Custom cam VS "shelf cam"

[DaveMcLain]

I'm not convinced that I/E ratio has much validity when it comes to how the engine responds to cam timing. While I know I've seen engines respond to having lots of additional timing on the exhaust side its usually on a large and or high rpm engine and I don't think it has anything to do with some ratio of intake and exhaust flow.

I think what might happen is that when you have a rather large cylinder open to the exhaust system it tends to change the exhaust tuning. The cylinder resonates at a changing and different frequency than the exhaust system. Having more duration gives it more "time" at each end of the exhaust stroke where the cylinder volume is not as large and that tends to make it more effective at high rpm. That's the only thing I can think of.... I don't think it has to do with flow otherwise what would you do if the engine was fitted with a supercharger? Give it 50 degrees more on the exhaust side?

[bigjoe1]

I also think like that.. I dont even look at the in/ex ratio BS



JOE SHERMAN RACING

[CamKing]

I also think like that.. I dont even look at the in/ex ratio BS



JOE SHERMAN RACING

Words are BS Joe. Math is Math. Math can not be bought. Words can be bought and paid for.

Ouch.

[IDT-572]

Start putting more split on a ford engine with a set of well ported Dooe-r's on it and see what the power does. And don't get scared and widen the sep because you think the over lap is getting out of hand either.

[DaveMcLain]

I'm not convinced that I/E ratio has much validity when it comes to how the engine responds to cam timing. While I know I've seen engines respond to having lots of additional timing on the exhaust side its usually on a large and or high rpm engine and I don't think it has anything to do with some ratio of intake and exhaust flow.

I think what might happen is that when you have a rather large cylinder open to the exhaust system it tends to change the exhaust tuning. The cylinder resonates at a changing and different frequency than the exhaust system. Having more duration gives it more "time" at each end of the exhaust stroke where the cylinder volume is not as large and that tends to make it more effective at high rpm. That's the only thing I can think of.... I don't think it has to do with flow otherwise what would you do if the engine was fitted with a supercharger? Give it 50 degrees more on the exhaust side?

Dave,
John Reed always said the most important tuning tool in the tool box is lash. It will tell you what the engine wants. Next time you have an engine with a known cylinder head that has a low I/E ratio, make a few pulls then tighten the lash on the exhaust.

I give lash adjustments many times a year so customers can see if a cam change will help. A few years back a customer with a single pattern cam on a Brodix headed 540 tried an aggressive lash adjustment of loosen intake .010" and tighten exhaust .010". Car picked up .5ths. I tend to follow this as I have seen it proven many times.

Your correct on blowers. You take a head with low I/E ratio and stick a blower on it, well yes it will need even more duration. I did one years ago that ended up on Al Parker's dyno in IA. It was 41 degrees more on exhaust then intake. It ran very well.

I always try to run some lash loops on the engine if possible. Sometimes it helps, sometimes it doesn't and the same goes with rocker ratio swaps. You never know till you try but either way I think that all of this has more to do with wave propagation through the engine system than some sort of flow ratio.

[dirtracr5]

I also think like that.. I dont even look at the in/ex ratio BS



JOE SHERMAN RACING

Words are BS Joe. Math is Math. Math can not be bought. Words can be bought and paid for.

And there lies the difference between guys that build street engines and guys that build class racing engines..

[hoffman900]

The problem with I/E ratios is it looks at cfm which has little baring of what is happening on the exhaust side. You're seeing the trend on really high level class stuff towards small fast exhaust ports that may not flow enough to reach some "magical" ratio with a really well designed and developed header.

[hoffman900]

From 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.

[hoffman900]

From 2005

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?

[hoffman900]

From 2012:

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.

[hoffman900]

From a few weeks ago:

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.

Read it over and over again and think about the above posts. Both posters (Darin Morgan and Calvin Elston) are some of the two most decorated, if not most decorated, industry people on here.

[novadude]

Words are BS Joe. Math is Math. Math can not be bought. Words can be bought and paid for.

Yeah, but all the math in the world isn't any good if your model is flawed. I'm NOT saying anyone's model is flawed HERE, but I've worked with enough Engineers that had the math right, but their model wasn't worth a damn, so it really didn't matter what the numbers said, even if the calculations were correct.

[paulzig]

With the pressure and temperature witnessed on the exhaust side of an engine, how in the world can an I/E ratio mean anything? Flow testing is done at ambient temps, in conditions that nowhere near simulate real world scenario .. Even if you had results that increased power on an engine dyno if you did the I/E stuff you would need real world testing to see how a reverse split cam would go as opposed to a 25° split on the exhaust .. Even then, it'd be hard to make a definitive conclusion ..

[Keith Morganstein]

I also think like that.. I dont even look at the in/ex ratio BS



JOE SHERMAN RACING

Words are BS Joe. Math is Math. Math can not be bought. Words can be bought and paid for.

And there lies the difference between guys that build street engines and guys that build class racing engines..

Joe Sherman has a flow bench and a dyno. I'm sure those tools help him pick his cams along with lots of experience with plenty of winning race engines.

[Warp Speed]

If this subject of valve events was nothing more than just math, there would be no need to test. Real world testing proves otherwise.........