Grant B

I am wondering why someone would do this? In theory, if you increase port cross-sectional area, valve size and flow in proportion to the increase in displacement, you do not need more duration. I understand larger valves might need more lift, but it seems like more duration would be necissary.

Is this done because intake manifold and port runner cross-sectional area is too hard to change on modified cars, and you need more flow?

Unaffliated Racing

iTrader: (15)

The larger the cubic inch of the motor, the more duration it takes to turn the same RPM. More cubes "swollows" up duration. Compression does the same thing. A 224/224 cam in a 346 motor may turn 6400rpm. The same cam in a 427 will probably only turn 5500-5800rpm.

Gary Z

A larger engine does need more flow but doesn’t necessarily need a “larger cam”. “Larger cam” is generally interpreted to mean some combination of more lift, duration and overlap, and to lead to increased power at higher RPM. A larger engine may be unable to benefit from a larger cam due an inability to safely reach higher speeds. Unless you are adding cylinders, the steps taken to increase displacement tend to reduce maximum safe engine speed.

grinder11

iTrader: (1)

An engine is an air pump. The more cubic inch displacement it has, the more cam it can handle. You can lift a valve 6 inches, but if it is only open 6 inches for one billionth of a second, it won't flow much. Of course, these are exaggerations, but you must get the idea. If you do the math, an engine that spins 6,000 times a minute (3,000 intake valve opening and closing events) would equate to 6,000 spins in 60 seconds, or 100 spins per second! this equates to the intake opening and closing 50 times per second!! Needless to say, you better hold the valve open for a little while, as it needs some amount of lift AND time to fill the cylinder, doesn't it? I don't know how many of the guys on the forum here (there are some, I know, as there are some SMART people on this forum, for sure) are aware or have seen an article that was in an automotive magazine (can't remember the name) recently that went into detail about the latest ultra high R.P.M. grand prix racing engines in Formula 1, I believe. They went on to say that the latest designs are abandoning camshafts altogether in favor of ultra high speed/controlled "electronic solenoids" for lack of a better recollection/wording.

02WS6Bird

iTrader: (2)

them puppies needa BREATHE!!! thats why

Grant B

I should have specifically asked about these engines only. I know they need to breath, but if you increase flow rate in proportion to the displacement increase (with larger valves, runners, ports, etc) I don't see why you would want more duration.

Unless you simply can't get the flow you need out of heads designed for a 5.7L motor. In which case I might wait for the LS7...

P Mack

iTrader: (6)

I think it is because the flow doesn't increase as much as the displacement does. I think you'd find that the cam duration on a factory big block would probably be similar to the duration on a factory small block, but i'm not sure about that.

cyphur

iTrader: (10)

More displacement eats up the cam, requiring a "larger" cam for higher cube motors. More cubes = needs more air. Cams open the valves, not heads, you know that Doesn't matter what the head flows if the valve isn't open long enough for the stroke/combustion cycle.

ALSO, port cross-sectional area isn't the end-all be all some think it is. Port DESIGN is tantamount, as AFR has shown with the smaller ports and runners, yet keeping the velocity up has enabled them to put out some killer numbers.

Whats the point in gouging out your heads and block at the costs of thousands of dollars, only to use a small cam that won't let it breathe?

P Mack

iTrader: (6)

What he's saying is if you have a 350 ci engine with heads that flow an average of 200 cfm, and you have a 700 ci engine (double the size for simplicity) with heads that flow an average of 400 cfm, then theoretically they should behave the same with camshafts of the same duration.

cyphur

iTrader: (10)

I suppose if you had heads that flowed @ 600cfm you could acheive that. Since most good ported heads flow ~300+. The cam would no longer be the deciding factor. However....since heads aren't about to flow like that....its a moot point. Thats kind of why I made the above post, b/c its not something that you can change right now.

grinder11

iTrader: (1)

This may be true in a perfect world, but that eludes us at this point. And even if we lived in that kind of world, it would only be true up to a point. If you had cylinder heads that had intake and exhaust ports the size of 55 gallon drums, it would do no good if the valves were welded shut. A small duration cam is like welding the valves shut. And the higher you spin the motor, the better the weld job. Even forced induction motors benefit from a custom grind cam. Otherwise, a Procharger would come with a smaller duration cam than my John Deere has. Not trying to be sarcastic here, just exaggerating it to be more clear.

P Mack

iTrader: (6)

Well i guess we agree then.

cyphur

iTrader: (10)

Thats what I was trying to say. Doesn't matter what the heads flow if you don't have the valves open long enough....and the valves need to be open longer for bigger cube motors.

nuzee

iTrader: (2)

The short answer to your question is that the engine is under-valved. The head's combustion chamber isn't large enough to accomodate LARGE valves without running into flow problems (shrouding). That is the bottleneck that creates the need for bigger cam duration & lift when using larger displacement.

Grant B

Thanks for the responses.

And BTW, I wanted to know the "perfect world" answer because I am wondering if it is worth waiting for the LS7 vs. a current LSx 427 (if the LS7 is indeed going to be a 427 at all). The new engine will undoubtably have bigger valves, ports, runners, etc. and be close to "perfect" for its larger displacement.

cyphur_traq, DOHC motors of varing sizes don't really change duration, so I don't think thats true.

grinder11

iTrader: (1)

You have made some very good, valid points!
I don't know if any of us has all the answers, but as far as waiting for the "427' if it is indeed that, that would be an interesting wait. I think that some parts would be very affordable, as the new LS2 intake most certainly is, as well as the 90mm TB. And they would be OEM quality, not a bad thing (Usually). If I re-do the 395 stroker I got to a 4.125 bore W/ Darton sleeves at around $3,000.00 or so, I wouldn't be a happy camper if after I am done G.M. decides to come out with a non-C5R-4.125 bore block for around the same price as the LS2's 4.00 bore ($1,000.00 or less!). Obviously, this would be a net loss of about 2 grand for me/us . If G.M. does do this, it would sure be worth waiting for, IMO! As far as the DOHC points you are talking about, I might have missed a post or something here, I don't know. I do know that for many years, I raced a 1,105 Yoshimura based Kawasaki motorcycle that I built myself. Porting, valve sizing changes, etc. The cams I ran were also Yoshimura brand and they came in 3 stages, with each stage offering a little more lift and duration. The first motor I built was a 998cc, that ironically the factory later went to in the KZ1000 model, right down to the bore and stroke. When I went to the 1105, I went up to the next stage cam with more lift and duration, so I am missing something here, as I said. However, to go along with your train of thought on this (DOHC engines of different displacement using the same duration cams), if you go to LPE's web site, they offer a package for the LT5 (ZR-1 Corvette, DOHC-4 valves per cylinder) that uses the stock cams, but they must make adjustable cam drive gears/sprockets as the only change I see is in the LSA, which on a DOHC is infinitely adjustable, a big inherent advantage they posess.

cyphur

iTrader: (10)

I can't comment on DOHC stuff, I don't know **** about it.

In a perfect world, lots of things would be possible. I was referring to the LS1 series. LS7 is different, I haven't read up on it much, so I dunno what will happen. But right now, as it stands, LS1 cams are the brains behind the beast....doesn't matter what the heads flow. Valves still need to open for long enough periods of times to accomadate cubes...