2007 mazda 3 vtec operation

ok,ill try to keep it short.ive got an 07 mazda 3 and just got engine mods to facilitate an inncreased nitrous shot(forged pistons 8.8 to 1,and eagle rods).I had it for about 2,ooo miles and notice that it doesnt feel like my vtec kicks in at all.it used to get a power spike from about 5200 to redline,and as soon as i felt that extra burst of power,my wideband went to 11.2 to 1 on a.f. ratio.now,with the rebuild and lower compression,my ? is should i expect this from the compression change?stock was 9.7 now 8.8.i still see the fuel richening up at 5200 but no feeling of a switch like vtec kicking in,just a broad pull all the way thru the rpm range.thanks  

Ok first off, your Mazda3 has variable valve timing, but its not VTEC. VTEC is a term used only for Hondas. Secondly, did you get a tune after you got your nitrous system installed? If you did, your tuner might have changed when your cams lobes switch profiles. If not, then maybe you should consider a tune.  

The higher the compression ratio the more horsepower and torque an engine produces...  

When you lower the compression ratio of your engine you actually lowered the power your engine make. And the situation is even worse with a variable timing engine, because they work by adjusting (retarding) the valve time timing or by increasing the valve opening time (and sometimes the lift) to produce more power at higher rpm's. But because you lower the compression ratio the variable timing is making the situation even worse because is adjusting too much for your compression ratio. For that reason you notice less power.

There is no reason to lower your compression ratio for Nitrous use unless it for a FULL TIME DRAG RACE ONLY race car that is not going to be used in the street, and that is going to be running with 2 or 3 stages of Nitrous. Nitrous can be run very safely with a 11.2:1 compression ratio if the mixture of nitrous and gasoline is correct, or slighly rich to be in the safe side (as long as you use forged pistons). Remember that Nitrous is only used for a few seconds, not like a Turbo that is creating pressure practically full-time. And that if you set the Nitrous with a slightly rich mixture you only loose some power. But that if the mixture is lean you are going to break your engine no matter if your compression ratio is 8:1.

For Drag Race Only engines we actually eliminate the variable valve timing and run with a "fixed" valve timing since the engine is going to be running at high rpm's only. For legal racing only Turbo or Nitrous is allowed (not both). But for bracket racing we usually run with Turbo plus 1 or 2 stages of Nitrous (or with 2 or 3 stages of Nitrous if it is a Nitrous only car).

Since you already lower your compression ratio your best shot is to install a Turbo charger since a Turbo charged engine works best with a lower compression ratio. Or you can go back to higher compression pistons and recover the power you lost by lowering your compression ratio. Of course you can advance the timing to recover some of the power lost, but your engine is not going to rev the same amount of rpm's, and still is not going to make the same power you were making with the higher compression ratio.  

what are your thoughts on this....ill run a fuel cell and external fuel pump and regulator.ill fill the fuel cell with 110 octane,run 93 in the fuel tank.then spray direct port.this is a street/strip car.my objective was to lower the compression ratio in order to convert my dry nos system to a direct port wet kit.i figure that i'm fine with a 55 dry shot(32 jet)on the wideband,and my old pistons and rings endured 20,000 miles of that and came out of the motor looking practically brand new.i'm planning to shoot 100,which is why i lowered the compression from 9.7 to 1 to 8.8 to 1.what do you think about that setup????  

Using high octane gas with a Nitrous engine is a must to diminish the chances of pre-ignition and breaking (or melting) a piston even with low compression.


Although I have seen many people running with only one injector (usually 1 NOS fogger) I always run with 2 or 3 NOS foggers per runner (2 or 3 stages Direct port wet), so I don't know the correct jetting for only one fogger.

Jetting depends on how much Nitrous and the fuel pressure you are using. When running only one stage I use .028 NOS with .032 gasoline, or .032 NOS with .036 gasoline or .036 NOS with .042 gasoline depending on the engine I am using(usually a 2.4 or 2.5L 4 cyl).

A Nitrous engine don't break because of your compression ratio as long as the pistons are forged and properly designed for nitrous use (lower rings position than normal), the Nitrous\Gas mixture is correct (or slightly rich to be safe), and your ignition timing is retarded to avoid pre-ignition. I usually run with the ignition timing at 4 degrees with .032 jetting, or at 0 degrees with .036 jetting.

The advantage of running with more of one stage of Nitrous is that you start with a lower amount of Nitrous for better traction and add more Nitrous as the car goes down and has the traction to accept more Nitrous. The disadvantage is the additional weight of the 2 or 3 bottles of Nitrous.  

well.happy to report that the n.o.s. is hooked back up.and ran the same 1/4 mile numbers,even though i went 1 compression point lower.also my wideband tells me that i am running 11.2 to 1.nice and rich.however with my stock pistons and rods,i was running at 14 to 1 on the wideband which is pushing it.i'm going to change the nos jet up to whatever size it lets me so i don't go any leaner than 12 to 1.my question to you is this.....if i didn't toast a plug or burn a piston or ring before running as lean as 14 to 1,do you think i could jet the dry shot that lean with the upgraded wiesco pistons and eagle rods??.common sense tells me that i'll have to use the colder plugs for the bigger shot,but the new pistons are forged and the rings,stainless with nitride coating.