yenkomike

iTrader: (2)

looking for some feedback from guys with borg 76/92 turbos.

this has been an issue sence we have had it on 2 different engines and complete hot side changes.

here is the setup as it sits

370" new engine flat top diamond pistons
ls9 gaskets fresh stock 317 heads 10.5 compression.
ljms stage 2 cam . degreed to verify cam timing.
truck manifold forward and down manifolds 2 1/4 piping nice and short
no leaks borg etr ho 476r 76/92 1.25 t4 open 4" downpipe.
no wastegate leak or any leak of any kind.

here is what we get.
with a 5 second wot trans break hit with the 2 step at 3900 rpm the engine flashes to about 2800 rpm converter flash then builds boost slowly up to the 3900 rpm setting . and will make 4.7 psi boost in 5 seconds.

lowered the 2 step to 3000 rpm and it comes up and hits the 2step fast but will only build 1.4 psi boost in 5 seconds.

the afr is in the 13.5 area before the launch control hits.

my question is this right seems damn slow to me . my cast 80/83 on my nova is a little slow but not like this 76/92.

we have ran it on 2 different engines 2 different hot sides and 6 converters. and if the converter is tight enuff to hold full boost it spools like this. thanks in advance.

yenkomike

iTrader: (2)

here is a pic of the new hot pipes.

coltboostin

What is your timing ret. and fuel enrichment on your 2 step?

yenkomike

iTrader: (2)

the launch control has timing pull to -10.00 and it cuts spark but not the injector for 2 step function.

when getting to the launch control rpm of 3900 it takes 4.7 seconds to build boost from 0 to 4.6 psi with no 2 step just wot and trans break at 12.6 afr on e85 and 26 degrees timing while building. . . I have triple checked everything and there are no exaust or boost leaks. this seems really slow for a 370" 10.5 to 1 engine.

it makes power it ran 10.32 last year at 137 mph at about 4300 to 4400 lbs race weight it is a 2003 rcsb Silverado. leaving at 5 psi or so on the break but still does not get to full boost until 2nd gear or 100 ft out or so.( old hot side and 6.0 with 218 jarrett cam. )

3 window

iTrader: (1)

Mike, do you have the ability to get up on the two step BEFORE you pull the timing out? Here's what I do; I footbrake it up to the point that the map sensor "stalls out" and/or won't build anymore boost. Then I hit my button which jerks the timing down from 32 to 15 degrees. Doing it like this builds boost fairly quick the way I have it tuned right now. I can build 7 psi in about 3 seconds. I haven't tried throwing more fuel at it or trying more or less timing as this way seems to work. If you pull the timing as soon as you hit the t brake button, I think it'll be hard to get it up on the converter quickly and build boost because of the lack of torque. Again, this may or may not work for you.

Nitroused383

iTrader: (10)

You need a ptc converter. 2800 rpm flash is pretty low but with careful tuning I've got my Neal chance to spool the same turbo from a 2500 rpm flash. I spent a lot of time tuning on the transbrake and I can make 8-15 psi at a 3200 2 step setting. I still ordered a ptc and can't wait to see how fast it spools. My Neal chance is a great converter with only 1-2% slip up top around 850 rwhp but it's stupid tight down low, it's also a nitrous converter so that doesn't help.

Best spool I could get before I messed with the tune alot.


After trying a bunch of strategies of when to pull timing and how quickly and how much fuel to add. I cut my spool time in half.

Nitroused383

iTrader: (10)

Also make sure you wastegate is not cracking open early that will kill your spool.

yenkomike

iTrader: (2)

this one is not to much different than your first video. it is not a real issue at the track. but street driving sucks it takes forever to get spooled from a roll. I do think the converter is pretty tight.

mkvamso

iTrader: (2)

i have a 347 with an s476r, 96mm turbine
with a ptc 9.5 inch converter i can make 12 psi in less than 2 seconds on the brake

yenkomike

iTrader: (2)

after some logging and testing it looks like the converter is right at 2800 rpm . it is a coan that ptc did a restall on. and seems to hold well up top with a shift drop down to about 5400 .

interesting think is we checked last years logs and the old sbe 6.0 with 9.5 to 1 compression and stock48 218 cam was a little stronger out of boost and was getting almost 3000 rpm converter flash.so even though the new engine is 10.5 to 1 compression the ljms stage 2 gives up a little power down low all things being equal.

sbcgenII

iTrader: (1)

I would expect a cam with a earlier IVC to be stronger down low.

Nitroused383

iTrader: (10)

When I get a chance to get on my laptop I will show you how to get it to spool on the trans brake. Mine spools pretty good while driving but it's not as quick as my precision 76mm. But I'm super impressed this billet s476 is a straight beast in my car @ 3600 lbs it has ran damn near an 8 second pass on 15 psi.

3rd gear spool

Forcefed86

iTrader: (4)

Sounds about right to me. The stg2 LJMS cam isn't setup to make power down low where that converter is stalling. Loosen the converter or put in a cam that makes power where your stall point is. (or 50 shot? )

LilJohn

Merged Hotside, Not fully divided if its on the hotside in the picture.

That right there is costing you 500 rpm of response.

Get rid of that converter and get a 9.5 inch from PTC.

yenkomike

iTrader: (2)

your saying a devided hot side one to each side of a t4 split flange will spool 500 rpm faster than my 2 1/4 hot side that merges to a single 2 1/4 ?

if that is what your saying I will build one and try that out.

coltboostin

Have you done a back to back on something to show this? I asked about this a while back, and we met with a bunch on internet scientist saying that "it work work right" since its basically impossible to "pair" the cylinders "correctly".


But, in practice I cant see how it wouldn't be a benefit.

RonSSNova

iTrader: (3)

This isn't really going to help because mine is a 76/87, divided hot side.
It actually spools too fast for my home track.

1CAMWNDR

iTrader: (21)

(2) 2.25"s merge into a SINGLE 2.25"? That is probably all of your problem right there. EITHER have split 2.125"-2.25" to each side of the divided housing, OR use a 3" merge pipe to the whole housing.

coltboostin

Good catch!

93camaro_zzz

iTrader: (13)

Dang, you have a single 2.25 feeding your T4 flange? If this were my car, I wouldn't be running a pipe that small. People can preach about the small pipe philosophy, but obviously there is a point of diminishing returns. I am willing to bet this is part of the problem.

Back when I had an undivided Garrett T6 turbo, this is what my hotside looked like. It was 2.5" pipe. Spooled fine. Now with the S480, divided turbo, I have remade the hotside to be true 2.5" divided. Still spools fine.