I was browsing thru the web when i stumble upon this thread from twincam forum.
Twincam Forums :: View topic - Progress on TODA 20V
Twincam Forums :: View topic - Progress on TODA 20V
Ok you can pick my brain, but since it all depends on what you want from your engine and what you are willing to spend i'll give you the information to help you choose the cam for your application. Sit back and relax, make a cup of coffee, this is going to be a long one.
*Please note when i refer to air, intake charge, or flow or anything to that effect flowing into the cylinder I am referring to the air/fuel mixture controlled by the fuel injection/carburetor system. I am in no way saying that a cam has any control over this parameter.
Lift
As I said earlier the lift is how far the cam "lifts" the valve, i.e how far the valve is pushed into the combustion chamber. Increasing the lift increases the curtain area around the valve and thus increases the flow of the port. This gives a power increase throughout the rev range with no real drawbacks to the drivabilty etc. But the increase in flow decreases at an exponential rate with respect to the lift thus adding more and more lift starts giving less and less gains. Thus I wouldnt recommend going much over 9mm lift on a 20V as its not worth the cost as you are getting close to maximum flow already. Other limits with respect to lift are valve clearances(pistons hitting the valves), valve springs (will require stronger ones to cope with valve bounce etc.) etc.
Duration
Duration is the amount of time in respect to crankshaft rotation in degrees that the valves are held open by the cam. The higher the duration the longer the valve is held open, put simply allowing more air to enter the cylinder. In fact it is far more complicated than that and is going to take some time to explain.
Intake Cam
On the intake stroke the the intake valve opens and the piston moves downwards, this creates a low pressure inside the cylinder below that of the air outside, and thus air is forced into the cylinder by atmospheric pressure. Now at really low revs, hypothetically speaking 1rpm the air will almost completely fill the cylinder and the intake valve will close. Things start getting much more tricky as the revs increase. At higher revs the air flowing into the cylinder through the ports is at a high enough velocity for it to gain a reasonable amount of momentum (known as flow inertia i believe, that is how i will refer to it from now on), this momentum is useful in that it can be used to force more air into the cylinder. This is done by holding the intake valve open longer, even as the piston is moving upwards on the compression stroke. With enough flow inertia the air will continue filling the cylinder even after the piston starts moving upwards, therefore more air is allowed to enter the cylinder, and more power can be obtained.
For peak power at a certain rpm the intake charge flowing into the cylinder will stop(due to piston on upward compression stroke) at the same time as the intake valve closes, thus maximum air has been allowed to enter the cylinder.
As the engine revs increase, the higher the port velocities and the higher the flow inertia. Therefore the intake valve can be held open longer and more power can be gained. The problem comes when you run a high duration cam to take advantage of this effect at high revs u get problems at low revs. At low revs with low flow inertia due to low intake charge velocites the air drawn into the cylinder on the intake stroke stops flowing into the cylinder before the the intake valves close and it actually starts getting forces back out again on the compression stroke, therefore you actually get less air in the cylinder and lose power.
Exhaust Cam
Exhaust cam benefits the engine in much the same way as the intake cam, on the exhaust stroke the piston moves upwards and the exhaust valve opens, this and the pressure created by the combustion of the air/fuel mixture on the combustion stroke forces the exhaust gases out at high velocity, thus it has a high flow inertia. Therefore at high revs holding the exhaust valve open longer even as the piston is moving downwards on intake stroke allows more exhaust gases to be scavenged from the cylinder. But again at low revs of the valve is held open too long some exhaust gases are actaully sucked back into the engine thus decreasing power.
Intake/Exhaust valve overlap
So far I have only been talking about the duration in respect to how late the valves are held open in the stroke, and havent referred to the fact that higher duration cams also open earlier. I will now explain the effect of this...
As I said in the exhaust cam section the longer the duration the longer the valve is held open, now with high duration, the intake valve is held open later in the stroke and the exhaust valve opens earlier in the stroke therefore the two openings overlap and the intake and exhaust valve are actually both open at the same time. This has its advantages(again at high revs) and its disadvantages (at low revs). At high revs the scavenging of the exhaust gases in the cylinder is aided by fresh air rushing in through the intake ports and pushing out the exhaust gases through the exhaust port, in turn the intake charge is aided by the flow inertia of the exhaust gases leaving through the exhaust ports drawing in more air through the intake ports.
At low revs though you end up with too much intake charge leaving through the exhaust port and many other problems.
This is where VVT comes in. Without VVT engaged a 20V intake cam is in a retarded position, thus its has more overlap into the exhaust cams timing, this is better for high rev power but not great for mid and low range power, that is why the VVT advances the intake cam 30degrees during the midrange revs to decrease this overlap and increase midrange power, it then switches off and the intake cam timing retards, the overlap increases and the topend power increases. However using longer and longer duration cams reduces the effectiveness more and more of VVT until it becomes useless as the overlap with higher duration cams becomes larger and larger and becomes too large to increase with VVT without decreasing performance.
Duration Conclusion
This is why the higher the duration of cam, the higher the peak power band of the engine, and the lower the bottom end power. The higher the duration of the cam the higher the revs you "NEED". Its pointless running a long duration cam in an engine that cant even rev to the optimum peak power band of the cam, as you will sacrifice even more bottom end without gaining topend.
When choosing a cam for maximum topend power decide on the max revs you are willing to rev to, or what revs you are going to mod your engine to be able to handle and choose a cam accordingly. Choose a cam with a peak power band 500-1000rpm below max revs as you dont want to have to change at peak power, it might give good dyno readings but will just be a waste for real life situations. (You dont want to change as you hit peak power, you want to use the peak power band, as many on this forum have correctly said you dont want to drop off the power/torque curve every time you shift).
The nice thing about the 20V is that it can rev really high stock, ive heard of 8500-9000rpm safely with a stock bottom end. This is very nice for cams as you should be able to get away with running up to 300 degree duration with those sort of revs.
But.........Now we get more complications when it comes to choosing cams. It is know as....
Dynamic Compression Ratio[/u]
Most of you have heard of compression ratio, what you most commonly hear of is Static Compression Ratio, e.g the Static compression ratio of a BT 20V is 11:1. Static compression ratio is the ratio between the maximum volume of the cylinder (Piston at BDC) and the minimum volume of the cylinder (Piston at TDC). This however is not really the important compression ratio when it comes to engines. Dynamic compression ratio is what we want to know. Let me explain. As i said earlier the higher the duration of the cam the earlier and later the valve opens and closes respectively. On the intake stroke the intake valve stays open even as the piston moves upwards on the compression stroke. The Dynamic Compression Ratio is determined by the ratio of air only once the intake valve has closed to the minimum volume (piston TDC) therefore the higher the duration, the longer the intake valve stays open, the further the piston moves upwards before the intake valve closes and therefore the smaller the maximum compressable volume of the cylinder. This means that the higher the duration of cam you run with a constant Static Compression Ratio the lower your Dynamic Compression Ratio will be.
So the higher the duration of cam you want to run the higher the Static Compression ratio you need to run, to bring up your Dynamic Compression ratio to get the maximum performance of the cam. On 95 octane you want to keep your DCR up around the 8.5:1-9.0:1 mark this is about the max to safely go without detonation. If you dont increase your SCR its like running a really low compression engine, you will get bad performance and wont get the results you expected. There are complicated formulas to work out the DCR with certain durations and lots of computer programs to help you do it too, as long as you have all the specs of your engine.
What I would recommend is to work out how much you are willing to spend to increase your Static Compression Ratio(Skim head, thinner head gasket, new pistons), choose where you want it and then choose a cam that will bring your Dynamic Compression Ratio to the desired amount e.g U wouldnt really want to run below 13.5:1 preferably more for a 300 degree cam. And it will be expensive to get such high compression on a 20V, so as I said choose how much you willing to spend on compression ratio, then choose cam to match, if you run a cam that requires more compression ratio than you are using its a waste as you will lose bottom end performance for little gains at top when compared to a milder cam that suits your CR more and will give you much better bottom end and similar top.
Conclusion
So id say if you are planning to change your cam, first figure out how much you are willing to spend, then choose the revs you are going to use, the SCR you are going to use and then choose a cam that suits. Also take into account the drivability you want.
Another thing, the larger duration the cam you want to run the more flow your engine requires. The bigger you go the more you will have to spend in this department, e.g. headwork such as gasflowing, valve seats, valves and obviously free flow exhaust. Another thing is when you go for larger cams ITB's or Side draughts are a must for the required flow and they allow the engine to properly with the more extreme cams.
Basically for NA applications almost all of your mods are built around the cam of your choosing. The amount of flow you need, the revs you need etc, these things give most gains in conjunction with the cam. Dont enlarge ports more than you need to flow enough for the cam of your choosing as you will just lose torque, basically choose what cam you want and build everything to compliment it, this way you will get most out of the money you spend.
Now with respect to the 20V, what is great about the engine is that its already got some very important stuff to run larger cams. The ITB's are a must in large cam applications and are insanely expensive yet the 20V comes with some very decent units. A high flow head is required and guess what, the 20V head stock has some amazing flow figures, with just a bit of smoothing in the head (no porting just removing some of the factory defects and scuffs) the head will have enough flow for some very good HP figures and long duration cams, and the 20V can seriously rev with a stock bottom end. So what I would recommend for good power without huge amounts of cash is something like this (This is theoretically a good way to go, and the way im going to go on my 20V, once i get it)
-Find out how much SCR you can get without changing pistons (Just skimming head and thinner head gasket)
-Choose a cam that compliments the CR and the Revs(If you end up with a SCR that allows you to run a cam with a power band above the revs dont increase the SCR so much(Though i doubt it will be this way round)
-Aftermarket Management
-Little bit of headwork(Can be done yourself there is so little to do)
-Free flow manifold and exhaust
Its all about matching everything to work together, you can get even more extreme and match the exhaust and intake manifold lengths to the revs you want to use to get even more bang for your buck.
Ok, im not a rocket scientist, so there are probably some errors, please correct me if im wrong, i hope this helps, and sorry im no writer, i run on in sentences alot, please try decipher it all Very Happy
Regards
4ageFTW!!!!!!
