VVT and cams - is it worth it?

[70XIN]

*This is a continuation of my old thread to some degree, but i have now ruled out compression problems - i have 200psi+*

As the title says, i'm wondering if bigger cams + VVT is always going to be a compromise of low/midrange power vs. top end, when compared to using an adjustable intake cam gear.

In my case, i have kelford 194C's - they aren't huge by any means, but still 'up there' (approx 288 degree, ~9mm lift). I currently make a respectable amount of power up to 6500rpm - after that, power flat-lines (or drops greatly, if VVT is left on past this point). Without VVT in the midrange, i lose about ~20hp through this area.

Most of the major camshaft manufacturers who do 20V cams, also sell 20V cams in a very similar spec ot mine (Catcams, TODA, and others). From looking at all of their cam cards, if you aren't running VVT, they suggest an intake centreline of anywhere between 100-105 degrees, and all of them use a ~105 degree exhaust centreline.

So for those of us who run VVT - when VVT is turned off you have an intake centreline of 120-125 degrees, and when VVT advances (30-35 degrees, depending on who you ask) your centreline becomes something like 85-95 degrees. From most of the dyno sheets i've seen on the web, retarding the intake cam seems to give you more high-rpm power (i must admit though, most of these dyno sheets are for large, V6/V8 SOHC motors). Is this not the case for small, DOHC motors? (Because otherwise, wouldn't 120-125 degree centrelines be awesome for top-end power? Or is this simply TOO retarded?)

I've been thinking about running an adjustable intake gear - if i was able to adjust my intake cam to the suggested centrelines, i would actually be running a centreline halfway between VVT on, and VVT off - so my midrange losses wouldn't be all that bad, but my high-rpm gain would (hopefully) be much better.

Unfortunately, most of the 'big 4AGE N/A' guys are unwilling to talk about it - as it's their business to make motors better than your average backyarder (me).

Thoughts?

*I don't have a free dyno to test all of my available options out, so please don't suggest that*

[andrew_mx83]

yes retarding the intake cam gives better top end power by allowing greater cylinder fill at high RPM -the cylinder continues to fill after it start moving up from BDC. this also drops your dynamic compression ratio allowing you to wind more timing into it. there is a cost down low but ive found a nice balance on my 7m so it pulls hard right up til valve float, and still has good driveability down low. i had it pulling like a train to 7500 but it was a pig unless you kept the revs up and made nasty noises in the head lol so i pulled the cam back a bit.


i make adjustable 16v gears, and had a look at making a VVT delete adjustable gear for the 20v. it should be fairly doable if people are keen.

[Celica RA45]

did you test a std motor like i asked you, and ask kelfords what the cams are at 50 thou

[70XIN]

Yep i did. Standard blacktop has 125 degree and 110 degree centrelines. I got the 50 thou specs, but kelford suggested just doing the centreline method the way they do it.

[Celica RA45]

so with vvt off 110 centerlines and 125 on .i need the 50 thou so i can see how hot the cam really is

[70XIN]

Yeah the cam size isn't really my concern, i'm just interested in the timing.

As kelford said to me "It is a pretty big cam and we generally see over 220 flywheel hp on that cam spec with a reasonably modified 1.7-1.8l engine."

So i'm still a LONG way off that HP, even if it is flywheel HP. If i had money to burn, i would buy an adjustable intake gear, time them to spec, and see how that goes - but i don't - and i'm interested in seeing if i can still retain VVT (or if that it is too much of a trade-off of midrange power vs. top-end).

[Celica RA45]

so is it 125 advanced or are you only giving me both of the cams specs ,if you wont my help you need to tell me things
i asked you what the std inlet cam was in off position and also what it was in on position ,if you cant give me that i cant help you

[70XIN]

VVT off is 125 degree centreline, VVT switched on is 95 centreline. Exhaust cam (stock position - and stock cams) is 110 centreline.

On my setup, VVT is switched on at 1500, and turned off at 6500-7000


And Kelford said, if i have both adjustable gears (no VVT), i want centrelines of:

Inlet : 100-103 degrees
Exhaust : 105 degrees

Kelford couldn't help with VVT specs, because almost everyone just ditches it.

[Celica RA45]

alright with the silver top vvt what have you got now more or less your current engine and didnt you say the silver top was 5degress less or more
or what are your cam specs in now with the 7age motor have you the same as std or not

[70XIN]

With silvertop VVT pulley, it advances timing 5 degrees (so 120 degree centreline with vvt off, and 90 degrees on). I haven't tried this on the dyno yet, as i just got a pulley a few days ago.

I haven't timed my exhaust timing to the kelford specs - mainly because my intake centreline isn't anywhere near right, so my LSA would be a bit wack. So when it was at the dyno, i just adjusted it both ways a degree at a time until i got the best result (4 degrees retarded on the cam = 8 degrees retarded).

[Celica RA45]

so you need to set the vvt to full advance and have it fully on for 105 and turn off at the high rpm which might be know 7000rpm so try to set it up how kelfords do it and you might have to retard the inlet cam 1 tooth from the std set up posi
ok before you do that how many teeth are on your cam pulley as add the up and devide the no off teeth and that will tell you how many cam degress each tooth is worth

[70XIN]

36 teeth, so 10 degrees per tooth, which = 20 degrees for timing purposes (relative to crank)

So in conclusion, you're saying i should just ditch VVT and be able to time it properly?

[Celica RA45]

no your saying 95 is full advance but you need 105 for it 2 sit in its best power range for this cam ,so you might have to retard the cam 1 tooth with the vvt to see where that puts you but be very carefull that you dont hit valves into pistons
so in theory you will now have fully turned on 105 and 135 turned off

so its the whole cam retard 1 tooth back ,so when you advance you should sit in the 105 range
100 is to narrow for this engine and cam would have to be over 260 duration at 50 thou to go in this range

if you are not to sure ring me