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Old 02-11-2009, 12:11 PM
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High Ratio Rocker Arms

I have a 5.3 Vortec....Cold Air Intake, Exhaust, and a Computer Tune. I am pretty sure this engine come stock with 1.7:1 Rockers. I have been thinking about putting 1.8:1 Rockers on it to get a little bit more out of it. I know I will have to retune the PCM, but my question is: Will I loose a lot of low end torque with the additional valve lift? I have calculated the valve lift with both rockers and the 1.7's are .466 intake/.456 exhaust, while the 1.8's would give it .493 intake/.482 exhaust. I couldn't find much information other than the lobe lift on the stock cam, and it is .274 intake and .268 exhaust. Any ideas? Thanks

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Old 02-11-2009, 12:17 PM
richard stewart 3rd's Avatar
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Hi,
I don't think there is enough difference to make a notable change, I doubt you will be able to tell.
Rich
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Old 02-11-2009, 01:15 PM
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Vortecs have 1.7 rockers?? Hmm, thats news to me. The earlier stock rockers are supposed to be 1.5 ratio and 1.6 is an upgrade. ??

What I do know about the Vortecs is that if you do use a higher ratio rocker you need to be careful because the lift of a stock vortec head is very limited due to the height of the valve guide bosses. I think you can use older style valves and retainers to allow a bit more lift, but most folks get the boses milled down to allow higher lift. With that said, more lift is a good thing and essentialy has the same effect as increasing the valve size.
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Old 02-11-2009, 01:27 PM
How fast is fast enough?
 
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Quote:
Originally Posted by 65smallblock
Vortecs have 1.7 rockers?? Hmm, thats news to me. The earlier stock rockers are supposed to be 1.5 ratio and 1.6 is an upgrade. ??

What I do know about the Vortecs is that if you do use a higher ratio rocker you need to be careful because the lift of a stock vortec head is very limited due to the height of the valve guide bosses. I think you can use older style valves and retainers to allow a bit more lift, but most folks get the boses milled down to allow higher lift. With that said, more lift is a good thing and essentialy has the same effect as increasing the valve size.
Wrong Vortec, I'm pretty sertain he mean the newer gen III 5.3L.
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Old 02-11-2009, 01:32 PM
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Quote:
Originally Posted by ap72
Wrong Vortec, I'm pretty sertain he mean the newer gen III 5.3L.
Ahh! So the LS series have 1.7 stock rockers?
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Old 02-11-2009, 01:39 PM
How fast is fast enough?
 
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Quote:
Originally Posted by 65smallblock
Ahh! So the LS series have 1.7 stock rockers?
That's what I understand, but I have no firsthand experience with the gens III 5.3
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Old 02-11-2009, 02:15 PM
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Quote:
Originally Posted by ap72
That's what I understand, but I have no firsthand experience with the gens III 5.3
Cool, that means I learned something today.
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Old 02-11-2009, 02:56 PM
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Quote:
Originally Posted by GunnerTHB
I have a 5.3 Vortec....Cold Air Intake, Exhaust, and a Computer Tune. I am pretty sure this engine come stock with 1.7:1 Rockers. I have been thinking about putting 1.8:1 Rockers on it to get a little bit more out of it. I know I will have to retune the PCM, but my question is: Will I loose a lot of low end torque with the additional valve lift? I have calculated the valve lift with both rockers and the 1.7's are .466 intake/.456 exhaust, while the 1.8's would give it .493 intake/.482 exhaust. I couldn't find much information other than the lobe lift on the stock cam, and it is .274 intake and .268 exhaust. Any ideas? Thanks
You'll give up some bottom end and get it back on the top end. Where bigger ratio rockers get you is that the amount of lift per degree of cam rotation increases, not just a change to max lift. This makes the engine act as if a bit more cam is in it. At low RPMs the incoming charge doesn't enough velocity to over come reversion pressures during overlap and reverse pumping of the piston as it starts coming up after BDC. So some incoming charge gets pumped back into the intake, this reduces the density of the charge to be compressed in the cylinder and is seen as a power loss. This continues till the velocity of the gases in the ports is sufficient to overcome these reversal effects on the gas dynamics. Usually takes about 4000 RPM to do this.

Porting has the same affect as well for the same reasons. Porting, more cam, more lift, bigger ratio rockers all are additive effects the more of these you do the more power is traded from the bottom end to the top RPMs.

Bogie
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Old 02-11-2009, 03:05 PM
How fast is fast enough?
 
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Quote:
Originally Posted by oldbogie
You'll give up some bottom end and get it back on the top end. Where bigger ratio rockers get you is that the amount of lift per degree of cam rotation increases, not just a change to max lift. This makes the engine act as if a bit more cam is in it. At low RPMs the incoming charge doesn't enough velocity to over come reversion pressures during overlap and reverse pumping of the piston as it starts coming up after BDC. So some incoming charge gets pumped back into the intake, this reduces the density of the charge to be compressed in the cylinder and is seen as a power loss. This continues till the velocity of the gases in the ports is sufficient to overcome these reversal effects on the gas dynamics. Usually takes about 4000 RPM to do this.

Porting has the same affect as well for the same reasons. Porting, more cam, more lift, bigger ratio rockers all are additive effects the more of these you do the more power is traded from the bottom end to the top RPMs.

Bogie
That would normally be true for most "hotrodder engines" but you're forgetting that his cam has short durations and an ungodly wide LSA. He probably won't loose any low end as those two combine to make minimal reversion, by the same token he won't see huge gains up top. Higher ratio rockers are great, but you probably want to consider a cam swap first.
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Old 02-11-2009, 04:08 PM
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Two data points I have seen over the years with simple rocker upgrades, either zero gain or 10 or so. Have never seen a loss. The engine either likes it or doesn't care. I doubt VERY much you need to retune the PCM just for rockers.
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Old 02-11-2009, 08:35 PM
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Alright thanks for your input guys
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Old 02-11-2009, 10:43 PM
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Quote:
Originally Posted by 65smallblock
Ahh! So the LS series have 1.7 stock rockers?
Quote:
Originally Posted by ap72
That's what I understand, but I have no firsthand experience with the gens III 5.3
Many of the Gen IIIs do indeed have 1.7 rockers. This includes the LS1, LS2, LS6 and the 4.8, 5.3 and 6.0 truck engines.

The LS7 uses 1.8s
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