Thread: Common Sbc Myth's

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I think he is full of bologna..............or some other substance..........

Originally Posted by 65ny

I think he is full of bologna..............or some other substance..........

If you are referring to my friend that I quoted earlier, he has been known to be FOS but generally is (seems) quite knowledgeable. I guess he is more convincing in person.

If your talking about the point where the lift of the valve becomes more than the lift of the cam is called loft ,not dwell and usually is not a good thing unless you are trying to fool the techs and will rebuild it all the time.Nascars use this for more power and to skirt the gray area,loft kills a roller cam.You could say the dwell time of the loft,but thats going to vary on rpm engine temp and will change its value after a couple hours of loft affecting the springs .I am not sure you could attach enuff instruments to a running motor to get a usable value for reference on the dwell time of loft.I would say the dwell time of loft is a variable and one could only say it improves high rpm performance ,not put a know value on it like .040 loft @ .550 lift @ 5500 rpm @ 110 lbs spring tention @ 1.98 installed height.Loft is also in a very narrow rpm band and once you go over that rpm the loft turns into seat bounce and kills all performance gains.The dwell time of loft is a variable and used to,{for lack of a politically correct word here}" cheating "or pushing the gray area and is abusive on the valve train components.EDIT I see what you are refering to and that calculation is not part of the normal specs on a cam and is mostly used by porters and cam grinders in very specific apllications and is used in conjunction with cfm numbers of a head at a certain lift ,the reason a roller cam is so much better in response than a regular flat tappet is the duration at which the cam is at max lift and the speed it achieves max lift.Dwell in this example is an exspression of time which is usually exspessed in duration or degrees of duration instead of dwell ,which is used more commonly on old distributors and refered to as dwell time or saturation time.

This is the ignition related dwell I was thinking of....(found online)

"The cam angle, or dwell, must also be checked when breaker points are replaced. Dwell is the angle or number of degrees that the cam rotates from the time points close until they open again. Increasing dwell reduces point gap and vice versa. Too little dwell causes misfiring at high engine speed, but too much dwell causes points to burn."

Like Shawnlee said: same term, different folks, different meanings sometimes...

I think I am on the straight and narrow now.

Originally Posted by 65cayne

This is the ignition related dwell I was thinking of....(found online)

"The cam angle, or dwell, must also be checked when breaker points are replaced. Dwell is the angle or number of degrees that the cam rotates from the time points close until they open again. Increasing dwell reduces point gap and vice versa. Too little dwell causes misfiring at high engine speed, but too much dwell causes points to burn."

Like Shawnlee said: same term, different folks, different meanings sometimes...

I think I am on the straight and narrow now.

You know that is talking about the cam on the distributor?right?Not the actuall cam shaft of the engine that moves the valves.

The dwell is ground during cam manufacture. I never said you could set it. The "Dwell" is part of the cam profile.

So you can have a cam that starts at the stock opening point and open faster with a longer dwell at the top of the ramp then closes faster to end at the stock closing point.

I ain't full of it. Been playing and racing 283s since 1969.

Bob

Stock Class Champion
1981 NHRA Summer Nationals ( Englishtown N.J.)
1981 NHRA U.S. Nationals ( Indianapolis In.)
1982 NHRA Summer Nationals ( Englishtown N.J.)

I think the person that started this thread is just jealous of the best engine ever built............The SB Chevy. Just because some kids say they can do this or that, doesn't make the engine a slug....It just makes those kids FOS

BTW, I turn mine up to 5400 rpm.....thats all I need it to.

Originally Posted by shawnlee28

You know that is talking about the cam on the distributor?right?Not the actuall cam shaft of the engine that moves the valves.

Camshaft !

We were talking about cams but I guess no-one here has ever heard that term. Every body talks about lift , duration and overlap and never stop to look at the dwell on the top of the cam lobe.

I see what you are refering to and that calculation is not part of the normal specs on a cam and is mostly used by porters and cam grinders in very specific apllications and is used in conjunction with cfm numbers of a head at a certain lift ,the reason a roller cam is so much better in response than a regular flat tappet is the duration at which the cam is at max lift and the speed it achieves max lift.Dwell in this example is an exspression of time which is usually exspessed in duration or degrees of duration instead of dwell ,which is used more commonly on old distributors and refered to as dwell time or saturation time.[/QUOTE]

I think you have it.

All cams have dwell as set by the designer. Race cams have more dwell than stock cams.

In days gone by racers had to go to roller cams because the cam ramp was so quick the regular lifter couldn't follow the cam profile with out tearing the lobes off .

If you go way back to post #16 I said that I ran a cam with more dwell and that's all I said. Then all of a sudden we are here.

I think what Bob is refering to is called lobe lift here in the US anyway.
Lobe lift X rocker arm ratio = max. lift.
An example would be .350 lobe lift X rocker arm ratio 1.5 =.525 net valve lift.

Originally Posted by erik erikson

I think what Bob is refering to is called lobe lift here in the US anyway.
Lobe lift X rocker arm ratio = max. lift.
An example would be .350 lobe lift X rocker arm ratio 1.5 =.525 net valve lift.

No the dwell is the # of degrees that the lifter is at the top of the lobe (degrees at .525 net lift).

Camshaft lobes have the opening ramp, then the dwell, then the closing ramp .

Hi Perf Cams are ground with more dwell to hold the valve at max lift longer.

Look at this link and read it all

http://www.pipercams.co.uk/NewPiperW...echAdvice.html

So, the valve "Dwells" at wide open postition longer.

There is also dwell for pistons in your engine , like how long the piston dwells at TDC or BDC. Things have to stop and dwell for a few dgrees then start to move the other direction.

Originally Posted by shawnlee28

You know that is talking about the cam on the distributor?right?Not the actuall cam shaft of the engine that moves the valves.

No, I assumed it was the horizontal one. Shows what I know about old ignition systems

Originally Posted by Bob W

No the dwell is the # of degrees that the lifter is at the top of the lobe.

Camshaft lobes have the opening ramp, then the dwell, then the closing ramp .

Hi Perf Cams are ground with more dwell to hold the valve at max lift longer.

Look at this link and read it all

http://www.pipercams.co.uk/NewPiperW...echAdvice.html

So, the valve "Dwells" at wide open postition longer.

There is also dwell for pistons in your engine , like how long the piston dwells at TDC or BDC. Things have to stop and dwell for a few dgrees then start to move the other direction.

Yes,rods will stay longer at TDC with a longer rod.
There are no cam companies in the US the recognize this "dwell" term.

Originally Posted by Bob W

... If you go way back to post #16 I said that I ran a cam with more dwell and that's all I said. Then all of a sudden we are here.

It seems like chaos, but the whole conversation would take about ten minutes with ten guys with 16oz cups in their hands standing around a stainless steel drum with manual siphon device affixed to the top to dispense liquid contained therein. Just seems crazy with everyone posting and quoting and replying to such...

So we have Canadian Dwell associated with the valvetrain and dealing with valve dwell due to cam lobe. Full roller cams understood to have a high degree (no pun intended) of dwell. (i.e. opens quicker, stays there longer)

And we have outdated American Dwell associated with ignition/distributor and the amount of spark (or rather, length of time a spark is) delivered at a precise time in any given combustion stroke.

The former being non-adjustable (in terms of degrees) and the latter being adjustable.

As far as the original post goes. It's fun to get off topic sometimes but let me say this: I think SBC's are bad to the bone and probably the best motor out there for hotrodders, but whatever you can do with a small block, you can do it bigger (hence, better) with a big block (oldest discussion out there, I know )...that's why I have a big block under my hood.

Originally Posted by poncho62

I think the person that started this thread is just jealous of the best engine ever built............The SB Chevy. Just because some kids say they can do this or that, doesn't make the engine a slug....It just makes those kids FOS

BTW, I turn mine up to 5400 rpm.....thats all I need it to.

I wouldn't call it the best engine ever built. it's a good engine with a lot of aftermarket parts made for it, which it makes it a good candadait for cheap power, the reason it's so popular