Advantages to Cam Timing

allansieben

allansieben

X1/9 Time
I'm in the process of rebuilding the top end of my 1.8 litre twin-cam engine in my Scorpion.
My friend who is carrying out the task,(The Doctor) is a master Italian car tech, who specializes in fiat and lancia. We are getting close to completion, and he is telling me that he wants to advance the intake cam one notch, and retard the exhaust cam one notch, for a longer burn in the cylinders.
My question is: has anybody on this forum ever done this? What is the true advantage? I'm not doubting my friend, but would like some input from someone who may have done this sort of thing.
Thank you in advance for any replies.
Allan
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Does he mean one notch on the cam pulleys, cause that's an awful lot to move the cams around. For reference, I played with the intake cam on my 1.8, advancing it 2-4 degrees added a little more below the peaks, and on my engine moved the peak down 200 rpm. I tried retarding the intake and it lost power all the way up to peak. Only after peak did it pick up a couple of hp. I never played around with the exhaust.
 
Yes

Yes. One notch on the cam pulleys. My friend says that the factory settings are more timed for emissions, and that his proposed settings will help increase HP.
 
Get adjustable pulleys

allansieben said:
Yes. One notch on the cam pulleys. My friend says that the factory settings are more timed for emissions, and that his proposed settings will help increase HP.
Click to expand...
Everything I've heard says that one notch is too much; AFAIR it is almost 7 degs. PBS used to fine tune the cam timing by redrilling the pulley 180 deg off. You can get adjustable cam pulleys and try.
 
A full tooth is a BIG change in cam timing, as mentioned cam timing alteration is usually limited to about 4 degrees either way... more than that and you change the complete camshaft to better suit the engine.

There are 42 teeth on the cam gear from memory, the engine goes thru 720 degrees of rotation for every rotattion of the cam, so each tooth at the cam is over 17 degrees of crankshaft rotation.

SteveC
 
Yeah Allan... I remember discussing this...

... at length years ago... and I believe we settled on 8 degrees with one notch.

Whether its 4, 7, or 8, its WAY too much on our SINGLE cam engines... and I would suspect the same on the twin cammers.

Everyone I've seen playing with this aspect uses the ADJUSTABLE pulleys.

Have fun... but I suggest get it running first with everything "straight up" and THEN start playing with cam timing. Best have a dyno or consistent test track and timers when ya do also! Lots of variables that yur playin' with here...
 
Not so Fast.

Stock engine maybe but a 40-80 cam shaft will run very well with 1 tooth retarded. In my case 1 tooth retarded gives a better bottom end and takes a bit off of the top rpm and HP. Moving to the centre will take a bit off the bottom and add to the top with a definate pull above 3500 rpm to 7500 rpm. So this now depends on what you want and where you are driving. Stop and go traffic is a bit tiring as the car has little bottom end, a lopy idle and wants to run hard. A dyno will give the best numbers in performance but may not suit where and how you drive the car.

TonyK.

Grimsby Ontario Canada.
 
This is impossible to answer definitively without an analysis of the cam lift, duration and shape /timing of both intake and exhaust lobes.

IF they have been re-ground what is the relation to the original timing mark? Gosh, there are so many questions to ask and answer before answering this.

I like Elgin's approach: http://www.elgincams.com/campaper.html what I got back was wayyyy different than what I sent him...and quite effective, it must be designed as a whole system.

Lets take it step by step, assuming stock cams and starting from original timing.

The purpose of the adjustment is to "fit" a cam so it maximizes intake charge and exhaust efficiency.

You are opening the intake valve 8 degree's sooner than the intake valve normally opens, you can't change the total duration of opening (set by cam), the piston draws the most air between several degree's past TDC and before BDC. If the intake opens too soon it will burp up exhaust gasses through the intake (bad). If it is too late the engine will fight itself. As a general rule opening earlier generally benefits higher RPM, but is wasteful at low rpm.

You are opening the exhaust valve 8 degree's later (especially while advancing intake) you are increasing overlap, which is good for high rpm but kills low rpm performance.

per elgin " The timing for Exhaust Opening is the least important of the four valve events. It can be anywhere between 50° and 90° BBDC, so its timing is easily adjusted to match the performance characteristics of that engine"

I interpret all of this to mean that you should time your intake cam for the best operation in the rev range you want, and then adjust the exhaust cam to compliment with an overlap that suits that rpm.

IF your buddy has done this before successfully I say go for it. If this is an experiment i say look at it a little closer.

All of this is a band aid for properly ground cams and I highly recommend adjustable cam wheels and a day at the dyno if you are serious about tuning for optimum performance

This should ideally be done on a dyno
 
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