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Cam and Rocker combo

4K views 20 replies 7 participants last post by  ap72 
#1 ·
ok. so i'm rebuilding a SB chevy almost from scratch. i have a 350 bored 30 over with a set of re worked 305 heads and new flat top pistons for increased compression at my altitude. along with a edelbrock performer air gap intake and performer 600cfm carb. my cam's lift is .462/.480 with 218/226 duration at .050. my problem is will it hurt to put on a set of 1.6 rockers or should i play it safe with the stock 1.5?
 
#3 ·
CrazyGearHead said:
will it hurt to put on a set of 1.6 rockers or should i play it safe with the stock 1.5?
Depends on a few things- including the valve springs.

Are they spec'ed for the new cam? If so, check the coil bind to see if there's enough room to run the added lift from the 1.6 rockers.

Be sure you also check the spring for installed height, seal to retainer, retainer to rocker, valve to piston, push rod to guide slot in head, rocker slot length (if using stamped/cast rockers w/pivot balls) and for proper geometry (push rod length). Prolly something else I'm missing.

A 'big' paper clip is right at 0.050" diameter wire- and can be used (after the thickness is verified) as a 'feeler gage' for the retainer to seal/boss and other clearances.
 
#5 ·
the change can help or hurt as briefly explained in this article:

http://www.popularhotrodding.com/enginemasters/articles/hardcore/0511phr_ratio/index.html

you want first class correct parts on the whole valve train with 1.6 because it drastically increases the valve acceleration speed (which increases valve mass inertia),,,which can cause valve float,,,it's not just more lift,,,its more faster lift to a further point closer to the piston....
 
#6 ·
red65mustang said:
the change can help or hurt as briefly explained in this article:

http://www.popularhotrodding.com/enginemasters/articles/hardcore/0511phr_ratio/index.html

you want first class correct parts on the whole valve train with 1.6 because it drastically increases the valve acceleration speed (which increases valve mass inertia),,,which can cause valve float,,,it's not just more lift,,,its more faster lift to a further point closer to the piston....

I've honestly never seen nor heard of any engine loosing power from a higher ratio rocker.
 
#7 ·
CrazyGearHead said:
ok. so i'm rebuilding a SB chevy almost from scratch. i have a 350 bored 30 over with a set of re worked 305 heads and new flat top pistons for increased compression at my altitude. along with a edelbrock performer air gap intake and performer 600cfm carb. my cam's lift is .462/.480 with 218/226 duration at .050. my problem is will it hurt to put on a set of 1.6 rockers or should i play it safe with the stock 1.5?

IMHO, running the 1.6 rocers with that small of a cam isn't really gonna be very benificial, you may gain a very little bit of power but will it be worth the extra stress on the rest of the valve train, another thing i would want to mention is that unless you're running the 4416 305HO heads then your flow numbers are going to drop off drasticlly after .450 lift, even with a ton of port work from someone who is a serious pro with these restrictive heads. one other thing is that you mentioned is that your running flat-tops (-5cc to -7cc valve relief pistons) with a 58cc or 60cc head, your compression is going to be very high for the street, without running it in a compression ratio calculator i would guess around 11.0+:1
 
#8 ·
AP,
different cylinder heads intake to exhaust flow ratio's vary so much is what can cause less power with higher lift ratio on both valves...

very often more lift on either the intake or exhaust "only" actually does add power on a particular head by adjusting the I/E ratio

they do make lower/slower ratios to adjust in the opposite direction for more power as well...

very wise words quote from Max Keith (a member): "ALOT more gooder is not necessarily more good"

my $.02:
only trying more and less lift combinations on a dyno can tell you good or bad,,,,it changes the interplay relationships of the whole rotating assembly
 
#9 ·
red65mustang said:
AP,
different cylinder heads intake to exhaust flow ratio's vary so much is what can cause less power with higher lift ratio on both valves...

very often more lift on either the intake or exhaust "only" actually does add power on a particular head by adjusting the I/E ratio

they do make lower/slower ratios to adjust in the opposite direction for more power as well...

very wise words quote from Max Keith (a member): "ALOT more gooder is not necessarily more good"

my $.02:
only trying more and less lift combinations on a dyno can tell you good or bad,,,,it changes the interplay relationships of the whole rotating assembly
I understand they make other ratios, I just hve never seen a case where a higher ratio produced less peak power.
 
#10 ·
Another great read on rocker arm ratios. Shows that a smaller cam with 1.6 rockers can be better than a bigger cam with 1.5 even though lift is about the same. The increase in duration of 4-5* is another bonus.

http://www.pontiacstreetperformance.com/psp/RockerArms.html

"Higher ratio rocker arms open the valve faster, higher, and hold it open for a much greater total period of time as compared to lower ratio units. Does this cause more stress on the valve train? There will be more pressure on the cam lobes due to the friction and pressure caused by the higher lift and resultant greater spring load. However, as compared to providing the same higher lift and effective longer duration with a more radical cam and even stiffer springs, the higher ratio rockers may create less total valve train stress. And such a cam lobe would be very aggressive and would require much heavier springs to keep the lifter from flying off the lobe. Very radical lobes will also add more side stress on the lifters/bores and could possibly cause lifter bore failure. The added pressure on the studs from either higher ratio rockers, or more radical lobe, will be well within the capabilities of modern after market studs."
 
#11 ·
shark,
darn good article...

but the chart also needs a heads I/E ratio line and a piston bore position per degree line based on stroke and piston acceleration/deceleration/speed line...
(math gets so complicated only a dynamic (dyno) test makes sense)

I sure wish articles would explain both sides,,,there is good and bad with any change....

a performance cam with normal lift 1.5 ratio already makes "yuch" for normal street rpms TQ output,,,compared to the oem cam...
(that's why you have to have a high slip convertor and/or deeper gears)

increase that ratio to 1.6 and you will have YUCH!!! TQ output at street rpms
(to gain say maybe a 1/10th quicker at the strip)
 
#12 ·
Run an "RV" cam with 1.6 rockers and you have more tq than stock and equal hp to a larger cam with smaller rockers.

Increasing the rocker ratio has the same effect on power as increasing the lobe's intensity, the difference is that an increase in rocker ratio is more stable and durable than increasing the lobe intensity.

Look at the VooDoo line of cams, imagine having all of the bennfits and none of the durability issues- that's what a rocker change can do.
 
#13 · (Edited)
ap

204/214 rv cam on a 8.5CR 302

stock 1.6 ratio on intake and exhaust
1k=142ft/lbs
2k=234
3k=290
peak = 290

1.7 ratio on both
1k=139
2k=233
3k=293
peak =293

you gained "squat" for actual TQ in the every day driving rpms range with a ratio change on those stock E7 style heads with a rv cam...

for kicks I plugged in 1.5's on both with the same cam
1k=142
2k=234
3k=289
peak=289

and finally,,,
1.5=210HP
1.6=215HP
1.7=220HP
(5HP is basically negligable gain accelerating a 3400lb car)

my $.02:
ratio change is the same as ign timing change,,,only testing combo's will tell you what the motor truly wants....
 
#17 · (Edited)
all these numbers are from my analyzer software which does reflect changes made darn close so far as how they relate before and after....
(especially when only changing one variable,,,the rocker ratio)

repeat last post info:
it is a stock 1987 550cfm EFI (HO) 302 with the factory "shorty's",,,
(rated 225/300 with the stock HO cam)
I only change the cam to a 204/214

first numbers were with stock 360cfm exhaust (cause it is only 240 total cfm at 3k on a 302 and my point is low rpms TQ gain/loss so the pipes were not a restriction)

plugged in 600CFM exhaust,,,,checked 1.6 and 1.7 (on both) intake and exhaust with the rv cam results,,,, from 1k thru 3k TQ numbers remain the same values for both as posted above (+,- 1ft/lb)...
(proves again big pipes don't do squat for street rpms)

my only point with the example is there is no "blanket" statement possible for what a ratio change is actually going to do...
 
#19 ·
ap,
nope, not me or the software,,,I've tested it to many times against published motor combo dyno results and tested it against another analyzer program....

the only "trick" I played on purpose was choosing the stock 302 heads,,,their stock I/E flow ratio and I/E valves flow effiency is why those results pop up...

PS: no software can give exact dyno results but they are darn good for guidance for answering "what if I bought",,,"what can I expect"
 
#20 · (Edited)
Using Engine Analyzer Pro, I got a small increase switching to 1.6 rockers on my set up. Honestly I was expecting more raising the lift from .525 to .560 on intake and .546 to .580 on exhaust. I guess at larger lifts the valve is already allowing high flow, further increasing lift does not have as big an effect. I guess it's more about duration at that point.

I tried it over 3 different compression ratios and got interesting results.

Compression..............1.5......................1.6
11:1..................542HP/545TQ.........547HP/542TQ
10.5:1................535HP/538TQ.........540HP/536TQ
10:1..................528HP/538TQ.........530HP/533TQ

Peak HP stayed right around 5750rpms and peak torque stayed around 4500rpms. However on the 1.6 rockers the peak HP was duplicated at 6000rpms before dropping.

What I thought was interesting was the difference in less than 1% increase in HP and simular torque drop. Doesn't seem worth the extra stress on the valve train for the extra lift. Of course with a larger duration cam it may make more of a difference.

I also may have some bad data as I don't see my combo making that much power. Maybe 475HP before spraying.

AFR195 heads on 383 with 11:1 compression all forged bottom end. Cam is hydraulic VooDoo .525/.546 with 284/292 advertised duration and 241/249 @ 0.050 lift, 110LSA 106centerline. 1 3/4 headers, air gap RPM and 750 vac sec carb.
 
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