03 Trailblazer Build.

Nothing super duper exciting to tell, ive been doing transmission tuning as well as fuel/spark tuning, optimizing the truck.

These atlass 4200s were quite ahead of their time for GM. They utilized VVT, known as variable valve timing... which is controlled hydraulically. If youve ever rode in or drove a honda with VTEC, its very similar. The VVT essentially manipulates cam overlap.

Its only on the exhaust cam, but it really allows the truck to FLY at the higher rpms, as the base VVT angle is set at 12° in the 4500-6500 rpm range. That stock setting alone feels like a mini turbo kicking in. I played with it a bit today and got it ramping in 17-22° along that rpm range and man does that truck rip.

My wideband o2 sensor for whatever reason is buggy, so i may just buy a new one from AEM. Its the old turbo lumina sensor/gauge, so i havent fiddled TOO much with higher rpm fueling and spark, as i have no way to see where i am in relationship to the commanded fueling... so i'l order one soon.

Im quickly running out of overhead on those stock injectors, theyre 26 lb/hr injectors and im presently at 90% duty cycle around 6k rpm.

Ive sent it on to 6500 and the thing STILL seems to be pulling. Perhaps i'll get brave soon and send it to 7,000... im just afraid of beating up my transmissiom too much... we'll see.

Im dying to swap cams in this thing or play with E85 gas, with the 10.2:1 c/r, ya gotta keep timing low at peak Tq, for fear of spark knock, where as some E85 would really liven it up... but my injectors would max out.

Perhaps i'll look into throwing a set of 60 pounders in ive got laying around.

Well ive been further playing with it, and today i finally maxed the injectors out (104.6%). I can probably enlean the fuel mixture at 6300 rpm, as im targetting mid-11's afr... which would spare a little injector duty cycle, but im running 89 octane and trying to keep the pistons cool.

Im sortof scared to go for more like 12.x afr (leaner) because of the 89 octane, but maybe i'll try it.

Either way, theres no real way to prove this without a dyno, but maxing the injectors out over 100% is approx a 300+ hp crank output.
Thats probably damn near close to the most i'll be able to wringe out of the motor without a power adder or E85. 20°-22° of spark advance. It seems to knock if i get too close to 24-26°

On the monitor, im closely estimating Torque by calculating cylinder airmass, so at 4800-5600rpm , i was seeing a consistent peak torque, somewhere around 275-300 at the crank. Even at 6300 cylinder mass just drops a little. This things a torque monster!

Try 92 octane for fun. Some engines don't care either way but some really react to it.

squaredeal91 said:

Try 92 octane for fun. Some engines don't care either way but some really react to it.

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Im thinking about it! If i ran 93, i could probably put a few more degrees of advance in at peak TQ and HP, and lower the fuel mixture a little leaner...

These things really like E85 as i understand, i just havent given E a chance yet, mostly due to the injector maximum.

I think this weekend im going out to a local junkyard and pulling a 06+ head, cams, and exhaust manifold.

Couple pics from the 4200 website .. 02-05 manifold next to a 06+

02-05 vs 06+ valve specs


The 02-05 stuff is inferior in terms of cfm they can move in comparison to the 06+ stuff stock, and evidently with some porting, you can get further performance with them. Im thinking stock its a decent hp bump if you just slapped them on and did nothing (no tune, no porting). Its even greater with a tune and some port work.
that coupled with the 06+ cams Will really, really liven it up. The major change on the 06 cams are slightly more exhaust duration, a good bit more intake lift, and slightly more exhaust lift.

Following along on this one.

You likely have run across it already, but what made me aware of, and interested in the 4200’s was Corey’s Checker cab he swapped one into:
It’s by no means a how to series, but some good data points on what can work with them.

Corey had sold it and the new owner took it too a sloppy mechanics dyno day. Made 388hp at the wheels on 10lbs of boost

Unfortunately it was wrecked a year or two ago, so no more turbo checker.

legopnuematic said:

Following along on this one.

You likely have run across it already, but what made me aware of, and interested in the 4200’s was Corey’s Checker cab he swapped one into:

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It’s by no means a how to series, but some good data points on what can work with them.

Corey had sold it and the new owner took it too a sloppy mechanics dyno day. Made 388hp at the wheels on 10lbs of boost

xc_hide_links_from_guests_guests_error_hide_media

Unfortunately it was wrecked a year or two ago, so no more turbo checker.

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I have not seen that! Sure does look cool. I wish the 55' was road ready, as i'd love to slap a 4200 in it with a spooli boy.

Right now im doing what i did with my turbo lumina. Running it thru stages,
First a stock tune, then bigger injectors, then some sort of factory upgrades, and lastly, a turbo

The biggest problem with a turbo on the stock TB is the front end shock tower is almost right next to the manifold, so i have the choice of putting a 1700$ artec performance manifold on (fits the tb stock), or welding up my own. I got some CAD concepts already sketched up on the computer for a home grown turbo manifold.. i guess i gotta decide what to do. I'd really like a 2nd motor or a bare block i could use for mock up

Something like this is what i have in mind. The shock tower is my first obstacle, the second is the AC lines coming out of the firewall. I'd like to be able to clear both of those...

DIY youre looking at <300$ in materials and alot of time

Vbb199 said:

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Just my ignorance showing, but is 43.5 the proper fuel pressure? I thought GM did 58 for most stuff of this vintage? If it is 43, that might be a cheap fix for maxed out injectors to swap in a 58 psi regulator (unless it's one that is regulated in the tank... ugh)

Hunter79764 said:

Just my ignorance showing, but is 43.5 the proper fuel pressure? I thought GM did 58 for most stuff of this vintage? If it is 43, that might be a cheap fix for maxed out injectors to swap in a 58 psi regulator (unless it's one that is regulated in the tank... ugh)

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Yea, the pressure sure is 58 or so, but most all injectors have a "rating" at which they flow, which is typically 43.5 psi... most of them anyways..

The injector data in the PCM is based off 43.5
Theyre rated as "26 lb/hr @ 3 bar (43.5psi)" from delphi or whatever

If i plugged 58 psi into the handy dandy calculator, that'd be 360hp @ 100% duty cycle , and i'd really have a hard time believing an unopened 4200 is putting that much out haha

Edit:
I guess i need to add, so you dont think im disagreeing

If you take 26lb/hr injectors, and put 58psi to them, thats 115% more flow than 43.5psi

The formula is sq-root ( new pressure/old pressure), which is 1.15

26 lb × 1.15 = 29.9 (30lb/hr)

If you take that 30lb/hr @ 58psi and put it into thr calculator, then you'd end up somewhere around 80% DC @ 315+ hp

Thats probably in actuality where i am on those injectors, as i hit 115% injector duty cycle yesterday on a steep hill climb in 2nd playing around and still didnt see any knock lol. The PCM's outputted duty cycle is based on 43.5 though, if that makes sense to you, i just hate running things so close to their max.

It could also be the pump i have in the tank. I had to put a stock pump back in... i was having issues with that bigger pump.
Theres also something regarding once you exceed 80% of idc, the linearity of the injector deviates and all your calculations can get fudged, and it starts putting wear on the PCM from the amperage draw i guess. They need a cool down time between cycles and larger pulse widths at higher rpms dont allow for that

theres also the concern of elevation
I live around 700ft here. So close to acceptable tuning conditions, but if i went elsewhere where the air was more dense or what have you that affects barometric pressure, i might could max these things out for real, and theres still not enough room to play with E85

Vbb199 said:

Yea, the pressure sure is 58 or so, but most all injectors have a "rating" at which they flow, which is typically 43.5 psi... most of them anyways..

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Thats 58 psi regulated @ the rail with a manifold referenced regulator

Hunter79764 said:

Just my ignorance showing, but is 43.5 the proper fuel pressure? I thought GM did 58 for most stuff of this vintage? If it is 43, that might be a cheap fix for maxed out injectors to swap in a 58 psi regulator (unless it's one that is regulated in the tank... ugh)

Click to expand...

I did forget something you just reminded me of though

Decapped, the 26 lb'ers will flow at 75 lb/hr
Im just worried about doing that

Vbb199 said:

Yea, the pressure sure is 58 or so, but most all injectors have a "rating" at which they flow, which is typically 43.5 psi... most of them anyways..

The injector data in the PCM is based off 43.5
Theyre rated as "26 lb/hr @ 3 bar (43.5psi)" from delphi or whatever

If i plugged 58 psi into the handy dandy calculator, that'd be 360hp @ 100% duty cycle , and i'd really have a hard time believing an unopened 4200 is putting that much out haha

Edit:
I guess i need to add, so you dont think im disagreeing

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That makes sense, just wanted to make sure you were accounting for that and didn't just forget to increase the fuel pressure or something. I'm following along and enjoying what you're doing, keep up the good work!

Hunter79764 said:

That makes sense, just wanted to make sure you were accounting for that and didn't just forget to increase the fuel pressure or something. I'm following along and enjoying what you're doing, keep up the good work!

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GM has/had some strange things in this PCM. Starting is the hybrid Alpha-N / Speed Density management type...
THe LS stuff and v6 stuff of the same era was speed density (MAP sensor based) or MAF based (im sure you know what the maf is) ... these 02-05 4200's use Engine Load vs RPM for their part throttle and high rpm tables, then they use MAP for idle and spark control

Engine load is defined in seperate parameters based off of throttle position which GM again didnt configure it like 10% throttle = 10% engine load; but instead 10% throttle meant something else... and so forth up to 100% throttle

Strange... given my experiences tuning mostly V8 and V6... this is difficult to tune, but ive got a pattern down now and know how to.

If thats not weird enough, the way the injectors are setup from the factory is strange

Theyre setup as 26lb/hr @ 43.5 psi, but the fuel flow table in the pcm is arranged as if the regulator had no return... even though the truck DOES use a return.. beats me why gm is doing that. Ive checked pressure at the filter (underneath the damn car is where the valve is) and it shows 58 psi

Ive changed the table since then to behave like a return-equipped regulator but as to why GM did any of this beats me