The intake manifold will be under pressure no matter the valve positions as long as the compressor is working and you're at WOT. I think I understand what OverZealous1 is asking, but I disagree. Even with the pressure exerted against the back of the valve, trying to push it open, it is going to be equalized by the pressure in the cylinder. Look at it this way, no matter how much boost you run, the same pressurized air is going into the cylinder on the opposite side of the valve. Obviously, not the exact same, but it won't be as though the boost pressure on the back of the valve is acting on atmo pressure on the face of the valve in the cylinder. When the intake valve first closes, the piston is traveling back to TDC and applying more pressure inside the cylinder than you're boosting anyhow. So no danger there. During the power stroke, pressures will go down, but not to less than the pressure on the back of the valve, so no issues there either. On the exhaust stroke will be when the pressure differential is probably the greatest, but there's valve overlap anyhow. If you're thinking that the pressure will prevent the valve from closing properly after intake, consider that the same boosted air that is in the manifold is also now in the cylinder, so again, it's not like that 15,16, or whatever amount of boost is acting against the valve under atmo, it's pretty equalized. Not evenly, but close enough to be a non-issue. So unless you're running some insane amount of boost that can prematurely push the valve off its seat during the exhaust stroke, I wouldn't consider the extra pressure on the back of the valve a problem.
Will

 

So in a nutshell: nothing to worry about?

 

i can't remember where i heard this and some of my books aren't here right now, but it was in a book from a reputable engine builder. maybe the effects just come more apparent at higher boost levels like 30+.

ok, next subject resolute, shoot............ hahahaa.


how do you feel about the leaf blower theory? i think sharif already has first hand knowledge of the positive effects of a couple black and deckers, hahha. oh with the precisely metered N2O injection used with it.

 

LOL! ^^^ Wholly crap has this thread become technical!!!!!!!!!! It's really good stuff though, I LOVE IT! And yes sharif, this has to be in the top 5 of all nerdirsh threads

Resolute, I knew you weren't the average guy when YOU started answering the way you did

It's funny how I originally stated (pst.#8 in lamens terms) some of these answers (below, if true?) to the orignal question, because that's all I'm capable of... LOL! Yea, I'll only take partial credit...

Exon Exec VVV

" and therefore more stress is on the engine making torque at the lower rpm."

So i guess we all agree on this now or where you just repeating what i was saying.

"the max torque developed from any one of our six "surges" is 400lb/ft of torque."

Your absolutely right. I dont know how i came to that in the first place.

The way I "visualized it" on a stock VQ motor ^, yes skewed a little as I stated later on, was the motor producing more "energy" early on was under more "stress". And more likely to be under stress more often given it's only 4k rpm if achieved at WOT from standstill. Like they say, you can't get something for nothing... but that something may be a WIN from the light! Totally worth it.

Zquicksilver

ps I hate to say it, because I'm considering a SC for my stock VQ motor, but a ST or TT with a proper launch settup will destroy a SC off the line. So much TQ with HP quickly following it can't compete. I would totally go with a turbo setup if the my motor was built to handle it, but it's not and never will be... so SC is as good as I'm going to get. It will be good enough to pass a few stock EVOs, C6s and M3s who tempt me. bbwhahahhaha!

 

Just wanted to add some thoughts after reading most of this rather lengthy thread.

People love to talk about cylinder pressures and about torque and how much "power" a motor can take before the internal components start to give out.

Something that has been overlooked this entire thread is that the most stress on the piston/rod comes during the EXHAUST stroke in each cylinder, NOT during the POWER stroke. During the exhaust stroke, the piston accelerates away from the crankshaft with nothing on the top side of the motor to slow it down. Piston speed and mass are huge factors during the exhaust stroke.

This is why revs will blow up a motor far more quickly at far lower power level than boost will.

(puts flame suit on)

 

so very true. forced induction actually "cushions" the intake stroke with the induction. but when the exhaust stroke happens, there is no buffer, and the rod takes the full brunt of the direction change. almost any rod will take the compression force (outside of detonation) made in a motor, but the true test is making it past tdc at high rpms when they try to rip in 2, or more pieces, lol.
i have heard of small blocks running 10k in near stock trim and making it through with out loosing a rod or rod bolt though. this is why i am wondering so much about the vq. with the larger bore/stroke ratio, this thing should rev to the sky with no probs especially with parts like pauter rods in there. not sure what rpm the drag racers are seeing there hp peaks at but we need to find out the bottle neck that is keeping the hp peak down. is it the heads, the cams, the valve springs, the throttle body, or a combination of all that are keeping the motors at this rpm peak? all of them i'm sure.
then we get into the head gasket issue that has been floating around. i have been trying to think of a girdle that could go around the tops of the bores, but i'm sure after machining, would be a far greater cost than sleeving the block and be a great mass-production challenge. with the open deck design on the vq, the top 15%or so of the cylinder is not adequately supported, which is right where the main force of combustion is. so the next project in nerdy thread #1. how to make the vq a 900whp street car!!

 

The dyno graph i am referring to is on page one or two and is an actual dyno graph of a Z at 7 psi

 

Here you go Quamen

I'm curious too... lets see what NASA says, lol.




Zquicksilver

 

your definately safe, you may even want to lean it out a little more at wot.

 

My head hurts after reading this thread. If you've ever taken this engine apart you will realize the weak links in it's stock form, the rods and the piston rings. Neither will last with any knock or excessive heat present. Yeah I know most engines won't last with heat or knock but more so when you boost this motor. That's easy to see when the only guy that's run 15psi on the stock block is using WI.

 

OverZealous,
It was not a question of A/F ratio safety. Remember that the richer the A/F the less power the car should make. That means that the cylinder pressures must be higher in order to make that much power with the rich of an A/F. Plus, if i am not mistaken, richer A/F leads to more heat in an engine. Sharif, please correct me if this statement is wrong.

I guess the best question would be would that power level be bad for the wrist pins.

 

well as discussed earlier, the stock rods will be fine for the compressive loads generated at that rpm (outside of detonation) at that hp level. don't worry about the wrist pins either.
leaner a/f's lead to higher heat in the motor not a richer a/f. more oxygen ratio=more heat.
edit- there is a point where detonation can become an issue from to rich of an a/f ratio also.

my reply about the a/f ratio is the fact it is even off the scale. so it could be dropping to 9/1 or more. would be best to lean it out a bit more. you are still in a very safe zone with the amount of power you are making on the stock motor if your timing is good too.

 

An excessively rich mixture can wash the oil off the cylinder walls and cause extra friction between the rings and cylinder wall. This friction would lead to extra heat..but we're talking REALLY REALLY rich A/F to cause this.

Too much timing retard can also cause more heat.

 

true. the heat is mostly because the mixture is still doing alot of it's burn in the exhaust, typically you need to be retarded alot though.