secondarries

[Corbusa]

Ok I'm no mechanic as far as the LT5 goes , and I wish I knew more . I personally do like the secondaries being switched because ( I think its cool ) , yet I also like as much power without hastles of vaccume leaks . At this point in time I'd probably keep them in place, port all I can to get some extra ponies, and find some sort of space shuttle,futuristic alien tech vaccume line materials and put them on the engine , hoping to settle the issues of vaccume leaks.
So I guess keeping it simple , If I can get close to 500 ponies with them in and not have a constant headeches with vaccume issues I'll leave'm in , If not I'll have Pete yank'em out when Its time. Then use the extra key switch for some NOS.

[FU]

And I thought that Pete took out the secondaries for weight saving's.....

[Hog]

Originally Posted by Pete

If it was for low speed torque then they didn't do a very good job.
My Z stock was peaking 330RWTQ @ 5200 now i make that at 2500 and peak is right at 400 @ 5200.
If you look at Z's with ported top end and no secondaries they make anywhere from 300-330tq @ 2500-3000rpm and close to 400 peak.
If you tune the car right you will loose nothing at low speed,my Z with huge cams for a 350 can cruise at 1300 rpm and get 24 MPG highway.
A lot more has been done with todays technology in the tuning department for our Z's.

If you ask most will post there before and after dyno sheets.

If it wasn't for guzler tax then it must have been for emissions,back in the old days when we had that thing they call emissions (rollers) i forgot my secondaries on it failed next day with no changes just kept secondaries off it passed.(same location)

On the C6 ZO6 mufflers there are half dozen for sale as we speak on top of the half dozen i have bought the most recent (2 weeks ago) the gentleman replaced them with B&B's,there's a few ZO6 guys that replace them,you might not be one of them.
Most car shows and dragstrips i go to i see most have replaced them.

Pete

Obviously Hib was talking about a stock application. Even Mark H admits torque loss under 1500rpm.

The secondaries are simply there to change the geometry of the intake manifold. Single small runner for low rpm torque, and 2 small runners for more upper rpm torque.power.

GM also used the variable geometry intake manifold on the 1992-95 CPI 4.3 V6. I'm sure there are other GM examples.

But of course ECM tuning will bring back some lost low rpm torque and throttle response.

In order to retain off idle low rpm throttle response, the single runner is used, this allows the engine to lumber around al low rpms, which allows for improved fuel economy and better emissions. Having low intake airspeed by having too much runner area (ie both runers open) at off idle rpms, in 4th gear(skip shift) will result in excessive throttle use and economy/emissions to worsen.

We are talking about 80's technology here, the LT5 ECM was very cutting edge with its injector control, but the calibration of the LT5 leaves much to be desired.
using todays ECM tuning and modern tuning techniques this can be overcome, but this ability wasnt available to the masses in the early 90's.
GM had to make bug power while meeting stringent emissions contraints. The engine must function in a variety of temps, humidities and altitudes and even in different countries with varying octanes and fuel qualities. All of these factors coupled with the technology at the time, all contributed to GM needing the primary/secondary runner/injector setup.
When the LT5 was being thought of, the wheezy TPI engine was top dog, it had great low/mid rpm torque, but was very wheezy in stock form, and only moderatley better on the 305.
GM had a choice, have a high revving engine, that had bad off idle/low rpm manners, would have poor emmisions and wouldnt satisfy the MAJORITY of drivers need for low rpm torque, or build an engine that did great at low rpm, but didnt have the power numbers that a flagship performance car needs.
The comprimise (if you can call it that) was the LT5 with its variable geometry intake. Decent off idle torque, with excellent high winding power numbers. Its like having the best of both worlds, being accomplished with what was available in 1980's engineering.

IMO the LT5 engineering challenge was akin to the STS-Space Shuttle program. GM knew they wanted a 400hp engine, but it had to fit in the Vette from underneath. Whereas, NASA wanted easy access to space, but were given parameters by the Gov. that the Shuttle had to fit. Like being more inexpensive(make it reuseable to be cheaper), in order for the Shuttle to get Airforce money, the shuttle needed wings to get cross range capability.
Lotus and NASA knew what had to be done, but were forced by outside influence to design a certain way.

Just imagine what the LT5 would have been if the car it was going into wasnt designed yet? There possibly could have been more room to fit in, therefore sprockets could have been larger, therefore increasing high rpm durability etc etc. Some big clues are found on the OBD2 version of the GEN 2 LT5. There was almost 10 years of tech. advancement between the 1st and 2nd GEN LT5 as well.
GM was able to satisfy power/torque needs, emissions(now OBD2) all without a variable geometry intake. IIRC they were using VVT in the GEN 2 OBD2 version of the LT5.

Sorry for the book, but I think that in a round about way, all of the reasons mentioned by EVRYONE previous are all correct reasons for the variable geometry intake manifold( dual runners/dual injectors per cylinder) that the LT5 uses.
Bottom line everyone is correct in some way because all the reasons mentioned are interrelated.

peace
Hog

[Pete]

Originally Posted by Hog

Obviously Hib was talking about a stock application. Even Mark H admits torque loss under 1500rpm.

The secondaries are simply there to change the geometry of the intake manifold. Single small runner for low rpm torque, and 2 small runners for more upper rpm torque.power.

GM also used the variable geometry intake manifold on the 1992-95 CPI 4.3 V6. I'm sure there are other GM examples.

But of course ECM tuning will bring back some lost low rpm torque and throttle response.

In order to retain off idle low rpm throttle response, the single runner is used, this allows the engine to lumber around al low rpms, which allows for improved fuel economy and better emissions. Having low intake airspeed by having too much runner area (ie both runers open) at off idle rpms, in 4th gear(skip shift) will result in excessive throttle use and economy/emissions to worsen.

We are talking about 80's technology here, the LT5 ECM was very cutting edge with its injector control, but the calibration of the LT5 leaves much to be desired.
using todays ECM tuning and modern tuning techniques this can be overcome, but this ability wasnt available to the masses in the early 90's.
GM had to make bug power while meeting stringent emissions contraints. The engine must function in a variety of temps, humidities and altitudes and even in different countries with varying octanes and fuel qualities. All of these factors coupled with the technology at the time, all contributed to GM needing the primary/secondary runner/injector setup.
When the LT5 was being thought of, the wheezy TPI engine was top dog, it had great low/mid rpm torque, but was very wheezy in stock form, and only moderatley better on the 305.
GM had a choice, have a high revving engine, that had bad off idle/low rpm manners, would have poor emmisions and wouldnt satisfy the MAJORITY of drivers need for low rpm torque, or build an engine that did great at low rpm, but didnt have the power numbers that a flagship performance car needs.
The comprimise (if you can call it that) was the LT5 with its variable geometry intake. Decent off idle torque, with excellent high winding power numbers. Its like having the best of both worlds, being accomplished with what was available in 1980's engineering.

IMO the LT5 engineering challenge was akin to the STS-Space Shuttle program. GM knew they wanted a 400hp engine, but it had to fit in the Vette from underneath. Whereas, NASA wanted easy access to space, but were given parameters by the Gov. that the Shuttle had to fit. Like being more inexpensive(make it reuseable to be cheaper), in order for the Shuttle to get Airforce money, the shuttle needed wings to get cross range capability.
Lotus and NASA knew what had to be done, but were forced by outside influence to design a certain way.

Just imagine what the LT5 would have been if the car it was going into wasnt designed yet? There possibly could have been more room to fit in, therefore sprockets could have been larger, therefore increasing high rpm durability etc etc. Some big clues are found on the OBD2 version of the GEN 2 LT5. There was almost 10 years of tech. advancement between the 1st and 2nd GEN LT5 as well.
GM was able to satisfy power/torque needs, emissions(now OBD2) all without a variable geometry intake. IIRC they were using VVT in the GEN 2 OBD2 version of the LT5.

Sorry for the book, but I think that in a round about way, all of the reasons mentioned by EVRYONE previous are all correct reasons for the variable geometry intake manifold( dual runners/dual injectors per cylinder) that the LT5 uses.
Bottom line everyone is correct in some way because all the reasons mentioned are interrelated.

peace
Hog

Hog we are saying the same thing,about old technology what they had to do with what they had at the time.

We are also agreeing about technology has gotten better,performance has been vastly improved from 20-30 years ago and tuning devices have been improved.

I might have to disagree on this i don't think there is a loss below 1500 rpm if so what are we talking about? Again tuning comes into play.

If you as much as graze my gas pedal heck if you sneeze on it jumps to 1500 i will put my throttle response over any stock Z's thottle response.

I do have to say that i do idle at 900 with the big Stage III intake cams .242 .450 with .224 .420 exhaust in a 350ci motor.

When i drive my Z to Bowling Green(highway) i basicly leave her in 6th anything at or above 45mph she's good anything lower i'm at idle speed.

Pete

[RHanselman]

Just one data point from the guys working on the ECM at MegaSquirt. They found that keeping the secondaries closed at the lower RPM's helped them increase low-end torque by 40-50 at the wheels on my stock Ruby. They spent lot's of time maximizing the torque curve playing with the secondaries. He said they keep them closed a little longer than the stock tune. They also turn on the secondary injectors when the primaries get to a 80% duty cycle and then start ramping up the secondary injectors. He said that when the secondaries do finally open the graph on the dyno took a turn to the vertical... Ben from MS said several times during his phone report that they are very impressed with the LT5...

I'm sure the'll have more on the specifics during their presentation at BG...

Cheers,
RH

[Pete]

Originally Posted by RHanselman

Just one data point from the guys working on the ECM at MegaSquirt. They found that keeping the secondaries closed at the lower RPM's helped them increase low-end torque by 40-50 at the wheels on my stock Ruby. They spent lot's of time maximizing the torque curve playing with the secondaries. He said they keep them closed a little longer than the stock tune. They also turn on the secondary injectors when the primaries get to a 80% duty cycle and then start ramping up the secondary injectors. He said that when the secondaries do finally open the graph on the dyno took a turn to the vertical... They (MS) are impressed with the LT5...

I'm sure the'll have more on the specifics during their presentation at BG...

Cheers,
RH

Again tuning comes in play you can't just yank the socendaries and think all is good.

So my understanding is if you were to port the top end on that Z it won't be driveable since we'll loose so much more low end.

Ron, my heads are 270cc intake runner plenum/IH's ported to 38mm,63mmTB,no secondaries huge cams my Z should be needing a push at every stop light like a Formula 1 car but i make more torque at 3000 rpm then most stock Z's make peak,so if i'm making 300 RWTQ @ 2500, 250 RWTQ @ 2000 and for arguement sake lets say a stocker makes 50 less at those RPM's so my dyno sheet should read like a turbo car dyno graph at 12-1500rpm i make 100-150 torque but as soon as i get to 2000-2500 it would double and shoot up to 300+ torque so it would be a straight vertical line.

Brotha,i'm not smelling what anyone steped in.

Again i will put my throttle response over a stocker anytime.

Pete

[RHanselman]

Originally Posted by Pete

Again tuning comes in play you can't just yank the socendaries and think all is good.

So my understanding is if you were to port the top end on that Z it won't be driveable since we'll loose so much low end,it would be like a Formula 1 car it would need a push to get it going.

Brotha,i'm not smelling what they steped in.

Again i will put my throttle response over a stocker anytime.

Pete

Just reporting what they found but I know it's apples and oranges... My car is stock... No port work... I know you have perfected your combination w/tuning... You may be able to do the same or better with the stock car with your techniques but we won't know until we compare the two side by side.

[Pete]

Almost forgot or just remembered.

Ok, here's the pudding on the secondaries.
Secondaries will open anytime you go WOT no matter what RPM your at.
From idle to 7000 rpm you go WOT secondaries are open.

I don't see too many guys complaining of how there stock Z's don't pull from a stop when they go WOT.

I don't know how they tested the low speed loss cause if they went WOT secondaries are open or did they just keep them closed etc.

Ron i didn't mean that comment towards you or the shop "not smelling what anyone steped in"
I meant it as to either i'm missing something or something is wrong and i could not put my finger on it,getting older and forgetful.

Pete

[Paul Workman]

My stock 90 LT5 WOT (w/ secondaries) torque vs. WOT after porting both runners and SPTs removed.

As Pete said before, I too can't see where torque was lost on the low end - at least not above 2000 rpm...



YMMV...

P.

[RHanselman]

Originally Posted by Pete

Almost forgot or just remembered.

Ok, here's the pudding on the secondaries.
Secondaries will open anytime you go WOT no matter what RPM your at.
From idle to 7000 rpm you go WOT secondaries are open.

I don't see too many guys complaining of how there stock Z's don't pull from a stop when they go WOT.

I don't know how they tested the low speed loss cause if they went WOT secondaries are open or did they just keep them closed etc.

Ron i didn't mean that comment towards you or the shop "not smelling what anyone steped in"
I meant it as to either i'm missing something or something is wrong and i could not put my finger on it,getting older and forgetful.

Pete

Ya Pete I know you're not directing at me... Just realize these guy's throughout the book (or never had it) on how the secondaries work. Their techniques for opening are nothing like GM programmed (from what they tell me). This is what's been taking them the longest. I'll share their technique as soon as I understand it or maybe we can ask some pointed questions at BG.