Guest DaneL24 Posted July 25, 2003 Share Posted July 25, 2003 I was just thinking if there were any serious performance tradeoffs for using a stock E31 head with a .460" lift 260/270 cam on my L28 motor...because it has about 10% less valve base area than an N42 or P79 head. Valve area increases exponentially (pi x radius squared) with increases in diameter, so this would make sense that small differences in valve diameter would make big differences in flow...but of course the effective flow area isn't actually determined by valve area. I realized that the effective flow area of the valve is the valve circumference multiplied by the valve lift (obvious, I know, but I hadn't yet realized it at the time.) I also calculated that the product of the valve area is actually much greater than the product of valve circumference multiplied by lift (real factors determining flow)...so great gains in flow can theoretically be gained by increasing lift without a larger valve size or porting. Of course all these factors are limited to the extremes by valve spring tension, combustion chamber size, and runner wall depth, but you see my point. So I calculated that my E31 head with small L24 valves and a .460" lift cam would have about 7% more peak effective flow area on the intake valves and about 6% more on the exhaust than a head with larger L28 valves (N42, P79, etc.) and a stock lift cam...and thats just effective flow area, duration not taken into account. And even if you did use a .460" lift cam on a L28 valve head, the difference in effective flow area would only be less than 5%...not enough to worry about if you are budget minded or aren't trying to break any big land speed records. I'm always hearing how smaller L24 valves won't support the flow demands of larger 2.8+ liter motors...but the math for effective flow area at the valves doesn't really support that the difference is really that significant. Thought I'd just put down a myth. So in conclusion: -Effective flow area at the valves equals valve circumference multiplied by lift, not valve area. -Percent increases in valve lift with aftermarket cams are much greater than percent decreases in valve size with L24 valves (although take every advantage you can get if you have the money...I'm on a budget). Anyways...have fun flaming, logically picking this apart, telling me this is totally obvious, or just realizing something you had never considered before... Quote Link to comment Share on other sites More sharing options...
Dan Baldwin Posted July 25, 2003 Share Posted July 25, 2003 Another thing to consider is the RATE at which the flow area is increasing as the valve opens. I.e., absolute maximum lift might not be as important as quickly providing flow area, and quickly closing that area off. Stuff to think about, anyway... Practice has repeatedly shown that high-flowing, big-hp engines want lots of valve area. I don't know of any direct L-series engine big-valve vs. small-valve comparisons, but it seems I've heard that L-heads are flow-challenged for big hp numbers. It only makes sense, if you're going to be modding for top-end power, to start with the bigger valves and mod from there. Taking into account flow boundary layers, the difference in effective flow area should be greater than a straight ratio would predict (smaller flow area => thicker boundary layer => less effective flow). But even if the bigger valves really do only flow 5% more than the smaller ones, I'd still call that significant. I believe most people would rather have flow area via larger valves than via a lumpier, low-end-torque-robbing cam. So, price vs. price, it would seem to make more sense to install a big-valve head rather than spending similar or more money on a cam for a small-valve head. But it IS a lot easier to install a mild cam than it is to remove and replace the head... yr Devil's advocate, Quote Link to comment Share on other sites More sharing options...
johnc Posted July 25, 2003 Share Posted July 25, 2003 Velocity, velocity, velocity... Forget ultimate cfm flow numbers, try to get the intake (and exhaust) charges moving as quickly as possible. Ramp profiles are often more important then ultimate lift because the valve spends way more time opening and closing then sitting completely open. Knowing that, I would spend my time and effort on finding a good cam as opposed to installing bigger valves, especially if this is a street car. An example is the changes Ford made to their street Boos 429 engine from 1969 to 1970 - smaller valves to make the beast somewhat driveable on the street. Quote Link to comment Share on other sites More sharing options...
Guest DaneL24 Posted July 26, 2003 Share Posted July 26, 2003 I see what you guys are saying about the ramp profile...if my valves flow 5% less around the circumference I will theoretically need a ramp profile 5% steeper to maintain the same effective flow area throughout the engine rotation. So anyways, I used the information I have on my cam grind to estimate the average slope of the cam lobe (change in lift / change in degrees rotation). Unfortunately, I don't have the specs for the lobe centerline which I need to setermine how many degrees of crank rotation occur before peak lift is reached. I do however, have the advertised valve timing specs and valve timing specs at .050" lift. On both the opening and closing specs for both intake and exhaust...20 degrees rotation occured before .050" lift occured, which leads me to believe the cam lobes are symetrical...otherwise there would have been more degrees rotation on one edge of the lobe before .050" lift were reached. Considering that the exhaust lobe has more duration, the same lift, but yet the same calculated average slope (20 degrees rotation before .050" lift) as the intake...I'm assuming they rounded those numbers off. So anyways, for the stock cam (I'm assuming it is symetrical too)...I figure out the degrees rotation before peak lift. 248/2=124 I then take the lift (.410") and divide it by the degrees rotation before peak lift. .410/124=.0033064 For every degree of engine rotation with the stock cam, lift averages out to 3.3 thousandths of an inch lift for every degree rotation. Now for the .460" lift 260/270 cam First the average slope for the intake lobe...I find the total degrees rotation before peak lift. 260/2=130 The lift is .460", so... .460/130=.0035384 Now for the exhaust... 270/2=135 .460/135=.0034074 So the intake lobe averages 3.5 thousandths of an inch of lift and the exhaust averages 3.4 thousandths of an inch for every degree of engine rotation. For the intake, that is about a 7% increase in average slope and a 3% increase for the exhaust lobe. Of course these are just average slopes on the lobe since the lobe isn't linear. So with about 5% smaller in circumference L24 valves (smaller than L28 valves) and with the 7% increase in average lobe slope on the intake and 3% increase on the exhaust...average intake effective flow area throughout the engine rotation will be slightly increased and exhaust flow area slightly decreased. Of course, this doesn't take into account the increase in peak lift and total duration...so overall flow with the L24 valves and bigger cam will still be greater than a stock L28 head with stock cam for both intake and exhaust. So there you go, I think thats about as deeply as I can analyse it. I should call Schneider for more detailed specs on the lobe profiles, especially the lobe centerlines to get a better idea of the ramp profile. Quote Link to comment Share on other sites More sharing options...
Guest midnitz Posted October 15, 2003 Share Posted October 15, 2003 It looks like you are on the right track... do consider this though.....the smallr diamter heads will support the airflow the 2.8 demands..however, you will reach peak airflow sooner in the rpm band...thus putting out more low end torque... The larger valved N42 head...supports good top end airflow...but this comes at a price....low end torque.... them 240z's are torquey little cars....my stock engine had good mid to top end torque... I would #1 port the 240z head and have a 3 angle valve job performed... then I would add some lift to the cam profile and extend duration a bit keep the lobe separation angle above 110. note:having the head ported roight will not only keep port velocity high, but it will enhance the engine's breathing capability in the upper rpm range. Quote Link to comment Share on other sites More sharing options...
pparaska Posted October 15, 2003 Share Posted October 15, 2003 The amount of flow during the intake and exhaust phases are a conglomeration of the exact cam profile (how fast and how much the valves are lifted throughout the phase), the head flow (at different lifts and different "depressions"), the stroke, bore, and rod length. That's not to mention the exhaust backpressure and the intake flow restrictions from the head port all the way to the air filter. One good rule of thumb is to make sure that the valve is opened as far as possible when the piston is at max velocity. That usually means some pretty fierce valve accelerations off the seat, etc. More total lift helps, as it provides more cubic feet of air (or air/fuel) at any point in the phase than a lower lift cam/rocker setup with the same lobe velocity. You can get away with a less lobe intensity (acceleration) if you have heads that flow well at low lift. Lots of trade offs. That's why it's always good to talk to the engine building gurus and cam and head manufacturers. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 15, 2003 Share Posted October 15, 2003 One good rule of thumb is to make sure that the valve is opened as far as possible when the piston is at max velocity. That usually means some pretty fierce valve accelerations off the seat, etc. Exactly. That's how "stock" cammed L6 engines are making over 200hp with unported heads, stock pistons, and SUs. Quote Link to comment Share on other sites More sharing options...
Zsane Posted January 2, 2004 Share Posted January 2, 2004 Wouldn't the rocker contact pad profile effect lift rates? Quote Link to comment Share on other sites More sharing options...
Guest freakypainter Posted January 2, 2004 Share Posted January 2, 2004 you need to look at how much valve overlap your cam will have also. if the intake valve starts to open while the exhaust valve is closing it will cause a vacuum in the combustion chamber and help to suck in the air/fuel mixture, you could run the smaller valves with more overlap but you need to make sure that the cam/piston combination will work together, to much overlap will bend the valves. Quote Link to comment Share on other sites More sharing options...
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