Quote:
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
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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