Camming a 10.3 to 1 350

[hoffman900]

I believe that it is a mistake to only go by the flow difference between intake and exhaust.

100% correct.
The exhaust duration has to be calculated off the exhaust port's ability to evacuate the mass in the cylinder. It has no relation to the intake port.

Except that the better the intake port is the more chance of there being more mass to evacuate. It won't be a direct % thing of course

Mike shared his thoughts on this some years ago here. I’ll have to dig up the post when I have time, and no, it’s not a linear relationship, and he acknowledged as such.

[statsystems]

J
100% correct.
The exhaust duration has to be calculated off the exhaust port's ability to evacuate the mass in the cylinder. It has no relation to the intake port.

Except that the better the intake port is the more chance of there being more mass to evacuate. It won't be a direct % thing of course

Mike shared his thoughts on this some years ago here. I’ll have to dig up the post when I have time, and no, it’s not a linear relationship, and he acknowledged as such.

Some day you'll have to organize all those threads in one place so you don't have to look to find them...you can just click right to them.

When you get the time of course!!

[cjperformance]

I believe that it is a mistake to only go by the flow difference between intake and exhaust.

100% correct.
The exhaust duration has to be calculated off the exhaust port's ability to evacuate the mass in the cylinder. It has no relation to the intake port.

Except that the better the intake port is the more chance of there being more mass to evacuate. It won't be a direct % thing of course

The intake port does not matter, all that matters is ^^ the mass in the cylinder ^^ that the exhaust has to deal with.
*this said i dont make cams, im simply a donkey that selects lobes and has them ground on a stick*

[digger]

100% correct.
The exhaust duration has to be calculated off the exhaust port's ability to evacuate the mass in the cylinder. It has no relation to the intake port.

Except that the better the intake port is the more chance of there being more mass to evacuate. It won't be a direct % thing of course

The intake port does not matter, all that matters is ^^ the mass in the cylinder ^^ that the exhaust has to deal with.
*this said i dont make cams, im simply a donkey that selects lobes and has them ground on a stick*

the inlet port and inlet lobe cam helps govern the mass of air that gets into the cylinder which then needs to be evacuated is all i was saying. the amount of mass you get in may not directly correlate with the CFM though

[GARY C]

How do people figure the mass going in without flow #'s?
If you only use head and valve dimensions but the head backs up .100 before max lift does that matter?

[statsystems]

How do people figure the mass going in without flow #'s?
If you only use head and valve dimensions but the head backs up .100 before max lift does that matter?

You can make virtually any head back up. That's one of the limits of a flow bench. Also, pick lift based on flow is not a good idea IMO.

I try and get all the lift I can that the valve train will reliably take for the application. At this point, .750 lift on some street applications is nothing. It 1980 that was near unheard of territory for race engine stuff.

[GARY C]

How do people figure the mass going in without flow #'s?
If you only use head and valve dimensions but the head backs up .100 before max lift does that matter?

You can make virtually any head back up. That's one of the limits of a flow bench. Also, pick lift based on flow is not a good idea IMO.

I try and get all the lift I can that the valve train will reliably take for the application. At this point, .750 lift on some street applications is nothing. It 1980 that was near unheard of territory for race engine stuff.

I was just curious how people figure mass without flow.
I have seen arguments claiming that a head that backs up on the bench will hurt power and other claims it doesn't matter.

[hoffman900]

Except that the better the intake port is the more chance of there being more mass to evacuate. It won't be a direct % thing of course

Mike shared his thoughts on this some years ago here. I’ll have to dig up the post when I have time, and no, it’s not a linear relationship, and he acknowledged as such.

Some day you'll have to organize all those threads in one place so you don't have to look to find them...you can just click right to them.

When you get the time of course!!

From: Exhaust Lobe Theory, 2016.
https://www.speed-talk.com/forum/viewtopic.php?t=46137

First I calculate the amount of mass in the cylinder at the max RPM you want to make power at, then I use the exhaust valve size, and exhaust port flow to calculate the "effective" choke diameter of the exhaust port, then calculate how much time(duration) that effective choke diameter will need to evacuate the mass in the cylinder.

In the 80's we used the exh/int flow percentage, and that worked well with the relatively low flowing ports we had in the 80's, but as the ports got better, the percentage didn't scale, and it was easy to see why. If you increase the intake flow by 20%, that doesn't mean you'll increase the mass in the cylinder by 20%, so you're not going to need to increase the exhaust by 20%.

Another interesting thread: HInlet to Exhaust Ratio Flow, 2008
https://www.speed-talk.com/forum/viewtopic.php?t=10814

I talked to a port designer that worked for Ferrari, and they didn't use a flow bench. They measured all the cross-sectional areas and calculated choke flow. That's more accurate, but a little time consuming.

Same at Honda

I like this from that thread as well

Darin

You mentioned that many convential headed BBC can benefit from smaller exhaust valves, why are Profiler's still coming out with 1.88 and 1.900 exh valves?

Because that's what everyone wants. Its up to the customer to custom order specialized heads or order the heads with no seat work so he can set his own valve sizes.

I cant go against what the market place and the overall majority of customers want. Sales will decrease if I do that. If they wish to consider or know my opinions on the proper way to approach a certain cylinder head combination all they have to do is ask. I am here to help in any way I can. 40% of people do it there way regardless of what I say, so who am I to argue. You give the people what they want, period.

and this one, Cam Profile in Relation to Airflow Curve, Harold? , 2006
viewtopic.php?f=15&t=4680

Jay,

I have delayed answering your question because the answer is very long, and there are many different interpetations of it. Eacfh Cam designer has his own theory, and often they contradict each other. Here are some of my thoughts and practices on cam lobes vs airflow.
First--AFAIK, there is NO airflow into the engine BTDC, except for various supercharged/turbocharged engines. BTDC in an unblown engine the piston is pushing things, ie--exhaust gases, out of the cylinder/combustion chamber. These gases have positive pressure(measurable back-pressure) and block the intake port as the intake valve opens. We call this REVERSION. I believe no intake airflow can start into the cylinder until this reversion is cleaned out of the intake port. This is why I design all my cams to minimize reversion, by delayed intake valve openings. I have done it this way since 1977, and I have seen that it works.
Second--I believe the part of the intake stroke that does all the 'heavy lifting' is the first 75* ATDC. Notice that max valve/lobe lift occurs 100* tp 114* ATDC--The piston has been slowning down for 25* to 39* before max valve lift; This is why max valve lift isn't the most important thing. What most people see with very high valve lifts is the actual valve movement from TDC to 75* ATDC, as mentioned before.
Third--The potential airflow(the airflow curve at various valve lifts) and the valve movement controlled by the opening side of the cam lobe generate the potential airflow into the cylinder---The rod/stroke ratio comes into play also, as the downward piston movement, and the rate-of-change of the piston, let the atmospheric pressure push the air into the cylinder.
So, mid-lift flow is very important, if not most important, in filling the cylinder.
The only caveat is when the port volume is very large(over 33%) of the cylinder volume, and the rod/stroke ratio is very long, 1.9:1-2.25:1. Then very fast ramped cams can move the valve faster than the air can follow the valve into the cylinder and the engine always acts over-cammed. I see this in NASCAR engines with flat tappet cams, it is not restricted to rollers.
I hope this has been informative and given you something to ponder. Even if port airflow hits a plateau and levels off, it is OK. Extra valve lift above that point is for dynamic control. High valve lift never hurts port flow, unless boundary air conditions start decreasing airflow above some valve lift. Then the port must be fixed, or the net valve lift stay belonw that point.

UDHarold

Bret,

Here are some thoughts I've had for a very long time......
Engines work off of pressure differentials--ie, differential equations......
In unblown engines we have a positive pressure(back pressure) in the exhaust pipe, and a negative (vacuum) in the intake port BTDC. The piston is moving upwards, pushing the exhaust gases out the exhasut port. The numbers aren't far apart, say 1 1/2 lbs backpressure, a few inches of vacuum, but it does not allow flow into the combustion chamber. Then TDC, and the piston starts down. As it moves downward, it is creating a volume where no volume had existed---and atmospheric air pushes in to fill that volume. This is even why cars run better on cold days that in the heat of summer, the cold air weighs more, and fills the cylinder faster because of inertia.
I'll be a little scattered myself, because of the lateness of the hour, but two points of interest.
First---The engine never has long-term memory. It only knows what is happening NOW, and what just happened. Here is an example:
Intake opening 40* BTDC--- Is this a
288* cam on 104 ICL?
292* cam on 106 ICL?
296* cam on 108 ICL?
300* cam on 110 ICL?
304* cam on 112 ICL? or a
308* cam on 114 ICL?
With symmetrical cams, all those intake durations and ICLs open at 40* BTDC.
Or, if it is an unsymmetrical cam, it might be a 288* cam on 101 ICL, like in my UD 288/296R6.
At .200", the 288 is as fat as many 300* cams, but the upper part of the cam lobe is way advanced compared to them. The engine sees the overall shape of the cam lobe, not some number we use to measure it with. The higher the port velocity, the better the inertia ram.
Second---Most engine builders, probably you too, have tried the same cam lobes ground on different LSAs, say like 106 and 108. If you dyno them in the same engine, and on the same Intake CL, both cams obviously have the same intake cam on the same ICL. Oddly enough, they have different power curves, and everyone says "Sure, because of the different LSAs!". Remember, they both had the same intake lobe on the same intake CL. The power curves differed because the effects of the exhaust back pressure and the resulting reversion were different for each test, and each cam filled differently because of their reversions. Once you start thinking on this, it leads all sorts of interesting directions......
Enough for me tonight, see you all tomorrow.....

UDHarold

PRI is next week. I will be around very little.....

Jay,

The intake backflow covers up to .250" valve lift, but the way the reversion works is like this:
Still using our 40* BTDC valve opening point.
Think of the insides of the combustion chamber at 41* BTDC---The piston is moving as fast as it's going to for the next 41*, it is slowing down and must hit 0 velocity at TDC. The burned exhaust gases are being pushed out the cylinder/combustion chamber, they have an ever-decreasing volume and backpressure.
Now we get to 40* BTDC and the intake valve is about .001" off the seat. The pressure differential is such that some of that exhaust gas goes around the valve into the intake port. The initial disturbance is the greatest disturbance, the piston velocity is dropping, and so is the exhaust volume and backpressure. A port designed to minimize that flow into the intake port will help the port recover sooner after TDC, and make it start airflow earlier than one that does not minimize reverse airflow. As I said earlier, the best airflow, that sets up the port velocity for later inertia-ram flow, occurs in the .250" to .600"-.650" valve lift range.
But I am not a head porter, and these are just my thoughts, from the cam design side. This is what I think happens, and I've been thinking it for a number of years. It is all simple physics, and what happens must agree with simple physics.

I hope this gets you all thinking......

UDHarold

Miscellaneous posts:
Darin Morgan on here in 2006:

The 75% intake to exhaust flow ratio is NOT correct and never was. I think some engineer pulled this out of thin air.
The engines RPM range, size and intended use as well as other design criteria will be the deciding factor as to valve size and placement but trying to hit some preconceived flow ratio with total disregard for other more important tuning variables is not an intelligent or well thought out approach. I will admit that the industry is full of these little snippets of pseudo wisdom but one should not believe everything they hear. When you try and optimize the valve area in an unlimited engine combination you always sacrifice exhaust valve size for intake valve size. Its much easier to cam and manipulate the exhaust system to evacuate the cylinder ( dynamic blow down and cam shaft events) than it is to manipulate the intake tract. Current Pro Stock and Comp Eliminator engines have intake to exhaust (size in diameter) ratios of 70% and flow ratios of only 58% to a maximum of 61%. These engines are producing 2.78HP/CID at over 5800fpm piston speed so the that kind of shatters the long held belief that the exhaust should flow 75%-80% of the intake. Could we make more power if the exhaust flowed more. No, we tried that. Could we make more power if we increased the exhaust size and kept the same discharge coefficient? Yes it does but there isn't any room left for a larger valve because we just stuck this big intake in there so its really a mute piont. I know thats a simplistic explanation of what is going on but I think it drives the piont home.

In 2005:

Darin Morgan wrote:
My personal opinion and from what I have learned about exhaust ports I have to say that small super fast exhaust ports make more power over larger high flowing exhaust ports except in the case of full exhaust systems such as Nextel cup engines. For some reason they like a slightly larger exhaust port but no where even close to what I would call large. Large and Small are ambiguous. In my book anything over about 110% of the valve area is large and anything under 105% of the valve area is very small but the exit velocity seems to play a role here as well. I try to adhere to the 105-108% in our pro Stock engines and it seems that I am not alone in my theory because many of the top notch heads I have seen are about the same or within about 2%. Another very big thing to consider in the tuning of exhaust ports is there sound or should I say the lack of sound. How smooth an exhaust port sounds and how quietly it can move the air are both very serious factors to consider. As the valve opens the sound of the ports should smooth up and get increasingly silent. The loudest portion of the exhaust flow on the bench is from .200 to .400 after that they should go increasingly silent with every lift increment. I have had exhaust ports that actually cracked and popped like fire crackers! With a little seat blending and chamber work I managed to smooth up the flow, gained a measly 2 cfm average and gained 26 horsepower and it still was not correct because the port was to big. The hardest thing I do is try and fix exhaust ports that are screwed up. Its much easier to fix intake ports!
Like an intake port, an exhaust port can be made to flow a great deal of air, Just make it big.

Some rules I live by.

(1) Exit area = 105-110 % of the valve.

(2) Exit air speed at a minimum of 300 and a max of 330 ft/sec mean.

(3) Smooth silent flow by at least .400 lift and absolutely by .500 lift.



On another note, I am not sold on the theory that the flow in the exhaust port goes Sonic. Anyone here care to prove this theory?

Calvin Elston:

If you keep dropping the pressure in the header, and that is not hard, you will eventually get to the condition which you are seeing which is overscavaging. Changing the camshaft around is effectual only to the degree that it was too large or too tight in the beginning.

Lets try stepping back further from the situation. There is a basic amount of duration you need to get to a certain rpm and power level with a given engine combo. There is a wall there that you hit when your just dropping header pressure and correspondingly reducing exh duration and overlap, (and all the other tricks there are). It does not allow you to really make too much more power with the same size intake valve. It is a diminishing return. The adjustment is to reduce the size of the exhaust valve. This allows you to keep the duration or cam timing where it really wants to be and still exhaust well enough. The smaller exhaust valve allows most engines to use a larger intake valve and this is the ultimate purpose in using a properly tuned header. The well tuned exhaust side allows a larger intake valve which is the easier way to move more air into the cylinders than sucking from the back door. If your header is not functioning well, you will not be able to get there.
Very few engine people get this.
I believe if you look at PS type 2valve engine development over the last 10 years, you will see a very large increase in power and rpm, but exh valves are the same or smaller in diameter and the intakes are larger.The merged collector and corresponding header and valve sizing has been a important factor.

I observed quite a long time ago when building both Cup and PS headers that the shorter I made the headers the more power, (more peak but not more average) you could make. (on the dyno anyway, that does not necessarily mean you can will be able to do more work with it)
The "header" design programs would always seem to agree with those observations but other observations tell me that is not the whole story.
First thing is the "tuned length" for a desired rpm band is based on the "lowest" rpm the engine is being designed to operate as you have to make sure that the "draw" reaches down to that lowest rpm point, protecting the engine from reversion. So understandably the highest rpm point "tuned length" is not optimal as we are trying to "spread" things over a given range. (multiple gear gearboxes) have helped to gloss over this reality. Stepping headers also is a way to flow more at the longer lengths needed for the longer powerbands, proportionately. So all of a sudden I am talking about flow, ok?

Here is another reality to me, the shorter the tubes the more power you make and it is not all about "tuned" length. The less time gases are moving in tubes, the less overall flow loss and guess what, this makes more power also. I also realized you do not have to have as large a tube diameter, ie, the shorter the header, the smaller the tube diameter needed. This pointed out to me and maybe some that would listen to me, that velocity of gases was important and on every part of your power curve. So about that time I did the 421 for the cup cars and they really changed things, not the least of which was my thinking about what might also be going on. It was when I did a couple variations that I learned that even 421's wanted a shorter primary tube and smaller diameter also. We could see that the shorter primaries are "tuning" upstairs and the secondaries protected the bottom end. You just had to make sure you made the back end big enough to make power upstairs...read "flow". The secondaries are helping to diffuse waves in the system. So in PS I kept making the primaries shorter on 421's, and overall length shorter depending on how much bottom end was needed. The problem is that the 421 is difficult to make equal length when the primary length is say 12 " and the motor is 24" long. The header gets too distorted and won't "flow" as well so I was at the limit of a viable 421 header for engines, (V8's anyway) that "start" at say 8500rpm. (talking 2valvers here).
Some might be attempted to say that you could make primaries bigger...well you could, and might make more up top but you lose overall.
So since PS and cup stuff had easily reached past 8500, a 421 was really not viable in most cases and short 4into1's are the ticket. I have built SB engines down to14-15" and BB's down to 15-16" and they always keep going upstairs. I have done unequal length 4into1's but have no reliable information on that direction.
This is also why you will see certain Cup venues running 4into1's as the rpm band is so high and tight that there is no longer a difference from a 421. ( It is dangerous to pull too much from Cup stuff as they have a pretty long system that moderates things) The work however is to learn how small or how quick to taper the tubes to match the flow requirements of that given engine. Keeping the velocity up is very important.

All the above has to be melded with the understanding that I was pioneering merged collectors with Jack Burns in the cup stuff from the start, the relationship between primary lengths (both 421 and 401) and merged diameter or (what I think is the best "term"), the choke, was a ongoing learning process also. I was fortunate enough to be able to explore these things across several professional venues at a time when NA American v-8's were undergoing extensive development by many organizations. I was able to observe all of them while found it difficult to convey what I was learning to those people across the venues. Headers are a very dark world for engine builders and tuners, they are expensive and there are many times a lot easier ways to make 10 more horsepower.

Getting back to the importance of flow vs tuning. Many readers here have read about the Log manifold stuff I did on a cup motor but a later extension of that work was in PS and the shorty or "stub" tests on a viable PS engine. 8" long stubs that ran on the dyno the same as the best normal 4into1. Readers might want to explore that as I think it is significant, and might raise some new thoughts. Did for me.

The reality I work with now, and after recent work with a couple different venues and anti-reversion apparatus, is the very important relationship between primary tube design and either merged collectors or AR devises.
The smaller choke at the end of the header functions the same way as a anti-reversion devise. It is able to reduce the magnitude of pressure waves trying to go back up into the header from both reflection and atmosphere. The shorter and smaller you can make the primaries, the smaller the choke you can use. The shorter the tube, the more "energy" available to expend into the venturi. Weaker pressure waves going the wrong way allow higher velocities and lower pressure. The result is the ability to control the overall or say the average pressure in the whole header as a result of higher gas velocity while allowing strong pulses going "out" and at the same time much less powerful waves going the wrong way.
This all work to decrease the average pressure on the exhaust side of the cylinder/engine. A proper design progression of a given engines exhaust system will not be able to progress if the tuner/designer is not willing to address other design parameters inside the engine that are based on a exhaust system that does not offer the same depression on the exhaust side. This is not uncommon.

I have said all the above on this thread as the topic is " exhaust design formulas". I have no problem with the reality of pressure waves, it is just that from my corner I see them as hurting more than helping. Further real gains from exhaust system design is not going to be found by figuring out reflected wave timing lengths. There is a lot more to figure out.

Nothing works in isolation. The engine is a system.

[CGT]

How do people figure the mass going in without flow #'s?
If you only use head and valve dimensions but the head backs up .100 before max lift does that matter?

You can make virtually any head back up. That's one of the limits of a flow bench. Also, pick lift based on flow is not a good idea IMO.

I try and get all the lift I can that the valve train will reliably take for the application. At this point, .750 lift on some street applications is nothing. It 1980 that was near unheard of territory for race engine stuff.

I was just curious how people figure mass without flow.
I have seen arguments claiming that a head that backs up on the bench will hurt power and other claims it doesn't matter.

Convert to any metric you like. Your still converting something thats non-dynamic.

[Warp Speed]

How do people figure the mass going in without flow #'s?
If you only use head and valve dimensions but the head backs up .100 before max lift does that matter?

Your concentrating on mass flow through an orifice, not static flow at 28".......

[cjperformance]

Except that the better the intake port is the more chance of there being more mass to evacuate. It won't be a direct % thing of course

The intake port does not matter, all that matters is ^^ the mass in the cylinder ^^ that the exhaust has to deal with.
*this said i dont make cams, im simply a donkey that selects lobes and has them ground on a stick*

the inlet port and inlet lobe cam helps govern the mass of air that gets into the cylinder which then needs to be evacuated is all i was saying. the amount of mass you get in may not directly correlate with the CFM though

I totally get what you mean, not discounting what you are saying and of course the intake port, valve and valve events help govern what can get into the cylinder.
Once that valve is shut, the mixture is burned etc then you have X ammount of waste to deal with , thats what the exhaust port and valve timing has to deal with, by this point it does not matter how that air/fuel got in that cylinder.
I would agree that with X head and its given port dynamics that you could say that for X application , THIS head and surrounding combo likes a XX% bias according to flow and use but that has has no relevence to a different head or surrounding combo.

[GARY C]

How do people figure the mass going in without flow #'s?
If you only use head and valve dimensions but the head backs up .100 before max lift does that matter?

Your concentrating on mass flow through an orifice, not static flow at 28".......

CGT... Convert to any metric you like. Your still converting something thats non-dynamic.

Maybe I am asking wrong... I know that mass is not the same as flow on a bench but what I am asking is how is it figured/calculated?

[Rimmo]

If you see 75-80% intake to exhaust ratio, some people will say perfect candidate for a single or reverse pattern, so you do the opposite, you smash 15-20° split in the bastard just out of spite...

So I have the wrong cam in my 421 ?
It has 78% ex/in ratio, but has 13* more ex duration.

Would it make more power and a better power curve with a single pattern cam ?

Randy

I think you might have miss read his post. If you read through the sarcasm he's saying to add 15* to 20* exhaust split in your situation, which you have done..

I'm familiar with paulie's cam preferences, you just need to catch him on a day when the beers are flowing less than the intake ports...

[paulzig]

I think you might have miss read his post. If you read through the sarcasm he's saying to add 15* to 20* exhaust split in your situation, which you have done..

I'm familiar with paulie's cam preferences, you just need to catch him on a day when the beers are flowing less than the intake ports...

I think he knows now... I dont drink much anymore really.. Lucky to have a few beers a month.

Have a look thru some of Randy, Rick and CGT's engine threads on here and you'll see the same trend on the exhaust lobe.

Seems to have worked... Some even have the dreaded 110LSA lol...

[Scotthatch]

100% correct.
The exhaust duration has to be calculated off the exhaust port's ability to evacuate the mass in the cylinder. It has no relation to the intake port.

I agree with this but how do you calculate the exhaust valve opening point when there is a major pressure difference in the end of the power stroke depending on the HP made and the drop as the piston descent decompress it in conjunction with a ongoing burn and whatever the exhaust system is like

I calculate the total time needed(duration) to evacuate the mass at peak RPM, then focus on closing the exhaust valve at the point the pressure below the exhaust valve equals the pressure above the intake valve. When I know the Exhaust closing point, and duration, I know what the exhaust opening point is.

I have been giving this some thought and am struggling with this idea .....

Any gas formula figuring mass flow over a restriction uses temperature , differential pressure and CFM flow over the restriction to figure mass flow...

With the exhaust cycle it's flow will be nothing like the intake side for starters the sonic choke point will be a lot higher do to temperature of the air .... at 1200 degrees you have almost twice the airspeed and the differential pressure on the valve is way higher since you are blowing down the cylinder from mean effective pressure not atmospheric pressure so the flow across the valve will be way higher but then since the air is heated it's volume is higher then when it came in ... the calculation to figure this would be multi tasking and degree by degree so I assume a computer would be the only way ...

That being said leaves me with just a rule of thumb or guideline .... I do not have nearly enough data to sort out the exhaust valve opening point .... I have moved this point out further than most do on the street engine build and seen better high rpm and carryover which makes sense from a time base standpoint and not really seen a loss in low end power like we are taught will happen ... and then moving further into the power cycle I get no change at all .... this I assume is because I have reached a sonic choke point in flow across the valve and at that point am just adding time not flow which is not a big change as we are talking a few degrees ... so I feel I have a good picture of the event but no way to say open at this degree....

I also have trouble with thinking backwards from the closing point of the valve .... though this point is important to the induction cycle it is the very end of the ex and from what I have seen the opening point is more important in getting the exhaust cycle compete at rpm then the closing point