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Old Bogie - Wisdom From The Past

 
tuffnuff
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Posts: 7841
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Posted: 03/03/13 07:04 AM

I thought I'd share some of Old Bogie's posts from quite a number of years ago.,. great info.,. hope it helps.


On a board to which I subscribe a guy said his ideal cam was 220/230, .600/.600 with a 112 LSA with 1.5 rockers. How would one expect to use this cam? What kind of advertised duration would be good to keep this cam barely in the smog legal range? Could this cam work as a flat tappet grind?  

(Most heads don’t show much flow improvement over a half inch, unless you’re a competitive racer such high lifts are not warranted, the extra machining is expensive and the wear is intolerable on a street machine. Rule of thumb is that lift should peak at 1/4th the diameter of the valve, research shows that the rate of increase in flow abruptly reduces once the lift exceeds this point. For example a 2 inch valve will show significant flow increases for every .1 inch it’s opened till reaching .5 inch, after that point the rate of gain becomes substantially less for each additional .1 inch of lift. This is very obvious when you read flow rates for heads that are documented out to .6 or .7 inch of lift.)

Does it matter if a cam has the exact same specs if it is a roller or flat, hydraulic or solid?

(If the measurement is at the valve it doesn’t matter what’s causing the valve to move. Roller cams can be, but don’t have to be, ground more aggressive that flat tappet cams. To some extent the same can be said for a solid follower cam versus a hydraulic. Typically for a street engine that needs to be emissions legal, you won’t be using a cam profile that comes anywhere close to optimizing a solid follower whether it’s a roller or flat design.)


Instead of more duration on the exhaust valve could you use more lift? Can you use more lift in general since the exhaust valve is typically smaller?

(Depends upon the port’s ability to flow a volume in a time period. If the port is at peak flow, adding valve lift won’t have an effect, however, spreading that flow over more time will help fill or empty the cylinder, therefore, more duration would be beneficial.)

What controls picking the right size valves? Is there a minimum/maximum ratio difference between intake and exhaust valves?

(RPMs where the engine’s power peaks and where it will operate controls valve, port and cam size. This is a case where one size does not fit all. The engine likes an intake velocity of about ½ the speed of sound. The design engineer attempts to hit that at the desired horsepower peak. A slow turning engine will have a mild cam, small ports and valves. This is relative to the size of the engine i.e. a 305 has smaller valves and ports compared to a 350 even though both may show peak power at the same RPM. Exhaust valves are close to 70 percent the size of intakes as tests have establihed this is sufficient. An engine that’s running a supercharger of some type or has nitrous injection may have a larger exhaust valve, a much longer duration cam lobe or both to compensate for more product being exhausted. A higher twisting engine needs bigger everything to keep the port velocities in the range of .5 Mach.)


Is it a safe assumption that larger valves require a larger chamber? Is there a way to quantify the intake runner to chamber ratio? Would it matter?

(This is a sticky wicket; chamber shape, engine operating range, sparkplug location, camshaft timing and lift, number of valves, etc. all comes into play. Basically a large chamber with a large valve and port will perform at higher RPMs. SMOG heads of the 60s through early 80s tend to have large chambers, smallish valves and a chef’s stew of port sizes. They don’t do anything well. SBC 350 heads tend to use a 1.94 intake with a 1.5 exhaust on standard output engines, but you can find the same casting number with the same port size using a 2.02 and 1.6 combination on a high performance version. Given the cam difference between these engines, one is stuck to question whether the increased valve diameter really provides any benefit without also porting the head. You can use the valve size to control the gas velocity in the port throat where a larger valve will slow the velocity around the corner from the port into the throat and past the valve which will allow pressure recovery past the valve which may improve local flow. This is the stuff that makes flow bench testing interesting. Notice all the adjectives and adverbs, this is place where little is absolute one way or the other. Small nuances make for big differences much is controlled by atmospherics, the set up of test equipment and the person running the test. of one day by one person often bear scant relationship to readings taken on another day especially by another person or machine. If this was reducible to a couple absolutely dependable equations a 100 years later we wouldn’t still be screwing around with flow-benches and dynamometers.)

What dimensions are required to determine the maximum lift you can get out of a cylinder head?

(The point of contact between the top of the valve guide and the bottom of the spring retainer is the final arbiter. In a way this and the length of the valve stem, assuming the dimensions of the head casting can’t be changed, establishes the available range of lift. The cam bearing diameter also sets a limit on how tall a lobe can get. Loads imposed back down the pushrod into the lifter and reacted between the lifter and the cam establish how much rocker ratio you can use which also places a limit on how much lift you can force with a bigger ratio rocker. Component loading goes up to the square of lift and RPM so small increases in lift net large increases in forces on the parts.)

How would a head/cam work if the cam was set to give you maximum lift up to about 3500 rpm and tapered down to about 75% of that lift by 6000 rpm?

(This is what is happening to cylinder filling without needing to do anything mechanically. The fight is to keep this from happening. The steady loss of cylinder filling above the torque peak is why power curves for both torque and horsepower peak over and decline at some point in the RPM range. Therefore, finding a mechanical means of reducing fill efficiency would worsen the problem of maintaining high RPM power.)

Would you need a big or small intake runner?

(If you reduced cam timing with RPM, you’d have to increase port and valve flow capability to maintain power. However, as speed of the flow is diminished in the port, as would happen with larger valves and ports, the power drops in proportion to loss of gas inertia. The best flow is achieved between .2 and .6 Mach.)

The lift couldn't be bigger at the end than the beginning could it?

(Conventional mechanical cam designs would not allow the lift or duration to increase. One can conceive of mechanical solutions that would add duration and lift as RPMs increased. This could be achieved by using a cam with a split lobe on concentric cores that could be individually controlled to provide a broader and perhaps taller lobe as rising RPM allowed use of increasing gas dynamics in the ports. But such a system would be complicated and thus expensive. Additional lift can be achieved with a variable ratio rocker, but again the complication, weight and cost make such arrangements impractical. The future of the 4 stroke piston engine probably includes electric valve actuation. This will allow tailoring duration and lift to speed and load at an affordable price with acceptable electro-mechanical complication.)

Does a lower head angle (23, 20, 18, and 15) work better towards a larger or smaller bore? Does the lower head angle allow for higher or lower lift?

(This is somewhat but not completely independent of bore diameter and does not have a direct effect on lift. The intention is to both tighten the chamber and provide better entry and exit angles between the ports and their valve pockets. The turn to and from the valves is very costly to flow efficiencies. The poster child for bad exhaust departure angles between the valve and port is the Nail Head Buick. The severe obtuse angle forced the engine to use considerable power to pump the exhaust around this corner. This reached the end of “Blow Down” theory where it was believed that the hot expanding exhaust gases would always find a way out of the cylinder. Ford’s 351 shares a similar trait with 50 horses being lost to the tight turn behind the exhaust valve. Unlike the Buick, it’s solvable with a milling machine and a hunk a aluminum where you hog off the exhaust ports and install a chunk of metal with a more graceful exit angle exhaust ports.)



At 100% volumetric efficiency are you getting all of the flow you can out of a head, getting as much flow as the velocity will allow, both or neither?

(Not necessarily, it is possible to design a porting system that can deliver greater than 100% flow at some point. This would not be consistent across the RPM band it would probably have a lot of peaks and ebbs in its flow as various dynamics of tuned waves came in went. It takes a lot of cam and RPM to do this, it's strickly the realm of full out racing engines.)

How do you match the intake manifold to the head to get the right flow/velocity?

(Not easy to do, manufacturers and sellers of manifolds don’t spend a lot of advertising space on flow volumes and dynamics. Manifolds, in and out, are mostly sold on power claims which you or may not be able to duplicate with your set up.)

Bogie

http://forums.chevyhiperformance.com/70/705486/general-chevy-technical-discussion/cam-low-and-velocity-questions/

Smile  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

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tuffnuff
Moderator
Posts: 7841
Joined: 12/09
Posted: 03/03/13 08:27 AM

More wisdom.

If the intake velocity is as important as the flow, how does one compute it? How do you determine how much flow and at what velocity you need it to accomplish x horsepower or torque?

(On the surface this looks easy to compute, but because of the compressibility of gases, thermal effects inside the engine, etc. it isn’t. The math gets real ugly real fast, if you’re a glutton for punishment start reading a few texts on Computational Gas Dynamics. Generally speaking, tests have concluded that .5 to .6 Mach within the inlet port is as fast as you want. Faster velocities lead to local sonic speeds which lead to shock waves that reduce flow in the port. The text book answer to this question looks something like Z = ((b/D)^2)(s/Ci a)) where Z= the speed of sound in Mach number: b = the bore in inches: D = the valve head OD: s = average piston speed: Ci = a flow coefficient that in my opinion is a really huge fudge factor intended to emulate volumetric efficiency: a = the speed of sound at the inlet air temperature and pressure. I’ve never been happy with Ci computations and I’m always wary of fudge factors that are so big they steer the outcome of the equation. Boy, will this statement tick off the Rocket Engineers that probably peer in here from time to time.)
(However, the back yard hot rodder can make some reasonably good approximations without having to get an engineering degree by assuming that at given RPMs the cylinder fills to 100 percent (or less, or more if you prefer) in some period of time determined by the duration of valve timing. This is my big fudge factor equivalent of Ci in the previous equation. The other big assumption possible with a compressible fluid is that the speed of the total flow is governed by the smallest area in the zone of total flow. This, for the intake will be the actual flow area of the valve opening. Typically a wedge chamber flows through about 1/3 the available open valve area. This is more true at high RPMs simply because inertia effects push the flow hard against the far wall of the valve pocket.   If you then calculate the average or smallest cross section you can now derive the length of the column of gas that must pass through a “pipe” to fill the cylinder. That will give you the speed the column has to travel at to fill the cylinder in the allotted time. Of course in the real world that speed may not happen, this is just an episode of #### and Jane playing with numbers.  Now keep in mind this is really a crude calculation, but it gives you an idea or a place to start. In the real world flow will mirror both the motions of the piston and valve and the resultant velocities of where the piston is in relation to degrees of crankshaft and movement in the bore. This ain't symetrical. So a cylinder of a 350 has to induct about 44 cubic inches of mixture. At 6000 rpm with a 220 degree cam, that’s 100 revs per second which is .01 second per revolution. There are 360 degrees in a revolution and the cam of 220 degrees would be 61% of that .01 second period which is .0061 second to induct 44 cubic inches. If the available area of the open valve is 1.04 square inches. 44 cubic inches divided by 1.04 square inches nets a column of mixture 42.3 inches long. To convert that into feet per second would be 42.3 inches divided by 12 is 3.52 feet which divided by .0061 sec nets a velocity of 577 feet per second. The speed of sound in the atmosphere at STP is about 1100 ft per second. So we have a Mach of 577fps/1100fps or .52 Mach at the carb inlet.  Of course in the real world Mach is affected by temperature and pressure both of which are not at STP ( Standard Temperature & Pressure) inside an engine. Plus the speed of sound in a mixture of air and fuel will be different from air alone. Add to that, without a supercharger, it’s highly unlikely that an SBC is getting anywhere near 100 percent volumetric efficiency at 6000 RPM.  But without taking you into a class on computational fluid dynamics (most of which is as full of voodoo as thermodynamics), this is about as good a way to hack at the problem as any. In Charles F. Taylor’s book “The Internal Combustion Engine in Theory and Practice” he states that a 283 Corvette engine with a 1.72 inch intake at .4 inch lift has a Z of .521 Mach. This book is published by the MIT Press, . There are two volumes, Volume 1 is rather harder to find than Volume 2. Costs vary widely from 40 to 150 dollars per volume. If you decide to get an engineering degree and build motors for a NASCAR team in some obscure mountain village, I’d suggest you get a copy of both volumes.)
(In reality the gas velocity is variable both to valve lift and related piston speed both of which are close to parabolic functions. This statement holds truer on the start of the intake cycle than the end, but in a running engine is influenced by gas inertia to a very large extent. If we could start an engine from standstill to 6000 RPM in the first revolution of the intake cycle you’d see the gas flow start slowly as the valve cracked open and the piston began to move from TDC. The piston would gain speed and the valve would open further with maximum mixture speed occurring around 90 to 110 degrees from TDC. At this point the piston begins to slow to BDC and reversal for the compression stroke. However, the column of incoming mixture has a lot of inertia at this point and a greater ratio of flow per amount of valve lift is occurring than when the engine first started this cycle. So if you plotted flow against crank degrees, there would be more flow as the crank proceeded in spite of the decreasing valve opening. A late closing valve takes advantage of this inertia which at high revs is so significant that you can hold the valve open while the piston is 40 to 50 degrees into the compression cycle. Now our magic engine has gone from standstill to 6000 RPMs in less than 2 revolutions so we can see what happens to the intake on the next open cycle.)
(Discounting the effect of pressure waves which add a whole other dimension of complication to this. What’s been happening while the intake valve was shut is that the inertia of the gas column in the port continued to slam mixture against the backside of the valve. With no place to go, pressure built up. Suddenly with exhaust ending, the intake reopens and this high pressure flow blasts into the combustion chamber scouring out the last of the exhaust gases that are lingering above piston travel in the combustion chamber. The exhaust closes, but the residual inertia from the previous intake cycle continues to flood fresh mixture into the cylinder even though the piston is barley starting to move down the bore. So compared to our starting cycle we are getting a lot of flow at very small amounts of valve opening. At some point the velocity of the descending piston reasserts control over port gas velocities and the remainder of the cycle looks like the first intake cycle.)

Bogie


Smile  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

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dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 12/07/13 05:46 PM

some of the statements on head exhaust flow makes me remember a couple of things, and a theory ive been thinking about...  old bruce crower was racing at indy and working with a naturally aspirated 4-valve offy 4- banger, he said he converted it to a 3-valve intake, with only one exhaust valve, and it picked up some power! while doing work for harley jim feuling got their engines to run better than it was running by using a SMALLER EXHAUST VALVE and PORT! my thinking is that on ALOT of engines when we reach about 250 degrees duration at 50 we begin to be able to start adding more intake duration than exhaust duration, starting with non dual pattern, and then a reverse trend dual pattern that favors the intake! like old turbocharger profiles! were talking naturally aspirated here and high rpm, because IT IS EASIER TO EMPTY THE CYLINDER THAN FILL IT at high rpm!!  

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Dave632
I mow my lawn and find Chevys
Posts: 2253
Joined: 07/08
Posted: 12/07/13 10:16 PM

Yes most naturally aspirated engines benefit more from intake improvements than from exhaust, since the exhaust is being pushed out by combustion pressure and the piston. Whereas the intake is only piston induced suction unless you add a supercharger of some kind.  
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dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 12/08/13 09:54 AM

rocket engineers tuff? do they drive pontiacs with drum brakes? maybe theyre NOT real men!!! they probably drive 660 plus horse disc brake equipped 2013 FORDS!!! LOL  

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dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 12/08/13 09:56 AM

Icon QuoteDave632:
Yes most naturally aspirated engines benefit more from intake improvements than from exhaust, since the exhaust is being pushed out by combustion pressure and the piston. Whereas the intake is only piston induced suction unless you add a supercharger of some kind.

home boy, ever notice cam catalogs DONT reflect this kind of thinking?!!  

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