Do I need adjustable cam wheels?

85AW20v · Post by **85AW20v** » Sun Dec 18, 2011 4:54 pm

I shaved 0.4mm off the silvertop head in the AW a while ago. Was talking to a mate at the Taupo playday last week who mentioned that I should probably put an adjustable cam wheel on the exhaust to get the cam timing right as taking 0.4 off would alter the exhaust timing quite a bit.

Thoughts anyone? And should I do the intake as well and not worry about the variable valve timing and fix the inlet in the advanced position?

touge_ae101 · Post by **touge_ae101** » Sun Dec 18, 2011 5:35 pm

will only get any gain if you are running a link. and even then gain with standard cams will be VERY minimal.

normally you would only do adj. exhaust cam gear when going to a set of bigger cams designed for vvt etc to make it easier to time in the cam in the right position.

leave vvt intake cam alone unless you are going big i.e. >288 ish. it works fine for a standard rebuilt motor.

RomanV · Post by **RomanV** » Sun Dec 18, 2011 9:46 pm

He means that the problem currently is that since shaving the head, the distance between the crank pulley and cam pulleys has reduced, pulling the cam timing is slightly out from the factory specifications when the belt is put on.

I bet it's bugger all though!

Mr Ree · Post by **Mr Ree** » Mon Dec 19, 2011 7:38 am

I have spoken to a few chaps about this in the past as I took a (tiny) bit off the head when I was going na-t.

The general concensus was that unless you had taken a large amount off it, it was unlikely to alter the cam timing enough to lose power.

Although your CR would be raised a little bit more as a result

Vertigo · Post by **Vertigo** » Mon Dec 19, 2011 8:45 am

^^ this.

Bazda · Post by **Bazda** » Mon Dec 19, 2011 9:37 am

0.4mm would not make much difference at all.

Changing from a stock headgasket to a TRD is more of a change!
1.2mm-0.5mm

When we use the 7a blocks which are approx 12mm taller or is it 15mm I cant remember, that changes timing by 10 deg.

So you could say for every 1mm change it will change the crank to cam timing 1 deg.
so 0.4mm would be 0.4 deg = bugger all!

85AW20v · Post by **85AW20v** » Mon Dec 19, 2011 4:33 pm

Probably should have said I've done the metal gasket as well which is - as Bazda says - another 0.7mm thinner than stock so all up about 1.1mm. It is definitely an interference motor now - thought I'd best try that before running it.

OK then - another question. Is the stock cam timing best for maximum power or is it a compromise for mass production/economy? Should/could the timing be advanced/retarded to get better power? It's in the racecar so economy doesn't even enter into it!!

Mr Ree · Post by **Mr Ree** » Mon Dec 19, 2011 4:56 pm

If you have a dyno to set up the cam gears, then there are small gains to be had. Normally the biggest gains come from having adj cam gears on really big bump sticks

pc · Post by pc » Mon Dec 19, 2011 5:05 pm

I would think that given the cost of dyno time you are better to change the cams and cam wheels at the same time and then dyno it.

It's probably not a good dollar:performance investment if you spend money on a dyno getting the cam wheels set right on standard cams.

Grrrrrrr! · Post by **Grrrrrrr!** » Mon Dec 19, 2011 5:16 pm

Why do only one? skimming head and thinner gasket throws both cams out by same amount.

crispy'86 · Post by **crispy'86** » Mon Dec 19, 2011 7:05 pm

Pretty sure the engine that was in my trueno when i bought it had .5mm off the head cause it went heaps better than any other 4age's i drove at that time with same mods. I put a .8mm TRD head gasket on it when i freshened it up, got it running, experimented a bit and it definitely went best at 12-13 degrees BTDC than the specified 10degrees. Tried this on different fuels, even tried playing with the cam timing but didn't really make much difference.

KinLoud · Post by **KinLoud** » Mon Dec 19, 2011 7:53 pm

Merry Christmas Simon! How you keeping?

0.4mm off head... my trigonometry gives me about 0.9 degree retarded

ahhh lets see

Cam wheel radius approx 50mm?
0.4mm shaved off head

0.4/50 inverse sin = 0.458 degree

cam turns at 1/2 crank speed - therefore multiply above by 2

= 0.916 degree cam retarded

Ken

RedMist · Post by **RedMist** » Mon Dec 19, 2011 8:17 pm

KinLoud wrote:
= 0.916 degree cam retarded

Ken

Ken hope you don't fly as inaccurately as you calculate.

It's 0.917 degrees retarded.

I cheated anyway. I just calculated it as a percentage of the circumference, then converted it to degrees.

.4/ (3.1415*100) X360 X 2
=0.917 3dp

KinLoud · Post by **KinLoud** » Mon Dec 19, 2011 10:30 pm

Yep - I must be retarded!

I worked it out twice... first the easy way then the hard (trig) way

Easy way = My flying rule of thumb 1 in 60 = 1 degree...

0.4/50x60 = 0.48 degree at the cam
multiply by 2 = 0.96 degrees at the crank

This was my dirty quick/rough proof of concept so I would know if I was in the ballpark when I used my very rusty trig skills

Then I did it the long way....

Ken

85AW20v · Post by **85AW20v** » Tue Dec 20, 2011 9:03 pm

Hiya Ken. Merry Christmas to you too!! All is well in this neck of the woods thanks.

Thanks for the replies guys. I can get an adjustable for a hundy so I think I'll stick it on and see how it goes. Then maybe think about some cams so at least I'm part way there. Spending litlle bits at a time is easier than a big lot all at once.

Mr Ree · Post by **Mr Ree** » Tue Dec 20, 2011 9:12 pm

RedMist wrote:
KinLoud wrote:
= 0.916 degree cam retarded

Ken

Ken hope you don't fly as inaccurately as you calculate.

It's 0.917 degrees retarded.

I cheated anyway. I just calculated it as a percentage of the circumference, then converted it to degrees.

.4/ (3.1415*100) X360 X 2
=0.917 3dp

Using pi to only 4 decimal places is probably the reason for the different result

RedMist · Post by **RedMist** » Wed Dec 21, 2011 6:00 am

Mr Ree wrote:
Using pi to only 4 decimal places is probably the reason for the different result

I'm not that redneck. Real calcs used the Calculator approximation of Pi. I just don't know how to create the greek symbol Pi in unicode.

Try it yourself. A small rouding error occured in Kens truncation to 3dp, when multiplied it equated to a huge 0.001 error.

Mr Ree · Post by **Mr Ree** » Wed Dec 21, 2011 8:16 am

Good point, now I really wish my keyboard had the Pi symbol

~
/\ theres my povo Pi

sergei · Post by **sergei** » Wed Dec 21, 2011 8:43 am

π : looks like cyrilic "п". which is read as Latin "p".

Flannelman · Post by **Flannelman** » Thu Dec 22, 2011 10:40 pm

Retard - open later, close later = INCREASE overlap.

The intake error is taken up in the tentioner