10 replies to this topic

[footprints]

hi,

I've started a thread sometime back about camshaft LSA and it has been really informative. I've bought a new set of cams and have some new questions about tightening the LSA with DOHC engines.

Since the LSA can be changed with DOHC engines, we can effectively increase or decrease the overlap period during TDC. question is, do we ****** the exhaust cam or advance the intake cam? Or do both to reduce the LSA?

We know that by advancing the intake cam, the overlap is increased, the valve opens earlier to start the intake charge movement, but it also means the intake valve is closed earlier and we will lose the potential ramming of the intake charge ABDC when the velocity is at its highest. Typically, this will mean higher cranking pressure, higher dynamic compression, torque is produced earlier, thus less high rpm power.

How about retarding the exhaust cam instead? It will increase the valve overlap and exhaust scavenging will help the intake charge, ofcourse it depends on exhaust back pressure and design of the exhaust header, it can help 'pull' in the intake charge. But does it make much difference to the opening of the exhaust valves BBDC? I think it makes a difference in torque because header designers will want to know exhaust valve opening BBDC to determine the best length for most torque.

So, my silly question here is, with regards to high rpm power, is overlap more beneficial or late closing of intake valve? Because since the duration is fixed, the only way is to swing the camshaft centerlines advanced or ********.

So it makes sense to just ****** the exhaust cam if i want to achieve more overlap?

My camshafts centerlines are

Intake 108
open 15 BTDC, close 56 ABDC
Exhaust 111
Open 55BBDC, close 19ATDC
events at 0.05 lift.

so LSA is 109.5

I can tighten the LSA to 105 for more overlap, but in theory, which will be a better way to achieve that? ****** the exhaust cam only, or also advance the intake?

I really wonder if overlap will make more power than a late closing intake valve. I know its all theory and the only way to find out is on the dyno, but i'm sure there is theory behind this.

I hope i make sense. Thanks!!!

[VAR1016]

I cannot help, but I have posted this to make sure that I don't miss any responses.

You can be sure that the "knowledge" is hereabouts!

PdeRL

[footprints]

thanks mcguire,

Now i understand more. Advance intake =more low end power. Is it because the valves will close earlier abdc thus limiting the effects of ramming in the intake and thus less cylinder filling?

Retarding the exhaust also = more low end power. Is the late opening of the exhaust valve the factor or the late closing? I can see why a late opening will make more torque low end, it keeps the cylinder pressure longer?

However on the overlap part, i think there is some confusion. I shall present it in my way of understanding.

Advance Intake:
Intake valves open earlier while the exhaust valves are still closing. So by advancing the intake cams, we are increasing the overlap. While the exhaust valves are still on the way close, the intake valves have been opened earlier, so more overlap. example 2 scenarios; intake opens 20btdc and exhaust close 15atdc, overlap is 20+15=35degrees. So if i advance the intake and it now opens 25btdc and exhaust close at 15atdc then the overlap becomes 25+15=40degrees overlap.

****** exhaust:

By retarding the exhaust, the exhaust opening bbdc is delayed, but the exhaust closing atdc is also delayed, so if the intake cam is opening for example 20btdc and the exhaust has been ******** to close from 15atdc to 20atdc, the overlap is increased from 35degrees to 40degrees.

So, we can tighten the LSA by either advancing the intake, or retarding the exhaust or doing both at the same time.

Now, the it gets really confusing and conflicting. Earlier we discussed that intake advance and exhaust ****** will have better low end, but my explaination tells us otherwise. I'm really puzzled.


We know that during overlap, the exiting exhaust gases will help pull in the intake when the intake cams opens. This will help VE and cylinder filling. More High RPM power.

We know that a late closing intake valve allows more time for the intake charge to ram in the high velocity charge to increase cylinder filling, thus increasing VE. More high rpm power.

Now, if we can only advance or ****** the intake cam with duration as a constant, which will be more favourable? Should i try to advance the intake to increase the overlap so as to take advantage of a exhaust scavaging but losing the late intake closing? Since the engine responds with better low end when we advance the intake, can i deduce that the intake valve closing is a more determining factor for high RPM power?


How does a camshaft grinder decide where centerline should be? How did they come up with 108 or 105 centerline?

[shaun979]

Quote

Originally posted by McGuire
I think you mean advancing the intake cam or retarding the exhaust cam will decrease the overlap, not increase it. Either will widen the lobe separation angle -- the distance in degrees between the intake and exhaust lobe centerlines. If the intake valve is opening and closing earlier, overlap is reduced.

I have to say this is the first time I've heard this. I can't picture how it can be so. Everything I've heard and read goes with footprint's understanding of advancing and retarding.

[McGuire]

That's because I totally botched the explanation, sorry.

[McGuire]

Quote

Originally posted by shaun979


I have to say this is the first time I've heard this. I can't picture how it can be so. Everything I've heard and read goes with footprint's understanding of advancing and retarding.

Thanks for the correction. My explanation was total gibberish. After looking it over I have decided there is no saving it, so I have removed it so no one will be misinformed.

[shaun979]

I actually thought you knew another way of looking at things that perhaps someone I hadn't read about had come up with and that I was about to learn something. No worries...

[m9a3r5i7o2n]

footprints;
I am sending you a sketch of the camshaft layout by PM as you print this out use 8.5" by 14" paper. I, for one, have trouble understanding the layouts until I have made out sketches of it and its effect on the timing. I believe you should do this also.

If it is clearer in the circular pattern just tell me so as it is very easy for me to transfer it to a 720 degree layout. Sometimes I do both just for clarification.

M. L. Anderson

[m9a3r5i7o2n]

footprints; Here is the linear layout of your cam timing.

A program to do this in a similiar fashion only using COREL is $180.00 at COMPUSA.
You can do it in either linear or circular fashion!


https://home.comcast...otprintsLSA.JPG

M. L. Anderson

[footprints]

thanks Mr Anderson, I've also analysed camdoctor specs of my cams but interpretation of the information is the tricky part.

I wonder how camshaft manufacturers workout the ideal camshaft centerlines during the design stage?

[Greenbird]

Quote

Originally posted by footprints
Since the LSA can be changed with DOHC engines, we can effectively increase or decrease the overlap period during TDC. question is, do we ****** the exhaust cam or advance the intake cam? Or do both to reduce the LSA?

Either or both will reduce LSA. Please note that LSA is not a cam designer's spec, it is a cam grinder's spec. (don't pick an LSA)

Quote

Originally posted by footprints
We know that by advancing the intake cam, the overlap is increased, the valve opens earlier to start the intake charge movement, but it also means the intake valve is closed earlier and we will lose the potential ramming of the intake charge ABDC when the velocity is at its highest. Typically, this will mean higher cranking pressure, higher dynamic compression, torque is produced earlier, thus less high rpm power.

Intake flow doesn't start until ATDC, no matter the engine or RPM. The intake valve is opened BTDC so it will be wide open by the time the piston stats down the bore.

Quote

Originally posted by footprints
How about retarding the exhaust cam instead? It will increase the valve overlap and exhaust scavenging will help the intake charge, ofcourse it depends on exhaust back pressure and design of the exhaust header, it can help 'pull' in the intake charge. But does it make much difference to the opening of the exhaust valves BBDC? I think it makes a difference in torque because header designers will want to know exhaust valve opening BBDC to determine the best length for most torque.

It is possible to over scavenge. Leaving the exhaust open too long can let the intake blow out the exhaust.

Header designs don't account for EO BBDC, as it's the same across all cylinders. Header designs are based on firing order, CID, RPM and VE.

Quote

Originally posted by footprints
So, my silly question here is, with regards to high rpm power, is overlap more beneficial or late closing of intake valve? Because since the duration is fixed, the only way is to swing the camshaft centerlines advanced or ********.

IC ABDC is the most important spec. too early and you won't make the high rpm power you want. Too late and the low rpm power will drop. The engine becomes more sensitive to changes in ABDC with increases in compression and rod ratio.

Quote

Originally posted by footprints
I can tighten the LSA to 105 for more overlap, but in theory, which will be a better way to achieve that? ****** the exhaust cam only, or also advance the intake?

I really wonder if overlap will make more power than a late closing intake valve. I know its all theory and the only way to find out is on the dyno, but i'm sure there is theory behind this.

First, what you have listed is not correct. Did you measure the centerlines or the valve events?
15BTDC+56ABDC= 110.5 ICL
55BBDC+19ATDC= 108 ECL

251 I dur. @ 108 ICL = 17.5BTDC+53.5ABDC
254 E dur @ 111 ECL = 58BBDC+16ATDC