million six gear ratios.

That Viper engine that was listed does make less hp/liter than a Ferrari F360. But it makes more torque at idle than the F360 does at peak. Think about the flexibility and "punch" that gives you at any speed, in any gear.

And funny, but they get about the same fuel mileage. When you make power, you use fuel, regardless of engine displacement. Modern American sedans, even ones with pushrods, get similar or better fuel mileage to direct competitors with OHC design engines.

 

[JBrady]

Good thread.

IMO the Lexus/Toyota engines are BUILT better than almost any other production engine. This creates the fantastic reliability that we L/T owners have come to expect. That said, the compexity of these engines means that repair WHEN it is needed is costly.

In addition to what is pointed out above we should ALWAYS consider power to weight AND power per fuel consumption. Another good consideration is power per physical dimension.

In the case of the LS1, LS2, LS6, LS7 family of GM engines vs. the 1,2,3UZFE family of Toyota engines the power to weight ratio winner is the LSx. The LSx is lighter and makes more power and torque.

In the case of BSFC (brake specific fuel consumption) or power per fuel consumed the combustion chamber burn speed and efficiency coupled with parasitic loss determines this. I do not have specific data but my guess is the xUZFE would win albeit by marginal amounts.

Power per physical dimension is important for packaging. The smaller the external dimensions the better the packaging options. The LSx is smaller than the xUZFE.

In stock form both (most of the variants) use aluminum block and heads. Both have 6 bolt main caps. Both have powered metal piston rods and hypereutectic pistons. The xUZFE has a forged crankshaft vs. the cast LSx. (again these are general statements as the LSx truck engines are cast iron as is the block on the 2UZFE)

Both engine families are GOOD. The xUZFE has more HP/TQ per liter potential but the LSx is BIGGER so it needs less per liter potential to remain more powerful. The xUZFE has a higher RPM potential but few exceed the RPMs turned by MANY LSx engines currently.

Honestly, I would and have considered and MAY yet swap in an LSx engine in the Toyota :bigeyes:

 

[xirforever]

What about reliability? In both stock forms of each engine family wouldn't the xUZ's be a lot more reliable? Ive never heard of an LSx going 500,xxx miles plus.... but then again you guys could probebly tell me more about it and/or end up correcting me...

But also the aim for Lexus was to be a nice powered motor to complement the CAR. I think in the camero's and whatenot the car just complemented the ENGINE, in other words.... GM went for all motor no doubt, as lexus did not.... but lexus ended up doing a great job without that even being their primary aim. Immagine if lexus/toyota made a 5.7 liter motor?