308 GT4 Wkshop Manual Cam Timing Error???

Today I put the degree wheel on my GT4 engine. In doing so, I referenced a page in the GT4 workshop manual, 1979 edition. I have placed that page below.

The text on the right side states that the exhaust valve Opens at 36 degrees before TDC and that the exhaust valve closes 38 degrees After BDC.

This neither makes sense, nor does it match the timing diagram. I believe the chart ought to read

Exhaust OPENS Before Bottom Dead Center 36 degrees.
Exhaust CLOSES After Top Dead Center 38 degrees.

This would make more sense, and would agree with the timing diagram. I searched and read a lot of cam timing threads (and some crazy arguments!), but was never able to find a reference to this oddity in the workshop manual.

Am I correct in my interpretation of this page, and do folk agree?

Jonathan
Denver, CO
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The owner's manual I have, listed as North American Version 1975, has the timing with the same numbers as my workshop manual, but with the correct nomenclature.

I do not know if there is actually an owner's manual for the GT4 except for the 1975 version - I'll have to look into that. Time to post on the 308 section.

I just checked all the technical bulletins I have, and the OM and the WSM and I only find those cam values, nothing else - but the WSM has the twin airpump info, not the single airpump info, so it's clearly older as well.

I'm off to search for the 1978 GTB/S owner's manual to see if I can find one and if the cam specs are different.

Good thing that I can't get to work on the car this weekend anyway...

Jonathan
Denver, CO
GT4 15302

 

Steve
While you were typing that link, I was there downloading the OM! Thank you.

The 1978 GTB/GTS OM gives the following cam timing specs:

Intake Opens 30deg before TDC
Intake closes 50deg after BDC
--I calculate a duration of 260 degrees


Exhaust Opens 36deg before BDC
Exhaust Closes 28deg after TDC
-- I calculate a duration of 244 degrees

There is no discussion of Lift in the OM, but I have read on other Fchat links that the early cams have more lift per people's measurements of their cams. I do not have a handy cross reference to those threads.


That's the same intake duration as the 1974/5ish data, but the intake cam opens and closes 4 degrees later.

The exhaust duration is 10 degrees LESS than the early cam - it opens at the same time but closes 10 degrees earlier.

This also gives 14 degrees LESS overlap than the earlier cam. No wonder the early GT4 I drove (JimShadow's at the time) felt so different!

I assume I should set the cams using these data points, since I have a 1979.

Hmmm... maybe I should set the intake cam as a 1974 spec cam, and set the exhaust cam per the 1978 spec? Much thinking to do about this...

Steve, thank you very much for pointing me toward that info.

Jonathan
Denver, CO

 

Steve
I have a North American version (I didn't notice the link DIRECTLY below the one I downloaded - that was stupid of me).

Via the 1978 North America OM cam timing data

Intake Opens 14deg before TDC
Intake Closes 48 deg after BDC
I calculate a duration of 244degrees

Exhaust Opens 50deg before BDC
Exhaust Closes 14deg after TDC
I calculate a duration of 244 degrees

This gives a calculated 30 degrees of overlap.

Yikes, what differences. 30 degrees overlap for the US/NA version, 58 degrees for the Euro version, 72 degrees for the earlier cars (probably euro spec). Plus the intake cam has dropped from a duration of 260 to a duration of 244, and the exhaust from a 254 duration to a 244 duration. Booooo.

Since the cams I have are the cams I have, I could move the exhaust timing to the euro spec of 36/28 to match the euro exhaust cam specs, but I can't make the duration of the intake longer just by changing the timing.

Much more thinking to do - probably best just setting the car up the factory way, since without a dyno I probably am at risk for making the car worse by screwing with the cam timing. Though if I only change it to a different Ferrari spec... better keep thinking and be careful here!

Jonathan

 

Well, I have measured and remeasured my cams (4 times each lobe, to try and eliminate user error). I have proper 1978 North American spec cams in my 1979 North American spec car. They measure out to 244 duration for both the intake and exhaust.

I'm going to set the intake cam to stock numbers for 1978 -- 16/48.

I'm going to set the exhaust cams to open at 36 degrees before BDC, which will close them at the 28 degrees after TDC that is listed in the 1978 euro OM. This will give me a little more overlap than the 50/14 listed for the 1978North America spec. IF the Euro owner's manual is a slight misprint, as SteveM suggests, I should still be ok. I'll have 44 degrees of overlap instead of the stock 30 degrees. My only fear is that will push the peak power up in the rev range - something that's not such an awesome need for a street engine up at altitude running crummy E10 "gasoline." Of course, I can always put it back.

Jonathan
Denver, CO