somebody explain..
06-19-2006, 09:35 PM
#1
somebody explain.. hey guys, i was wondering if somebody could actually tell me exactly what modular means? and also how is the performance of dohc vs. sohc? and exactly how 4v helps in performance? i know the 99-04 stangs dont use pushrods.. but why is that exactly and what kind of performance does that do?
06-20-2006, 03:43 AM
#2
TECH Addict
Part 1 answer
Originally Posted by TXZ28LS1
hey guys, i was wondering if somebody could actually tell me exactly what modular means?
Modular is a term used as in theory all the different variations should be interchangeable. Therefore any head/block combo should be possible to meet the needs.
Examples are:
-cast iron 4.6 block
-aluminium 4.6 block
-cast iron 5.4 block
-aluminium 5.4 block
-2 valve per cylinder SOHC heads
-3 valve per cylinder SOHC heads
-4v per cylinder DOHC heads
The cranks, sumps and other items like intakes where all supposed to be interchangeable. They are for the most part but it does have limitations.
GM Gen III small blocks are all pretty much interchangeable as well from 4.8, 5.3, 5.7 and 6.0 so nowadays its more of a marketing thing for Ford but back in the early mid 90's when the modular engine was being developed it was a fairly new concept for American automakers.
Originally Posted by TXZ28LS1
and also how is the performance of dohc vs. sohc? and exactly how 4v helps in performance? i know the 99-04 stangs don’t use pushrods.. but why is that exactly and what kind of performance does that do?
SOHC stands for single over head camshaft. This means in a V8 you have one cam per bank of cylinders which sits on top of the cylinder head. This operates the valves either directly (most common) or indirectly. The advantage of this is less rotation mass in the valve train which allows for more stable and safe high rpm use. These are 2 valve per cylinder engines as the norm although there are a few 3 valve variants.
A regular OHV push rod V8 uses a single cam in the centre of the block and has push rods this operate each valve. Typically (99.99%) OHV engines are also 2v per cylinder. This means in terms of power a SOHC has no advantage on this basis. The only way one will produce more power is due to higher rpms. But the limitation of 2 valves per cylinder will limit the operational range of the engine just the same.
DOHC is Double Over Head Cam. So it has two cams per cylinder bank. The main advantage here is a thing called curtain area. This is where two smaller valves will essentially open up a larger area quicker than one large one. (remember in a 4v setup two valves for intake and two for exhaust and a 2v setup only has one for intake/exhaust).
Example (not using real valve sizes):
2v setup has a 2" valve and a 4v setup has two 1" valves. Opening the two 1" valves by 20% will flow more than the one 2" valve open the same amount.
So a DOHC has many advantages, most to do with valve train. Because of curtain area a DOHC allows you to run a "milder" cam which means it will need lower rated valve springs and which in turn means less valve train resistance and wear.
Adding a large aggressive cam to any 2v setup (SOHC/OHV) requires heavy valve springs which lead to increased wear rate and less throttle control at lower rpms.
Another advantage with multivalve setups like DOHC's is operational range, e.g.
1500-2000-2500-3000-3500-4000-4500-5000-5500-6000-6500-7000-7500
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx - 2v OHC/OHV
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxx - 4v DOHC
Now if you add a wild cam to an engine like an LS1 it will rev fine but you'll loose the bottom end because it's only 2v per cylinder, the same would occur on a 2v SOHC
----------------xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx - cammed 2v
4v setups allow smooth, fuel efficient setups in low rpms but still able to hold power in high rpms. Essentially the best of both worlds.
Another advantage to multivalves in DOHC design is modern system like Honda's VTEC and Rovers VVC (Variable Valve Control). These essentially change the profile of the cam as rpms increase (well the effect anyhow). They are very clever system and would not work on OHV very easily if at all. This allows such cars as the Honda S2000 with it's 240bhp 2.0 engine to be smooth and drivable at low rpms with good fuel efficiency and emissions control yet have what is very much a "race" setup at high rpms. 20 years ago this would not have been possible.
So from a technological point of view for maximum potential DOHC multivalve engines are superior. Most forms of major motor sport (meaning world and not unique to North America) such as Rallying, Formula 1 and so one tend to use DOHC engines.
There are even more fancy DOHC setups such as the Ferrari 5 valve per cylinder unit. From only 4.3 litres they manage to get 493bhp STOCK with great drivability, emissions and fuel efficiency. These are a different breed of V8 though and use a single plain crank which gives them an even firing order as opposed to an uneven like most V8's. This is why they sound so different.
F1 engines rev to 19500 rpm in some cases. A regular 2v setup would not be able to breath at such rpms and a push rod setup would just not be strong enough in the valve train.
The down side to SOHC/DOHC is they are generally physically larger engines and more complex to manufacture. Which may lead to higher costs. This isn't always the case though, Jaguars AJV8 is very compact and weighs the same as a LS1.
Sadly there are a lot of ignorant people out there who believe the pinnacle of OHC V8's is the Ford modular. This is totally FALSE, while the DOHC is pretty decent the SOHC is a dog and physically they are the same size as a big block V8 of old and with the cast iron block weigh similar.
For more proof of how successful SOHC/DOHC is just look at the world market. ALL 4 cylinder petrol engines in over 25 years + are SOHC/DOHC this is because they are far more efficient and produce better bhp/litre. Most V6's are also DOHC's and even the majority of V8's. OHV is really an American thing. Although many diesels including 4 cylinder ones use push rod and OHV technology. This is because OHV tend to prefer lower rpms due to valve train rotational mass which tally's in nicely with low revving diesels (typically max rpm is ~4500rpm)
This leads onto another grey area where people get confused (especially with the Ford modular V8). Although ALL OHC valve trains will be fine at higher rpms it doesn't mean ALL OHC engines are. This is due to the engines stroke (how far the piston has to move back and forth). Long stroke motors typically don't like high rpms due to the inertia involved where as short stroke motors are the complete opposite.
Long stroke will tend to offer more low end torque which when combined with OHV setups make a perfect combo such as many traditional American V8's are.
Short stroke will often make less low end torque and prefer to rev, so the obvious answer is to run OHC valve train to exploit it. This is not always the case and many times a long stroke motor is used with OHC valve train so it just doesn't use the inherent benefit of the OHC design.
Almost there
Now even though this all sounds great that doesn't mean push rod engines are old hat and out dated, because they are NOT.
Advantages in OHV are most evident in V engines such as V8's (or even V6's) and that's due to packaging. Having the cam in the middle of the block means the cylinder heads can be compact. So most of the time a OHV push rod engine has the ability to be smaller and lighter than it's equivalent OHC counter part. However as already noted this is not always such an issue, but it does depend which ACTUAL engines you compare.
OHV also makes a great street engine, because they generally favour lower rpms they are often long stroke motors which means they produce a lot of lower rpm torque with a broad power band. This makes them ideal for street use.
06-20-2006, 03:43 AM
#3
TECH Addict
Part 2 answer There have been some fancy multivalve setups for OHV engines but these are generally complex and have a high wear rate by comparison. At present their best bet is just in normal 2v format.
The biggest thing OHV has is DISPLACEMENT. Because most OHV V8's are American most have a large displacement. This means that they don't need to be as efficient to produce big power.
Example:
For street use most DOHC engines are able to produce 100bhp/litre specific output and still remain as driveable as ever while meeting fuel, noise and emissions standards for a production engine.
So if you target is of 400bhp then in theory a 4.0 litre DOHC engine will be able to achieve it. TVR offer a 4.0 S6 with 406bhp from the factory meeting ALL requirements of a production engine.
With a OHV design you will not achieve 100bhp/litre and still remain driveable (this is curtain area and rpm operational range). Also noise, fuel and emissions would all be compromised or simply not achievable as a production engine. Typically OHV tend to be in the region of 50-70bhp/litre going from standard tune to more extreme.
The current 6.0 LS2 in the C6 Corvette produces the same 400bhp as the TVR does, but its V8 is only making 66bhp/litre specific output. So it takes 2.0 litre more the achieve the same goal as a PRODUCTION engine.
I stress production because we all know that with aftermarket parts BOTH engines can produce more HP. In the end the LS2 is actually quite efficient for a OHV, far better than any Chevy small block before it so ultimately it probably has enough capacity to out power the TVR unit. But considering the displacement difference it wouldn't be by much.
The alternative way to look at it, is a 4.8 LQ Chevy V8 even though it's still larger displacement than the TVR engine it would not likely even get near 400bhp in form of reliable streetable setup and certainly would be a long way past emissions and noise restrictions.
So the advantage in performance is questionable but only due to other parameters not the physics itself.
Think of it this way, a DOHC motor is like one man with a M16 machine gun where as a OHV motor is like TWO men with shotguns. Both are very effective and both have the ability at being superior but it relies on other factors to make the final decision.
What is interesting is when the odd LARGE displacement DOHC engine makes it into production. This is very very very rare and at present there are few real candidates.
The most notable is probably the TVR 7.7 litre V12 DOHC this was built as a hand built production engine with the intention of being used as a race engine in much the same way as the current Chevy LS7 was.
Well as where are here lets talk LS7. A FANTASTIC engine and with 7.0 litres it manages 512bhp as a LEGAL production engine. But it is hand built and requires lots of exotic materials to do so. For a OHV engine this is probably the current pinnacle of what can be achieved for a PRODUCTION engine.
The LS7 has a specific output of 73bhp/litre, so above the norm for a OHV engine. But its displacement is really what gives it the headline figure. And it easily beats the Dodge Vipers 8.3 litre engine in terms of specific out put. The Viper requires 1.3 litres more displacement to achieve slightly less bhp (500bhp). Which gives it only 60bhp/litre specific output.
Now remember I said DOHC can achieve a far higher specific output yet still meet all of the criteria for a production engine. Well the TVR engine was actually rated at 880bhp which gives it an incredible 114bhp/litre specific out put.
When the best of DOHC technology is combined with the displacement of OHV I think the results speak for them self. But as I said this is rare. There are reports that GM is developing a new 6.4 DOHC V8 to replace the LS7. So with less displacement it will be capable of producing a lot more HP.
However, there really is no solid argument to say this engine is better than that just because of the valve train setup. This is due to many factors already highlighted (stroke, displacement, power to weight, specific output).
At the end of the day BHP is BHP and it doesn't really matter what produces it as long as it meets the demands and criteria laid out beforehand.
Sorry for so long but your question(s) where not quick to answer. Hope this has helped.
BTW - In case you are wandering I personally own 3 cars at present and ALL have OHV push rod engines
But in the past I have owned SOHC V12 and DOHC turbo 4 cylinder engines as well.
The biggest thing OHV has is DISPLACEMENT. Because most OHV V8's are American most have a large displacement. This means that they don't need to be as efficient to produce big power.
Example:
For street use most DOHC engines are able to produce 100bhp/litre specific output and still remain as driveable as ever while meeting fuel, noise and emissions standards for a production engine.
So if you target is of 400bhp then in theory a 4.0 litre DOHC engine will be able to achieve it. TVR offer a 4.0 S6 with 406bhp from the factory meeting ALL requirements of a production engine.
With a OHV design you will not achieve 100bhp/litre and still remain driveable (this is curtain area and rpm operational range). Also noise, fuel and emissions would all be compromised or simply not achievable as a production engine. Typically OHV tend to be in the region of 50-70bhp/litre going from standard tune to more extreme.
The current 6.0 LS2 in the C6 Corvette produces the same 400bhp as the TVR does, but its V8 is only making 66bhp/litre specific output. So it takes 2.0 litre more the achieve the same goal as a PRODUCTION engine.
I stress production because we all know that with aftermarket parts BOTH engines can produce more HP. In the end the LS2 is actually quite efficient for a OHV, far better than any Chevy small block before it so ultimately it probably has enough capacity to out power the TVR unit. But considering the displacement difference it wouldn't be by much.
The alternative way to look at it, is a 4.8 LQ Chevy V8 even though it's still larger displacement than the TVR engine it would not likely even get near 400bhp in form of reliable streetable setup and certainly would be a long way past emissions and noise restrictions.
So the advantage in performance is questionable but only due to other parameters not the physics itself.
Think of it this way, a DOHC motor is like one man with a M16 machine gun where as a OHV motor is like TWO men with shotguns. Both are very effective and both have the ability at being superior but it relies on other factors to make the final decision.
What is interesting is when the odd LARGE displacement DOHC engine makes it into production. This is very very very rare and at present there are few real candidates.
The most notable is probably the TVR 7.7 litre V12 DOHC this was built as a hand built production engine with the intention of being used as a race engine in much the same way as the current Chevy LS7 was.
Well as where are here lets talk LS7. A FANTASTIC engine and with 7.0 litres it manages 512bhp as a LEGAL production engine. But it is hand built and requires lots of exotic materials to do so. For a OHV engine this is probably the current pinnacle of what can be achieved for a PRODUCTION engine.
The LS7 has a specific output of 73bhp/litre, so above the norm for a OHV engine. But its displacement is really what gives it the headline figure. And it easily beats the Dodge Vipers 8.3 litre engine in terms of specific out put. The Viper requires 1.3 litres more displacement to achieve slightly less bhp (500bhp). Which gives it only 60bhp/litre specific output.
Now remember I said DOHC can achieve a far higher specific output yet still meet all of the criteria for a production engine. Well the TVR engine was actually rated at 880bhp which gives it an incredible 114bhp/litre specific out put.
When the best of DOHC technology is combined with the displacement of OHV I think the results speak for them self. But as I said this is rare. There are reports that GM is developing a new 6.4 DOHC V8 to replace the LS7. So with less displacement it will be capable of producing a lot more HP.
However, there really is no solid argument to say this engine is better than that just because of the valve train setup. This is due to many factors already highlighted (stroke, displacement, power to weight, specific output).
At the end of the day BHP is BHP and it doesn't really matter what produces it as long as it meets the demands and criteria laid out beforehand.
Sorry for so long but your question(s) where not quick to answer. Hope this has helped.
BTW - In case you are wandering I personally own 3 cars at present and ALL have OHV push rod engines
But in the past I have owned SOHC V12 and DOHC turbo 4 cylinder engines as well. 
06-20-2006, 10:08 AM
#6
TECH Addict
Originally Posted by TXZ28LS1
damn..lol..thanks alot. that was very informative.. that has helped me with a little more understanding..but im sure its still complicated than that..lol..
