BPU Limits

Hi All

So I'm nearly full BPU now, decatted, FCD'd, 3 inch exhausted and have a HKS VI EVC IR BC which is going to be programmed to take it to the final 1.25 bar shortly by a tuning company. My car is a JDM 93 TT Auto.

I've always been told the limits on BPU on JDM is the turbos, so if I acquire a set of UK TTs or US TTs can I then just increase the boost more to their safe levels as they arent ceramic? Or is there something else stopping me? I cant think of anything and I also have CW brake upgrade too so I will be able to stop. I know finding a set of twins will be hard but I should be able to do it if I search long enough or dont they fit on my block or something?

Also I've been told several times on here the BHP limit on a Supe Auto Box is about 500 - 550 but I recently spoke to someone who said TTs were fitted with two different boxes in the course of their lives, one handling this much and another which could handle waaaaaaaay more. How do I tell which one I have and what is the other called if I have the lower one?

Hi All

 

So I'm nearly full BPU now, decatted, FCD'd, 3 inch exhausted and have a HKS VI EVC IR BC which is going to be programmed to take it to the final 1.25 bar shortly by a tuning company. My car is a JDM 93 TT Auto.

 

I've always been told the limits on BPU on JDM is the turbos, so if I acquire a set of UK TTs or US TTs can I then just increase the boost more to their safe levels as they arent ceramic? Or is there something else stopping me? I cant think of anything and I also have CW brake upgrade too so I will be able to stop. I know finding a set of twins will be hard but I should be able to do it if I search long enough or dont they fit on my block or something?

 

Also I've been told several times on here the BHP limit on a Supe Auto Box is about 500 - 550 but I recently spoke to someone who said TTs were fitted with two different boxes in the course of their lives, one handling this much and another which could handle waaaaaaaay more. How do I tell which one I have and what is the other called if I have the lower one?

What bhp are you currently running and what is the aim, an auto box and tt auto boxes are the only two fitted as far as I’m aware

The tuner said there were 2 boxes in TTs (although maybe he meant one was an aftermarket one) one which is about 500 and the other capable of handling much more power. I had a single conversion in one of my previous supe and whilst you get more power they are much more jerky in day to day driving so I wondered what power I could get out of a JDM block with US/UK TTs fitted?

At the moment when the BC is dialled in I reckon I will be somewhere around 420 as all my mods have been bought to be the best for power including 3 inch decat, 1 bar ring, CW FCD, Tein Adjustables, and my engine has been fully rebuilt and bored out to plus point 5 so is basically new and the best it can ever be. If I could get to 550-600 on Twin Tubs that would be amazing but not sure if I would be aiming too high.

Injectors may be your limiting factor? but then maybe something can be done with the fuel pressure? outside my knowledge really, but I do remember as a rule of thumb for the supra being 1bhp per injector cc, ie jspec's with 440cc injectors 440bhp is about the safe limit, uk cars with 550 injectors 550bpp.....

Obviously an injector size change takes you into the ecu piggyback or standalone area and mapping etc costs/complications.

Much above 1.3 bar I think it was sort of found/thought that you end up just heating the air intake charge, certainly diminishing returns.

It's been a while since people pushed the TT boundaries as lots go straight to single, but I think water injection used to be popular for protection and reducing intake temps/increasing power.

Jspec turbo - 1.2 ('safe' limit) after this you're creating mad heat

UK/EU or hybrid turbos - generally handle 1.4 bar but then you're maxxing out Jspec injectors

MK4 TT only had one type of autobox, you can get upgrades and aftermarket ones, also run a gearbox cooler (I use a SRD one) at BPU, then 2 coolers are advised above 500hp. Its the heat that kills them.. SRD do uprated boxes, a lot of the other ones I've read about seem to have harsh behaviours

The limit for any turbo car is really the exhaust, the UK spec downpipe, EGBV etc are 3 inch so offer a better base than the J spec.

Ok I guess with UK TTs and bigger injectors and a piggyback I am looking at getting mid to late 500s then on a stock autobox? That would be fine for me. I'm getting a lot of difering info on the safe limit for BPU too, a lot of people are saying 1.2 is absolutely safe whereas some people seem to run about 1.25 which the car will naturally spike up to 1.3 briefly then settle down to 1.2-1.25, I assume this is the bst approach when you have a fancy dandy BC? Or should 1.2 be considered the absolute ceiling for safety?

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Ok I guess with UK TTs and bigger injectors and a piggyback I am looking at getting mid to late 500s then on a stock autobox? That would be fine for me. I'm getting a lot of difering info on the safe limit for BPU too, a lot of people are saying 1.2 is absolutely safe whereas some people seem to run about 1.25 which the car will naturally spike up to 1.3 briefly then settle down to 1.2-1.25, I assume this is the bst approach when you have a fancy dandy BC? Or should 1.2 be considered the absolute ceiling for safety?

Personally. I wouldnt use boost alone as a predictor.

You need to remove restraining parts of the setup to allow for a better power outcome without using boost alone.

These would be intake temperature reducing items such as larger pipework and inter cooler. Exhaust restrictions such as pipe size and also cams for same boost more power outcomes. Try some 272 cams and you'll get more hp with the same boost but it will indeed move your power band. I've never seen anyone utilise big cams with stock turbos.

I've seen people remove the restrictor ring and control boost with standalone.

Fuel system would be recommend to upgrade but oversize on all of the above and it will definitely help you.

I dont think enough people go big enough on their exhaust. I have a 4inch downpipe to a 4.5inch system. Contributed towards allowing me to hit 501hp with 0.8bar on a 62mm turbo on my NAt.

uk turbos are actually smaller then the jspec but handle more boost due to being steal mid to late 500 on stock turbos is quite a ask and they normally have to be put in TTC which makes it laggy as hell , and is not worth the power gain in a auto as the TC stall point is 2000rpm and the turbos in TTC come on around 4000rpm will you have more power yes will your car be faster defiantly not , drag racing a full bpu in sequential auto with around 410bhp was beating mine and other single turbos with 500-600bhp and the single turbos spool faster then the TTC stockers .

I have had my auto stock , bpu , stage 3 hybrids , single turbo 524bhp. and if im honest I liked it best when full bpu for day to day driving as a weekend toy it is now single is good fun but still left standing until spooled up which I don't like too much on the street as your doing speeding ticket speeds by then.

lots of factors to think about its not just power its putting that power to the ground and how that power comes on

Yes these are good points, I've heard many people say as an actual driving car full BPU at early 400s is about as good as it gets as a compromise between power and smoothness and of course no lag which makes fun driving possible. I am in two minds to see if I can get the best possible TT power output as I intend to keep my Supe forever situation permitting and just going BPU and then thinking about handling and laying the power down and leaving the engine at full BPU. I have been down the big single path years ago and yes you get that boost but it was clicky and jerky and by the time it kicked in the other car was a dot, and then you caught back up again whereas the twins seem to have the power there at the right time and of course cornering with predictable twin power across the rev range is a delight but I guess we all have that burn for more hp.

So do we agree 1.2 1.25 or 1.3bar is the safe max on BPU? If I tell my tuner to put it up to whichever and see if I like it then maybe get active suspension and see if I need more power that would be the best option? Also I heard the HKS accesses both first and second turbo for better power and acceleration which is why I got it, is this true and anyone have the HKS EVC VI IR like me to confirm this is possible to set up?

I wouldnt be pushing more than 1.2 bar.....unless your prepared to take a chance on failure

Anymore power required i would be going small single setup.....but thats just me

Your tuner shouldn't be pushing anything with whatever you've asked for, he should be monitoring temps, knock, fuelling etc etc, and set everything at the correct levels for the highest gain, but keeping it within normal operating limits.

Mark Dance (Pulley) achieved great gains with his custom free flowing setup, but you'll never see anywhere near the same power on stock twins, as you will with a decent single setup.

That being said, the way the twins deliver power is not to be sniffed at. I actually miss mine at bpu sometimes, as the car was a pleasure to drive. I went single to simplify everything, as I knew that 20 year old components would eventually fail.

You've also got to factor in the auto transmission. If you want to push past bpu, I would strongly advise you shim the box, run an additional cooler with a thermostatic switch, and do a full flush and fluid change.

Defo worth tweaking, with a good boost controller (or via a bleed valve setup) the first turbo boost pressure. I think the stock boost pressure on the first turbo is 0.7 and when you then hit 1.2 you get a vicious transition. Just getting the first turbo to 0.8/0.85 makes a nice difference in the lower revs (and let's face it we spend a lot of the time) and smoothed the transition, it's like a stock car just more everywhere.

When I had the bone stock and the bpu one originally the bpu one was disappointing on the first turbo.