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Boost threshold and cam questions.


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My plan was to move it one, drive it, then move it to the next if need be. I guess the one hole will do it, since he told me about 4deg.

Save yourself some time and go straight to 3. If the power peak becomes too low in the rev range for your taste, then go back to 2, but I doubt you will. 4 deg (one hole) isn't that much of a difference.

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I'm not going to have any interference issues with 8deg cam advance, am I?

I never experimented with advancing it when I ran that cam so can't answer that one, although I don't think so - I only tried retarding. I was running a 4.11 with an 81-83 5 speed so only cared about power at higher revs.

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  • 2 months later...

Update:

 

Advanced the cam to to the 3rd hole, or 8 deg. Now I have a lumpy idle around 500rpms and low rpm stuttering. I'm planning on going back to hole 2, unless there is a solution to my problem within the stock efi. Should I have adjusted my ign timing as well?

 

On a side note, why does everyone seem so scared of the timing chain? Took 1hr tops, and it was my first time.

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What do you mean by boost threshold? If you're referring to the rpm when the turbo begins to build boost, then both RPM values sound too high. But if you're (improperly) referring to creating full boost, then the cam, along with the cam timing, is definitely a big part of it. You should verify the cam timing is correct, or even advance it some with an adjustable cam gear to generate more boost at a lower rpm if you prefer that.

 

Keep in mind that this cam should make good power to at least 6500 rpm, which is significantly higher than for the stock cam.

 

Zmanco, your definition is incorrect. He CORRECTLY defines "Boost Threshold" as the point in the RPM range where the turbocharger give FULL BOOST. This is the DEFINITION of "Boost Threshold"

 

Boost Threshold has NEVER referred to when the turbocharger 'starts' making boost, it has ALWAYS referred to the point when FULL BOOST is available.

 

Turbo Lag is the time, ABOVE BOOST THRESHOLD when the engine is given WOT that it takes for the Turbocharger to go to FULL BOOST.

 

The universal incorrect assumption is that 'boost threshold' is when the turbo makes ANY boost at all, and then by continuation, the time between production of boost, and hitting of full boost is then incorrectly referred to as 'lag' --- it is NOT, this is improper driving technique.

 

The application of the turbo to the ZX was an effort to aid in terrible driving habits of Americans (and drivers in general) who expected power before the STOCK camshaft 'came on' around 3000-3500 rpms. By boosting at 2500, the car pulled harder below the stock cam torque peak and acted like a much larger engine. For the European Market, due to the higher speeds driven, the stock 0.63 A/R hotside scroll was substituted for a 0.82 A/R with correspondingly higher boost threshold.... no longer did it have full boost available at 2500, it was higher in the rpm range, as European drivers have more experience with 'proper gear for speed' driver's instruction and universally did not have an issue keeping the engine "on the turbo" in the same manner they didn't have a problem keeping the car "on the cam" by driving the engine properly in the correct gear for speed to allow instant response upon WOT.

 

THAT BEING SAID, the delayed boost threshold is an issue... I would have checked the timing of the cam to the cam card provided, and then played using an adjustable gear (Tomei) on the dyno to find where the sweet spot was for the engine.

 

Remember that valve adjustment plays a big part in the stock EFI at idle, and if you have overlap on the cam, your vacuum goes to hell, causing a rich condition at idle from excess fuel pressure and from reversion. Especially with a turbo corking the exhaust side.

 

Give the engine more air at idle with more idle bypass screw opening (if its stock turbo, take the cap off the AAC and idle it up using that screw, that is what it's there for! If it's an NA Converted, open up the idle air bypass screw and idle it up.) With a cam you may want to idle at closer to 900 for proper vacuum and idle fuel pressure.

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Zmanco, your definition is incorrect. He CORRECTLY defines "Boost Threshold" as the point in the RPM range where the turbocharger give FULL BOOST. This is the DEFINITION of "Boost Threshold"

 

Boost Threshold has NEVER referred to when the turbocharger 'starts' making boost, it has ALWAYS referred to the point when FULL BOOST is available.

 

Tony, I was using Corky Bell's definition from his book, "Maximum Boost".

 

From the glossary on page 235:

Boost Threshold or Boost Point: This is the lowest engine rpm at which boost from the turbocharger will increase power over the engine's atmospheric equivalent. More simply, the lowest rpm at which noticeable boost (usually 1-2 psi) can be achieved.

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Have you had a chance to take a look at the wastegate yet?

 

It seems to me that the symptoms you're describing could be caused by a wastegate that is ever so slightly open at rest. I'm not familiar with stock 280ZXT wastegates, but I imagine there is a threaded rod from the canister to the valve. If not, then a bendable one.

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Checked the car out this morning. Removed that b*tch cotter pin, and found the wastegate was being held 1-2mm open. Tweaked the bracket a bit so the arm would line up with the wastegate being closed, and put a simpler cotter pin in.

 

I get max boost at about 28-2900rpm. I forgot how fast this car was! Boost still comes on a little soft, so this weekend I'll be adding a poppet mbc and referencing the manifold.

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The one I recommend is no longer in print. And when it was, it was $184.00 as an undergraduate text for engineering students.The last time I mentioned the title someone posted a link to a used books site that had it available for somethng like $745.

Simple title, produced as new technology including GM electronic boost control PIDs, and Rally Anti-Lag...now terribly outdated (As Corky's book was back in the 90's) by advancement in five axis milled impellers and anti surge wheel profiles emerge.

"Turbocharging the Internal Combustion Engine"

I loaned my copy to someone I trusted and the MF absconded with it. And if I ever run across him, I will kick the living shite out of him for ripping me off! That book at the time represented close to 25% of a month's salary.

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BLOODY HELL $949!!!

Follow that link, and there is probably a likely supplanting text that looks promising, only $135...

I would say its better than Mr. Bell's book, which was published first around the publishing date of the one I referenced... And it was chock full 'o 70's Technology from an American Viewpoint...

***CRAP! Reading the reviews, most guys say that the new book (link below) is somewhat hurried, and that the original book by Watson should also be reviewed and if possible used as companion texts. The guy that wrote it carries on Watson's work in the short seminar circuit on turbocharging...

My suggestion, buy it (damn I'm going to...) AND THEN DON'T LET ANY F****R borrow the damn thing and then find out 20 years later the book you bought from OPMAP Technical Books (it was 7 or 8 years in print when I bought it in 89 or 90) is now selling for a GRAND!!!!! Buy this one now, before it's a grand, too! Or $749...

The New $135 Book

 

Edited by Tony D
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My biggest beef was the perpetuation of the American-Style of performance being applied across the board to engines that generate excitement.A good example is most VG 30's out there, power peaks in the 5800 rpm range and "just boost the hell out of it" thinking. All well and good I suppose it gets the job done.But for the L - Series in Particular, I feel the L24 has a more exhilarating power delivery than an L28. The Japanese stressed porting and camshaft selection on all turbo cars, similar to my exposure to VW's being turbocharged before I was into Z's.

The Bell Book just wants you to throw boost at the L28, and IMHO makes for a slug of a boring engine.

Yes, you can have a 380HP engine using all stock parts like suggested in The Bell Book. It IS a cheap way to do it, pump it to 20-25# and there you have it.

But the same L28 making 380 HP on 8# of boost will be a totally different car to drive than the one that takes 25# to do it.

Basically I'm of the opinion that if I wanted a Corvette, I'd buy one. Or put a V8 in the car for that kind of driving.

Where I give credit to Mr. Bell is he made a damn nice compressor bypass valve. But the rest of the kit as we have seen through subsequent analysis, is a bit hamfisted in its approach. It works, but it can assuredly work better.

The sad part for me is the " finesse" of the application is lost within Bell's book. Ad since its the newest one out there, it's looked upon as some sort of "Bible" and that is definitely is NOT!

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  • 3 weeks later...

Wow tony, do you know something about compressors or something? It's almost as though you do this stuff for a living.... :-P

 

28-2900 sounds a lot closer to what it should be for boost threshold for your setup, but I'd say you should be able to get it even lower than that with simple tweaking.

 

Lower the backpressure with a 3" downpipe with 45 degree bends instead of the stock 90 and run 3" all the way back while you're at it. And make sure it's done with mandrel bends, not crush bent as a 3" crush bent is more like 2.5" mandrel bent. Run a straight through muffler. It's going to be loud. Deal with it, it's a damn turbo car, be proud of it. :-P Then run as close to rich best torque as you can get, even a bit extra if you can down low off idle for WOT.

 

Fight detonation with as much heat management as you do spark management. Detonation is directly related to heat in your air charge, be it from chamber temp, ambient temp, or compressor outlet temp. Get temps under control and detonation follows suit. This will allow more aggressive timing, and faster spool times.

 

If you have your exhaust manifold out, port it. Port it. Port it. There's lots of excess meat in there that can be smoothed out and not just help overall flow, but direction of flow into the turbine.

 

If you want to get fancy, add water/meth injection for WOT use. Not only will it massively bring down charge temps, but it will add volume, and thus mass, to your exhaust charge, exerting more force on the turbine wheel equating to faster spool. I've noticed switching to E85 can have as much as a 10-15% reduction in boost threshold for a given setup. Water/Meth can have similar results.

 

 

Quick story... I took a ride in one of my best friends STI soon after converting to E85. We went for a spirited run and were taking a u-turn to head back. The light we were at was 2 lanes each direction and we'd been stopped at the light. He puts it in 1st, pulls away from the light casually getting up to about 2,500rpm at MOST, drops to 2nd, mashes the throttle, and snaps the clutch all in one quick smooth motion. The car almost INSTANTLY slid ALL FOUR WHEELS and we went sideways a full lane plus! Yes... a 2.5 liter made enough power with a stock STI turbo to move a 3500 pound load sideways, overcoming the traction of 4 wheels with limited slip, all while coming off of NO LOAD!!!!

 

Turbo lag my arse. And that turbo isn't even ball bearing. It's single scroll. The only reason it can do as mentioned above is that it is pretty much DONE at 500hp, a mere 200hp per liter. The other benefactor is that it was E85. There's also a third, often overlooked, factor in this scenario... everything was STOCK, and therefor WELL MATCHED as a SYSTEM. Stock intercooler, stock exhaust manifold. He was running larger exhaust turbo-back too.

 

I've often felt that a well prepped L28 with a T3 should be able to reach 1psi in 1st gear by 1750rpm. It just takes the right setup. And I also understand that a setup that does that probably won't reach over 350whp very easily. But who cares? When I finally get my S30 back on the road my goal in the first year is just 250whp with a 2300lb curb weight with a full tank. That's more than enough for my stock brake calipers and rotors and tiny 215 treads. Eventually I DO want to build a 500whp monster, but I also want to do it with a setup that nobody else I know has done. We'll see if I ever have the money for it though, and if I still want to once I do have the money.

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My bud Keith had a Pulsar GTi-R that wasn't boosting... Called me over to diagnose it. Signal Out line was off the HKS EVC.

I put it back on, double zip-tied it in place and told him take it for a ride. He wanted me to do it... Nope bud, been here done this, take YOUR car out for a spin. He did similarly, short shifted to second and was feathered because of oncoming traffic, as he got into second he went WOT...FORGOT He had left the EVC in "Scramble Mode" and 1.5 KgC slugged those wheels hard...car stopped moving and started crabbing sideways burning the tires in an intersection till it started bouncing on the rev limiter.

 

After that, we realised something, EVERYBODY who drove that car, suitably warned, seemed to have their grin attached to the tach in second gear...when boost came on you just started grinning like an idiot wider and wider as it went to redline. Then it just stayed there for the rest of your sprint.

 

Man, that was a fun car...

 

Gollum, PM me before you spend your Money on the setup nobody has done! I'll betcha a buck someone has at one time or another!

 

Oh, and 1750 was where my T3 Hybrid with the 0.48 Hotside A/R could make 17 psi! All naysayers aside, if you do a "Bell Build" using the smaller A/R and modern wheel cuts, you aren't out of turbine flow till over 6000 rpms, and have the ability to boost like crazy almost off idle. I had one Track Instructor repeating at the MSA Auto-X how he "don't like turbo cars because they have a non-linear power delivery" but once we lift the line he kept repeating "this is a turbo car?" And by the end of the course it was more like "THIS is a TURBO car? I GOT to dive this car!!!"

 

Remember JeffP had the same exhaust backpressure as inlet manifold pressure at 650+HP and 7,000 RPMs on his T3 0.63 Hotside Turbine. That is something the "old" books says only happens in F1 (dating their production/printing timeframe!) if Jeff can make 700HP on a 0.63 A/R turbine with that kind of backpressure... Seems clear to me larger A/Rs simply narrow your power and if you are unwilling to cross the mythical 7500 rpm zone. Jeff realises now there is more myth than facts out there regarding Turbo-L's and craves the day his forged bottom end goes back into the car to test his actual power peak on his setup. His terminal HP was turbo airflow limited. He was making similar power at 7,200 at 17psi as at 7,000 on 21 psi, or 6800 at 25 psi.

 

"And then the X-Series came out with more flow in the critical 18-25psi portion of the map allowing 100 more HP to be produced" and Jeff started looking for excuses to replace his current GT35R with one of those new X-Turbos...

Edited by Tony D
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