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What compression ratios are you running on your TT Set-ups?

Any of you that are planning on building a higher performance TT set-ups (like an APS TT 600-800whp setup)?

Peter (aps), what do you recommend (are you using) for your high end set-up???

Thanks,
Nick
 

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I am running a 8.5:1 ratio. I would say that you should run between 8.5-9.0:1 for any boosted application.
 

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Discussion Starter #3
Originally posted by Zxrage@Nov 7 2004, 12:04 PM
I am running a 8.5:1 ratio.  I would say that you should run between 8.5-9.0:1 for any boosted application.


Ok, I kind of asked a loaded question because I wanted to stimulate some technical/theory conversation.

First, Zxrage, I just want to say I've been reading some of the threads from you guys at MRC, and I am intriged.... you APS TT project is swaying me into getting a 2005 Z. Starting an arguement is the last think I am trying to due. I want to stimulate constructive converstation for my benefit was well as the rest of the members of 350z-tech.

Thank being said..... to some extent.... I dissagree.

Here is the thing.... Obviously, engine power is derived from cylinder combustion pressure. Period. Everything else is a proponent of trying to maximize combustion pressure, maximizing the duration of high combustion pressure, and achieving it as efficently as possible.

One direct way to increase cylinder pressures (and in turn combustion pressure) is through compression (increased pre-combustion cylinder pressure).

Now, if you think about what FI essencially does, it really just increases the maximum pre-combustion cylinder pressure. And in turn, increases the combustion cylinder pressure.

For example:

N/A 10:1 Compression:

(Atmospheric pressure) x (Compression) = 14.7psi x 10 = 147 psi (maximum pre-combustion pressure)

10:1 Compression with 10psi boost:

(atmospheric pressure + boost) x (compression) = 24.7psi x 10 = 247psi


Now, if you reduce the compression, you will lose some pre-combustion cylinder pressure. (say 9:1 compression and 10 psi of boost)

24.7psi x 9 = 222.3 psi

Obviously, the result is a 10% drop in pre-combustion cylinder pressure.


Now, on the flip side. If I have 10:1 (static) compression and am running 12psi of boost, this would result in 267psi of pre-combustion pressure.

If I were to drop the (static) compression to 9:1, I would need to run 15psi of boost to achieve the same pre-combustion pressure.

And the difference will increase as we increase our boost and desired power output.


So, I would think that being able to run higher compression and lower boost (and resulting in the same pre-combustion pressure) would be to our benefit. Lower required boost means you could probably spec out a turbo that will spool faster since we are not worried about big boost levels (as big anyway). And I would think we could achieve a more linear/better power band and torgue curve (closer to that of a SC while still gunning for the higher hp). I would think this would be more thermal efficient. Overall resulting in a fastener car.

I know running higher compression with moderate or better boost levels creates detonation concerns. But I am lead to believe this is more a result in inadequate tunning and fuel/timing control. With the continued development of better fuel/time management and better tunning (as with a good dyno with an electronic retarder), I would think we can better manage the tunning and provide safer high compression and boost levels.

I know that there are many other factors to consider (and some factors may hvae a greater impact that I realize), but I would think that this would be a more efficent approach that just dropping the compression way down and turnning the boost way up.


Anyways..... this is just my two cents. I would like to see some more engine theory discusion on this forum. What can I say, being an engineer brings this out of me.

I hope to gain lots of comments, opinons, and insight on this theory.

Thanks,
Nick


PS I'm an engineer, not an writter.... so forgive the spelling and grammer.
 

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Comparing Static CR directly to Dynamic CR is an oversimplification.
Effective boosted CR has the benefit of an intercooled charge and Static CR does not. From a pure power increase perspective you can get more return by raising boost vs, raising Static CR. The APS turbos are already sized towards the larger side. So with these snails a target CR between 8.5 - 9.5 will probably yield the highest possible power output. Of course inorder to yield the max potential out of the APS system the fuel system requires upgrade to support boost psi above ~12. The CR strategy taken falls mostly on variable such as how much boost and available octane. If you were always to run 100+ race gas , you would benefit from a higher CR (~9.5). If you were attempting to get the most out of 91 , you would benefit more from a lower CR (~8.5). If you don't have it already, Corkey Bell's book "maximum boost:" is a really good read.
 

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Another thing to consider is our relatively small rev range. Supras and Skylines can run lower compression pistons and they will still have a large on-boost rpm range because they rev so high. As we dont rev as high if you run pistons with too low a compression pistons then you will need more RPM to build up boost cutting into your on-boost rpm range. It will change your power band to be more like a single turbo's, with less power down low and a steeper climb to peak power, which equals less time on-boost. Peak numbers are good on the dyno but for a daily driver there are more things to consider.

I believe somewhere around ~9.5:1 would provide a better comprimise, depending on your requirements of course. The forged pistons and slightly lower compression should enable more boost to be run with only a minor drop in off boost performance. If the stock 10.3:1 units can handle 400rwhp, then forged steel ~9.5:1 should be able to handle a fair amount more while still retaining good off boost power, fast spool characteristics and large on boost rpm band which equals more time on power puling ahead.
 

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Not exaclty , if you look the shape of th eTQ curves produced at SGP racing @ 8.6 cR on 93 pump , It comes on stronger and lower than a comprable peak rwhp single turbo Supra (infamously peaky). The VQ has extra TQ to spare in the sub 4k rpm range and does not require ultr high RPM to produce very good low and mid range TQ. 9.5 is too high IMO unless you wnat max power on race gas.
 

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I was using supras etc as a general example to illustrate that they can get away with longer spool up times because there rpm range is greater than ours. Obviously it depends too much on the individual setups of the cars to make a direct comparison like you mentioned. Yes a low compression TT Zed might have more torque sooner than a Supra but it would have less low rpm torque than a stock compression TT or NA Zed, which is what we have become accustomed to. Im not worried about having more power sooner than a Supra but I am worried about loosing any noticable power that I have become used to.

As I dont plan on using race gas, higher compression pistons will actually lower my potential max power because they are more prone to detonation at those higher power levels.

It comes down to what you want to achieve, I have heard from guys with 8.5:1 compression that they can definately notice less power off boost and longer spool up times. For a daily driver I personally want to retain at much of that low rpm power as safely possible as most driving and shifts occur in these low rpms. I want my car to feel quicker over the largest rpm range as possible and im happy to sacrifice some POTENTIAL PEAK hp to do so, as I dont think I will ever be reaching that potential max peak power anyway (APS ~8-900hp).
 

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Originally posted by mchapman@Nov 10 2004, 01:52 AM
I was using supras etc as a general example to illustrate that they can get away with longer spool up times because there rpm range is greater than ours. Obviously it depends too much on the individual setups of the cars to make a direct comparison like you mentioned. Yes a low compression TT Zed might have more torque sooner than a Supra but it would have less low rpm torque than a stock compression TT or NA Zed, which is what we have become accustomed to. Im not worried about having more power sooner than a Supra but I am worried about loosing any noticable power that I have become used to. 

As I dont plan on using race gas, higher compression pistons will actually lower my potential max power because they are more prone to detonation at those higher power levels.

It comes down to what you want to achieve, I have heard from guys with 8.5:1 compression that they can definately notice less power off boost and longer spool up times. For a daily driver I personally want to retain at much of that low rpm power as safely possible as most driving and shifts occur in these low rpms. I want my car to feel quicker over the largest rpm range as possible and im happy to sacrifice some POTENTIAL PEAK hp to do so, as I dont think I will ever be reaching that potential max peak power anyway (APS ~8-900hp).

I think 800~900 is no problem with low comp built Z's. I wouldn't be surprised if the bottom end could be made to support 7500~8000 rev limit comfortably. Anyone know what the Nismo crank with appropriate rods and pistons for boost will support. The arguement about compression versus boost is an old one, but 8.5:1 should be able to handle 20 psi on pump gas. We do it all the time on other cars. We've seen plenty of 21+ psi on properly tuned 8.8:1 cars with 93 octane. It will take more than a piggyback for the Z, though.

Mark
www.Quantum-Racing.com
Home of the Dynapack Dyno
937.642.5400
 

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What do you think the max amount of boost or max power from our 3.5L engine could be run run with 9.3:1 compression pistons on ~91-93 octane fuel?

With stock pistons and a compression ratio of 10.3:1 our stock motor can handle around 9psi or 450rwhp. Bare in mind that aftermarket pistons would be superior to the stock Nissan units.

Please only use detonation as the limiting factor and assume all other support systems have been taken care of.
 

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"Bare in mind that aftermarket pistons would be superior to the stock Nissan units."

In strength yes, in detonation reduction , not necessarily. Note that at
~9.3:1 , one shoud place more attention to the piston dish design
as to maintain the high knock tolerance as the OEM does.

So basically the amount of boost on pump gas at this static "higher" CR can greatly be effected by piston dish design.
 

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g3po,

Once again I agree with you completely; this is oversimplified.

NWN,

Let's not forget the effect ignition timing has on peak cylinder pressure and engine responsiveness. I wish I had more time to elaborate, but I am at work.

Nick
 

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It is over simplified but just as QuantumZ was able to give ruff number on setups they had dealt with before it should be possible for him to make some assumptions and give a ruff estimate. Im not after a actual figure that I will then boost through my car, tuning will determine that actual real life figure.

Ignition timing does have a very large impact, this another reason to go with slightly higher compression than 8.5:1, as you can always take out timing at the top end. On a stock setup I think this should be considered manditory to reliably prevent from damage.

Assume a boost friendly / detonation resistant dish design like most of the aftermarket pistons available for the Zed and timing can be the same on both setups if this makes it easier tro give an estimate or as im actually interested in knowing what is potentially achievable, the timing can be what ever.
 

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Originally posted by NWNewell@Nov 7 2004, 06:21 AM
What compression ratios are you running on your TT Set-ups?

Any of you that are planning on building a higher performance TT set-ups (like an APS TT 600-800whp setup)?

Peter (aps), what do you recommend (are you using) for your high end set-up???

Thanks,
Nick

Sorry guys I mised this thread so I will keep this post short as most of the real issues have been covered previously.

Seems to me that the comp ratio that is correct for the Z engine will vary from car to car, owner to owner, depending on,

1) Available fuel octane

2) Intercooler efficiency

3) Turbocharger efficiency

4) Engine management (precise ignition timing control)

There is no one answer for the the best/correct comp ratio for the Z engine, rather a number of different answers depending upon the the intended application and level of technology in the TT system involved.

That said for a street car I always prefer a higher static comp ratio (with less turbo pressure and the best ignition timing strategy possible) as this delivers the best off boost partial throttle response and excellent fuel economy.

I hope this makes sense to you guys and of course there are always applications which may alter my above comments. :thumbsup:

Happy Boosting

Peter
 
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