FLATDC and FLBTDC for mixed cams

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Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Sat Apr 02, 2016 6:41 pm

Rob, no..

you wrote,

1. "..so to reduce FL ATDC I move CLOCKWISE the Inlet cams and to reduce FL BTDC I move Exhaust cams COUNTER-CLOCKWISE. (The "crown" of pulleys remaining at its position i.e. the belt does not move) .."

CORRECT SO FAR...


2. "the consequence is an increase of the overlap..."


No it is REDUCTION OF OVERLAP...!

As you move the FL positions in opposite directions relative to each other the include-angle at TDC gets smaller, not bigger. The bigger the angle the more overlap. More overlap = more top-end power (in fact large overlap can generate a FLAT power 'curve' for over 1500 rpm (or more).. Less overlap = more bottom-end torque.

There are reasons for this.

I recommend that you make card cut-outs of the cam timing diagram I posted and move them around and you will understand immediately.

You can (within Protocols) pretty-well post what you want. Whether it may be 'boring' for some is of no importance, all that matters is that the person who posts gets the help he (or she) needs.

G
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Guy Croft
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Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Sat Apr 02, 2016 6:43 pm

Charles, thanks for the update!

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Robert
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Re: FLATDC and FLBTDC for mixed cams

Post by Robert » Sat Apr 02, 2016 8:25 pm

What a problem of ... communication ! I understand what is wrong.
We are not speaking of the same thing. The "what" and the "how"... I try to explain.

The "What":
If we refer to the diagram (I used an excel file instead of card board and you know it Guy, I sent it to you 10 years ago... and I see the picture in your 2nd book), the FL After TDC reducing implies increase of overlap.
Ex: take a 40-80/82-38 cams. So, In Open Before TDC = 40, In Close After BDC = 80 and Ex Op Bef BDC = 82, Ex Cl Aft TDC = 38
The In FL After TDC = 110°and the Ex FL Before TDC = 112( I don't give the calculation well known and present in you books Guy)
The overlap being IO B TDC + EC A TDC = 40+38 = 78
If these figures are wrong, I definitely nothing understood up to now and I can go to bed directly !

So now I modify the setting on the drawing, I turn Counter Clock Wise the Inlet of 2°
I obtain :
IO B TDC = 42, IC A BDC = 78 and I FL A TDC = 108° (reduced). The overlap is now IO B TDC + EC A TDC = 42+38 = 80 and well increased.
Then if I modify the setting and I turn Clock Wise the Exhaust of 2°
I obtain:
Op Bef BDC = 80, Ex Cl Aft TDC = 40 and E FL B TDC = 110 ° (reduced). The overlap is now IO B TDC + EC A TDC = 42+40 = 82 increased once again.

Conclusion: on the drawing showing the engine cycle the overlap increases if the FLs are decreased.
The "How":
Now how to perform a FL reduction when we use a vernier pulley ?
I think if you don't move the belt (so the crank) and just rotate the cams, the right direction is CW for Inlet and CCW for Exhaust. Thus the contrary of the "drawing".
Difficult to me explaining why... I try. If you rotate the Inlet cams CW without moving the crank, the lift is modified and takes another value. To get the previous value for the FL you have to go back with the crank...
If you rotate the Ex cams CCW without moving the crank, the lift is modified and takes another value. To get the previous value for the FL you have to go forward with the crank...
Hoping it is clear enough but I cannot do better ...

It is the "how" I would like to confirm. (I know on the pulley there is a symbol for advance and retard but I prefer to speak about CW or CCW in order to avoid confusion).

Thank you for your concern and attention. Have a good night !!!

Robert
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Guy Croft
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Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Sat Apr 02, 2016 8:52 pm

You need to visit GCRE!!


If you change the inlet full-lift from 110 to 108 you get more overlap.

If you move the ex cam timing from 112 to 110 you get more overlap.

TC cam timing should ALWAYS be done on the head OFF the engine, otherwise you will bend a valve for sure.


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Robert
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Re: FLATDC and FLBTDC for mixed cams

Post by Robert » Sun Apr 03, 2016 6:25 am

Dear Guy,

I think that if Lincoln was not so far North to me I had already visit your workshop !

OK for the tip. But if you have enough clearance (in the worst case where the valve-piston distance is minimal) you have the possibility to move the cams without risk to hit the pistons, no ?

Robert
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Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Sun Apr 03, 2016 9:20 am

If you want to change cam timing on the TC with the head fitted you must - before fitting the head - record the valve lift vs degree around the TDC position for all the FL timing settings you think you may use!

Then do a dry-build at your first setting and measure the clearance valve-piston. You must have 2.5mm vertical clearance. If you don't have enough you have to change the cam timing or do something about the depth of the VRs (valve reliefs).

If you want to change the cam timing, the data you have recorded will guide you as to whether it is safe or not. Accuracy of TDC position of the piston and the cam timing itself will mean the difference between success and destruction of the engine and 1/2 deg movement of the cam can make a big difference.

I have done this procedure once or twice, it is not fun to do. Especially with a big-valve unit where there is a risk of valve clash on the overlap (where they hit each other).

GC
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Robert
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Re: FLATDC and FLBTDC for mixed cams

Post by Robert » Sun Apr 03, 2016 12:17 pm

Hello Guy,

I measured the valve clearance in the past with the 130TC cams, 43.5 mm valves and the pistons I am using.
The minimum clearance is 3.64 mm for inlet and 3.70 mm for exhaust.
By calculation with the GC3a inlet cams I obtain 2.52 mm.

So I just tried this morning and went to the road. I had to increase fuel (A/F monitored with a Lambdameter) on several location of the map, specially at wide open throttle position. The engine runs up to 6800 rpm (shift light) easily. I put the cut off at 7200 rpm but did not try.
Tomorrow afternoon I go to dyno bench and will give you the results with the map updated completely.

Many thanks for the customer support, it is still comforting to get thought from others.

Robert
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Guy Croft
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Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Sun Apr 03, 2016 12:24 pm

thanks Rob,

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