From 7685 to 8685

[khezz]

Hello.

my R32 is ready to start but I received a message which has thrown a small spanner in the works. I currently have everything built to hold 1600hp. Engine (3.2L), fuel system, sequential drive train. However the Turbo is a gen 2 7685 T4 1.12 rear (1350hp). I have an offer of doing a straight swap for a gen 2 8685 with the same rear housing (1500hp). this will put the turbo in line with the rest of the setup regarding HP potential. 

main purpose of the car is drag and roll racing. I will be driving it on the street as often as I can. If anyone has any real work experience with a turbo that size, what’s it like on the street. Would love some dyno graphs. I would even welcome opinions. I found a few dyno graphs of these turbos on 2JZ engines which is a reasonable comparison but there is no rpm reference on the graph. It’s all MPH based. 
 

thanks to everyone in advance. 

[Predator1]

Which sequential? Do you know the ratios? Assuming your first will be super long as well, making it seem more laggy. Plus you're going to be running a massive turbo, and its a precision, so will be laggy as hell on the streets. Expect to be gapped by prius's. Precisions dont do compressor maps which kinda sucks.

What work have you done to your head? Please make sure your engine builder degree's your cams!! We just played with the cams on mine and its alot more responsive and made a wee bit more power.

[khezz]

Thank you for the reply.

Head has the works done to it. Port polish, oversized valves. Biggish cams. I will spend as much time as needed on the dyno getting this thing right  

the only real wold result I’ve seen is WARGTR dyno where he made 1410hp but the line was a perfect slope. 
 

I realise a 6466 will be a street weapon but that’s not really the purpose of the car. I guess I’m still remembering old bush bearing Japanese turbos with dyno graphs like Mount Everest  

I won’t be racing people on the street (much). Occasional hwy push or a scare of a passenger on a cruise maybe. Planning lots of track time. Just don’t want the turbo to completely choke the engine. From what I see with all the US supra videos, doesn’t seem to be the case. 

[Javier_Z31]

 

I know is not exactly the precision 8685 but you will get an idea of it . Actually they talked about it around the 12:25 of the video. Hope it helps . 

[khezz]

Great video. Thank you.
 

I actually got a few messages back from guys that use 8685. So far 100% thumbs up. Almost all of them went from a 7675 or a 7685. sentiments were along the lines of “once your balls grow big enough, you’ll win lots of races”. All said that it is still very drivable on the street. Being a 3.2 with 9-1 comp ratio. Sequential box. Light tail shaft. It will spool nice and even. Will have all of the power in the 5500-8500 range where you shift. 

[r32-25t]

Most people are going 8085 these days and making more power everywhere 

[khezz]

This was a free upgrade. Well just the cost of postage. Going 8085 meant forking out thousands of dollars and having an extra turbo in the shed hoping it sells.
 

next Gen is still very new and there is not allot of comparison data to Gen 2. I haven’t seen any videos where new 8085 clearly beats the old 8685 for outright HP. Even though it is meant make 100hp more. A couple of people I have spoken to that have gone next gen say it makes similar power but does make it a little earlier in the rev range. Best example is RHM R34 gtr. Romanos does a back to back run with that exact combo. He makes the same power and he would max out both turbos for sure. All he said is he gained some torque. Not allot, just some. And went Pro Mod immediately after. I am not in the racing catagory where 500-1000rpm will make a difference to my results. I’m sure my GTR with an 8685 will make me shit my self for years to come. By the time I say “I need more power” there will be something better out anyways and I will be going billet block and getting divorced. 

[Lithium]

1 hour ago, khezz said:

Not allot, just some. And went Pro Mod immediately after. I am not in the racing catagory where 500-1000rpm will make a difference to my results. I’m sure my GTR with an 8685 will make me shit my self for years to come. By the time I say “I need more power” there will be something better out anyways and I will be going billet block and getting divorced. 

I was reading through the post and the comments and was basically going to say something along these lines - feels like you answered your own question really

I haven't had direct experience with the exact kind of setup you are talking about here, but my observations and involvement with things of this level basically surmounts to - once you are talking about 80mm+ turbos on "small" engines the lag etc all becomes a bit academic.   You are going to NEED launch/rolling start antilag, you're going to need to rev the hell out of it and a fast shifting trans with shift cuts are ideal if you want to keep progress happening well.

At this point if you want more power and you have an option, the only thing to think hard about is "Do I have the trans, fuel system, and engine spec to ensure I can use the upper end of this turbo?".  If you don't, then probably a waste of time... the extra lag without being able to get the treats is pointless.  If you can use pretty much all it has to give, may as well.   

Edited January 22, 2024 by Lithium

[khezz]

I was wondering when you would through some wisdom in. 
 

everything is built to withstand 1500-1600hp. Turbo was the only part that was slightly shy. If you can call 1350hp shy. 
 

Car will be driven often on the street. Can call it a daily but it’s not a street racer. If it was I would have gone 64xx or 68xx. This is mainly to go low 8s on the quarter and win roll racing. Sequential with FBW/flat shift. Two step, light tail shaft, front and rear drag spec diffs. 
 

I was hoping for some seat of the pants experience. Anthony (WARGTR) and Romanos (RHM) were very forthcoming coming. Both said that there is very little difference between 76 and 86. On the street you change under 4K anyways so it’s a nice quiet street car. The 3.2 with its counter weighted crank revs very well and has enough capacity for town driving. If you do decide to do a squirt, you would shift down and from 4500 onwards there is nothing that can keep up with you. 

[Lithium]

14 minutes ago, khezz said:

I was hoping for some seat of the pants experience. Anthony (WARGTR) and Romanos (RHM) were very forthcoming coming. Both said that there is very little difference between 76 and 86. On the street you change under 4K anyways so it’s a nice quiet street car. The 3.2 with its counter weighted crank revs very well and has enough capacity for town driving. If you do decide to do a squirt, you would shift down and from 4500 onwards there is nothing that can keep up with you. 

Yeah makes sense to a degree, like you have a "strong" 3.2litre engine anyway.  As soon as it starts getting any boost the displacement advantage gets compounded and you start picking up speed (and with that, rpm) quicker than the poor beggers with 2.6s and even 2.8s.

As soon as you get it up and cooking its going to be insane, but its only going to be useful in races you're allow to get up on launch control or at least involve very illegal speeds on the road but you were already there with the 76mm. 

[r32-25t]

I’ve said a few times if I was starting again I’d go 3.2, but I had the stoker kit before I knew about the block being stuffed 

[khezz]

I’d like to hear this sad tale please…

38 minutes ago, r32-25t said:

I’ve said a few times if I was starting again I’d go 3.2, but I had the stoker kit before I knew about the block being stuffed 

 

[r32-25t]

1 hour ago, khezz said:

I’d like to hear this sad tale please…

 

Basically slowly kept modifying a standard engine and ended up with a 6466 on it and then ended up with coolant spraying out of the over flow bottle on boost, thought it had just lifted the head and was going to throw a 2.8 kit into the factory block and keep going.
 

Got the head off and found the block was cracked in 4 spots (3 in the deck and one in the side) and because I already had the 2.8 kit I bought a billet block for that to go into. 

[khezz]

Well that’s not so terrible. How’s the billet block working out? 2.8 rev like crazy. What you loose in capacity you gain in revs. 
 

is it running now? 

[r32-25t]

No it’s not running yet, I got it running briefly and something was wrong with the block, I don’t know what it was and I haven’t got it back yet 

[khezz]

Ok that’s more sad. I hope you get it back soon. Are you still running a 6466?

who is fixing the block?

[r32-25t]

I went up to a 6870 im hoping I won’t regret it

the machine shop/engine builder has it at the moment, I don’t know the exact details of what happened apart from water was getting into the oil and after they did a pressure test it was coming from behind a sleeve 

 

[usmair]

Couple of comments:

The 8685 is laggy. You will definitely feel the difference between a 7685. 

With a manual don't expect anything to really happen before 6,000rpm. You'll probably need to rev to 9,000 to get the most out of the 8685.

I've had friends with 8685 and a Samsonos, 3.2 and it is still laggy/doughy.

I've had both 8685 and the 8085 and there is certainly a noticeable difference between the two. The 8085 does make slightly more power but my convertor doesn't want to show it....

If it's roll racing then you will need to ride the fk out the clutch to get it on. Make sure you have something that will cop the punishment otherwise you'll get left behind. The direct clutches 9 inch what everyone in Sydney is using and that is taking the punishment. Everyone in Sydney is now using gas to help get out of the hole too. You might need nos.

I don't know how long the track is at Perth but you'd want to be comfortable that you have enough track to really use the hp from the 8685.

If your main focus is drags and RR where the extra power can be used the go the 8685. There is no chance of putting down 1500hp on the street (not to mention that it is dumb!).

If you want an all rounder then you may wish to rethink the 7685.

 

[khezz]

Wow. You are the first to give the 8685 a thumbs down (of sorts). I am using an OS quad plate clutch. I would rather stay away from gas if I can. Personal preference really. Sydney RR is probably the most competitive in the world. I have never been but by all the videos and written accounts, if you aren’t pushing 1500+ hp, you aren’t even fighting for a podium. That’s insane for such a young competition. With Perth, 1000hp puts you close towards the top. We have a nasty turn at the end of our straight. Scary as hell. 
 

on the street, I will be using 98 octane and keeping the boost down. I’m thinking 700-800hp no more. Anything above that is not really usable. Most of my street duties will see the engine in the 2500-5000 rev range. Drags and RR is the main purpose of the car. Maybe a dyno comp from time to time. 
 

cheers for the info. 

[usmair]

OS quad won't survive riding the clutch at 1500hp. Not as much as the Sydney boys do anyway.

If you need 1000hp then you don't need an 8685. It's a 1400hp turbo. The 7685 is more than enough for 1000-1200hp.