cwylie

iTrader: (1)

Stock motor with a 3.12 Vortech kit?

Jeremy@Altered

7100 should be fine on a non revup motor.

UpRev

iTrader: (5)

We generally stick to the stock Rev Limit, unless you've got a built motor with better rings. Or you're willing to deal with catastrophe, then we'll raise it. Sometimes racing means your equipment gets beat half to death.

ashtrojan2008

iTrader: (1)

i lowered my limiter from 7k to 6700. Dont want to float a valve. I'll raise it up considerably once i address valvetrain. I've got a built motor with stock heads/cams lol

UpRev

iTrader: (5)

Honestly it's not valve float, it's the stock pistons. The piston speed on the VQ motor is crazy to begin with. Then add in the fact that the piston top to ring clearance is so short on the factory pistons. They do that to help reduce emissions, and it works, it's just not a good thing when you want to raise the piston speed or add power beyond stock ratings to the motors.

I'm doing my damnedest to find a non-SAE article showing the difference between how the stock low emission pistons rings are set, and the average aftermarket performance pistons are set. The SAE articles describe what I'm talking about very well to weird geeks like me but they are very difficult to follow for most people.

Basically the shorter the distance between the top surface of the piston, and the first ring (aka compression ring) the less unburnt fuel you will have and the lower the hydrocarbon emissions will be. Thats great for a stock motor with stock rev limits and no forced induction or other performance additions. The problem is that you end up with reduced piston thickness and heat becomes a big issue on the piston surface edges. Secondly because of the extra heat and thin material holding the ring in place, the more flutter you can get and the lower the compression and the higher the leak down will be. Meaning more blow by and less power you'll get. Also because of the high piston speeds and over-square dimensions of the VQ motor you end up with way too much side load and no stability above the compression ring.

That said, put the rev limit to 9,000rpm if you want, but expect to rebuild your motor soon. Leave it at a stock rev limit and don't beat the every living sense out of it and it should last you a while as long as your tune is safe.

And for the geeks out there. According to calculations by Cosworth Engineering. The stock VQ35DE motor has a piston speed of 0.002717647058824 Meters per second.

6700 RPM = 18.21 m/s
7000 RPM = 19.02 m/s
7300 RPM = 19.83 m/s

Thats almost another meter the piston moves for every 300 RPM you raise the limit. I don't have the mass of the stock piston handy or I'd calculate the amount of stress you're adding per 100 RPM but it's insane once you add in the start/stop cycle of a four stroke motor and the distance it's traveling. I won't even get into side loads, because thats the sort of math that puts everyone to sleep.

Add the amount of force exerted with the extra air and fuel in a forced induction situation, and it just really begins to break down the idea of raising the rev limits when you're using stock pistons. If you have forged pistons with lower set rings, and you have forged rods that will also handle the stress then yeah raise the rev limit. The heads flow really well and it should remain somewhat reliable.

Z_HighSpeed

iTrader: (5)

6600

UpRev

iTrader: (5)

I wish I could just give simple answers.

OCG35

iTrader: (2)

I'm 7500 with cams and upgraded rod bolts & oil pump – was at 7200 with all stock (for 2+ yrs)... wasn’t aware of potential hazards with OEM pistons... I rarely get into the rev limit - but after reading post, I'm a "bit" concerned when reaching limit at the track... is it Jared or Rich that posted?

UpRev

iTrader: (5)

Rich. You can tell because I spell things correctly.

SergEK

So what about REVUP motors?

24v GTI Guy

iTrader: (2)

Yea, what about us?? I wanna learn about my engine now!

Z_HighSpeed

iTrader: (5)

simple questions, simple answers...

didn't know I had to post a phd theory with every response