After Vct Info

[MRHD66]

Hi I'm looking for R33 RB25VCT info I know it moves 20degress but which way ?

[Hanaldo]

It advances the inlet cam 20 degrees.

[MRHD66]

So if it's a std cam it goes from 240 to 260?? Cheers mate

[MRHD66]

So if it's a std cam it goes from 240 to 260?? Cheers mate

That's wrong it changes the LSA by 20 degrees so if it was 112LSA it would be 92LSA yes??

[89CAL]

That's wrong it changes the LSA by 20 degrees so if it was 112LSA it would be 92LSA yes??

What?

[The Dan]

So if it's a std cam it goes from 240 to 260?? Cheers mate

That's wrong it changes the LSA by 20 degrees so if it was 112LSA it would be 92LSA yes??

WOAH!, headspin

No it doesn't change the duration, yes it does adjust the LSA.

[MRHD66]

WOAH!, headspin

No it doesn't change the duration, yes it does adjust the LSA.

I sat there thinking how can it change the duration for a hr and I was like it can't !! The LSA gets closer like I said yes?? Edited June 7, 2012 by MRHD66

[MRHD66]

What?

cos it gets closer 20 degrees closer to the exhaust cam

[discopotato03]

The best way to look at is the inlet cam phasing only changes . The exhaust cam timing is fixed in relation to the crank and therefore piston cycles .

By changing the inlet cams position in relation to the exhaust cam you can vary the overlap timing .

Valve overlap occurs when a pistion is a the top of its fourth or exhaust stroke , at this point all of a cylinders valves are open but by this I mean the exhausts are closing but not on the seat/s and the inlets are opening but not very far off their seats .

You possibly need to look at a cam chart because a good one gives you the opening and closing points of the valves in your four stroke cycles .

You need to know what these terms BTDC ATDC BBDC and ABDC mean . Basically - Before Top Dead Center / After Top Dead Center / Before Bottom Dead Center / After Bottom Dead Center .

If someone said to you my cam/s give these valve timing numbers 20-60 60-20 , it means the inlet valves open 20 degrees Betore Top Dead Center and close 60 degrees After Bottom Dead Center . The exhaust valves will close 20 degrees After Bottom Dead Center and open 60 degrees Before Bottom Dead Center .

To work out the overlap duration add 20 BTDC (inlet valves) to 20 degrees (exhaust valves) and get 40 degrees overlap duration .

For engines to work smoothly and over a wide range its good to be able to vary this overlap duration to suit the engines speed .

Shorter overlap duration works well at low engine (low gas speeds) and longer overlap at high engine (and gas) speeds .

Volumetric or cylinder filling efficiency is greatly affected by valve timing and some of this is because the right amount of overlap timing encourages good scavanging (the ability of the exhaust gasses velocity and momentum creating an area of low pressure at the closing exhaust valves which allows the exhaust side to actually help draw clean air through the chambers to purge the last of the exhaust gasses out) .

Ok so when you can open up the cam centers by changing the inlet cams position the overlap duration decreases which helps to keep the gasses moving in the right direction at low revs .

When the revs get up a bit the gas flow volume and speed is greater so to make the engine scavange well at medium to high revs you close the cam centers up which increases the overlap duration .

As a hypothetical example look my earlier 20 60 60 20 valve timing numbers . This is fixed timing like say an RB20 or RB26 has but if you could vary the inlet cam phasing it could for example look like say 10 70 60 20 with the cams opened up and 30 50 60 20 with them closed up . Only the first two numbers which represent inlet valve timing change because only the inlet cam has changed its position .

It goes without say that the inlet cam lobes effective profile has not changed so the inlet cam/valve duration can't change .

Look back to those inlet valve timing numbers . They can be 20 60 or 10 70 or 30 50 , add any pair together and you still get 80 . You add this 80 to 180 and you will always get 260 degrees inlet duration .

Now lets say you have an RB25DET with 280 degree cams and no VCT , assume for a sec that its valve timing numbers are 30 70 70 30 making the overlap duration 60 degrees . Just to prove the 280 duration add the inlet , or exhaust , timing numbers together and add 180 so 30+70 =100 +180 =280 . An engine like this WILL be cammy and a bit breathless at low revs compared to one with tamer cams and less overlap timing . A better overall compromise might be 260 or 270 degree cams and 20 degrees of inlet cam variation - VCT .

This is probably hard to fathom unless you have a good understanding of how a four stroke engine works and how valve timing works in relation to it .

Its all out there if you search for it , cheers A .

Edited June 7, 2012 by discopotato03

[MRHD66]

The best way to look at is the inlet cam phasing only changes . The exhaust cam timing is fixed in relation to the crank and therefore piston cycles .

By changing the inlet cams position in relation to the exhaust cam you can vary the overlap timing .

Valve overlap occurs when a pistion is a the top of its fourth or exhaust stroke , at this point all of a cylinders valves are open but by this I mean the exhausts are closing but not on the seat/s and the inlets are opening but not very far off their seats .

You possibly need to look at a cam chart because a good one gives you the opening and closing points of the valves in your four stroke cycles .

You need to know what these terms BTDC ATDC BBDC and ABDC mean . Basically - Before Top Dead Center / After Top Dead Center / Before Bottom Dead Center / After Bottom Dead Center .

If someone said to you my cam/s give these valve timing numbers 20-60 60-20 , it means the inlet valves open 20 degrees Betore Top Dead Center and close 60 degrees After Bottom Dead Center . The exhaust valves will close 20 degrees After Bottom Dead Center and open 60 degrees Before Bottom Dead Center .

To work out the overlap duration add 20 BTDC (inlet valves) to 20 degrees (exhaust valves) and get 40 degrees overlap duration .

For engines to work smoothly and over a wide range its good to be able to vary this overlap duration to suit the engines speed .

Shorter overlap duration works well at low engine (low gas speeds) and longer overlap at high engine (and gas) speeds .

Volumetric or cylinder filling efficiency is greatly affected by valve timing and some of this is because the right amount of overlap timing encourages good scavanging (the ability of the exhaust gasses velocity and momentum creating an area of low pressure at the closing exhaust valves which allows the exhaust side to actually help draw clean air through the chambers to purge the last of the exhaust gasses out) .

Ok so when you can open up the cam centers by changing the inlet cams position the overlap duration decreases which helps to keep the gasses moving in the right direction at low revs .

When the revs get up a bit the gas flow volume and speed is greater so to make the engine scavange well at medium to high revs you close the cam centers up which increases the overlap duration .

As a hypothetical example look my earlier 20 60 60 20 valve timing numbers . This is fixed timing like say an RB20 or RB26 has but if you could vary the inlet cam phasing it could for example look like say 10 70 60 20 with the cams opened up and 30 50 60 20 with them closed up . Only the first two numbers which represent inlet valve timing change because only the inlet cam has changed its position .

It goes without say that the inlet cam lobes effective profile has not changed so the inlet cam/valve duration can't change .

Look back to those inlet valve timing numbers . They can be 20 60 or 10 70 or 30 50 , add any pair together and you still get 80 . You add this 80 to 180 and you will always get 260 degrees inlet duration .

Now lets say you have an RB25DET with 280 degree cams and no VCT , assume for a sec that its valve timing numbers are 30 70 70 30 making the overlap duration 60 degrees . Just to prove the 280 duration add the inlet , or exhaust , timing numbers together and add 180 so 30+70 =100 +180 =280 . An engine like this WILL be cammy and a bit breathless at low revs compared to one with tamer cams and less overlap timing . A better overall compromise might be 260 or 270 degree cams and 20 degrees of inlet cam variation - VCT .

This is probably hard to fathom unless you have a good understanding of how a four stroke engine works and how valve timing works in relation to it .

Its all out there if you search for it , cheers A .

cheers mate that makes sence and you explained to very well !!

[The Dan]

The best way to look at is the inlet cam phasing only changes . The exhaust cam timing is fixed in relation to the crank and therefore piston cycles .

By changing the inlet cams position in relation to the exhaust cam you can vary the overlap timing .

Valve overlap occurs when a pistion is a the top of its fourth or exhaust stroke , at this point all of a cylinders valves are open but by this I mean the exhausts are closing but not on the seat/s and the inlets are opening but not very far off their seats .

You possibly need to look at a cam chart because a good one gives you the opening and closing points of the valves in your four stroke cycles .

You need to know what these terms BTDC ATDC BBDC and ABDC mean . Basically - Before Top Dead Center / After Top Dead Center / Before Bottom Dead Center / After Bottom Dead Center .

If someone said to you my cam/s give these valve timing numbers 20-60 60-20 , it means the inlet valves open 20 degrees Betore Top Dead Center and close 60 degrees After Bottom Dead Center . The exhaust valves will close 20 degrees After Bottom Dead Center and open 60 degrees Before Bottom Dead Center .

To work out the overlap duration add 20 BTDC (inlet valves) to 20 degrees (exhaust valves) and get 40 degrees overlap duration .

For engines to work smoothly and over a wide range its good to be able to vary this overlap duration to suit the engines speed .

Shorter overlap duration works well at low engine (low gas speeds) and longer overlap at high engine (and gas) speeds .

Volumetric or cylinder filling efficiency is greatly affected by valve timing and some of this is because the right amount of overlap timing encourages good scavanging (the ability of the exhaust gasses velocity and momentum creating an area of low pressure at the closing exhaust valves which allows the exhaust side to actually help draw clean air through the chambers to purge the last of the exhaust gasses out) .

Ok so when you can open up the cam centers by changing the inlet cams position the overlap duration decreases which helps to keep the gasses moving in the right direction at low revs .

When the revs get up a bit the gas flow volume and speed is greater so to make the engine scavange well at medium to high revs you close the cam centers up which increases the overlap duration .

As a hypothetical example look my earlier 20 60 60 20 valve timing numbers . This is fixed timing like say an RB20 or RB26 has but if you could vary the inlet cam phasing it could for example look like say 10 70 60 20 with the cams opened up and 30 50 60 20 with them closed up . Only the first two numbers which represent inlet valve timing change because only the inlet cam has changed its position .

It goes without say that the inlet cam lobes effective profile has not changed so the inlet cam/valve duration can't change .

Look back to those inlet valve timing numbers . They can be 20 60 or 10 70 or 30 50 , add any pair together and you still get 80 . You add this 80 to 180 and you will always get 260 degrees inlet duration .

Now lets say you have an RB25DET with 280 degree cams and no VCT , assume for a sec that its valve timing numbers are 30 70 70 30 making the overlap duration 60 degrees . Just to prove the 280 duration add the inlet , or exhaust , timing numbers together and add 180 so 30+70 =100 +180 =280 . An engine like this WILL be cammy and a bit breathless at low revs compared to one with tamer cams and less overlap timing . A better overall compromise might be 260 or 270 degree cams and 20 degrees of inlet cam variation - VCT .

This is probably hard to fathom unless you have a good understanding of how a four stroke engine works and how valve timing works in relation to it .

Its all out there if you search for it , cheers A .

You need to get laid

[r33_racer]

Then combine an engine with lots of overlap and a greater then 1:1 exhaust to inlet pressure ratio and you get a whole lot of exhaust gas reversion and residuals left in the chamber!

If your hot side is going to be restrictive then don't opt for cams with large overlap windows. It ain't an atmo v8 here where large overlap is required for high power/high rpm.