affects of high lift and duration cams on forced induction engines

[RobV]

Hi ,

im in the process of building a 4agze engine for my awll mr2, i have nearly completed the bottom end, and had the head ported, oversized valves ect, recently i aquired a fully built head, that included bucket over convertion,TRD double valve springs titanium retainers, and 300 degree 11mm lift cams, i have ben setting the engine up running a twinscrew blower ex E55 AMG, originally i was planning to run with cams of around 270 degrees 10mm lift, i thought it would be worth experimenting to see what will happen with overlap ect, this is a track, mototsport vehicle, so fair amount of stroppyness ok, any wise advice be much appreciated, cheres

[knightrous]

I'm just going on what I have heard in a few discussion on this previously, so take it with a pinch of salt . The overlap between intake and exhaust valves is generally an issue with forced induction because you end up blowing boost straight out the exhaust during the overlap period. The overlap however, helps a natro because the flow of the exhaust gases helps pull a bit more air into the cylinder from the intake.

[jonathan yee]

the overlap as said will mean you'll waste a lot of fuel but you'll have a clean charge and make more power.

so more power horrible economy. be pretty awesome with a SC, maybe if it was turbo you may have problems with unburnt fuel igniting in the manifold/turbo housing when it gets real hot.

but surely 270 cams would have a decent amount of overlap anyway and you can reduce the overlap with cam gears/timing...NO?

[Sam_Q]

I agree with the above, besides you could probably get a decent price if you sold your old sticks

[Steve M]

High lift is a good thing. I read somewhere ages ago that high duration in a forced induction engine pushes your max power range further up the rev range than the same cams would in a n/a engine.

N/A multivalve engine with 300deg cams would have all its power in a very high rev range anyway, I think using them with forced induction might be taking it out of a usable range and into dangerous rev territory for the supercharger and the engine.
This may be more the case for turbo engines (due to different spool up characteristics) rather than blower engines. I really don't know.

I have not played with forced induction engines, that is only what I've read from other peoples experiences.

I'd be using the 270 cams and selling the 300s toa N/A engine owner.

[Radar]

Just from the research I've done, if you want to twinscrew, you gotta go for as little overlap (112-116LSA) as possible with an exhaust that will flow like crazy (0 back pressure)!!!
If you have overlap and exhaust back pressure you'll be up for yuckness..
I.E. If you have 14psi going IN and the exhaust has 7psi back pressure, your only seeing 7psi going in.
This is only while the overlap is occurring, once the exhaust valve closes you'll be back to full boost but have pretty bad combustion temps/AFR due to the slight back flow of exhaust gasses that got caught in the chamber.
And with the big duration cams, you'll have to make sure your static compression is correct(measure ABDC @ .050") to suit other wise you'll be driving a DOG off boost with horrible fuel economy + dick all power till mass boost levels.
Its better to have less boost with a supercharger due to outlet temps, so the more compression to start with the better from what I understand.
(I think.. Just from what I've been reading)

[Radar]

Just on from what I was saying, It depends a lot as well as to what your after.
Track or Dyno queen?
If your after a track car, the wider the LSA(112~120) the more usable your power band will be but the downside is you'll make less peak HP.
If your after MASS kw... An LSA with (106~110) is great for mass choppy idle,
crap bottom end and HUGE power in the upper RPM. Good drift setup also I'm thinking?
Overlap - Decreased (wide)/Increased(tight)
Low End Power - Higher / Lower
High End Power - Lower / Higher
Detonation Potential - Lower / Higher
Intake Events - Later / Earlier
Exhaust Events - Earlier / Later
Overlap - Decreased / Increased
Manifold Vacuum - Higher / Lower

*Note on intake events, the later the ABDC the higher the compression to keep the dynamic cranking pressure up*

[beerhead]

My 2c-
If your building a race spec motor and want a car that has the exact power band you want, start with the biggest lift and duration cam you can get - like what you have and if they've both got dizzy drive cogs I'd do the following.

Have them profiled down incrementally after driving the car. Happy with the drive - don't change it, but if you're not have each cam profiled down one at a time starting with the exhaust cam. Get it ground to 280-290 duration and see if you like it. Still not enough bottom end? Swap that reground cam to the intake and get the other 300duration machined down to 270-280 duration and install in the exhaust side. At a good shop it should only cost you $80 - $100 a cam for a light machine to change the duration, make sure you tell them what you want to do and they'll be very accommodating.

If only 1 has a dizzy drive you can do similar to above too. BTW I'd have a search on ebay USA for Yamaha shims. There's massive packs for motorbikes to make this kind of thing easier - just not sure if the diameters match.

[jonathan yee]

what's the ramp up rate of the cams?

cuase that'll also have an impact. ie the overlap may only be when the cams are almost shut if they have a long ramp.

whats the duration @ 0.5mmlift?

[Talasas]

I too have been wondering this, particularly with turbo engines as the common information around the net tends to suggest using shorter duration cams (with little mention of lift) for turbo engines and absolutely zero in reference to dialling in the cam.

I just wonder, how one get nice rapid spool, well balanced power with a turbo on a 4AGE without the need to go over the factory rev range.

I any case, the more information the better, keep us informed.

[jonathan yee]

if you increase the duration it will push the peak power figure higher up the rev range.

not nessisarily higher than the stock rev limit tho. i mean most stock engines are loosing power and drastic amounts of torque by the time you get to the factory revlimit.

but lift and a fast ramp rate will allow more power without as much sacrifice at low rpm.

with spool of the turbo that comes down more to the turbo and the timing of said cams.

but ultimatly if you want big HP+revs you go big duration and lift

[the_random_hero]

For FI without massively pushing your rev limit up you want big lift and close to factory duration. Reason turbo guys often run bigger cams is they need to be making more revs to make more boost (as the turbo is often mismatched), so within the factory powerband they might only be making 3/4 maximum boost.
If I was you I would be talking to a couple of cam manufacturers and seeing what they are suggesting.
You are actually having the same car/engine setup as I'm going to eventually have, so let us know how you go.

[Hen]

I can add very little to this discussion, but I remember reading the specs of 6boosts 1G in the AE71 wagon and the cams took me by surprise. The motor makes silly power and runs to 10,000 or so, yet the cams were only 250degree but 9.x mm lift. I'm unsure if that was 250deg @ xx lift or 250 deg advertised, and don't take my sketchy memory as gospel, but it might be a motor worth investigating as it seems to work pretty well.

Hen

[Radar]

My cam is a 288 with 254 duration at .050 with 13mm lift.. And i'm advised of 8500+ rpm.
So 250 duration with 9mm lift per intake valve (two or more) on a twin cam could quite easily make 10,000 with the correct bottom end.
From what I read lift after a certain point, doesn't really gain huge amounts of flow and just adds extra stress on the cam and valve train due to higher lobes and stiffer springs.
So basically it pays to get your head ported and flowed so lower lift valves can be used.
(I think)