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Safe boost on 98ron


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My GTR, if it ever gets finished, will be my daily driver. In Perh e85 is still quite rare so I'll be using 98ron and go e85 when I go racing.

Looking at all the dyno results, there is a huge variance in boost levels on 98ron fuel. My builder doesn't want to go much over 14-16psi. I see some people running close to 2bar. 

The engine is a 3.2L nitto with a precision 7685 running 9.0 comp ratio. 

What are people's theories, experiences, views, opinions on what is a safe boost level. 

Im not chasing 1000hp+ for street but I don't want a lazy 400hp car either.

Edited by khezz
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Interesting question...if your builder says not to go over 14psi and you go 20psi and it breaks, who pays to rebuild it?

Boost isn't really the issue, and you tuner will be the expert for this. Timing is the key thing you can run more of on e85 because the biggest difference between it and pump 98 is resistance to knock

Also keep in mind that a bigger turbo flows more air at the same boost, so boost alone is not a good yardstick.

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Your tuner will be the best person to determine what boost target is safe for 98RON.

He or she would need to attached good knock detection equipment and load hold the motor and listen for knock and determine what timing and boost is good for street use on 98RON.

Also, on 98 - I personally prefer to run a bit more boost and lazy timing then throw heaps of fuel in to keep it cool and happy (although this is not an efficient way to extract power).

Of course, a full flex tune would be the minimum requirement here.

Disclaimer: DYOR, this is not tuning advice :D 

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My gut feeling was "about 15 psi" before I opened the thread.

You gotta keep in mind you're not talking between 1000hp monster or a 400hp lazy engine, a better term is a 1000hp melted engine, or a 400hp reliable motor.

Turning up boost is burning the candle faster, always. There is no 'safe' amount because what you're really asking is personal, how bright do you want the candle to be, vs how long it will last. Only you really know that. This is a really big/serious setup but tbh I'm not sure it really matters much with regards to how many kms of aggression the motor will actually last.

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On 10/10/2021 at 6:47 PM, Kinkstaah said:

My gut feeling was "about 15 psi" before I opened the thread.

You gotta keep in mind you're not talking between 1000hp monster or a 400hp lazy engine, a better term is a 1000hp melted engine, or a 400hp reliable motor.

Turning up boost is burning the candle faster, always. There is no 'safe' amount because what you're really asking is personal, how bright do you want the candle to be, vs how long it will last. Only you really know that. This is a really big/serious setup but tbh I'm not sure it really matters much with regards to how many kms of aggression the motor will actually last.

Yeah but a 7685 on 15psi on 98 real world?

My opinion ?

Car will be still mostly asleep until 6000rpm even on a 3.2. With that turbo on low boost it'll be chopped on the street by a Yaris. Not a GR, a normal Yaris. This is a 1300hp + at wheels turbo - low boost and 98 is going to be shit.

Not trying to shit on such an expensive combo but this setup is all about the top end and nothing else. It is never going to be a well rounded package.

Still keen to see the actual outcome though - It needs E85

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No shit taken. I realise and accept that this will be very different on e85. More fuel, more gas, quicker spool, lower temps, more power. 

Everything going well the car will be on run in tune before Christmas. I guess it is all just a guess until it's on the dyno/street. 

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On 11/10/2021 at 9:19 AM, khezz said:

No shit taken. I realise and accept that this will be very different on e85. More fuel, more gas, quicker spool, lower temps, more power. 

Everything going well the car will be on run in tune before Christmas. I guess it is all just a guess until it's on the dyno/street. 

Remember with E85 it's potentially an 8 second car though 👍

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Listen to whatever the tuner/engine builder says. In my opinion For run in? 15psi is fine. For an actual tune? I'd look elsewhere. 

A large turbo with a decent sized hotside and more than likely aggressive camshafts all to keep it at a 2:1PR and on 98ron? I wouldn't even bother building such a setup for that. 

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The question is how much performance matters for you on pump gas vs E85. Take measurements to find scavenge ratio and where you are in the turbo's operating map for both turbine and compressor. Personally I would rather have the right sized turbo for pump gas and just use E85 to get more timing out of everything but I'm not building a 3.2L drag engine.

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On 11/10/2021 at 10:53 PM, Super Drager said:

Listen to whatever the tuner/engine builder says. In my opinion For run in? 15psi is fine. For an actual tune? I'd look elsewhere. 

A large turbo with a decent sized hotside and more than likely aggressive camshafts all to keep it at a 2:1PR and on 98ron? I wouldn't even bother building such a setup for that. 

The setup is for drag and roll racing. But it will also double as my daily. I would like to see around 600-700awhp on 98 Ron. That'll give me and my son the giggles on the freeway. I can't see any dyno results with a similar sized turbo on 98ron. Or any videos of one on the road either. If e85 wasn't an hour rou d trip, I'd use it all the time.

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It's all guesswork, but you'd have to expect that you should be able to use 20 psi on 98 without problems, given the nature of your build. And I think achieving 700whp shouldn't be too hard. Should be trivial. I mean, hell.....20 years ago I was peripherally involved in a JZA80 that easily made 900whp on much older turbo hardware, on 98. It was a "HKS" engine, which really just meant forgies and big cams, and still 3L.

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Going back to 2010 I was pushing 21psi through a t51r on 98 with a built 2.6 with a pretty decent head to make 600hp.

at the time I was told by the tuner that it will take more but with 98 that’s all they really wanted to push through it

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On 12/10/2021 at 12:55 PM, UWISSH! said:

Going back to 2010 I was pushing 21psi through a t51r on 98 with a built 2.6 with a pretty decent head to make 600hp.

at the time I was told by the tuner that it will take more but with 98 that’s all they really wanted to push through it

Nice. What was it like to drive on the street?

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On 10/12/2021 at 5:49 PM, khezz said:

Nice. What was it like to drive on the street?

Terrible. It was laggy as.

but once you got it up n going or launched it, it was great fun.

e85 was still a fairly new thing back then but if it was more freely available to me I 100% would of had it on e85 only as a car with almost identical set up was making 650kw atw on 30psi

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My 6466 on the 32 is 450kw/600hp + on 98 at 22-23psi. It's massively faster than the old T51 was at 29psi on 98 ron at over 520kw at wheels, and is safer in the knock department.

You can definitely make 700, 800, 900hp on 98 at up to 30psi - but how safely ?

The Precision 7685 is a massively, MASSIVELY more power productive turbo than a T51. You're going to reach that damaging knock point earlier at comparative power as cylinder pressures will be higher comparing pound for pound of boost.

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I got a bit of momentum writing a ramble about this but then decided that it was probably a waste of time, the short of it is that you should be trusting your tuner to make the call - there are WAY too many variables to ask a bunch of randoms on a forum about.  Your tuner should have all the variables and the wisdom to choose what best suits your interests and setup, if you don't trust him to make that call then you should probably have a different tuner.   

In regards to power, a mate just tuned an RB26 with a 7675 Gen2 on NZ BP98 last week and stopped at this point (it will be wound up more on E85, but it's a flex fuel setup):
image.thumb.png.8385f7e075335d2fe585720072738dd2.png

That would still be >700whp on a typical Oz rolling road, so realistically with a 7685 on an RB32 you'd think 600-700whp should be quite doable if the right combo has been put together in the right way to allow it but there are plenty of things which could get in the way of that being achieveable.

Edited by Lithium
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Thank you everyone for your advice. Spoke to my tuner again. He took me for a drive in an r34 that is a nitto 2.8 with a 6870. Making 600hp on 98ron. It was so much fun. Very little lag. His words were "forget boost. We'll make 800 easy". He said that he keeps boost low because it gives the client the abity to use any 98ron fuel. If you can stick to one type, say bp ultimate, we will cater a tune for that.

I guess after a 5 year wait and more money spent then I ever thought possible, I don't want to be disappointed. I am pretty sure I won't be.

Thanks for the advice fellas.

Edited by khezz
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