half/half 1.5/1.6 rocker arms
08-10-2009, 09:59 PM
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half/half 1.5/1.6 rocker arms a buddy gave me a set of rocker arms half 1.6 half 1.5 where do the 1.6 ones go and where do the 1.5 ones go?
08-11-2009, 02:53 AM
#4
Agreed. The exhaust side of the heads flows less than the intake so the added lift helps out in that department.
A split duration cam already accomplishes this however. I would do all 1.6's and sell the 1.5's. The added lift on the intake side will only benefit.
A split duration cam already accomplishes this however. I would do all 1.6's and sell the 1.5's. The added lift on the intake side will only benefit.
08-11-2009, 09:30 AM
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wow this is something new to me. i never knew about running a bigger rocker on the ext side of the house. learn something new everyday i suppose
08-11-2009, 05:41 PM
#12
Wow. I have NEVER heard of running 1.5's on the intake and 1.6's on the exhaust, and I've been doing this for a while. Dyno test after dyno test shows that the increased lift is only beneficial on the INTAKE side. On the exhaust side, the effect is negligible.
I've run 1.6 intake / 1.5 exhaust several times and it works quite well. In fact, that's what I have in my turbo car now. My cam is 224/236 and lift is .605/.593. I could have run 1.6 across the board, but didn't want the increased spring load at .625 lift on the exhaust for no power gain.
And, you can't compare intake versus exhaust flow numbers. The intake and exhaust stroke processes are completely different.
I've run 1.6 intake / 1.5 exhaust several times and it works quite well. In fact, that's what I have in my turbo car now. My cam is 224/236 and lift is .605/.593. I could have run 1.6 across the board, but didn't want the increased spring load at .625 lift on the exhaust for no power gain.
And, you can't compare intake versus exhaust flow numbers. The intake and exhaust stroke processes are completely different.
08-11-2009, 05:46 PM
#13
Wow. I have NEVER heard of running 1.5's on the intake and 1.6's on the exhaust, and I've been doing this for a while. Dyno test after dyno test shows that the increased lift is only beneficial on the INTAKE side. On the exhaust side, the effect is negligible.
I've run 1.6 intake / 1.5 exhaust several times and it works quite well. In fact, that's what I have in my turbo car now. My cam is 224/236 and lift is .605/.593. I could have run 1.6 across the board, but didn't want the increased spring load at .625 lift on the exhaust for no power gain.
And, you can't compare intake versus exhaust flow numbers. The intake and exhaust stroke processes are completely different.
I've run 1.6 intake / 1.5 exhaust several times and it works quite well. In fact, that's what I have in my turbo car now. My cam is 224/236 and lift is .605/.593. I could have run 1.6 across the board, but didn't want the increased spring load at .625 lift on the exhaust for no power gain.
And, you can't compare intake versus exhaust flow numbers. The intake and exhaust stroke processes are completely different.
08-24-2009, 08:54 PM
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my engine builders are doing this on my lt1 so ill let you guys know how it works for me... guy buildint my engine is very experienced and has a great reputation so i trust them.. and yea its 1.6 on the int and 1.5 on exhuast... i was a little weary at first too.. but engineermikes post made me feel a little better... i have my cam already and the lifts were a little strange to me (custom grind comp cam for my setup)... but hopfully it all works out well... i mean these guys arent lt1 specialists but lt1 is really just an updated SBC and the know those in and out... why whould you run 1.5 int. and 1.6 exh... that wouldnt make sense if more exhuast was exiting than air entering.... correct??
08-24-2009, 09:49 PM
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Basically two factors: The intake to exhaust flow ratio of the cylinder heads and how restrictive the car's exhaust system is.
If the exhaust flow on the cylinder heads is less than 70% of intake flow, then the exhaust needs either a longer exhaust duration and/or more exhaust valve lift. Many guys consider 70% as the cross-over point.
Stock manifolds, a single CAT and quiet (restrictive) mufflers also create more exhaust flow restriction, so the same thing to the cam's exhaust lobe can be of benefit. Best way to find out is through trial and error on the dyno or by comparing the MPH at the track.
On a 415 L98 running a ZZ9, I had 1.6/1.5s and tried them both ways. I could definitely tell the difference with 1.6s on the intake and the 1.5s on the exhaust. However you must remember the ZZ9 has quite a bit more exhaust duration than you normally find in many off-the-shelf cams. Many OTS cams have a 6 degree duratiom split, while the ZZ9 has more like 14 degrees of split.
Jake
West Point ROCKS! NATION'S TOP COLLEGE per Forbes Magazine!
If the exhaust flow on the cylinder heads is less than 70% of intake flow, then the exhaust needs either a longer exhaust duration and/or more exhaust valve lift. Many guys consider 70% as the cross-over point.
Stock manifolds, a single CAT and quiet (restrictive) mufflers also create more exhaust flow restriction, so the same thing to the cam's exhaust lobe can be of benefit. Best way to find out is through trial and error on the dyno or by comparing the MPH at the track.
On a 415 L98 running a ZZ9, I had 1.6/1.5s and tried them both ways. I could definitely tell the difference with 1.6s on the intake and the 1.5s on the exhaust. However you must remember the ZZ9 has quite a bit more exhaust duration than you normally find in many off-the-shelf cams. Many OTS cams have a 6 degree duratiom split, while the ZZ9 has more like 14 degrees of split.
Jake
West Point ROCKS! NATION'S TOP COLLEGE per Forbes Magazine!
Last edited by JAKEJR; 08-24-2009 at 09:58 PM.
08-25-2009, 03:51 PM
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i still dont know why my builders are doing this... im gonna ask them this week when i go in there... but like i said they are very experirienced and have built many race motors much grander than mine soo i trust them...
08-25-2009, 04:03 PM
#17
back when the LT1 was "new" (94 for most applications) and cam/valvetrain choices were limited many did the Crane 227 cam with 1:6/1:5 split RR with the 1:5's on the exhaust side.
Main reason was with 1:6 the exhaust lift was on the edge for the press in studs (iron heads) and what was typically used 10309 spring kit
newer cams and valvetrain componets typically don't require a split but there are "reasons" a split is beneficial on some builds.
ymmv
Main reason was with 1:6 the exhaust lift was on the edge for the press in studs (iron heads) and what was typically used 10309 spring kit
newer cams and valvetrain componets typically don't require a split but there are "reasons" a split is beneficial on some builds.
ymmv
08-26-2009, 09:27 PM
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back when the LT1 was "new" (94 for most applications) and cam/valvetrain choices were limited many did the Crane 227 cam with 1:6/1:5 split RR with the 1:5's on the exhaust side.
Main reason was with 1:6 the exhaust lift was on the edge for the press in studs (iron heads) and what was typically used 10309 spring kit
newer cams and valvetrain componets typically don't require a split but there are "reasons" a split is beneficial on some builds.
ymmv
Main reason was with 1:6 the exhaust lift was on the edge for the press in studs (iron heads) and what was typically used 10309 spring kit
newer cams and valvetrain componets typically don't require a split but there are "reasons" a split is beneficial on some builds.
ymmv
08-23-2011, 09:05 PM
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hello folks. digging up an old thread I know but I ran across it so... The 1.6 intake/1.5 exh rockers on older sbc are used as a crutch for aftermarket cams, in place of a custom grind, or to make a streetable cam flow a little more. Many (not all) companies don't design exhaust lobes, they just slap a larger intake lobe on a smaller cams exhaust. You can look at cam specs to see what I mean, they're getting better on the whole these days.
In order to have the need to do this you'd need heads that flow well above a certain lift lets say they flow well to .600, and you want a more streetable/better idling cam with more vacuum. So you choose a cam that fits somewhere in the neighborhood of .52-.54 intake/exh lift and then you throw a 1.6 on the intake side to get a bit more lift and take advantage of your head flow. Changing the lift with rocker arms will not change the duration of the cam, but does increase valve overlap flow. You can get real technical here but its just an example. Above a 170 duration for a sbc or similar (lt engine) 1.6's are going to hurt you rather than help you. I don't know the science behind it but some old engine gurus found this out. Theres a high positive pressure when the exh valves open so they dont need the larger lift, but you will notice most sbc cams have a larger duration on the exhaust.
Some cams are meant to give a certain lope at idle and go one step further with the exhaust lobe, take a look at the specs of comp cams thumpr hyd rollers, they have a huge exhaust duration compared to the cams made for power.
One downside to using two ratio rockers is you're going to wear the intake lobes or their lifters associated with it faster than the exhaust.
In order to have the need to do this you'd need heads that flow well above a certain lift lets say they flow well to .600, and you want a more streetable/better idling cam with more vacuum. So you choose a cam that fits somewhere in the neighborhood of .52-.54 intake/exh lift and then you throw a 1.6 on the intake side to get a bit more lift and take advantage of your head flow. Changing the lift with rocker arms will not change the duration of the cam, but does increase valve overlap flow. You can get real technical here but its just an example. Above a 170 duration for a sbc or similar (lt engine) 1.6's are going to hurt you rather than help you. I don't know the science behind it but some old engine gurus found this out. Theres a high positive pressure when the exh valves open so they dont need the larger lift, but you will notice most sbc cams have a larger duration on the exhaust.
Some cams are meant to give a certain lope at idle and go one step further with the exhaust lobe, take a look at the specs of comp cams thumpr hyd rollers, they have a huge exhaust duration compared to the cams made for power.
One downside to using two ratio rockers is you're going to wear the intake lobes or their lifters associated with it faster than the exhaust.