Importance of matching piston dish to kidney shaped chamber of P90 head

[TimO]

Does anyone have experience regarding the importance of matching the piston dish to the kidney shaped combustion chamber of the P90 head? ...."importance" discussed below as hp, torque and engine response.

 

Background; my question relates to an L series turbo non stroker currently making approximately 450-475+ rwhp. Fully ported P90, aggressive cam, ITB’s, GTX35r, custom turbo header, arp throughout, TEC3r engine management system, etc. Currently running 88.5 mm JE forged round dished pistons. For various reasons I will be freshening the engine this winter. I may be converting to a stroker if the killer deal on a LD28 crank comes through. If I build to a stroker, no question, I will run forged pistons matching the quench area of the P90 head. If the LD28 deal falls through, I’m asking myself, should I sell my current JE round dish forged pistons and replace them with forged pistons that have a dish matching the P90 closed chamber? 

 

I understand that by matching the piston dish with the kidney shape combustion chamber, the flame front doesn’t have to travel so far allowing optimal ignition advance due to the more efficient quench area. But I don't know to what degree performance is enhanced by this on an L series motor.

 

So here’s the question - On an L series making 450+ rwhp on 91 octane California pump gas, all things being equal except for the round dish piston design, does anyone have experience/knowledge regarding increase in hp, torque, and engine response by going from a round dish piston design to a piston dish design matching the kidney shaped combustion chamber of the P90 head? 

 

Thanks guys.

[Xnke]

Depends on how much boost you want to run, and if you are going to run methanol or not.

 

Lots of extreme boost engine builders want what they call a "soft" chamber, with no quench other than a ring on the piston above the ring lands themselves. Others, for less extreme boost, want a dish matching the chamber, and yet there are others who want to split the two.

 

In the end, it comes down to application. I would say you could get the required timing advance for NO boost down to about 28* with a flat-top piston or a piston with a matched dish in it, as in you do not need more timing to make more power...this could be helpful, it may not be.

 

Talk to your tuner and ask him what your engine wants. You need real dyno data to be able to make good choices at this level.

[JMortensen]

I remember reading an article in a magazine about a turbo build where they cut out the quench areas of a 4V head so that they could reduce compression ratio and crank up the boost more. The impression I came away with was that quench is FAR less important in a turbo motor in terms of ultimate power. I think stony or TimZ or one of the high hp 6 guys had a writeup on a similar build here.

[Tony D]

JeffP has both stock ZXT Pistons, and Pistons cut to match the P90 on his 3.0. He could probably expound on the differences in what amount of Methanol / Ignition Advance he was running on the two different setups.

 

I know on C16 he was making 475RWHP with the L28ET GT35R 17psi with his combination of head/cam/etc

[TimO]

I've talked with both TimZ and JeffP.  E-mails pending to a couple other guys. When I get a definitive answer I'll post it. Thanks Guys. 

 

Tim

[Xnke]

I'm not tooo terrribly sure that it matters at Lower Boost pressures...higher flow, lower boost...but when you have no option but crank the boost to it, then quench is less useful. Get the flow rate up as high as you can before you start working on boost alone.

[TimO]

Xnke, thanks for your input. 

 

I understand that power from a turbo motor is a function of flow. The same with an NA motor. Turbo PSI is air resistance, flow is volume of air. My engine has huge amounts of flow with a fully ported head, very aggressive cam, ITB's, custom exhaust header and a relatively high flowing GTX35r turbo.  Flow is not really the root of my question. My question has more to do with combustion efficiency where a piston's dish matches the head's combustion chamber vs a piston where the dish does not match the head combustion chamber. More specifically how much does this combustion efficiency differential affect hp, torque and engine response?

 

I'm checking other sources. Hopefully when I get a definitive answer relating specifically to Nissan L Series motors,  I'll post it. Thanks again guys.

[Tony D]

I think the design consensus is a quench area promoting tumbling homogenization in the combustion chamber and faster burning is a good thing. More power with less ignition lead, important at higher rpms.

[Gollum]

We can speculate till the cows come home. Want a definitive answer? Find a tuner that's willing to use a pressure sensor in the chamber and record data at various loads, intake PSI, AFRs, and spark timings and find out what YOUR engine is doing with YOUR setup.

 

In all reality we still know VERY little about your engine, and can't give you an informed answer from an uninformed seat across the internet. This is why financial specialists on the radio say "you should see a financial planner and talk about your specific details to get better advice" because they simply can't get into the nuances in a few minutes.

 

Want real advice? Find a professional or treat us like professionals and write a 2,000 word post detailing everything you know about the engines setup and its intended use and goals.

 

I plan to run flat tops on a MN47 (closed kidney shaped chamber that's smaller than the P90) with a huge turbo, but I also wouldn't recommend anyone do that unless they knew what they were doing any why. Just because you have an L series with a P90 doesn't mean we know the answer. Engines aren't that general of a science.

[Sam280Z]

When I built my very similar engine, I used flat top JE pistons with valve reliefs sourced from Dave Rebello on his advise. I was originally looking to get combustion chamber matched pistons. FWIW.

Sam

[Xnke]

Tony, the quench issue is only in play at low and moderate boost pressures. As manifold pressure rises, the importance of quench diminishes and pretty soon you will make more power from a "soft" chamber than the quench-chamber head will make. This is independant of flow rate or horsepower...it's purely related to manifold/cylinder pressure.

 

This is coming from some of the local high-boost racers in my area; they're pushing 50lbs of boost through SBC's and are finding that the least compromise results in an open chamber aluminum head with a full-circle spherical dished piston, leaving the only quench pads as a circle above the top ring land. I'm told that at less than 30lbs of boost, it's a wash...and below 20lbs of boost you will get better results with the high-quench style combustion chamber.

 

To the OP, to help we really need numbers.

 

Saying high-flow fully ported head means absolutely nothing. In order to give good advice, we need to know the real numbers, for both intake and exhaust flow rates, combustion chamber diameter, valve sizes, cam events and lift, duration at 0.025" and 0.050", preferably duration at 0.200" lift as well. Need bore diameter, stroke length. Need intake manifold and exhaust manifold details.

 

Give us real numbers, we can give you closer to real answers.

Edited August 2, 2014 by Xnke