If I go on the brake right now the engine will rev to about 2800 and just sit there at zero vac/boost. I've swept the timing from 0 to 40 while on the brake, and it doesn't make much difference. Always sits at about 2800 with zero vac/boost.

A dump or looser converter would let it come up to more RPM which would obviously help bring the turbos online.

With the nitrous on, the engine makes enough power to push past 2800 RPM while on the brake. I've been on the brake at 3500 RPM against the two step, with the nitrous on, and it still sits at zero vac / boost. I was afraid to try winging the timing around on the two step while spraying it.

I think I could go to 4500 on the brake with the nitrous on, so maybe that'll be where I wind up. I'll let everyone know how it goes.

 

Sorry, but this statement is a bit confusing so just to make sure I understand what you did: The 2-step Rev Limiter for Launch Control is activated and the engine is "stuttering" at 2800 RPM, Correct?
If not, it should be.

In order for this to build boost the 2-step needs to be limiting the RPM so the motor is stuttering at 2800RPM.

If all you can do is 2800RPM then activate the 2-step rev limiter to 2700/2800RPM.
Now when you get on the brake the engine will start to stutter at 2700RPM. This alone "MAY" create some amount of boost, but it may not. So:

WHILE the rev limiter is limiting the RPM the TIMING is changed to 0* to 20* After Top Dead Center (ATDC) NOT Before Top Dead Center (BTDC).

Having the timing firing AFTER top dead center will cause the explosions to happen with the exhaust valve open and all this explosive energy spins the turbos faster thus making boost.

This is what the car should be doing when on the brake:


****
My car also has a 2800 stall and I can only foot brake it to 2800/2900 but when "Launch Control" is active the 2-step stutters at 2800 and the timing goes from +30* of timing to -20* After top dead center NOT Before top dead center. With this going on I can get about 4psi of boost. Not much and not enough for me but I'm sure it's b/c of the low 2800RPM stall. IF I could get around 3500-3800 stall I could probably get a lot more boost.

With this said the car goes to the dyno in a few days for a re-tune and I'm going to see of the tuner can figure out how to get more boost while on the 2-step.

Sorry if all this is trivial but I wasn't sure from your comment if you had the timing set correctly while on the brake.

***

I would also bet that IF the nitrous is flowing while the 2-step is activated with the timing around 0* to 20* ATDC there would be a LOT of fire/flames out the exhaust to spin those turbos.

Some people will say that shooting flames out the exhaust can hurt the motor or turbo but I have also been told that in order to do damage it takes a minute or two on the 2-step to do that kinda damage so a lot of racers don't worry about it.

KS

 

I'm using a Holley Terminator X, so I'll define some terms using their terminology.

Rev limit - Max RPM, this is 7200 in my car.
Rev limit 1 - This is the rev limiter that is engaged when the transbrake is on. Often called the two step.

Things I've done with the car and the results - If I don't mention timing adjustments, timing is approximately 25 degrees during all phases of each test.

Foot brake - about 1500 RPM before it starts to do a burnout. Rev limit 1 inactive.

Transbrake - rev limit 1 inactive. No two step. No stuttering. At WOT it sits at 2800 RPM, zero boost / vac.

Transbrake - rev limit 1 set at 2800. At WOT it arrives at 2800 and stutters against rev limit 1. Zero boost / vac. It's not hitting rev limit 1 very hard.

Transbrake - rev limit 1 set at 2500. At WOT it arrives at 2500 and stutters pretty hard against rev limit 1. Zero boost / vac.

Transbrake - rev limit 1 set at 2500. Timing set to zero degrees once it engages rev limit 1. At WOT it arrives at 2500 and stutters pretty hard against rev limit 1. Zero boost / vac.

Transbrake - rev limit 1 set at 2500. Timing goes to 40 degrees once it engages rev limit 1. At WOT it arrives at 2500 and stutters pretty hard against rev limit 1. Zero boost / vac.

Transbrake - rev limit 1 set at 2500. Timing is 40 degrees when rev limit 1 is not engaged, and goes to zero 1 second after rev limit 1 is reached. At WOT it arrives at 2500 and stutters pretty hard against rev limit 1. Zero boost / vac.

Now with nitrous, 125 HP dry shot, Holley is adding 44 lbs per hour of fuel, and using closed loop correction. Closed loop fuel correction is turned off when rev limit 1 is reached.

Transbrake - rev limit 1 set at 2800. At WOT the nitrous engages. It arrives at 2500 and stutters pretty hard against rev limit 1. Zero boost / vac.

Transbrake - rev limit 1 set at 3500. At WOT the nitrous engages. It arrives at 3500 and stutters pretty hard against rev limit 1. Zero boost / vac. In this configuration the car goes 1.5xx sixty foot and runs 10.0's at about 140 MPH. Peak boost is 16 PSI. The nitrous is programmed to shut off at 10 PSI of boost. This happens 2.5 seconds into the run.

Transbrake - rev limit 1 set at 3500. At WOT the nitrous engages. Timing is increased from 25 to 30 degrees before and during rev limit 1. It arrives at 3500 and stutters pretty hard against rev limit 1. Zero boost / vac.

Transbrake - rev limit 1 set at 3500. At WOT the nitrous engages. Timing is set to 30 degrees before rev limit 1 is reached. Once rev limit 1 is reached, the timing goes to zero. It arrives at 3500 and stutters pretty hard against rev limit 1. Zero boost / vac.

You keep bolding your timing recommendation. I understand what you are saying about firing the spark after TDC. In the Holley software, they use a negative timing number to reflect that. -5 for example would spark 5 degrees after TDC. Just letting you know that I'm tracking with what you're saying.

I haven't tried going to negative timing yet.

I have also seen where you can throw some extra fuel at it while using negative timing while on the rev limit 1 and try to create a backfire in the exhaust. I have not tried this.

 

As far as the theory of the boost builder strategy, I think I have a good understanding.

Going to extremely low or negative timing starts the burn later in the 4 stroke cycle. That increases the amount of burn happening when the exhaust valve opens. That is supposed to add pressure to the exhaust system, which should help bring the turbos online.

The next phase would be adding some fuel, along with the negative timing, whilst bumping against a spark style limiter. The theory is that can / will create an exhaust backfire. In the case of a turbocharged engine, the backfire puts quite a lot of force on the turbine. Instead of the burn happening in the cylinder and the turbine getting a puff of the aftermath - the cannon fire method is igniting the fuel from dead cylinders created by the rev limiter inside the exhaust system, the resulting backfire puts more energy from the burn into the turbine - wicking up the turbine speed so that the compressor comes online and makes some boost.

I'm reasonably certain that's the idea behind the boost builder strategies we are talking about. If I'm wrong, please let me know!

 

You got it..

I just wasn't sure if that was how you tested it.

ks

 

I haven’t tried negative timing yet. Looks like that’s the route if I want the S369’s to work.

Also, I don’t think this is trivial. I appreciate yours and everyone else’s time on this deal. I think it’s cool that you felt like mentioning and apologizing for a perceived triviality, to me that reflect positively on your character; but as far as I’m concerned no apology necessary!

 

Re: nitrous and flames:

I agree that nitrous flames should be present and should spin the turbos up pretty quickly.

I’ve had the car on nitrous, against rev limit 1 at 3500 RPM, with zero total timing. The car has full exhaust, so I don’t know that I’d see the flames. None of my friends mentioned fire out the back, so I don’t think the candles were on enough to make it through the entire exhaust.

There are two boost activated cutouts about 18 inches from the turbine outlets. They open at about 4 PSI boost. If I could get to 4 PSI on the tram brake, the cutouts would open and there may be nitrous flames.

The old set up would shoot a little fire out of those cutouts during a pass or dyno run, but it had to be at night to see it. There was no nitrous on the old set up.

 

I haven't read everything you've posted the past few days, but 4 psi is a big number when you're trying to spool large turbos. Have you tried with the cutouts open from the start?

 

I would be curious to see what happens when you start with open cutouts like Mnlx said and also go negative on the 2-step timing. Maybe creep up the limit 1-200 rpm at a time until you get something you like?

 

Run shop air to the cut outs on the Dyno?

 

Solid. I was gonna tie them open with a metal zip tie, lol

 

just out of curiosity what valve springs are you using, did they change with the cam or are they still the same as before?

 

The springs are new. I went to TFS 205 heads for this build; new heads and new springs.

 

Really just seems like it needs more RPM. You've only got X amount of airflow through a 5.3L. You need a certain amount of air to get the turbines spinning with enough energy to push air on the compressor side. I just don't think 2800 will do it. Hell, I don't think 3800 will do it. You've got to move quite a bit of air and it seems reasonable to think it'll take 4500 or so to do it.

 

I agree that the turbos are gonna want rpm, there's really no way around that, but the nitrous is like adding 100+ cid at 100% ve. And it has some decent head flow. It seems lazier than it should be.

 

Couple points, not sure if they will be helpful.

1.) Def. a difference between mechanical cam timing and syncing the timing to the ECU. I’ve had issues with both. If the mech. cam timing was excessively retarded your cranking compression would likely be really low. Mine was in the 140’s and was 8.5* retarded from the cam card when installed “dot to dot”. After I degreed the cam in at +1, I then saw 190-200psi cranking compression and picked up 12mph at like boost (24lbs). I don’t think its cam timing, but you should still check.

If the timing is not sync’d to the ECU correctly you would likely be excessively retarded. You def need to do this! If it was crazy advanced you’d have melted down my now. I was about 12* retarded. So I was basically running 0-2* timing while in boost over 10psi. The obvious tell there was the glowing manifolds during a run. I picked up about 200whp after it was sync’d. I’ve built a ton of cars and never had one more than a degree or 2 off sync wise….

2.) Your converter is too tight still IMO and your launch RPM is too low. The reason you still 60’s so well IMO is the TH400’s crazy 1st gear. So do what ever works for your combo. Still, once it flashes and you hit 6k or so you should make boost instantly. And esp. with N20 assist. So I still think something is mechanically wrong… leaky bov? Piping etc..


3.) Unfortunately I’ve yet to see a set of SXE S369’s run well. The cast S366’s outrun them and then some. There is something goofy about that design. Its like they aren’t designed for low pressure ratio applications like twins on a V8… Where as they tend to shine on high pressure ratio small engine setups running high boost levels.

4.) Your spool times are NOT normal at all. Neither is the time it takes for you to hit peak boost. My cheapo cast 78/75’s with .96 T4’s on a lazy SBE 5.3 (STG2 summit turbo cam) will easily hit launch boost (what ever I want) in about 4 seconds. 1-1.5 sec into the run I’m at PK boost. Best of 5.50, but only at 15lbs now. A 17 stator is WAY too tight IMO. I run a 20-5 on my 9.5 PTC. I leave at 4000rpm flash to 5600-5700 and shift at 7500. I also pull timing at the hit to limit power. ( glide trans) 1.2x 60 as well. You don’t need 5000+rpm.

IMHO don’t band-aid the issue with nitrous, dump valves, boost builders etc. Get the right converter on there to start with and once you find your issue, you won’t need any bandaids. Can you send that 17 stator out for a reflash? My 20-5 has zero issues on the street. I’m not saying that will solve your issues but it needs done regardless.

Also unfortunately I’d get rid of those turbos. As mentioned, I’ve yet to see an SXE line perform well on a V8 of any sort. I wouldn’t down size the turbos with that goal. Just get the right ones. 78/75’s are cheap and have been bottom 7’s with room to grow. They are also available in mirror image which is kinda cool. Jose at forced inductions may also be able to rework your turbos or take them on trade for a set of billet S366’s. They have hit your goal and are proven. May give him a call.

As a quick example. This is a 5.50 run on 15lbs. 1.30 60'. I could have started spooling up on the first bulb but i wasn't racing anyone here. This is an 8lb 2 step on 4k pulling 10* out over 2 seconds. (non intercooled setup as well)

Might also check backpressure and pressure drop across the IC.

 

Turbo John came by and we messed with it some. With and without nitrous, we weren't able to achieve any meaningful boost on the brake at 3200 RPM.

I hit the drag strip with it last Sunday. I set the two step at 4K, and sprayed it while on the transbrake. Still no meaningful boost, about 2 PSI when I left the starting line.

Unfortunately, it lost a head gasket about 5 seconds into the run. I took it apart earlier this week, the block and head are both damaged pretty badly, so I'm sidelined until I get it all repaired.

 

Thanks for chiming in, I appreciate the advice!

I'm going to make some changes to the combination while it's down for head & block repairs, hopefully the next version performs better. GT35's are in my future, and maybe a cam shaft change. I'll report back once it's back up and running.

 

Just my .02, but I wouldn't go with GT35's. the 62mm exh wheel is too small. I ran a set on a 5.3. Not saying they can't make some power, but they run out of steam and stop making great gains per psi very early. You'd be overspinning the hell out of them for your goals.

Something was defiantly wrong with your setup. A 50 shot on the brake would light off my turbo in less than a second with a PTC 14-0 stator and a 4.8/s480. I was on the spray for less than 1 second until it hit 6-7psi and shut off. Once lit, it would make boost on its own from there. Don't under turbo the car for fear of not being able to spool a healthy set of twins. I wouldn't go any smaller than a 65mm exh wheel. 67/65's spool great. Also they have some new 70/70 t4 units. or even the 72/65.

https://turbo4less.com/product/vsr-70-70-billet-t4-copy/

 

They don't if you size them correctly. Someday you guys will start to see that a/r varies the turbos exhaust swallowing capability.

Not every turbo needs the smallest a/r housing on it.

With to small of a housing it chokes out and becomes the bottleneck, not the turbine wheel.