FAO Stu @ MSD or Chip
25-11-2005, 11:13 AM
#1
FAO Stu @ MSD or Chip
I have enjoyed reading all the technical posts you have put up over the last few days 
Please could you do one on camshafts for the YB.
What cams to use for 400bhp and over and why you can mix cams, ie BD14 inlet and BD10 exhaust. What does this do ?
Many Thanks
Steve.
Please could you do one on camshafts for the YB.
What cams to use for 400bhp and over and why you can mix cams, ie BD14 inlet and BD10 exhaust. What does this do ?
Many Thanks
Steve.
i dont have enough experience with the YB to write that one im afraid mate.
26-11-2005, 08:58 AM
#5
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you did one over 2 years back stu.....
Its not rocket science but its different for every engine.
You need to see peak lift at the point your piston is travelling at the fastest speed.
This obviously varies from engine to engine so go and get a pen and paper and do some drawings
Retarding from here will generally move peak horsepowqer higher and advancing move peak horsepower lower.
Overlap will improve spool up but drop peak power.
Heres a basic rundown of the 4 critical timing events:
VALVE TIMING EVENTS IN ORDER OF IMPORTANCE:
The LEAST important is the exhaust valve opening. It could open anywhere from 50 degrees to 90 degrees BBDC. If it opens late, close to the bottom, you will take advantage of the expansion, or power stroke and it will make for good emissions, but you will pay for it with pumping losses by not having enough time to let the cylinder blow-down.
You must let the residual gas start out of the exhaust valve early enough so that the piston will not have to work so hard to push it out. Opening the exhaust valve earlier will give the engine a longer blow-down period which will reduce pumping losses. But, if you are only interested in low speed operation, say up to 4000 RPM, you can open the exhaust valve later.
The next least important timing point is the exhaust valve closing. If it closes early, say around 15 degrees ATDC, you will have a short valve overlap period.
Less overlap makes less emissions, but it does not help power at the higher engine speeds. Closing the exhaust valve later, in the vicinity of 40 degrees ATDC, will mean a longer valve overlap period and a lot more intake charge dilution that will translate ultimately into poor low-speed operation.
Some compromise must clearly be made to determine just how much overlap one needs to use. Many factors such as idle quality, low speed throttle response, fuel economy, port size, and combustion chamber design must be considered in making this choice.
A somewhat more important timing event is the intake valve opening.
Early opening allows for a greater valve overlap period and adds to poor response at low engine speeds. Now, for the high performance enthusiast, low engine speed could mean 3000 RPM, but I would not consider such an engine as appropriate for normal road use!
If you are not concerned about emissions, then early intake valve opening will help the power output of the engine. That is, earlier valve opening will have the valve open further when the piston reaches maximum velocity and that, in turn, will increase volumetric efficiency.
Now, the last timing event is the most important, and the most critical to engine performance - THE CLOSING OF THE INTAKE VALVE.
This event governs both the engine's RPM range and its effective compression ratio. If the intake valve closes early, say about 50 degrees ABDC, then it limits how much air/fuel mixture can enter the cylinder. Such an early closing will provide very nice low speed engine operation, but at the same time it limits the ultimate power output as well as RPM.
Another problem with early intake valve closing that most people do not consider is that if you have a high compression engine, say 10:1 or higher, you will have more pumping loss trying to compress the mixture. This might even lead to head gasket and / or piston failure!
These observations suggest that if you close the intake valve later the cylinder will have more time to take in more air/fuel and the RPM will move up.
That seems simple enough, doesn't it? The later the intake valve closes the higher the RPM and therefore the more power, MAYBE?
In reality if the intake valve closes past 75 degrees ABDC, you could lose most of your low-speed torque and if your static compression ratio is only 8:1 like std cosworth, the engine will not be able to reach its horsepower potential. This should give you a better understanding of why the intake valve closing is the most important timing event.
So go work from that lot folks
Best Regards,
Stu
Its not rocket science but its different for every engine.
You need to see peak lift at the point your piston is travelling at the fastest speed.
This obviously varies from engine to engine so go and get a pen and paper and do some drawings
Retarding from here will generally move peak horsepowqer higher and advancing move peak horsepower lower.
Overlap will improve spool up but drop peak power.
Heres a basic rundown of the 4 critical timing events:
VALVE TIMING EVENTS IN ORDER OF IMPORTANCE:
The LEAST important is the exhaust valve opening. It could open anywhere from 50 degrees to 90 degrees BBDC. If it opens late, close to the bottom, you will take advantage of the expansion, or power stroke and it will make for good emissions, but you will pay for it with pumping losses by not having enough time to let the cylinder blow-down.
You must let the residual gas start out of the exhaust valve early enough so that the piston will not have to work so hard to push it out. Opening the exhaust valve earlier will give the engine a longer blow-down period which will reduce pumping losses. But, if you are only interested in low speed operation, say up to 4000 RPM, you can open the exhaust valve later.
The next least important timing point is the exhaust valve closing. If it closes early, say around 15 degrees ATDC, you will have a short valve overlap period.
Less overlap makes less emissions, but it does not help power at the higher engine speeds. Closing the exhaust valve later, in the vicinity of 40 degrees ATDC, will mean a longer valve overlap period and a lot more intake charge dilution that will translate ultimately into poor low-speed operation.
Some compromise must clearly be made to determine just how much overlap one needs to use. Many factors such as idle quality, low speed throttle response, fuel economy, port size, and combustion chamber design must be considered in making this choice.
A somewhat more important timing event is the intake valve opening.
Early opening allows for a greater valve overlap period and adds to poor response at low engine speeds. Now, for the high performance enthusiast, low engine speed could mean 3000 RPM, but I would not consider such an engine as appropriate for normal road use!
If you are not concerned about emissions, then early intake valve opening will help the power output of the engine. That is, earlier valve opening will have the valve open further when the piston reaches maximum velocity and that, in turn, will increase volumetric efficiency.
Now, the last timing event is the most important, and the most critical to engine performance - THE CLOSING OF THE INTAKE VALVE.
This event governs both the engine's RPM range and its effective compression ratio. If the intake valve closes early, say about 50 degrees ABDC, then it limits how much air/fuel mixture can enter the cylinder. Such an early closing will provide very nice low speed engine operation, but at the same time it limits the ultimate power output as well as RPM.
Another problem with early intake valve closing that most people do not consider is that if you have a high compression engine, say 10:1 or higher, you will have more pumping loss trying to compress the mixture. This might even lead to head gasket and / or piston failure!
These observations suggest that if you close the intake valve later the cylinder will have more time to take in more air/fuel and the RPM will move up.
That seems simple enough, doesn't it? The later the intake valve closes the higher the RPM and therefore the more power, MAYBE?
In reality if the intake valve closes past 75 degrees ABDC, you could lose most of your low-speed torque and if your static compression ratio is only 8:1 like std cosworth, the engine will not be able to reach its horsepower potential. This should give you a better understanding of why the intake valve closing is the most important timing event.
So go work from that lot folks
Best Regards,
Stu
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