Autocross turbo and gearing setups. (R32 Skyline GTS-T)

So I have a project car i've had since 2014, it's a Nissan Skyline GTS-T so it has a RB20DET engine. Originally when I got it and put together a "build" on the car I was just following what people say is the normal on the Australia Skyline website. The thing is down there people, at least with this car, only care about drag racing or track. At the time I didn't really care much about autocross so I followed their guides and listened to what they said. I ended up building a car that dynos 320whp and because of how light it is the thing is fast. Easily able to pass and keep up with modern v8s, which in 2014 was my goal. 

The thing is this engine has serious issues with turbo lag and on top of that any ~300whp turbo builds boost extremely late, after 4000rpm. 

I've gone through a lot to get this car to be reliable and in good shape but I considered selling it at this point and starting with something else. Especially something that would be classed better in autocross. The thing is I did everything there is to do on this car and it's been good for a year. I don't want to have to go through doing that again on ANOTHER car especially when I am not someone who sees themselves ever wanted to travel for autocross or go to nationals or anything like that. 

So right now i'm trying to figure out what I need to do. This is kind of a me thinking out loud bench mod post but if anyone has any input please do!

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Before doing these mods I autocrossed this car and the stock turbo is basically perfect. At the time I wanted MAXIMUM POWAH and the stock turbo's internals are made up of exploding bits and so can't be adjusted for the kind of PSI I would need to make it. Ironically I would be perfectly ok with the power it made now (got a little over 220whp from it) but wouldn't be too keen on the reliability. 

Gearing on the car is:
1st: 3.321 
2nd: 1.902 
3rd: 1.308 
4th: 1.000 
5th: 0.759 
Reverse: 3.636
final drive: 4.363

255/40/17 rear tire

Dyno graph of a stock RB20DET:

My current turbo would start hitting 16psi around 4200-4300rpm. Which according to this below if I could get that by around 3000-3500 as the stock turbo setup I could have power at ~15mph slower speed. Which is a HUGE difference. 

To accomplish this I would need to get a twin scroll manifold (like from 6boost) and a twin scroll turbo of somekind. This would be probably a EFR. 6258, 6758, or 7163. The final one is what full-race recommended me. I have see dyno sheets of the 6258 and 6758 on the early SR20DET and BP mazda engines and the 6258 seems safe but small. I compared numerous dyno sheets to twinscroll builds on both of those to RB20DET and they are the most similar. I have a hard time interpreting those mystery turbo graphs so this is the method I use.  Twin scroll is considered an expensive build overseas where these cars are like SN95 mustangs so there is no decent dyno graphs. There is for RB25 and people overseas would recommend a swap. I do not intend on doing that. First off I've put a lot of time/effort/money to get the current engine setup to be reliable. Second if I do this I would like to try the Ultimate Street Car series again but this time in this car in GTL which mandates 2000cc or less turbo engine. The power I have now is fast enough for me, I just want it sooner. 

The downsides to this is:

If I were to ever have transmission issues the usual route is to then upgrade to the RB25 trans. Which has longer 1st and 2nd gears which may cause this problem again, if that time comes. 

$$$

I can't use the Skyline in any reasonable autocross class. So if I get the competitive itch i'm basically digging myself into a bigger hole. 

GTS-T is a narrow body in terms of R chassis skylines. I can only fit 235/45/17 in front and 255/40/17 in rear which is how I have it set up now. If I wanted to I could get a R33 GTS-T right now which can fit much wider, or change to something more modern like a Corvette or Evo IX or whatever else and not have any of these problems lol. However this car has sentimental value at this point.... that or i'm fooling myself and I just don't want to give up. I CAN make this work but if I had a FD RX7 which is the same weight and power I could fit much more tire on that AND be able to be in *SP or 'easier' class. Then you have the starting from scratch when I already know/fixed or am aware of all the problems in the car I have. 

I'll have to clean up a number of other things on the car if i'm going to keep going down this path. Work I wouldn't have to do if I got something newer. 

Upsides:

I know the car, and after owning it for so long and fixing so many issues I know R32s in general and all future paths for any potential mods or issues. 

I have a very good stand alone engine management setup made for Skylines working properly complete with MAF delete and all that. Link G4+. The biggest downside of going to anything else would be losing this and having to set it up again. 

Another thing that makes me reluctant to sell is that I found some amazing early 90s Nismo LMGT2 wheels and had them restored for this car. It took me forever to hunt them down. The only other car they would work on is a S13-S15. 

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Of course I could always wait until 2020 and get a R33 GTR but IDK if I want to go through this E36 M3 again with another sight-unseen JDM import lol. I wish my local rotary expert didn't leave for florida or MK4 Supra Turbos were still available under $30k. RX7 or MK4 Supra are the only two cars I wouldn't care about selling this car for. 

I don't have the "This Thread is Worthless Without Pitchers" pitcher so I'll just say, "this thread is worthless without pitchers".

Actually, it's very interesting. I just wanna see pitchers of your R32 Skyline.

Pic I posted to oppositelock a while ago once I finally got it working properly. 

I dunno man, my rb20det powered s13 only had a s15 turbo on it and it was still a lag-tacular experience. It was the way manufacturers did turbos back then. 

That said, I would aim for 2nd gear reaching 60-70mph. Which I KNOW there is a few different Nissan diffs that’ll give you that. Particularly the z32 r200. If you don’t have to shift, you make better time. And the only time you’ll be out of boost is the REALLY, REALLY tight 180’s. 

Edit: according to the chart you posted you look fine. Boost come on around 4,000rpm. In second gear that’s 40mph. Downshift to first if you need lower. 

Double edit: if you want to have fun, go do it. If you want to be competitive, sell the car and buy a Miata. 

In reply to _ :

2nd gear reaching that speed is not an issue at all with this car. Current tune has a 7500rpm redline and with the gearing i have setup 2nd gear will be 67mph according to the chart I posted.  Tuner said it would be save to near 8000 but current turbo makes no power at 7500 as it is so no point. 

I know 1.8l mazda engine can get 16psi by 3300rpm with a 6758 twin scroll setup already. I'm sure that the RB20 can't be any worse than that and I likely would be going 6258 which would see power even sooner. I am going to be very conservative with turbo selection this time unless I can be proven that the other two would make power where I want. 

Right now as it's set up on a slow autocross course I have literally no power for most of it. On a faster course it's OK. 

Downshift to first if you need lower. 

I would need to do this often lol. I rarely had to do this in any other car I own so maybe I need to change the way I drive but still. 

In reply to kanaric :

Then you aren’t driving right. In autocross it isn’t about “power delivery”, it’s about being smooth through cones. That means not downshifting or upshifting as often and possible, not upsetting the chassis. I’ve never ran an autocross to date that needed me to shift out of third or into first. I would rather ride second gear with less engine torque and more geared torque, instead of switching to another gear, upsetting the chassis, and making the engine hit its sweet spot for that brief moment, which actually turns into a slower lap time. 

I suppose you could get the longer z32 gearing, but that’s not going to help much. You could do a custom long gear so you can use first the whole time, but that would SUCK to drive any other place. Or you could swap transmissions, lololol. That wouldn’t be wallet friendly. 

Could it be possible to use a more modern turbo?  Turbo tech has changed drastically in the past few years.  Turbos are basically able to flow as much as turbos a whole frame size larger used to be able to.  You get all the flow with none (well, less) of the lag.

One of the more interesting setups I've heard of was for an FD RX-7 for autocross.  It had a GT35, which is tiny for a 13B.  It made what most people would consider pathetic horsepower for a single turbo FD (rotaries usually make about 20-25% less power for a given turbo size due to all the inefficiencies) but it was meant for a controllable yet explosive midrange.  The tuner said they didn't even care about what it did over 7000rpm tuning-wise as long as it didn't go lean, as if it ever saw that high it was just a momentary thing to prevent having to upshift on a long straight.

Lag time in a turbocharger is directly related to the turbine wheel size, design, and the A/R of the housing. What is your current A/R and is there a numerically smaller one available? 

Javelin said:

Lag time in a turbocharger is directly related to the turbine wheel size, design, and the A/R of the housing. What is your current A/R and is there a numerically smaller one available? 

It's a little more complicated than just the turbine side.  For instance, the K24-7400 in my Volvo basically flows air like a K27 because the compressor wheel has about 10 years of design and mass-production capability progress, even though it's a smaller inducer and exducer.  This means the turbine doesn't have to work quite so hard to spin up the compressor in the first place, as there's less mass in the whole system.  The end result is an engine that makes power like a Sport Quattro (a similar-displacement watercooled engine that used a K27) while having a tractable, seamless torque curve instead of something that hits hard and late.

And that is with a 15 year old mass-production wheel.  There are billet compressor wheels available today that fit the same compressor snail that will flow more, AND have less mass.

(and the '7400, as well as the K24-7200 from the Audi RS2, are technically K26s, as they use K26-sized bearings and shafts and not the even-smaller K24 stuff.  But the K27 bits were HEE-YUUGE, both in wheel size and in bearing size)

What is the current turbo on the car?
What does the car weigh?
What size courses are most common for your local region?

If you have small courses locally, 67mph is pretty good for the top of 2nd. I wouldn't worry about that. Pick a turbo that can spool early and not choke at 7500rpm. You are on the right track looking at modern EFR options.

If you have larger, more open courses, I would look at dropping some rear gear. 4.36 is pretty steep and those trans ratios could work with something in the 3.6x-3.8x range to get a higher top speed in 2nd, depending on vehicle weight.

In my 3200lb Tbird with 2.3T I ran a small turbine housing; spool early to get the weight off the corners, but giving up some HP up top. I moved the same engine into my truck at 2100lbs, and loosened up the turbine housing. The lighter truck moves off the corner easier and the extra useable RPM from the bigger turbine housing lets me stretch the gearing further. I run 74mph at the top of 2nd gear using a 3.73 rear gear set, but we have a couple big open sites. If I had to run tighter lots more often I would likely swap to a 4.10 gear.

What turbo is on it now?  The EFRs are remarkable.  Transient response is greatly reduced with the lightweight turbine wheel, which doesn't show up on most dynos.  I have a 7163 on an SR20 and it's way more responsive than the previous GT2871R.  What's the deal with the RB20.  Poor head flow?  They don't seem to make great power and are definitely kind of a redheaded stepchild in the Nissan engine family.  That said, I would think that with all of those exhaust pulses and a twin-scroll manifold, a small efr would really rip.  If you really aren't looking to make more than low 300 wheel horsepower, I don't see any reason to go with a bigger turbo than the 6258.  There seems to be very little spool penalty going with the 6758 from what I have seen, but if the smaller turbo will hit your maximum power goals you might as well maximize your spool and response.

As an aside, how was your overall Skyline experience? I have been playing with the 240SX cars since college and have had a bunch of them. Last few have been primarily track cars. Most of them were S14 Zenkis, so I have a lot of sentimental value for that rounded mid 90s style. I've thought about getting an R33 GTR when they're available stateside.  I think most would agree that they are the least loved of the modern generations, which should keep them a bit cheaper. But they're also much closer to the R34 than the R32 in terms of tech.  R34 looks so angry and badass, but the price premium for them is really quite ridiculous.  However ultimately my reservation is that any of them are fairly heavy cars, right hand drive, and replacement body parts etc are not going to be easy to come by here. Doesn't make for a very sensible track car.

In reply to turtl631 :

Borg Warner EFR turbos are amazing. The responsiveness of the turbo for the size they are still boggles my mind. The FP Miata I codrove last year had one and it felt more like a supercharged car. The transient response was awesome for a 400hp 1.8L. 

As as for the gearing, 65ish mph on the limiter in second gear is where I would want that car (hell, most cars) for autocross purposes. 

In reply to gumby :

Car has a Hypergear SS1.5 which is a turbo specifically built for the RB25DET, it's considered a fast turbo for a RB20. When it's on power it's actually not that bad lag wise, it has good response. The problem is actually more like WHERE it makes the power. 

The car weighs slightly south of 2800lbs. I could get it to weigh less but it won't be 2100lbs like your truck unless I started swapping out things for FRP. Car is from 89 and since it's not a GTR it's all steel. 

Where I am there are two kinds of autocross venues but usually we have faster courses. On this car i've never had to go to 3rd gear. One thing i'd want to do to get a drop in gear is trying to find a S15 torsen gear set. IIRC it's 4.1 or 3.9 and it's supposed to be a good lsd as well. Swaps into my housing. I think the rear end ratio is fine on this car though. I would just want the diff lol. 

What turbo is on it now?  The EFRs are remarkable.  Transient response is greatly reduced with the lightweight turbine wheel, which doesn't show up on most dynos. 

When it's in it's powerband the response isn't that bad. If that's how it was during rpms where I needed the power I would be fine. 

 I have a 7163 on an SR20 and it's way more responsive than the previous GT2871R.  What's the deal with the RB20.  Poor head flow?  They don't seem to make great power and are definitely kind of a redheaded stepchild in the Nissan engine family. 

Poor head flow is definitely a problem with this engine. Another thing is the exhaust manifold has a LOT of room for improvement. Even a non-twin scroll manifold like a HKS sees amazing results. 

That said, I would think that with all of those exhaust pulses and a twin-scroll manifold, a small efr would really rip.  If you really aren't looking to make more than low 300 wheel horsepower, I don't see any reason to go with a bigger turbo than the 6258.  There seems to be very little spool penalty going with the 6758 from what I have seen, but if the smaller turbo will hit your maximum power goals you might as well maximize your spool and response.

The way i'm thinking is I probably can make at or near 300whp on this same dyno with that. If I make the same as I do now that's a bonus. Where I am you only have 91 octane available but there is the E85 option.

As an aside, how was your overall Skyline experience? I have been playing with the 240SX cars since college and have had a bunch of them. Last few have been primarily track cars. Most of them were S14 Zenkis, so I have a lot of sentimental value for that rounded mid 90s style. I've thought about getting an R33 GTR when they're available stateside.  I think most would agree that they are the least loved of the modern generations, which should keep them a bit cheaper. But they're also much closer to the R34 than the R32 in terms of tech.  R34 looks so angry and badass, but the price premium for them is really quite ridiculous.  However ultimately my reservation is that any of them are fairly heavy cars, right hand drive, and replacement body parts etc are not going to be easy to come by here. Doesn't make for a very sensible track car.

The first 3 years were a pain in the ass. Waiting weeks for parts and all that. A recent development this year is that parts availability has been good. It's not go down to the junkyard and find parts good but there are multiple US sources for parts now. Like Terra Firma for example is a website in the southeast that has a lot of skyline parts. And it's rare parts as well that are becoming more common and MUCH less expensive. In part because Nissan is producing GTR parts again and MOST GTR parts fit coupe-Skyline GTS-Ts

I've thought about getting a R33 GTR as well. It's my favorite one. The difference between an R33 and R34 is negligible. Both have an outdated and slow AWD computer, the R34 less so, but companies like Do-Luck make replacements that work for both and make them pretty much equal.

The weight of them is about the same as my Audi S3. The R32 is the real Godzilla and the winning car in motorsport for a reason though. The later cars are pretty much 2nd fiddle. 

One of the reasons why I picked R32 GTS-T is because it's an uncomplicated car, relatively, to the GTR and I wanted to make sure of how things like parts would be. The first few years it failed the test but today? I might consider a GTR. In fact GTR parts are MORE readily available.  R33 and later the GTS-T equivalants share much more parts with the GTR than my car does and GTR has better parts availability. 

As as for the gearing, 65ish mph on the limiter in second gear is where I would want that car (hell, most cars) for autocross purposes. 

If this car had like the RB30DE engine from the Tommy Kaira M30 car it would basically be a perfect autocross car lol. Like a E36 M3 with perfect gearing. Unfortunately I have to figure out how to make due with this RB20. 

Do the manifold and turbo, then run it and see how your current gearing works with the new combo. If you still never need third with 4.36 rear gears, great!

Don't be surprised if bringing boost in almost 1000rpm sooner results in climbing the tach fast enough to run out of gear even on familiar sites that were fine before. Not a bad problem to have and it sounds like you have good options to rectify the situation when it arises.

He still thinks he ain’t got punch for autocross in second year... I don’t know what to tell ya man. Almost Everyone here autocross was and they will tell you what I already have. At 35mph, you’re at 4,000rpm, which is still near power band.