BigPlanTransAm

why is it that a DOHC engine would supposedly make more power than a pushrod engine of similar displacement? also, i have noticed that most DOHC engines are 32 valve. are the extra valves responsible for the extra flow(power) and if so what would stop someone from making a pushrod motor with 32 valves? sure more moving parts arent always a good thing but a 32 valve pushrod motor would have less parts than a DOHC motor would.
i understand that DOHC engines tend to make more power in the upper rpm's at the expense of the low end but i dont understand why.
if anyone has any facts/theroys feel free to jump in, i know next to nothing about this subject.

jrp

iTrader: (16)

https://ls1tech.com/forums/showthrea...light=DOHC+ls1

TeeKay

I read the thread that JRP refered to. I really gotta say that after having been involved with and hot rodded about 5 different platform/engine combo's over the years (BB Pontiac, BB & SBC, 3800 Buick, 4.6/5.3 DOHC Ford) that everyone seems to have tunnel vision about THEIR engine combo being the most _________ (fill in the blank). 4 valves per cylinder is the absolute best N/A technology available today, CI for CI of displacement!

A 5.7 L DOHC designed today for the same considerations as the LS1 would make an easy 450 HP stock. The other post compared the Ford DOHC to the LS1 - apples and oranges. The 4.6 Ford is 281 CI (4 CI less than the original 1955 SBC at 265 CI and 140 hp SAE) and still makes well over 330 crank HP stock (N/A), while the 5.7 L LS1 (347 CI) makes about 380 crank stock in the same configuration (N/A). Note that neither figure is advertised by the factory, but what is actually on the street. You go figure the average per liter of cubic inch of displacement for yourself. Besides that, the DOHC (Ford or GM) can easily go to 7500 rpm without straining the valve train. The pushrod LS1 (as fine an example of the OHV configuration as you will ever find) would have trouble up there without major changes from stock. Now, don't start giving me lip, I have personally taken a 383 70's era SBC to 8 grand reliably, but it wasn't simple! And, boy, did it sound different!!

Sorry, I didn't mean to make this a tutorial! What was the question? Oh, yeah, how does it do it? Well first the intake runners don't have to dodge the pushrod guides; second, the runners are individually smaller and naturally easier to get smooth flow and velocity directly at the valve mouth; third, the individual valves are smaller (and lighter) and use the 25% principle easier. The 25% principle says that the flow efficiency improves drastically at about a lift of 25% of the valve diameter. There's other stuff, but suffice it to say that the stock DOHC head's flow is roughly equivalent to a comparable Stage II or III OHV head! IOW, the ***** can flow bigger numbers for it's size, and that makes power!

Benjamin Russick

iTrader: (1)

They have the potential to rev higher...

OWENMUSTANG

iTrader: (2)

RPM and valve area. however, this does make it more expensive, consume more
availible engine compartment room, and in most cases require alot more gear
to keep it happy. i guess the bottom line is this. if you like peaky, sport compact
style of power. 4v's are for you. if you like throttle response, torque then 2v's the motivation of choice.

Old SStroker

Another benefit from DOHC is the ability to vary the valve advance/retard (phasing). You can vary intake and exhaust separately, or just one, like the variable exhaust phasing of the Vortec 4200 I6. This is great for shaping your torque curve, and also for emissions and fuel efficiency.

Of course, you could stack two cams vertically in an LS block and have one open 2 inlet valves with one pushrod, and have the other open one exhaust valve. It's probably the intake valve low lift efficiency that makes multivalve engines flow better. Both cams could have variable phasing. You'd have a intake and exhaust VVT pushrod V8 with 2 cams, very few additional parts and 3 valves per cylinder. The 2 cam-in-block architecture could probably be added to the LS engine family with the parts made on the same lines now used for single cam engines.

FWIW, if the upper cam was the intake, push rods would be shorter and stiffer, so higher rpm would be easier.

My $.02

ChrisB

But the problem is, in the end, cubic inches is irrelevant - we are interested in them for the effect on other more direct metrics (such as economy, power, emissions, size (physical), weight, etc.) - but in and of themselves and are irrelevant (unless you are racing in a class that artificially restricts them, but let's talk about a street type motor).

Using the example of the 4.6 DOHC vs. LS1 - sure, the 4.6 has less cubic inches, but what does that matter? Let's look at other factors.

1) LS1 makes MORE power/torque than the 4.6
2) LS1 get's the Same/Better mileage than the 4.6
3) LS1 get's better emissions thant he 4.6 dohc (per ls1's epa status)
4) LS1 is physically (total size) smaller than the 4.6 DOHC - easier packaging
5) LS1 is physically lighter (less weight) than the 4.6 dohc
6) LS1 costs less than the 4.6 DOHC.

Sure, the 4.6 can rev higher, but it *has* to to just to make less power than the ls1, and it does it at the cost of more fuel, weight, and size.

so why?

The biggest problem I see with large 4v heads (large bore) is the poor mixture motion you will be getting. This is one of the big factors that hurts fuel mileage and economoy (not as much of a power issue, though still there). With 2v heads you can get massive amounts of swirl, with 4v heads you just may get a bit of tumble.

Now throw a blower on there and the 4v really begins to look better (which is what ford finally figured out) - but the LS1 itself does pretty well with FI, so I would just call it more of an evening.


Now all imho, but obviously I am not a 4.6 fan

Excal

but you have to thinkg that if the ls1 had 32 valves..it would b emaking more power.

specially if it had some type of variable cam timing.

there's no reason why they can't have BIG cis AND big revs.

BigPlanTransAm

so really the only differences between a 4v(per cyl)DOHC and a 4v pushrod is that the DOHC has no pushrods to get in the way of airflow(although i didnt know pushrods went through the intake), it can rev higher due to the reduced weight of having no pushrods. and i'm not sure if i read correctly but it seems like someone was saying there is a cam for the intake valves and another for the exhaust valves. so is there an intake and exhaust cam for each bank of the engine(1,3,5,7 and 2,4,6,8 cyls) or just 2 cams in the whole equation?
Now what if GM produced a 32 valve LS1/2/6/7? it seems to me that it would make huge power at low(er) rpm and have smaller size/weight compared to a DOHC, and be simply amazing on FI.
what do you guys think? is there anything i missed?
and i am reading the linked post now.

SS00Blue

Chris, you're an over-pretentious, self-assuming, self-proclaimed know-it-all, but you're certainly not humble (ie - IMHO), but a dumb-*** in the making.

The 4.6 4V lacks the essential ingredient in a quick combination, which is cubic inches. A 5.6-to-6.0 litre 4V Ford would kick the stuffing out of any LS1/LS2/LS6/LS7 Gen III Chevy. It is simply a better design, though the pent-roof of the Ford does leave some emissions considerations on the table, and the additional weight of the valvetrain does factor in.

That aside, the efficiency of the air management certainly rules out the EPA considerations. Quench is only essential in a lean-burn engineering exercise. The bottom line is air + fuel ===>> power, and the four and five valve designs are always going to accel over the decrepit two-valve in air flow, given the same volume.

SC-

BigPlanTransAm

and excal, the thing with big CI's and big revs is that a bigger bore means you need
a larger piston which means more mass and thats bad for high rpm's. or you would need a longer stroke which would mean a higher avg piston speed and that combined with more revs is also a bad thing. now if you were to add more cylinders with smallish bore/stroke then thats annother story.... but thats neither here, nor there. atm im more curious about the diff cam configs.

Cheatin' Chad

iTrader: (3)

I find people like yourself absolutely hilarious. You're the same guy you see at a street race saying : "I wouldn't have lost if my car had a turbo" or if "My car had some nitrous things would be in my favor" ad nauseum. You sir are the idiot.

The fact of the matter is the LS1 currently kicks the snot out of the 4.6l ford powerplant in their stock N/A configurations. You can "what-if" and "if I" all ******* day and you still don't have a foot to stand on.I say this because you don't have a single bit of your argument based in reality.The 4.6 Ford is not 5.6-6.0 liters in displacement. The argument can go no further without you reaching into la-la land to fuel your side of the debate.

Concerning the superiority of 4 valve per cylinder engines.YES, 4valves per cylinder *CAN* be better than a two valve configuration. This is assuming the 4 valve head is a good design in the first place. Merely placing 4 valves in a combustion chamber will not magically make it better than any two valve on the market.

Now before you get all uppity and make some feebleminded attempt to attack my character and knowledgebase. Pull up your panties and realize you are the one that took this to such a base level by calling names and spitting forth your conjecture.If you'd like to return this to a rational,intelligent discussion I am all for that.

If not. Oh well. I don't place much value in the opinion of idiots and ********...

gojo

iTrader: (12)

One of Chris' points, which is major, is that the DOHC setup requires more engine compartment space and weight for the same CI. A major reason for the LS1 was that it would be part of the Corvette total platform and fitting a small engine compartment was a goal.

jmX

Dunno if you guys have looked in a DOHC cobra engine bay in the last 5-10 years, but its not an engine bay I'd want to work on! Headers/spark plug access look awful, the thing looks like it was shoehorned in. Its a big freaking engine, that makes less power, and consumes more fuel. ChrisB summed it all up quite nicely I thought.

66ImpalaLT1

The 5.6-6L comment is pretty funny, maybe they could make power at that size if Ford didnt have their heads up their collective asses when they came up with that bore spacing. If they didnt cram the cylinders so close together maybe they could have built a 500hp NA engine for the Ford GT?

LS1derfull

BullSHiT!! The Triton 5.4 is close enough in size to 5.6 Liter LS1(yes 346"is actually 5.6 liters) and it does nothing even close to LS1 in any way! Fords design is a total LOSER and Chris B couldnt have said it any better. Lack of mixture motion, and swirl, parasitic intake port runner surface area(extra friction to airflow) and very poor combustion chamber(centrally located plug pent roof) makes the Ford design inferior in any measurable way! Ford doesnt even have higher rpm for its peak power than any LSbased engine, so much for taking advantage of DOHC or SOHC! Get a life dude Fords cant rev because they will come apart, so theres no use having ability to rev with any of those engineering embarassments. As for clean running can you say"Add extra Cats to get fords to pass EPA standards" LOL!!!

DANSLS1GTO

Isn't the LS2 a 24 valve motor?

restoman45

iTrader: (2)

what does a good set of ported 4.6 dohc ford heads flow?....a good set of ported ls6 heads flow ~330-340....?...just curious...and i just have to add in the fact that the pushrod C5R vette seems to be holding its own/kickin butt against dohc v12 ferraris...

LS1derfull

Yes i believe it is.

LS1derfull

For those who didn't realize 4.6 Frod has been around since 1992, thats 12 years and i've yet to drive or see anything impressive about this motor at all normally aspirated. Now this has changed and latest 2005 Mustang is finally pushing 300hp with its latest redesign 4.6. I am also a big fan of this all new mustang with its longer wheelbase and no "extra parts bin" styling of the last 10 years or so.