FLATDC and FLBTDC for mixed cams

Question and answer direct with GC. Competition engines and 'live' projects only. Good photos to illustrate your post are expected.
Robert
Posts: 86
Joined: Sun Aug 20, 2006 8:17 am
Location: France
Contact:

FLATDC and FLBTDC for mixed cams

Post by Robert » Thu Mar 31, 2016 9:10 am

Hello Guy,

I would like to know the best setting for “mixed” camshafts:
Up to now I used the Ritmo/Strada 130TC “as is” (FLATDC Intake = 112°, FLBTDC = 111.5° Exhaust. Original setting) .
I am now using the GC3a Intake cams with the 130TC Exhaust cams. My actual setting (measured for intake cams) is 111°/111.5°.

Because I can modify these angles with vernier pulleys could you please indicate here the best compromise (I and E) for such a configuration.
I’m sure this information is useful for others members.

The main characteristics are:
2 liter engine
Fuel Injection/Static ignition mapped in an ECU
Pistons from 105TC giving about 10.7 CR
130TC head, OE valves but back grounded by GCRE, “3angles” seats, GCRE valves guides.
Intake manifold GCRE
TB 45mm (Lancia Montecarlo 20° tilt)
Intake tract (trumpets) 40mm
CSC exhaust manifold 4-2-1 (Lancia Montecarlo)

Many thanks

Robert
GC_131

Guy Croft
Site Admin
Posts: 5003
Joined: Sun Jun 18, 2006 9:31 am
Location: Lincoln, UK
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Thu Mar 31, 2016 10:19 am

Sorry Rob,

I cannot answer your question, I have only ever run standard cams at standard cam timing.

There is no reason why you could not try inlet FL 106 and ex 108, or 104/106 - I suppose they will respond like any other cam setup, ie:more overlap = more top-end power and less bottom-end

G
Guy Croft, owner

Robert
Posts: 86
Joined: Sun Aug 20, 2006 8:17 am
Location: France
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Robert » Thu Mar 31, 2016 12:28 pm

Guy,

If inlet GC3a is 107 and exhaust standard remains 111, is it worthwhile or not ?
Inlet closure earlier but overlap not so great... The idea is to keep a "good" bottom end.

Thanks.

Robert (apprentice)
GC_131

Guy Croft
Site Admin
Posts: 5003
Joined: Sun Jun 18, 2006 9:31 am
Location: Lincoln, UK
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Thu Mar 31, 2016 1:01 pm

Use 108 inlet, 100 ex

G
Guy Croft, owner

Robert
Posts: 86
Joined: Sun Aug 20, 2006 8:17 am
Location: France
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Robert » Thu Mar 31, 2016 7:48 pm

Guy,

106/108 gives an overlap of 75° and 108/100 gives 81°. A big difference no ?
Is it really 100° E you recommend ?

Robert
GC_131

Guy Croft
Site Admin
Posts: 5003
Joined: Sun Jun 18, 2006 9:31 am
Location: Lincoln, UK
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Thu Mar 31, 2016 9:01 pm

very sorry Rob,

that was a typing error, thank you for questioning my comment,

I meant 110 ex

G
Guy Croft, owner

Robert
Posts: 86
Joined: Sun Aug 20, 2006 8:17 am
Location: France
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Robert » Fri Apr 01, 2016 6:16 am

OK Guy, I guessed a mistake.
Thank you. I let you know the result next week.

Robert
GC_131

Robert
Posts: 86
Joined: Sun Aug 20, 2006 8:17 am
Location: France
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Robert » Sat Apr 02, 2016 1:26 pm

Guy,

I really don't want make a mistake. I have thought so much to the problem that I am puzzled...

I'm in front of the engine, pulleys side. They are equipped of vernier and index. Assuming I know the initial/current inlet FLATDC and the initial/current exhaust FLBTDC.
If I want to decrease FLATDC (passing from 111 to 108°) I have to rotate the cams in which direction ? CW or CCW ?
If I want to decrease FLBTDC (passing from 112 to 110°) I have to rotate the cams in which direction ? CW or CCW ?

"It is better one who knows than two who are searching" (as my grandmother said)

Thank you once again !

Robert
GC_131

Spider 1969
Posts: 122
Joined: Sun Sep 22, 2013 7:21 pm
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Spider 1969 » Sat Apr 02, 2016 3:32 pm

Hi Robert,

Presuming you want to reduce LATDC for both:
As the cams rotate CW and the Inlet has FL after TDC you have to rotate CW to decrease LATDC.
Exhaust has FL BTDC so rotate CCW to decrease LATDC.
This wil also decrease overlap.

Hope this is of help.
Best regards,
Charles
GC_26

Guy Croft
Site Admin
Posts: 5003
Joined: Sun Jun 18, 2006 9:31 am
Location: Lincoln, UK
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Sat Apr 02, 2016 5:23 pm

Correct, well-done Charles..

btw how is your motor?

G
Guy Croft, owner

Robert
Posts: 86
Joined: Sun Aug 20, 2006 8:17 am
Location: France
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Robert » Sat Apr 02, 2016 5:28 pm

Hello Charles,

Thank you for that. I confess I am comforted because it was what I though.
Just a remark, for exhaust I will decrease FLBTDC. The overlap will be increased (if both cams FL are closer to TDC I suppose the overlap is higher)

Best regards
Robert
GC_131

Guy Croft
Site Admin
Posts: 5003
Joined: Sun Jun 18, 2006 9:31 am
Location: Lincoln, UK
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Guy Croft » Sat Apr 02, 2016 5:31 pm

To visualise what happens when you change FL (say from 108 to 110 ATDC inlet) - rotate the blue or red sections...
Attachments
cam timing.jpg
cam timing.jpg (51.28 KiB) Viewed 1008 times
Guy Croft, owner

Robert
Posts: 86
Joined: Sun Aug 20, 2006 8:17 am
Location: France
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Robert » Sat Apr 02, 2016 6:23 pm

To me:
FLATDC : Full Lift After Top Dead Center for Inlet (blue line on the drawing)
FLBTDC : Full Lift Before Top Dead Center for Exhaust (red line on the drawing)

So to reduce FLATDC I move CW the Inlet cams and to reduce FLBTDC I move Exhaust cams CCW. (The "crown" of pulleys remaining at its position i.e. the belt does not move). The consequence is an increase of the overlap.

This is what I understand and I need confirmation.

Hoping to don't be boring for the professionals.

Robert
GC_131

Spider 1969
Posts: 122
Joined: Sun Sep 22, 2013 7:21 pm
Contact:

Re: FLATDC and FLBTDC for mixed cams

Post by Spider 1969 » Sat Apr 02, 2016 6:23 pm

Hi Robert,

Rotating the Inlet CW and Exhaust CCW decreases overlap. It get's less as the Exhaust closes earlier and the Inlet opens later. Visualize blue and red line moving away from each other from TDC in the diagram.


Hi Guy,
Motor is ready for first start up. Tommorrow I'll be testing ignition and fuel system. If time permits I'll fire it up and do the cam bedding in. Quality before time.
I'll take a day off somewhere this week to synchronize the carbs and tune idle mixture.
If all goes well it's off to the rolling road whenever they have a slot for bedding in and optimizing.

Sincerely,
Charles
GC_26

Guy Croft
Site Admin
Posts: 5003
Joined: Sun Jun 18, 2006 9:31 am
Location: Lincoln, UK
Contact:

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
Guy Croft, owner

Post Reply

Who is online

Users browsing this forum: Bing [Bot] and 3 guests