1.5 or1.6

[NMike]

i'm going to build a 500+ inch motor. eddy heads, mild cam. build for cruising, NOT racing. should i even bother with 1.6 rockers, or just stick with 1.5.?

[Ghoste]

If it were me, and I wanted to feed 500 cubes through Eddy heads with a mild cam I'd use the 1.6 for that little bit of extra lift.

[firefighter3931]

It really depends on the cam profile. 

I'm not a fan of fast rate/high lift profiles with 1.6 rockers...spring pressures need to be increased to keep the valvetrain stable at higher rpm's which in turn increases the load on the valvetrain. The Comp XEHL and Engle fast rate cam profiles don't need a 1.6 rocker to work well. 


Ron

[Mike DC]

 
Good point.

The higher rocker ratios and the modern fast-rate-of-lift cam grinds would basically be doing the same thing.  The spring pressure starts to be a problem as soon as anything in those situations.

   


Hmm . . .

But I wonder if the 1.6 rockers + fast-rate cams might go well together if you also used a lower ultimate lift number on that cam to account for it?  It seems like that might work to artificially increase the cam's rate of lift even farther without pushing the final lift figure through the roof in the process.  (The whole shebang would probably function at dang near roller-cam profiles by then.)

[Ghoste]

So it comes down to what you define as a "mild" cam NMike.  What specs are on the stick you're using?

[NMike]

i need to pass an emissions test, so the cam won't be that big. it has to idle pretty clean and smooth.

[Ghoste]

Will you be able to do that with a 500 cube engine?  What are they testing for?

[NMike]

limit is 500ppm HC, 3.5% CO. hence the need for a smooth cam. i think i might just put in a stock 440 to get through the test, then in the big motor.

i am trying to get "Collector" insurance,(savings of about $1000) and here it must pass the test once in order to qualify.

[firefighter3931]

 

But I wonder if the 1.6 rockers + fast-rate cams might go well together if you also used a lower ultimate lift number on that cam to account for it?  It seems like that might work to artificially increase the cam's rate of lift even farther without pushing the final lift figure through the roof in the process.  (The whole shebang would probably function at dang near roller-cam profiles by then.)

Mike, some of the fast lift flat tappet cams are on par with the milder street roller profiles and in some cases are more agressive. The problem arises when trying to keep the valves out of float at high rpm's when using the 1.6:1 rockers. The spring pressure needs to be increased significantly but that can also wipe out the cam....it's a catch 22. 


My  : if the flat tappet cam profile/rocker arm combination requires more than 400 lbs of spring pressure then you should be looking at a roller cam...assuming you want it to live sucessfully on the street. High spring pressures with flat tappet cams and low engine speeds (traffic/idleing) is a recipe for failure.




Ron

[Mike DC]

I agree with you in general. 

But I'm not entirely sold on why a faster-lifting cam would need more spring pressure than a slower-lifting one.  If we're assuming the same final result (same rev limiter RPM, same ultimate lift number at the valve itself), I don't see why the spring pressure would need to be any different. 




Just theoretically speaking here. 

I agree that faster rockers probably aren't a great way to squeeze more lift out of a motor and try to hit 7500 rpm.  Seems like if you want more valve opening THAT badly then you just need bigger heads/valves in the first place.   


But suppose you're building a fun 5500-rpm street tire ripper, and you want a basically mild cam duration to keep the street manners, and you're trying to give it a midrange throttle-response from hell . . .
 

[firefighter3931]

I agree with you in general. 

But I'm not entirely sold on why a faster-lifting cam would need more spring pressure than a slower-lifting one.  If we're assuming the same final result (same rev limiter RPM, same ultimate lift number at the valve itself), I don't see why the spring pressure would need to be any different. 

Mike, as valve speeds increase more VS pressure is required. Compare a solid cam to a roller and it becomes obvious when you look at the spring requirements. You won't see a mild roller with less than 200lbs on the seat and 450 lbs over the nose. Those spring pressures would eat a flat tappet cam in short order. 

So, an agressive flat tappet cam with a 1.6 rocker is going to approach roller cam velocities and now you need to keep it out of valve float...the only way to do that is with more valve spring. But....those higher spring pressures will wipe out the flat tappet stick, so you're fubarred. 

I still maintain that a fast lobed flat tappet must be used with a 1.5 rocker or it will go into valve float with the recommended spring. Upping the springs to keep the valves stable will push the pressures into the danger zone.

Bottom line....spec the right cam in the first place and don't try to re-engineer your valvetrain. Custom cams aren't that much more and it's easier to do it right the first time than having to do it over a second time. 



Ron

[Mike DC]

 
Okay, I got it that time.  Makes sense.

If we had been talking about a bump in the road ramming a wheel upward into the wheelwell at various speeds then I would have totally gotten it right off.  But for some reason I wasn't visualizing the valvespring operating enough like a real spring.




Hey, was it you who argued on a thread a while back against running a roller cam on a Mopar BB street wedge? 

I remember a thread like that.  Somebody was making the case that on a mild motor, rollers weren't always worth the money & side effects that come into play when compared to sticking with a good modern flat-tappet grind.  (Not a theoretical problem with rollers, just a practical issue with the current parts & prices for converting Mopar BB wedges to run rollers.)