Paul Workman

An interesting comparison of a 355 ci 500+ hp 2-valve SBC vs. the 4-valve DOHC 500+ 350 LT5 at idle. The ability for the 4-valve configuration to breathe without the overlap that sacrifices drivability - is pretty evident. (I'm not faulting either...I'm just sayin' the distinction is striking!)

The 500 hp SBC...
http://www.youtube.com/watch?v=05Tc7...eature=related

and the 500+ hp LT5 (with stock bottom & stock cams)
http://www.youtube.com/watch?v=y1YFa8ruh8A

It is in the teens outside just now. Dang, I hate winter!

P.

PLRX

Thats a huge difference on idle. The LT5 acts like a normal V8 idle, I guess the 16 breathers make a difference.

93Rubie

An engine is an air pump, the more air you move the more HP, you make. 4 valves will almost always be better than 2 valves. However, other factors come into play other than airflow (packaging, etc...) and thus is why we have the excellent LS-X engines. Which no one makes fun of, only raves about. So do you really need 4 valves? Yes and no.

earthquake68

I need to swap my intake, but my car idles like the first one.

cv67

I highly doubt that motor is 500+hp (everyone on the web says theirs is) but yes the idle characteristics are night and day. Nice car though.

Personally I love a rowdy *** cam that winds to the moon but thats just me. As others said depends what you want, liked the linear powerband of the LT5 but it was just a little tame sounding.
Great motor though and for stock runs pretty darn strong.

dizwiz24

I read an interview from smokey yunick (famous engine builder) and he said that 4 valves were for fools.

So what I interpreted from that is 2 big valves can flow as much (or more) air as 4.

But maybe im wrong?

bb62

4 valve engines are certainly not for fools. If that were so, then virtually every Formula 1 winner in the last 60+ years would be a fool for driving a 4+ valve per cylinder engine. I think you are quoting Mr. Yunick out of context.

cv67

Some like less moving parts ease ofworking on etc some are willing to deal with working on a 4v deal they both have their + and -

4v gives good longevity as you dont have to open the valve very far or very long being as there are two of them probably one of the reasons those Z cams dont seem to go flat theres hardly any spring pressure, valves are lightweight and can take some rpm.

earthquake68

Maybe in Trans Am series where the courses are tight and engine RPM's don't get to go that high. I think the idle comparison speaks volumes on the subject. Which engine is under more strain? I would say with two valves, you have to have two valves open farther and longer to flow the same amount as a four valve. By design, the the two valve won't live as long. The spring pressure needed to hold up to that puts tremendous strain on the valve train. Not to mention, the two valve engine will bleed off compression because of overlap in the cam profile. So now you need to run a higher static compression ratio to compensate for that.

The small block Chevy is a very inefficient design. They came up with it because they were cheap and easy to manufacture back in 1955. It stayed around because they were able to tweak it to keep up with performance demands with very little changes to the production line, thus keeping the cost down.

RedLS1GTO

There are a whole lot of things causing the difference between those 2 engines, not just 4V vs. 2V. You can't generalize all 2V engines by an old school SBC. The LT4 in my track car sounds ferocious, has a big overlap, idles like an earthquake, and would be a bear to drive on the street. With all of that, it has <100 hp more than the LS6 in my Z06 that is as friendly a car to drive as you can get.

I have seen plenty of 500+ hp LSX engines with a very smooth idle. Go start a stock C6 Z06; 505hp and an idle as smooth as can be. I have also seen plenty of 600-700++ hp LS7 2 valve engines that idle and run very very smoothly with zero sacrifice in drivability (other than the constant urge to shred expensive tires).

87 vette 81 big girl

I think it has to do with Refinement in each overall engine package.

From the drawing board to production.

The LS series engines are a very good example of a pushrod engine that makes fantastic power into the 21 st century and purs like a kitten smoothly.

DOHC should have taken over in all engine designs by now in 2011, but haven't yet.

Pushrod V-8 engines will be around for some to come.



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earthquake68

Yes, this is understood. I made a rash generalization for comparison purposes. Head design, flow rates, etc. all have plenty to do with performance. Bottom line is, the more air and fuel you can flow, the better the performance. You will ALWAYS be able to flow more air through four valves than you can with two valves. (all things being equal)

I hope I haven't made my point too unclear with this.

RedLS1GTO

Obviously if thangs are equal and the valves are the same size, more valves = more air but that doesn't necessarily mean that a 4V configuration is going to make more power in every application. Depending on what kind of space you are working with you can come up with configurations that flow more with a single intake valve vs. 2. For example a single 1" valve flows more volume than two .5" valves that take up the same head space (.79ci volume vs. .39ci). The use of 4V in F1 engines was brought up as a very valid point. If we are going ot go to that level it should be said that the 8,000+ hp Top Fuel engines and for that matter virtually every big power drag racing engine in existance are a 2V design.

Top Fuel:


3,000 hp Hemi (that I am lucky enough to get to work with):


I'm not arguing which is better, just adding to the discussion. They obviously both have their advantages for certain applications, especially when you start playing with the big $$ versions.

earthquake68

No disputing the hemi. My point was for internal strain on parts. Sure you can make a two valve flow like a 4V engine. Can you run that hemi for 24 hrs straight without an issue? What I was trying to get at, with a 4V head, it's easier to make power that won't tear itself apart as fast. Most 4V engines have a hemispherical combustion chamber anyway. It lends itself very well to that shape. My Mom's Hyundai is a hemi, too. They just can't call it that because of the trademark from Chrysler Corp.

I go back to the videos. Which car is more likely to have a failure at the track?
The one with 300 psi seat pressure closed to support the higher lift lobes with a thin wall piece of steel tubing pushing them open from somewhere in the middle of the engine, or the one with 150 PSI seat pressure, lower lift and the followers riding right on the valve stem? With the 4V engine you don't need the compression or the aggressive cam profile like the 2V engine to get the same performance.

Now after all that........

#1: I'm not arguing, just sayin'.
#2: throwing a picture of a dual plug race hemi head up there is cheatin'!

I'd like to come out and watch you guys run that in the spring. Horsepower is cool no matter how you get there. One more question, I see the head is O-ringed. Do you guys run an O-ringed block as well, or just the head?

rocco16

Top fuel engines are apples and oranges. Start putting pressure in the intake (super- and turbo-charged) and you can get good breathing with tiny little valves. Normally-aspirated engines are a different story.
Also, it isn't valve area that makes the 4-valve superior to a two valve engine, it is intake valve circumference in total. I.e. two small intakes in the same combustion chamber have more total edge area (circumference) than one larger valve. This equals more airflow at anything less than fully open....and a valve is partially open much more of the time than it is fully open. Hence, more flow for the 4-valve head.

Larry
______________
not easily impressed....

RedLS1GTO

A lot of times we have issues getting it to run for 2.4 seconds.

I never said anything about reliability.

Seeing that we have to pull the top end apart after every 1/8th or 1/4 mile run I'm going to go ahead and say that the 500hp DOHC is more reliable.

I had to throw the "not arguing" part in there... it seems that people have a tendency to take opposing views in an insulting way on here most of the time and I didn't want to come off that way.

...and yes, if you ain't cheatin', you ain't tryin'. Top Fuel Hemi head FTW. It's usually hard to argue with 8,000hp.

Since most of the Outlaw Dragster events are going away (funding) the car is actually for sale so not sure if it will be running this season or not. If it doesn't sell, I'm sure it will be racing somewhere although perhaps with a different configuration than it is now.

The block is not o-ringed (use copper head gaskets).

http://www.racingjunk.com/category/1...-Quick-8-.html

RedLS1GTO

I agree... but somebody threw out an F1 engine as an example of a 4V which pretty much opened the discussion to ALL engines.

I realize that 4V engines certainly have a lot of advantages, especially at high rpm and that forced induction changes things entirely. My only point was that as stated above, the most powerful internal combustion racing engines just so happen to be 2V. Basically... just being a pain in the *** (because I can). I am all for high tech and efficiency, I'm an engineer by degree and am always interested in different engine technologies.


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rocco16

I disagree: F1 engines are not supercharged.

RedLS1GTO

They might not be supercharged but I don't think that your run of the mill DOHC motor is built with such precision and tolerances that it can make 780hp and turn 20,000+ rpm.

We'll take the supercharged engines out of the equation. 2V pushrod small blocks make 800 horsepower, turn 10,000 rpm and survive for an entire weekend worth of racing in NASCAR. The Corvettes that absolutely domininated GT1 endurance racing to the point that every other factory team quit (and still dominate in the hands of privateers) are all pushrod 2V engines that run for 24 hours with no problem.

The point isn't that 2V are better than 4V engines. I don't think that at all and have never said that, however making it sound like DOHC engines are built by Gods and that 2V engines are archaic and incaple based on a random big cam SBC in a video is just plain wrong.

earthquake68

Very true. I actually feel a little hypocritical. All my engines are of the push rod type. Actually, my 'Vette sounds just like the one in the video. I love the way it sounds. I just need to lose the Flowmasters before I lose my mind from the noise.

Would I rather have an LT5? Yooooouuuuuu Betcha!