Advancing intake cam timing

I am wondering what is the maximum safe advance on the intake cam timing and how much that would shift the powerband? Can I use cam timing to shift the powerband from 7500 RPM to highway cruising RPM of 6000?
P.S. I see stock cam specs on megacycles as 236/232 but what is the LSA?

 

P.S. my bike is an 85 XJ700

 

I would also like some information on degreeing cams? I bought another set to experiment with.
Cheers, 50gary

 

How much can you alternate timing on the intake camshaft without modifying anything?

 

Well here we go with this. You are correct that cam timing generally will influence, within the boundaries of the engine under them, the characteristics of the engine. The boundaries can be airflow limit,rpm limit, valve sizes, gearing to a degree, as well as proximity to adjacent valves and piston. On top of this any changes to the engine characteristics is likely to require a change in jetting, much the same as adding straight through 4 into 1 exhausts, but to a varying degree.

So what’s to know? Well, advancing timing can improve bottom end, as can reducing overlap. It’s unlikely you will gain somewhere without losing somewhere else, assuming Yamaha got it near right. If you’re trying to turn your 8 valves into a screaming 20 valve motor you will be disappointed, but with some head work it’s probably possible to make some improvements - note that the fj600 used larger valves with thinner stems - this wasn’t to save money..
Ultimately though, I would recommend you look for a book called desktop dynos, it came with some simple windows software which basically looked at timing, valve sizes, cc’s, rpm, and allowed different changes to show effect. Even if the numbers aren’t right, it shows what is likely to happen with each change. Carburetion is not covered, you melt your pistons it’s your problem...
Picture of a couple of pages inserted, hopefully it doesn’t contravene copyright laws (shouldn’t rally as I’m promoting it).

 

I am fairly knowledgeable of the theory so I am asking if anyone tried advancing the intake camshaft in order to improve low/mid range.

 

The two concerns with advanced timing I see that prevent me from playing myself with timing are:
1. increasing compression (good) but then knock (bad). When would that happen and require premium gas? Since static compression is high on my engine I suspect that dynamic compression would build quickly too when advancing the cam.
2. too much advance would cause intake valves to collide with piston, when would that happen? Since I see tuner cams with longer duration and lift I suspect this is not as much of a concern, there must be room in there.
I am guessing it is safe to try advancing intake cam by 2 to 4 degrees and see observe if knock happens.

 

In my experience you won’t measure the effect of 2 degrees, 4 yes, more likely. You are right about trapping efficiency increasing dynamic cr at low rpm, but is it likely to cause a problem? I don’t know.
You could make a tool to push the valve down at or near dc to check piston clearance - usually the valve will “chase down” the piston, but there are many things that influence this - rod length being just one of them.

 

What is the intake/exhaust cam overlap? I was reading some other posts and it looked like over 100 degrees? Can we just move the intake cam one cog over instead of drilling holes?
I am counting 32 cogs so 1 cog over would be 11.25 degree advance. Sounds like a lot but if the overlap is over 100 degrees it should be ok.

 

32 teeth? in the cam sprocket? I never countedthem so don't know. However, if it is 32 on the cam, jumping one tooth gives 11.25 deg at the cam, which is 23 deg of engine rotation....

 

So glad there are smarter/more experienced people in the forum than me.
P.S. My bike is the 700x, the 5-valve engine, I don't think I made that clear.

 

So do you know the lobe center angles and valve overlap?

 

This is what I see for the Stock XJ-650 but it is probably different for my MaximX:
.335" 236°
.305" 232°
No lobe separation angle though. If I have to guess 108 meaning overlap of 18°.

 

Do you guys know, if I wanted to modify the intake cam sprocket, do I need to just remove the valve cover? Do I need to replace therefore buy some gaskets?

 

You can try using the same valve cover gasket and donuts and see if there is any oil leakage. If there is leakage from the gasket you can try methods such as spreading a thin film of RTV on the mating surface. If all fails, buy the necessary gasket and possibly donuts if needed.

 

Hi guys, looking at camshaft sprockets for the xj700x they seem to have 2+2 holes on each sprocket. The holes are spaced out about 2 teeth which seems a lot to be used for adjustment. So what are these extra holes for?

 

Since I couldn't find a mechanic to fool around with the intake camshaft timing, I changed strategy and started fooling around with intake runners. I bought some silicone hoses Ф 51 mm and a new set of pod filters to match them. I started out with as long runners as would fit with the airbox removed. The inner two runners are straight and around 10 cm long. The outer two have a 45 degree bend and are 15 cm.
My initial test showed surprising results. Torque increased significantly between 3000 and 6000 RPM. But the engine will not rev past 6500-7000 RPM at all!
I chopped off a third of the lengths of the runners and tomorrow will try again. My goal is to have the peak torque around 6000 RPM and be able to rev to 9000 RPM.

 

Update: with 7 cm central intake runners and 10 cm side runners with 45 degree bend the powerband is 3000 to 7000 rpm and torque is awesome in this range. Max RPM is around 7500-8000 rpm.

 

Do plug chops.

 

I think the 5 to 7cm intake runners are the best . Great low and midrange, revs out to 9000 RPM fine. Only issue I had is during a long ride through a mountain pass in the rain the bike was sputtering a lot. When the weather cleared it was fine again. Then another rain storm and sputters again. I think just water hitting the pod filters caused issues.