[Mark1200]

I am trying to understand increasing rocker ratios.
If u are starting from scratch, should u buy a cam with say 480 lift useing a stock 1.5 rocker ratio . Or is it better to use a 450 lift cam and a 1.6 ratio. In a gen 1 350 chev.
I am trying to understand the pros and cons of adding ratios and why u would change it.

 

[curtis73]

The benefit of adding lift with rockers is that it tends to leave more room in the lobe for safe ramp speeds. It does add stress to the rocker and pushrod.

For a given cam lobe profile, you have a max ramp speed which is determined by the type of lifter and your needs for durability. Too fast and it destroys flat tappets, or pushes roller rockers hard into the side of the bore. If you increase the lift at the lobe, you are potentially increasing the ramp rate. If you re-grind the lobe with less ramp rate for your lift, you are abnormally slowing down the effective lift rate. (reducing the area "under the curve") The answer is a happy medium; you need "enough" lift from both the cam and the rockers to hit your valve lift numbers that match the flow in the heads.

 

[curtis73]

To expand a little bit... Although every cam lobe is different, let's say you have two otherwise identical cams. One has .320 lobe lift with 1.5 rockers (.480 valve) and the other has .300 lobe lift with 1.6 rockers (same .480 at the valve). Since the ramp rate can be the same for each lobe, the .300 lobe can get to peak lift faster than the .320 lobe. In this case (assuming they have the same ramp rates) the .300 lobe with 1.6 rockers will potentially make more power because the valve will open faster and have more dwell at higher lifts.

The downside is that for a given valve spring, the lobe-to-lifter interface will see higher loads compared to the 1.5 rocker. Usually not a big deal with proper lube... or with roller lifters.

 

[Mark1200]

The benefit of adding lift with rockers is that it tends to leave more room in the lobe for safe ramp speeds. It does add stress to the rocker and pushrod.

For a given cam lobe profile, you have a max ramp speed which is determined by the type of lifter and your needs for durability. Too fast and it destroys flat tappets, or pushes roller rockers hard into the side of the bore. If you increase the lift at the lobe, you are potentially increasing the ramp rate. If you re-grind the lobe with less ramp rate for your lift, you are abnormally slowing down the effective lift rate. (reducing the area "under the curve") The answer is a happy medium; you need "enough" lift from both the cam and the rockers to hit your valve lift numbers that match the flow in the heads.

I Assume that it is easy to find a cam for my mid manored 350 at a 1.5 ratio, So would there be any benefit to using 1.6? It sounds like I could accomplish the same thing both ways?
Do guys normally just start with 1.6? For a mid powered 350 ? Is there a norm?

 

[curtis73]

For a mild 350, you likely won't notice a difference. Pick a cam you like and use the ratio that matches your head flow. More lift at the valve isn't always better. Take a look at your heads' flow numbers. There will be a lift number at which the flow no longer increases or starts to decrease. No reason to go beyond that lift.

Many folks switch to 1.6 rockers because many factory cams are so mild that they're leaving some undiscovered ponies by not lifting high enough. Since you're selecting a new cam, it would be wise to take a look at flow numbers of your heads and decide on rocker ratio that way.

 

[Mark1200]

To expand a little bit... Although every cam lobe is different, let's say you have two otherwise identical cams. One has .320 lobe lift with 1.5 rockers (.480 valve) and the other has .300 lobe lift with 1.6 rockers (same .480 at the valve). Since the ramp rate can be the same for each lobe, the .300 lobe can get to peak lift faster than the .320 lobe. In this case (assuming they have the same ramp rates) the .300 lobe with 1.6 rockers will potentially make more power because the valve will open faster and have more dwell at higher lifts.

The downside is that for a given valve spring, the lobe-to-lifter interface will see higher loads compared to the 1.5 rocker. Usually not a big deal with proper lube... or with roller lifters.

Thanks for your reply.
So it makes more sense to use the higher ratio whenever possible then,right?
Since the 1.6 will have a faster ramp rate ,similar lift, and marginally more duration,how different would it look on the dyno? Would it touque in a particular Manner?

For a mild 350, you likely won't notice a difference. Pick a cam you like and use the ratio that matches your head flow. More lift at the valve isn't always better. Take a look at your heads' flow numbers. There will be a lift number at which the flow no longer increases or starts to decrease. No reason to go beyond that lift.

Many folks switch to 1.6 rockers because many factory cams are so mild that they're leaving some undiscovered ponies by not lifting high enough. Since you're selecting a new cam, it would be wise to take a look at flow numbers of your heads and decide on rocker ratio that way.

I was thinking about the Promax 183 heads.
Flow #s @
100, 2, 3, , 4, 5, 600.
68 137 198 241 249 250
Looks like they slow down at 400 and stop at 500.
Should I b looking for another head? I assumed I would start with a cam in the 48o lift range, but these seem to max out there. Can I have a head that has room to grow into without sacrificing bottom end?

 

[ericnova72]

If you want a ProMaxx head to grow into, the Maxx 200cc would be a better choice.

It doesn't necessarily hurt power if you push valve lift above where the heads stop gaining significant flow, as long as the head flow above that point doesnt drop off badly.
If the head is good, pushing the valve open above the peak flow lift of the head means that that the valve actually will spend more time with the meat of the flow curve flowing through that bigger open window than it would with a lobe that only opens the valve enough to just barely reach the peak flow lift of the head.

Looking at the 183cc numbers, peak flow lift appears to be somewhere in he .500" to 600" lift range, it is starting to slow gain around .500"....so if I was camming this, i would want to be in the .530" tp.600" lift range.

SBC's will often also react well to a 1.6/1.5 ratio split set of rocker arms, using the 1.6 on the intake to increase lift and flow window, 1.5 on the exhaust because it is a high pressure blow-down situation, it doesn't need the help from increased lift.
Intake does because it is operation under just atmospheric pressure.

 

[Mark1200]

If you want a ProMaxx head to grow into, the Maxx 200cc would be a better choice.

It doesn't necessarily hurt power if you push valve lift above where the heads stop gaining significant flow, as long as the head flow above that point doesnt drop off badly.
If the head is good, pushing the valve open above the peak flow lift of the head means that that the valve actually will spend more time with the meat of the flow curve flowing through that bigger open window than it would with a lobe that only opens the valve enough to just barely reach the peak flow lift of the head.

Looking at the 183cc numbers, peak flow lift appears to be somewhere in he .500" to 600" lift range, it is starting to slow gain around .500"....so if I was camming this, i would want to be in the .530" tp.600" lift range.

SBC's will often also react well to a 1.6/1.5 ratio split set of rocker arms, using the 1.6 on the intake to increase lift and flow window, 1.5 on the exhaust because it is a high pressure blow-down situation, it doesn't need the help from increased lift.
Intake does because it is operation under just atmospheric pressure.

Thanks for the advice. This needs to have good street Manners,run smoothly on the highway and oull hard in the lower ROM's. I thought the smaller port heads would suit that better. Is a 180 port the best choice for this? As for growing into, I don't amagine I would ever exceed .500-550 lift.

 

[ericnova72]

The 183cc would work nice on a 400-450hp street engine, and maybe 520-ish on a racy strip 350.

If you thought you might go changing it into a 7000 rpm ripper, or build it into a 383 and needed that room to grow in the future, then the 200cc would be a better choice, even though it would sacrifice some low end on your current plans.

 

[Mark1200]

The 183cc would work nice on a 400-450hp street engine, and maybe 520-ish on a racy strip 350.

If you thought you might go changing it into a 7000 rpm ripper, or build it into a 383 and needed that room to grow in the future, then the 200cc would be a better choice, even though it would sacrifice some low end on your current plans.

Do to your advice so far. I will look more closely and compare the flow #s on more heads.
Thanks.

If you want a ProMaxx head to grow into, the Maxx 200cc would be a better choice.

It doesn't necessarily hurt power if you push valve lift above where the heads stop gaining significant flow, as long as the head flow above that point doesnt drop off badly.
If the head is good, pushing the valve open above the peak flow lift of the head means that that the valve actually will spend more time with the meat of the flow curve flowing through that bigger open window than it would with a lobe that only opens the valve enough to just barely reach the peak flow lift of the head.

Looking at the 183cc numbers, peak flow lift appears to be somewhere in he .500" to 600" lift range, it is starting to slow gain around .500"....so if I was camming this, i would want to be in the .530" tp.600" lift range.

SBC's will often also react well to a 1.6/1.5 ratio split set of rocker arms, using the 1.6 on the intake to increase lift and flow window, 1.5 on the exhaust because it is a high pressure blow-down situation, it doesn't need the help from increased lift.
Intake does because it is operation under just atmospheric pressure.

when deciding on split ratio rockers, would the cam company recommend this depending on the chosen grind? or is this something i would pick based on the percentage of exht vs intake flow?

 

[johnsongrass1]

look at it this way.
the engine cars not about the cam profile.
It’s only sees the valves open and close.
The closer to ideal opening and closing will make the most power.
Staggered rockers are usually chosen as a crutch to a less than opening/closing.
When valve train weight is a concern, more ratio is better given the parameters of packaging. On a standard 23 degree head head 1:7 is pushing the limits.
for a R07 wide bore with 11 degree heads 2:1 is possible.

 

[Mark1200]

look at it this way.
the engine cars not about the cam profile.
It’s only sees the valves open and close.
The closer to ideal opening and closing will make the most power.
Staggered rockers are usually chosen as a crutch to a less than opening/closing.
When valve train weight is a concern, more ratio is better given the parameters of packaging. On a standard 23 degree head head 1:7 is pushing the limits.
for a R07 wide bore with 11 degree heads 2:1 is possible.

"Staggered rockers are usually chosen as a crutch to a less than opening/closing"

Sorry but I don't und your statement.

 

[Ianstaine1]

Yeah keep in mind Velocity when researching port volume and flow numbers. If you want good low end.. and at 450 lift a 180cc head matches a 200cc head then the 180cc will be a better choice due to higher velocity etc. and if the 200cc flows a little higher at .550 or .600 you may not notice it much if you aren't running in a high powerband. Another thing to think about using a 1.6 rocker ratio is you can get the lift you want out of your valve to match head flow and keep your duration lower so you would need less or no stall converter. I usually think about what stall speed i want along with matching flow and it will help figure out the rocker ratio.

 

[BogiesAnnex1]

To some stent the big advantage if using a higher ratio rocker is it produces faster and more lift at the valve without the inertia expense of having to lift the tappet and push rod as well. Not that it’s a fee lunch as there are greater forces from the spring, retainer and valve but at least that isn’t all the parts.

When building really serious pushrod engines you can also use different ratio rockers on to help balance out manifold induced feeding differences between each cylinder.

In general using a 1.6 over a 1.5 rocker will allow the cam to act as if it is bigger producing a bit more power than the ratio difference alone would indicate. It is reasonable if your going to hit the rev limiter a lot to go to 7/16th studs, or stud girdles on the 3/8ths studs along with aluminum rockers as this beefs up the rocker mount and removes some of the mass forces. Not that this is needed with a mild cam on the street, but ya know it always good to be prepared.

Different heads react a bit differently to this as will a for instance is the ProComp SBC head which is not a super good flowing head at low lifts but carries really well in the high lifts so here getting the valve open faster and with more over the top allows to head to spend more of its duration time at lifts that are useful to the port flow.

So these and several other tricks are hiding in this rocker ratio bag.

Bogie

 

[Mark1200]

To some stent the big advantage if using a higher ratio rocker is it produces faster and more lift at the valve without the inertia expense of having to lift the tappet and push rod as well. Not that it’s a fee lunch as there are greater forces from the spring, retainer and valve but at least that isn’t all the parts.

When building really serious pushrod engines you can also use different ratio rockers on to help balance out manifold induced feeding differences between each cylinder.

In general using a 1.6 over a 1.5 rocker will allow the cam to act as if it is bigger producing a bit more power than the ratio difference alone would indicate. It is reasonable if your going to hit the rev limiter a lot to go to 7/16th studs, or stud girdles on the 3/8ths studs along with aluminum rockers as this beefs up the rocker mount and removes some of the mass forces. Not that this is needed with a mild cam on the street, but ya know it always good to be prepared.

Different heads react a bit differently to this as will a for instance is the ProComp SBC head which is not a super good flowing head at low lifts but carries really well in the high lifts so here getting the valve open faster and with more over the top allows to head to spend more of its duration time at lifts that are useful to the port flow.

So these and several other tricks are hiding in this rocker ratio bag.

Bogie

Looks like I need to do a lot research on the flow of all the heads before I go any further.
Ian.
Can u expand on the stall converter selection. I dont5 know anything about them really. This is a 2500lb. ModelA with a 700r and we will start with a 373 rearend.

 

[jaw22w]

I just built a 350 +.030 for my '26 Ford model T. My car has specs very similar to yours. 2500#, 700r4, and 3.80 rear. I used the ProMaxx Maxx 183 heads with 1.5 rockers. 10:1 CR. I was only looking for about 350 HP in this engine. I know it is probably capable of 450HP at least, but I really had no need of that much HP in a 2500# car. 350 HP will pretty much spin the tires at will. My advice on the cam is to call one of the custom cam grinders, give him all your specs and wishes and let him design you a custom cam. I happened to use Straub. There are others, such as Mike Jones and Bullet. My custom cam cost ended up being very competitive with what I could have done at Summit. I've not had the new motor on the dyno, but experience tells me that it is right around 350 HP, maybe a little more. The hydraulic roller cam Straub supplied is 215/224 @.050, 284/296 advertised, .470/.490 w/1.5 rockers on a 112 LSA.
My convertor is something Dan, my transmission man, came up with that has a couple hundred more stall rpm than the stock convertor. Seems like he said it was out of a Vega, but I could be very wrong about that.
I'm pretty happy with the end result.

 

[Mark1200]

I just built a 350 +.030 for my '26 Ford model T. My car has specs very similar to yours. 2500#, 700r4, and 3.80 rear. I used the ProMaxx Maxx 183 heads with 1.5 rockers. 10:1 CR. I was only looking for about 350 HP in this engine. I know it is probably capable of 450HP at least, but I really had no need of that much HP in a 2500# car. 350 HP will pretty much spin the tires at will. My advice on the cam is to call one of the custom cam grinders, give him all your specs and wishes and let him design you a custom cam. I happened to use Straub. There are others, such as Mike Jones and Bullet. My custom cam cost ended up being very competitive with what I could have done at Summit. I've not had the new motor on the dyno, but experience tells me that it is right around 350 HP, maybe a little more. The hydraulic roller cam Straub supplied is 215/224 @.050, 284/296 advertised, .470/.490 w/1.5 rockers on a 112 LSA.
My convertor is something Dan, my transmission man, came up with that has a couple hundred more stall rpm than the stock convertor. Seems like he said it was out of a Vega, but I could be very wrong about that.
I'm pretty happy with the end result.

I have had your build in mind for sure. I was not going with a roller cam for cost saving but I like your build as we have basically the same car.
Did u do a shift kit?

 

[johnsongrass1]

It's hard to understand sentences that don't have all the words in it.
What I meant, was staggering ratio's are a way to crutch a cam that's less than ideal. Such as a more lift on the exhaust to help out a poor flowing exhaust for instance or crutch a poor flowing intake tract on the intake side of things.
It's far easier to add a half set of rockers than change a cam. Up to the point where the rocker ratio interferes with a good geometry anyway.

 

[johnsongrass1]

"Staggered rockers are usually chosen as a crutch to a less than opening/closing"

Sorry but I don't und your statement.

It's hard to understand sentences that don't have all the words in it.
What I meant, was staggering ratio's are a way to crutch a cam that's less than ideal. Such as a more lift on the exhaust to help out a poor flowing exhaust for instance or crutch a poor flowing intake tract on the intake side of things.
It's far easier to add a half set of rockers than change a cam. Up to the point where the rocker ratio interferes with a good geometry anyway.

 

[Mark1200]

It's hard to understand sentences that don't have all the words in it.
What I meant, was staggering ratio's are a way to crutch a cam that's less than ideal. Such as a more lift on the exhaust to help out a poor flowing exhaust for instance or crutch a poor flowing intake tract on the intake side of things.
It's far easier to add a half set of rockers than change a cam. Up to the point where the rocker ratio interferes with a good geometry anyway.

I don't have the in-depth understanding of this so I was confused
Now that makes perfect sense to me .lol
.
Thanks.
Am I right in saying that I should start with 1:6 rocker ratio ,and a cam to suit, then adjust ratios if necessary to adjust for head flow ?