Here is my questions/problems:

Since I already have a Vortech and I don't expect it can be pushed much further without upgrading the compressor, wouldn't I see a significant decrease in performance if I just switched pistions? About 30% decrease?

If this is not the case, please explain.

I also don't understand why it is benifical to use low compression pistons with extreme FI. It seems to me that if you are going to increase boost, you are increasing compression. Right?

So if someone was going to go to 20psi with an 8.5 piston, then why wouldn't useing 10.5 pistons at 15 psi accomplish the same thing? For the sake of arguement lets assume that these pistons are of the EXACT same quality. The only difference is the compression. Just trying to learn here.

Please no flaming!

 

you need lower comp. in the motor, as you are upping the boost VIA the blower....

instead of a blower, if you're N/A, you'd try to raise the compression some (thinner head gasket, etc)

 

Peter from APS says...

 

I still don't get it.

FI increases compression to get more power, right? So, hypothetically speaking, lets assume we have two engines that both create a final compression of says 13:1. One engine uses low compression pistons and more boost from FI and the other uses high compression pistons with less boost. Why is one method better than the other? How does the engine know there is a difference? Doesn't the engine feel the same stress and have the same detonation issues in both instances? Obviously, I am missing something, since everyone says you need to change pistons for extreme FI.

I need enlightenment.

What would happen is someone like me ( w/ a Vortech) changed pistons that are say 11.5:1 instead of the stock 10.5:1. Would things go Kaboom? Or would I say about a 10% gain.

 

Your question revolves around the fact that you need to lower your compression to increase your boost without auto detonation (i.e. knocking). If you have 10.5 compression and you want to run say 15lbs of boost, you will run into the chance of knocking which is bad for your engine, unless you use race gas with a high octane (the gas is more prone to extreme pressure before igniting itself), you won't be able to get around this problem, even with retarding the timing. The reason tuners today lower their compression is because it allows for them to run higher boost with the gas they have available to them on a daily basis, (93 octane for example) without knocking.

If you increase your boost, you are increasing your chances of knocking because the fuel you would run in your car like 93 octane would eventually start to knock. And for the record knocking is bad........ um kay.

 

With lower compression you have more volume in the combustion chamber- you can push more air into that area-therefore your motor will move more CFM- making more HP (in effect) its like more cubic inches.

10.3 to 1 is a high compression ratio to start out with for a stock car.
Most factory turbo motors are about 8 to 1

Most Top fuel Rails are about 6 to 1 or so and make upwards of 35 pounds of boost and add on top of that Nitro

The Vortech sc trim Supercharger will support 680 HP it can move 1000 CFM @ 20PSI. On a stock vortech kit/350z you are using maybe 60% of the blower.

TimRod

 

I was over spinning my V1S to ~55k, seeing 13-15psi of boost depending on the weather on a stock VQ30. Using the J&S it worked fine. Run the **** out of the Vortech blower, tune it right and dont worry about the compression.

For turbo its a different story because you can put out allot more boost.

 

TIMROD-

i agree that the vortech has more room left, but......

perhaps you can share the formula (calcs) in the dubb digit post?? (helps calc for the three pulley options...)

thnx bro

sms

 

ZxRage,

I am aware of the detonation problem with high compression and low ocatane. What I don't understand is why would the likelyhood of dentonation be less with low compression pistons if the engine ultimately generated the same amount of compression due to FI (see my hypothetical senario above)

Timrod,

Okay now we are getting somewhere. I'm not sure if this makes sense yet, but we are getting there.

I thought compression was a measure how compressed the air in the cylinder chamber is at the peak of the compression stroke. If this is true, then I don't understand how this would allow more air into the chamber, since the intake valves would be closed at this time. It seems to me that the FI is what gets more air volume in the Chamber.

Is a lower compression piston thinner than a high compression piston? I ask this because I am trying to figure why you said a low compression piston will give me more air volume

 

with lower compression, there is more room (area) (volume) in side of the combustion chamber vs High compression less room

Look at a high compression pistion (Domed or pop-up) vs a low compression pistion. (dished)


You see what I'm talking about.

TimRod

 

Houston..good questions.

I dug up my old treatise on this topic.....lots of good answers to all of your questions from members more knowlegable on this subject than me.



Hth,

https://my350z.com/forum/showthread....rged+internals

 

MardiGrasMax...

my similar experiences (albeit years ago) mirror the same in regards to the blower...

can we maybe get vortech to offer a H.O. kit for the VQ?? (like the 5.0l mustangs of the early 90's had)...

the HO vortech kit could consist of:

**v2 sc compressor (same)

**belts (cog same, serp ½" shorter rating)

**312 or 287 pulley (287 needing 92.3 rated belt)

**190lph walrbro drop-in replacement pump (or mayube the t-rex inline pump, if its any quieter than the older days!

**six injectors (one step???) above stock

**DIFFERENT SS maps loaded, OR a [b]ZERO map loaded[b], so it FORCES the end - user to get custom - made dyno tuning (or have one emailed from a friend... not my idea!)

**low temp thermo, to help keep the cool , well - um , cool


i think this could be a very good H.O. kit, with very impressive (and safe ) gains..... (if i knew what the 287 already does, id say go w/ that one, as i feel the 312 should be the oem pulley size anyway)

 

Thanks to everyone for their comments so far.

GQ,

As normal, you made an outstanding contribution. I have read the first 30 or so post from the thread you linked and it is right on point.

Thanks again

 

Well running 13-15 psi on 10.0:1 compression is not a great thing for longevity. While there are a few doing it, you will also find they are going through engines from burning up the rings or worse. It is a combination of detonation, heat and cylinder pressure. An early VQ30 is cheap, so most don't care and they just keep throwing in new motors. A few even week spare motors around expecting them to go. A VQ35 is not anywhere near as cheap because there are very few in salvage yards. A J&S is deinitely not a fix. It only retards timing when detonation starts. If it is working, you are still killing the rings and pistons...just at a slower rate. You are not reducing the cause of detonation and by retarding timing, it reduces engine power.

With the stock components the worst problem is the rods. VQ35 rods have thin arms and extremely weak rod bolts. The stock cast pistons are not that tolerable of heat. But decreasing the compression does a lot more than you think. In actuality, every point drop in compression ratio will reduce power 10%. That is if you are buring the same amount of air and fuel. Since the combustion area in the engine is increased, you have a much greater area to fill. If you use more air to fill the additional area to the same cylinder pressure as the higher compression engine was using, you actually make a lot more power. You increase the amount of air in the cylinder by increasing boost. Remember that boost increases air density, allows more air to fill a smaller volume of space. The larger the cylinder volume, the lower the heat of the charge from the ignition and the longer the burn to make more power. On an engine with 10.3:1 compression and 10 psi of boost actually has much higher cylinder pressure and stress on components than the same enigine with 8.5:1 compression and 20 psi of boost. The higher boost engines actually makes 70% more power.

Since you went the Vortech route, I actually think you are wasting your time on the engine upgrades. I think the Vortech is a great route for a stock engine. There is person making 396 rwhp with jost a couple bolt-ons and tuing and another 427 rwhp with cams. Both run a stock pulley. Yes you will have to upgrade to a new compressor, but if you are going to spent the money to rebuild the engine then you are obviously looking for the greatest power. Because of the lack of drivetrain losses, turbos is a much better way to go.

 

timrod and loren definitely seem to be explaining things better HoustonG35. The cylinder can only hold so much volume with each cycle. The F/I works to compress more air into the same space, this not only improves your VE but also increases the temp of the cylinder. People go with a lower c/r piston to improve design, withstand cylinder temps, etc as others have said. If you are running low boost in some application, going with an extremely low c/r piston may not be what you need. The guys looking to build bottom ends to sustain, heavy cylinder pressures go with these low c/r pistons, stronger rods and work with head design to help improve ve, drivability, life etc... so forth.

 

I think I figured out why a lot of the explanations did not make sense to me.

What I could not figure out was in a theoretical setup, why would one engine be better than the other that created the same internal pressure, but did it in different ways.

There were two variables, the amount of FI and the net cylinder volume at TDC.

Let me see if I can explain this with an example, and someone tell me if I am right. One car has a 5:1 compression and another has 10:1 compression. The 10:1 compression car at TDC will have exactly 1/2 the space in the cylinder chamber as the 5:1, Right?

Now, if we add an FI application that creates a cylinder pressure of 200psi at TDC in both cars, then the 5:1 car has double the amount of air molecules than the 10:1. Thereby increasing the amount of fuel that can be added and theoretically there should be double the power. Since the pressure is the same, then the likelihood of detonation is also the same, but power is doubled in the 5:1 engine. Of course, to get 200psi at TDC for the 5:1 car takes a much larger blower because it has to force in twice as many air molecules into the cylinder during intake, in order to achieve 200 psi at a 5:1 compression. I think I got it!

This discussion has been very on point and it has answered the WHY question. So often, I read posts from well intending members that just stated the observed EFFECTS of particular phenomena, but they never get to the underlying cause. THANKS AGAIN FOR ALL THE CONTRIBUTIONS!

My conclusion is that, if I am going to stay with the Vortech (I probably will) I should stick close to my stock compression level and get forged components. I say this because, I feel that the V-2 trim level I have will not safely create high boost levels (perhaps 10 to 13 psi). So to maximize my overall power, I need to keep my compression fairly high. I also like the low end torque of the higher compession.

 

Another advantage to low compression pistons is temperature. By offloading more of the compression to the turbo, that partially compressed air passes through the intercooler and approaches ambient. This provides for a lower cylender temp than if the piston did all of the 13:1 compression. Basically with different pistons you end up with 13:1 compression at the temperature of 8.5:1 compression, which means denser charge, far less detonation, etc.

I was musing a few days ago about connecting a turbo's compressor directly to its exhaust inlet and adding some kind of fuel pump. Viola! A home made jet turbine. Actually I'm sure the impeller design is waaay off, but the concept would be interesting to try out.

 

Lower compression pistons actually create more cylinder heat and increase EGT's...all things being equal. Becuase the burnt and unburt gas charges hang out in the cylinder longer...due to the lower compression.