Chevy Nova Forum banner
21 - 32 of 32 Posts

·
Registered
Joined
·
1,057 Posts
I've always ran the same on intake and exhaust. I was just wondering why. Has anybody tried both and seen performance gains by running a smaller lift on the exhaust or is it pretty much a wash performance wise. I have bumped both to 1.6 instead of 1.5 just to have more lift.
 

·
Registered
Joined
·
1,246 Posts
I've always ran the same on intake and exhaust. I was just wondering why. Has anybody tried both and seen performance gains by running a smaller lift on the exhaust or is it pretty much a wash performance wise. I have bumped both to 1.6 instead of 1.5 just to have more lift.
What I can say, is the 1.6's across the board doesn't HURT anything. Just requires the spring and piston to valve clearance to use it since the piston is chasing the valve on the exhaust side. I have not seen a 1 .6 exhaust LOSE power. But I have seen small gains with the 1.6 intake and no gains with the 1.6 exhaust. I mean, at the time I think it was 10-15 hp. What we were able to do, is not lose the compression by flycutting the piston to clear the extra lift on the exhaust side in a few cases. In a street car with a flat top piston this means really nothing. Years ago I ran my 427 big block, and I thought I needed a 1.94 exhaust to go with the 2.25 intake. The car didn't really show much gain. Later on I found that the head would have been happier with a 2.30 intake and 1.88 exhaust.
 

·
Registered
Joined
·
184 Posts
Discussion Starter · #23 ·
It's a lot harder to get it in, with nothing but atmospheric pressure filling the cylinder.

On the exhaust side, you've got the ability to open the valve when pressure is still high, and then let the outrushing high pressure exhaust help evacuate itself with the draw it puts on the header pipe and port at the end of the exhaust event.
It not just the rising piston pushing it out, actually the rising piston does very little of the entire exhaust event.

Put bluntly, the intake side needs the help, the exhaust side doesn't....at least on an SBC.

Typically putting the 1.6 rocker on the exhaust hurts low end torque for a slight high rpm gain.
I've had some suggest I'd have been better to use 1.7 on the intake . What's your opinion on that ?
 

·
Registered
Joined
·
255 Posts
Probably depends on valvetrain stability really. At a certain point I'd think it would be better to go with a different lobe than a bigger rocker ratio.

That's why I ended up ordering a custom cam. Pay someone else to make those decisions for me.
 

·
Registered
Joined
·
1,246 Posts
Probably depends on valvetrain stability really. At a certain point I'd think it would be better to go with a different lobe than a bigger rocker ratio.

That's why I ended up ordering a custom cam. Pay someone else to make those decisions for me.
I agree with the thinking of if you need that much ratio, the lobe is wrong. The heads were designed for 1.5, if you start changing the ratio too much you start creating new issues with alignment and clearance. In an all out race engine when you can’t make the lobe bigger, then sure, you do what you have to other places. But for a street motor I wouldn’t.
 

·
Registered
Joined
·
1,057 Posts
What I can say, is the 1.6's across the board doesn't HURT anything. Just requires the spring and piston to valve clearance to use it since the piston is chasing the valve on the exhaust side. I have not seen a 1 .6 exhaust LOSE power. But I have seen small gains with the 1.6 intake and no gains with the 1.6 exhaust. I mean, at the time I think it was 10-15 hp. What we were able to do, is not lose the compression by flycutting the piston to clear the extra lift on the exhaust side in a few cases. In a street car with a flat top piston this means really nothing. Years ago I ran my 427 big block, and I thought I needed a 1.94 exhaust to go with the 2.25 intake. The car didn't really show much gain. Later on I found that the head would have been happier with a 2.30 intake and 1.88 exhaust.
I'm never too old to learn. That is why I was asking. I was thinking that for years and years I tried to pick up ET by tuning at the track. It never dawned on me to try a smaller lift on the exhaust. After a while there simply isn't anything left to make a lot of difference so you have to start looking for smaller chunks of power. If I would have thought about it I would have tried it. I did always try to stay pretty square even on my camshafts. My 65 does have a split duration and lift hydraulic roller in it but the splits aren't large.
 

·
Registered
Joined
·
703 Posts
I've had some suggest I'd have been better to use 1.7 on the intake . What's your opinion on that ?
1.65 is as far as I've used personally, so I can't say on the 1.7.

If it was a shaft set-up and you had talked with a cam designer to get a cam designed to utilize that ratio I wouldn't have a problem with it.

Don't forget that when you increase rocker ratio you don't get just a lift increase, you also get an increase in effective duration measured at the valve(which is the only place it counts!)....like you added a larger duration, higher lift cam lobe.
 

·
Registered
Joined
·
157 Posts
I agree with the thinking of if you need that much ratio, the lobe is wrong. The heads were designed for 1.5, if you start changing the ratio too much you start creating new issues with alignment and clearance. In an all out race engine when you can’t make the lobe bigger, then sure, you do what you have to other places. But for a street motor I wouldn’t.
It is more about valve velocity. I can create more velocity with rocker ratio than I can with lobe profile. The cam can only have so steep of a ramp before we lose control. I have built a few SBC with 2.0 and 2.1 ratio intake rockers. Need to be careful on the exhaust as a person can actually over scavenge the intake charge and pull it right back out the exhaust.
 

·
Registered
Joined
·
184 Posts
Discussion Starter · #29 ·
It is more about valve velocity. I can create more velocity with rocker ratio than I can with lobe profile. The cam can only have so steep of a ramp before we lose control. I have built a few SBC with 2.0 and 2.1 ratio intake rockers. Need to be careful on the exhaust as a person can actually over scavenge the intake charge and pull it right back out the exhaust.
Yes I agree and more velocity is what I need I think because I'm running a 2.08 valve which is supplying more than the demand of my engine which could potentially result in and engine that may be a little lazier than if the square inches of valve were appropriately matched with the demand of the engine but as you said higher lift can create more velocity .
That is one of the aspects of my engine combo that I'm second guessing now..
Lift . I think I could have lifted atleast 1.7 .
I sunk quite a bit on Money into shaft rockers because of the potential valve train stability issues that could arise due to the very aggressive steep lobe ramp cam that I chose.
I do have 1.6 on the intake but again I could have or should have used a higher ratio .

What is your oppinion on using a 2.08 valve on a 385 cubic inch SB ?
 

·
Registered
Joined
·
184 Posts
Discussion Starter · #30 ·
1.65 is as far as I've used personally, so I can't say on the 1.7.

If it was a shaft set-up and you had talked with a cam designer to get a cam designed to utilize that ratio I wouldn't have a problem with it.

Don't forget that when you increase rocker ratio you don't get just a lift increase, you also get an increase in effective duration measured at the valve(which is the only place it counts!)....like you added a larger duration, higher lift cam lobe.
Ok I see and doesnt using higher ratio rockers open the valve faster too ?
 

·
Registered
Joined
·
703 Posts
Yes I agree and more velocity is what I need I think because I'm running a 2.08 valve which is supplying more than the demand of my engine which could potentially result in and engine that may be a little lazier than if the square inches of valve were appropriately matched with the demand of the engine but as you said higher lift can create more velocity .
That is one of the aspects of my engine combo that I'm second guessing now..
Lift . I think I could have lifted atleast 1.7 .
I sunk quite a bit on Money into shaft rockers because of the potential valve train stability issues that could arise due to the very aggressive steep lobe ramp cam that I chose.
I do have 1.6 on the intake but again I could have or should have used a higher ratio .

What is your oppinion on using a 2.08 valve on a 385 cubic inch SB ?
Ok I see and doesnt using higher ratio rockers open the valve faster too ?
Seems like you are confusing two different things using the word velocity.
High ratio rocker increases the velocity of the valve moving open or closed in the valve guide. Quicker rate of lift at the valve.
This has nothing to do with airflow velocity in the intake port as far as being able to speed up the airflow velocity.....that would be a function of port volume and port shape.

Nothing wrong with a 2.08" intake valve on 385".

You wouldn't find any miracle amount of power with 1.65 or 1.7 ratio rockers, any gains would be small increments.
Without Spintron test data, you could easily put yourself into a situation where it eats springs or beats up valve seats because the aggressive lobe and additional ratio throw things out of control.
Don't sweat the choice of 1.6.
Get it running.
 

·
Registered
Joined
·
184 Posts
Discussion Starter · #32 ·
Seems like you are confusing two different things using the word velocity.
High ratio rocker increases the velocity of the valve moving open or closed in the valve guide. Quicker rate of lift at the valve.
This has nothing to do with airflow velocity in the intake port as far as being able to speed up the airflow velocity.....that would be a function of port volume and port shape.

Nothing wrong with a 2.08" intake valve on 385".

You wouldn't find any miracle amount of power with 1.65 or 1.7 ratio rockers, any gains would be small increments.
Without Spintron test data, you could easily put yourself into a situation where it eats springs or beats up valve seats because the aggressive lobe and additional ratio throw things out of control.
Don't sweat the choice of 1.6.
Get it running.
Thanks .
It's close. Very close.
Machine shop guy who is putting it together is stalling a little because hes swammped
It's cool. I'm just ridin with it without breaking the guys balls because hes good and a real straight shootin honest guy .
Realistically I should have it on a dyno within 2 weeks
 
21 - 32 of 32 Posts
Top