Discussion about turbo cams, overlap, boost and reversion

Zombie · 06-11-2009, 01:22 PM

I see this subject come up almost daily now and it makes me curious as to why it's such a hot topic. I've decided to run some math and get some hard numbers. I don't have a whole lot of cam experience so I'd like some people who know more to chime in on this topic.

At 3000 rpms it takes 40mS to complete a 4 stroke cycle (720 degrees of crank rotation and 360 of cam rotation). 6000 rpms take 20mS for a complete cycle.

If your cam has 10 degrees of overlap at .050 then here is the time the exhaust valve is open per cycle based off RPM.

3000 rpms 1.11 mS
4000 rpms 0.83 mS
5000 rpms 0.66 mS
6000 rpms 0.55 mS
7000 rpms 0.47 mS

These are very small numbers, for reference and average blink is 300-400 mS, these times are hundreds of times shorter.

There are 3 main stages of exhaust pressure from spool up to full boost of a turbo charged engine.

1. You have what is essentialy very low back pressure when not in boost
and for a very short time (right before making boost) you have equality of pressure in the exhaust.
2. During spool and full boost you have higher than 1:1 back pressure, often 1:2 and higher.
3. As RPM increases under full boost you often have rising back pressure due to the higher mass flow of the engine.

With such short overlap times I don't see how it can make much of a difference by "having boost blow out the exhaust" during spool up and it's impossible under boost.

My very basic understanding of overlap is that it's for used for aiding the evacuation of the exhaust out of the cylinder and also has an effect on scavenging created by the pulses made in the header (if equipped with headers). I can understand how the cam affects spoolup by making more power when not in boost though.

I can see where having no overlap in a high pressure differential (3:1+) of drive pressure to supplied intake pressure (back pressure vs boost) might be beneficial by not allowing the exhaust gas to revert into the combustion chamber polluting the intake charge during overlap, but it's a very short time duration.

Anyways, lets discuss and see if we can get some good info. If there are any errors in my examples, please let me know.

CamaroCoby · 06-11-2009, 01:31 PM

This flew over my head like a airplane. I would like to learn though as i have came to a fork in the road on making a cam choice. I hope some guys could chime in on some usefull information to assist me in making one.

98Z28CobraKiller · 06-11-2009, 02:09 PM

I'm not sure that looking at it from the stand point of time is the right way to do as EVERYTHING is happening fast. I think that a better way to look at it is in percentages. So in your example, the valves are both open to some extent for 3.6% of the time. This isn't alot but the bigger cams that guys are using have 20*+ of overlap or over 7.2%+ of the time.

Now, depending on the flow of the heads and the backpressure on the exhaust side vs the boost pressure on the intake side (for us turbo guys) you can determine how much overlap is acceptable. It is my understanding that a free flowing exhaust is required in order for scavenging to work properly and realize the gains of a very high overlap cam. The Blower guys can take advantage of scavenging where I don't think that scavenging is as big a factor for us turbo guys because of our exhaust restriction, headers or not.

engineermike · 06-11-2009, 02:52 PM

Interesting thread.

Vizard once wrote that turbo motor cams should "forget about the dynamics, and worry about the pressures", or something like that. His turbo cam was all about managing the pressures in the cylinder. In other words, you basically hold the intake valve closed until the piston moves down enough in the cylinder to expand the exhaust gasses down below boost pressure. This totally prevents reversion into the intake port. The result was something like 200/234-124 LSA, 10 deg retarded.

However...consider that the amount of exhaust gas remaining in the chamber is the amount you're stuck with. Whether or not it stays in the cylinder or reverses into the intake port seems irrelevant because it's all going to wind up in the cylinder anyway. The advantage to opening the intake valve early is that when flow INTO the cylinder does actually start, the valve is far open and out of the way so the restriction is minimized.

Validating this 2nd theory was INTMD8, who tested at least 4 cams in his twin turbo LTx. His conclusion was something like, "everytime I added overlap, I got more power". In all fairness I don't think he went as high as even 10 deg @ .050, but I think he did invalidate the theory that 35 deg of negative overlap is good. Furthermore, I have NEVER heard anyone say that they added overlap on a turbo car and lost power.

Mike

ls1290 · 06-11-2009, 04:11 PM

Quote:

Originally Posted by engineermike

The result was something like 200/234-124 LSA, 10 deg retarded.

That is the most interesting turbo cam I have ever heard of......

Shawn @ VA Speed · 06-11-2009, 04:22 PM

well i'm no expert,but i can tell you what works and what doesn't.What works for one setup doesn't work for another.
Just about any cam will make good hp in a turbo motor,but the right cam will make power and tq where you need it.
I can tell you the cam in Fireball's motor doesn't look anywhere close to any of the cams talked about here.You would probably laugh when you saw the specs.But it is making close to 1800hp out of 366cid and makes 8lbs of boost at 4000rpm.

engineermike · 06-11-2009, 04:26 PM

Quote:

Originally Posted by ls1290

That is the most interesting turbo cam I have ever heard of......

Exact specs were 200/234 duration, 126 deg LSA, 9 deg retarded

It's in Vizards "SBC Camshaft and Valvetrain" Book, published in 1992.

engineermike · 06-11-2009, 04:34 PM

Quote:

Originally Posted by Shawn @ VA Speed

...I can tell you the cam in Fireball's motor doesn't look anywhere close to any of the cams talked about here.You would probably laugh when you saw the specs.But it is making close to 1800hp out of 366cid and makes 8lbs of boost at 4000rpm.

In all fairness, you're probably running turbo's that approach 1:1 exhaust:intake pressure. In those cases, you basically cam it as though it's naturally aspirated because the dynamics are nearly the same.

RAACCR · 06-11-2009, 05:16 PM

Vizard's stuff looks strange, but if you look at the valve events, and the theory behind it, it make perfect sense. That would be a total pussy cat cam that would drive like a stocker, be unaffected by high back pressures, and pull good through a wide rpm range. If you were choking a motor to death with a tiny turbo, Vizard's grind would be close to ideal.

Then people started putting big diesel turbos on their hot rods, and reverse splits became popular. How else are you going to spool up one of those big, heavy monsters? The "impact wrench" effect of the short exhaust durations got the job done, and the long intake durations filled the cylinders with positive intake pressure past BDC.

But, things have progressed. Wheels are lighter and more efficient; housings have likewise come a long way from their industrial base. More and more, the ideal turbo cam looks pretty similar to a naturally aspirated grind. If you have sized both sides of your turbo(s) correctly, you should be pretty close to a 1:1 I:E pressure ratio. I ran 14* overlap at .050" on my last turbo street car, and might go even bigger this time around.

Fireball · 06-11-2009, 05:31 PM

Quote:

Originally Posted by engineermike

In all fairness, you're probably running turbo's that approach 1:1 exhaust:intake pressure. In those cases, you basically cam it as though it's naturally aspirated because the dynamics are nearly the same.

doubtful...1.15 AR with 1800hp worth of air is definately backing up. I have to run 40psi of co2 on top of my 8.5psi WG spring to get to 27psi

Pro Stock John · 06-11-2009, 05:33 PM

As much as I love to make sweeping generalizations about cam specs, and I have over the years, I've found that veteran cam designers will change the specs based on factors like twins vs single, rpm range, engine size, and size of turbos.

Louis · 06-11-2009, 05:44 PM

Quote:

Originally Posted by Pro Stock John

As much as I love to make sweeping generalizations about cam specs, and I have over the years, I've found that veteran cam designers will change the specs based on factors like twins vs single, rpm range, engine size, and size of turbos.

And Weight, use of car, purpose, etc.

engineermike · 06-11-2009, 06:19 PM

Quote:

Originally Posted by Fireball

doubtful...1.15 AR with 1800hp worth of air is definately backing up. I have to run 40psi of co2 on top of my 8.5psi WG spring to get to 27psi

What turbo's are you running?

Fireball · 06-11-2009, 06:21 PM

Quote:

Originally Posted by engineermike

What turbo's are you running?

Forced Inductions' FI-98x

engineermike · 06-11-2009, 06:28 PM

Quote:

Originally Posted by Fireball

Forced Inductions' FI-98x

With all due respect, I believe a FI-98X on only 366 cid would have a quite favorable backpressure ratio.

mike13 · 06-11-2009, 06:34 PM

cam experts are like torque convertor experts, their universe travels around a different sun. They figure out the best combination and it works but how they got there is beyond my world.

Fireball · 06-11-2009, 06:35 PM

Quote:

Originally Posted by engineermike

With all due respect, I believe a FI-98X on only 366 cid would have a quite favorable backpressure ratio.

with a 1.4x a/r turbine housing...yes...

explain the 48.5 lbs of spring to hold the wg closed...

Zombie · 06-11-2009, 06:43 PM

Quote:

Originally Posted by 98Z28CobraKiller

I'm not sure that looking at it from the stand point of time is the right way to do as EVERYTHING is happening fast. I think that a better way to look at it is in percentages. So in your example, the valves are both open to some extent for 3.6% of the time. This isn't alot but the bigger cams that guys are using have 20*+ of overlap or over 7.2%+ of the time.

I thought about describing it in percentage as well, but it doesn't seem right to me since it's a dynamic system measured in CFM and has a time unit. Dealing in percentage makes me think of a static situation (which it is when based off of the 360 degrees the cam has). The flow through the valves is constantly varying due to the amount of back pressure present in the exhaust and the amount of pressure coming in through the intake, VE varying with rpm, etc.

Maybe someone could clarify which would be the better way to look at this.

Quote:

Originally Posted by engineermike

Interesting thread.

Vizard once wrote that turbo motor cams should "forget about the dynamics, and worry about the pressures", or something like that. His turbo cam was all about managing the pressures in the cylinder. In other words, you basically hold the intake valve closed until the piston moves down enough in the cylinder to expand the exhaust gasses down below boost pressure. This totally prevents reversion into the intake port. The result was something like 200/234-124 LSA, 10 deg retarded.

However...consider that the amount of exhaust gas remaining in the chamber is the amount you're stuck with. Whether or not it stays in the cylinder or reverses into the intake port seems irrelevant because it's all going to wind up in the cylinder anyway. The advantage to opening the intake valve early is that when flow INTO the cylinder does actually start, the valve is far open and out of the way so the restriction is minimized.

What kind of results did Vizard get with that type of cam over a more conventional design? Were there any dyno's done to see the difference?

Out of boost it seems like it would be best to treat the car as a NA setup and then manage the pressure differentials as they came up. What's better though when you can't only have one or the other? A more conventional NA cam or one that manages the pressures better in a high back pressure system?

Quote:

Originally Posted by Fireball

doubtful...1.15 AR with 1800hp worth of air is definately backing up. I have to run 40psi of co2 on top of my 8.5psi WG spring to get to 27psi

Fireball, Have you ever measured you turbos drive pressure?

Louis · 06-11-2009, 06:43 PM

It would be interesting to see what your pressure ratios are Brian. It wouldnt be that hard.... Want a 0-150psi sensor to log it with?

What gate do you run? The larger the gate, obviously, the harder it is to keep closed.

Zombie · 06-11-2009, 06:46 PM

Quote:

Originally Posted by Fireball

with a 1.4x a/r turbine housing...yes...

explain the 48.5 lbs of spring to hold the wg closed...

What is the diameter of your waste gate valve? Would be interesting to figure out how much pressure is actually pushing on it. Is the waste gate opening and regulating the boost at 27psi or is that all the turbo will supply with the gate held 100% closed?