zrracer

I have looked at some threads on a few forums. Most threads discuss the how to aspect but not the benefit side of doing this. Is it to get rid of some spaghetti and possible leaks or is their a driving benefit attached also? I also saw somewhere that I will need to get into the computer to turn something off. Will I need a tune as a result of this?.
TIA,

ZR-71

ccmano

Do some searching on this forum and over at the ZR1.Net forum. There are volumes written about this.

This should get you started.
https://www.corvetteforum.com/forums...d-systems.html
H

zrracer

I did see that how to.
I am looking for information on why to though. Maybe someone who has done this can tell me the results, the plus and minus exc.
Thanks for the link.

ccmano

For a stock displacement motor there is really no performance advantage. It simply eliminates the complexity of the system and potential problems. I am not aware of dyno results showing improvements. As these unique secondary parts become more scarce it becomes an alternative to keep the beast running. The effects on emissions are not clear. I have not heard of any California owners experiences with this mod. For the large displacement highly modified engines it makes fuel delivery and tuning simpler as I understand it. Again, in and of itself there is no performance increase but it aids in optimizing other performance mods like
Cams and bigger displacement..

At least that's my understanding, I'm sure much more qualified poeple than I can chime in.
H

zrracer

Thanks for that information. That is what I am looking for.
I have a 368 that makes 568. I do have 4 new cams, intake valves exc....
Am I looking at retune if I do this?
Thanks for the good info.

ZR-71

Then my question is, what advantage do the secondaries provide and why were they part of the LT5 in the first place?

ccmano

As I understand, it was done to meet fuel economy standards. Most importantly to avoid the "Gas Guzzler Tax" of the time. It's not just simply the fuel economy of the vehicle in question, it's also about the affect on corporate fuel economy of the entire mfg's offering. Seems to me this was addressed in the book "Heart of the Beast".
H

ccmano

Not sure I'm qualified to answer that question. In my opinion yes, a retune will be in order. Several others on this forum who have gone down this road may be able to provide you with more specific info.
H

Paul Workman

I've deleted my SPTs some 7(?) years ago, and never regretted it for a moment!

Initially, according to the book Hans mentioned, the dual runner was in vogue at the time; designed to provide increased torque in the bottom and mid rpm ranges, along with fuel economy and emissions.

My reason to delete the SPTs was reached after my 2nd (or was it the 3rd?) trip under the plenum to address a leak or check-valve in the actuator circuit. So, at that point I opted to remove the system entirely, and Marc Haibeck provided me with a chip calibration which was necessary to tell the ECM that ZERO vacuum at the MAP sensor switch was NORMAL and to operate both injectors when the throttle was just off idle.

As maybe you saw in other posts that throttle response is improved. It is, which is slight (but nice!) bonus in addition to eliminating issues with reliability.

Some leave the SPT mechanics in place by simply securing the blades in the open position. This has the advantage of facilitating the return to stock configuration later, especially if the SPT vacuum circuit stuff was NOT removed; just reconnect the wire to the vacuum pump and the vacuum hose to the MAP switch, and swap the chip back to stock. Done!

Far as emissions compliance goes, the fuel and timing calibration can be compromised slightly to meet the California requirements (BUT fresh cats are definitely part of the solution).

Marc Haibeck told me once that tying the SPTs open has only a slight impact on HP performance. However, if nothing but the best will do, removing them - especially if the porting exceeds 36mm - is very easy to accomplish.

Tuners and experimenters have ported the LT5 induction system to the extreme; "Siameseing" the runners into one has proven to be effective, especially in big inch modifications. In fact, the dual runner scheme was completely eliminated by LOTUS in the GEN-III LT5. There are NO SPTs or dual injectors to be found in the gen-III motor (tribute to or at least validation of the experimentation results achieved by LT5 experimenter/tuners? Yes...IMO!).

I'm NOT a tuner myself (yet, anyway). But, some different nuances in tuning technique may exist among those that are, or perhaps it is also something germane to the particular year.

IF there is a downside to eliminating the SPTs in my 1990, the only (slight) annoyance is when the clutch is held in while coasting to a stop, the idle rpm will "hunt". But, as soon as I come to a stop, the hunting quits and it idles fine.

I don't know if it is related to the calibration for the secondary delete, but during deceleration from speed when weather conditions are just right, the motor will suddenly run rough and maybe attempt to stall.

Far as these two very minor distractions go - one I'm sure is related to the calibration, and one I'm not sure - I can all but eliminate them by adopting techniques:

the idle "hunting" is mitigated by downshifting and not depressing the clutch until the last moments. It wouldn't be worth mentioning were it not for the fact that on rare occasion the motor will stall completely. But, that is very rare: a good thing.

the stalling during decel (from speed, usually) is preceded with rough running/sputtering. However, the motor responds instantly to a blipping of the throttle, and then all is well.

But, on decel, on rare occasion the motor may stall completely when the clutch is depressed and my right foot is on the brake (not "toe-heeling" at that moment) before I have a chance to blip the throttle - like when downshifting. But, soon as I let the clutch out and blip the throttle, all is well as tho nothing had happened. (This sounds much worse than it actually is. The need to deal with it is so rare, and by now I am accustomed to the symptoms and enact the "blip remedy" w/o even thinking: it is only a minor annoyance. AND, for what it's worth, I'm not even sure the decel/stalling is even related to the secondary delete.

Bottom line: Nothing not to like about deleting the secondaries. Far as the two quirks I mentioned, I should also mention that my wife's 1991 ZR-1 has neither of these quirks - which leads me to believe the tuning maybe the answer, in the case of my 90?

Dominic Sorresso

are u saying u have a 368 w new cams or that u just eliminated the secondaries? If u have just installed new cams you will need to do a significant tune for part throttle, idle etc. If u already have a tune and now need to eliminate secondaries, that's relatively simple altho its likely u will want to update the VE tables as well.

zrracer

WOW,
Thanks for the great answers and concerns.
The car is tuned to the 368 mods.
I am not worried about being stock. This is a go for me I think.
I remember that Jeff Jeal was doing the plenum work years ago I think. Is anyone doing them anymore? Any before and after results? If I am going to need a tune for secondaries it might be worth doing the plenum at the same time.
Thanks for the great answers.

Dominic Sorresso

just to clarify, u have cams (not stock?) and a 368 CID but the plenum is NOT ported? How about the injector housings?
If so, u will need a remap of the fuel tables plus other items.

XLR8TIN

Mine are removed, gained RWHP , love it, I think 1 member had a screw come loose from throttle blades and ate up the engine, thats more than enough info to remove

zrracer

Dominic ,
Thanks for the concern but everything has been done to the engine to make it the best it can be. Except maybe the Secondaries.😄
I am going to go ahead with this. I am bringing the car back to life after years of sitting. I am hoping the car,myself and the grandkid can have adventures with it. I am hoping when he gets it he will love it and think off me. Anything I can do to keep it KISS will help as the years go by.
Thanks for all the help.

Dominic Sorresso

It wasn't clear specifically from ur post how the motor had been modified. That's why I asked.

Rkreigh

I talked to marc and the throttle blade open only costs a couple of cfm so it's not much. but removing it opens up porting opportunities which is where the real benefit lies. for certain, the lt5 used the flapper for "torq, emissions, and drive by" and also the "valet" capability

from my experience with the mod, there was zero loss in low end torq below 2k so why worry about it

what I did see it the car would get with the program quicker, about 8 ft lbs of increased mid range torq, and no difference in the top end

now this was on an LPE 390 LSV so your results may vary but in general this is a great mod.

If you'd like to explore this on your 368 I highly recommend it. the tune update is easy, and if you can, update the porting at the same time. Either way it's affordable and we make this a "standard" on the LT5 as parts left out don't break, way much, or get in the way.

I've been able to pass my parts down to those that need them, I don't need no STEEENKIN canisters or snakey vacuum mess under there.

Haven't had a single problem since doing this mod. Remember to ask the tuner to "delete the air pump" That needs to be done at the same time as it's no longer needed and you can help out another brother by passing it down. durn thing makes a racket anyway!

PM me for more info, your car would really benefit from this mod in my opinion based on how much I like it.

the one issues as pointed out is emissions. the flapper being close increases the port velocity which makes the engine run cleaner which the flapper is closed (slightly). I think the fresh cats might be enough to fix this minor difference and my car is a bit dirty on HC with the LPE cams and bigger valves. so if you are in an emissions area, just be advised, this might hurt emissions a bit.

I love the reliability of this mod and an added benefit is that the secondary injectors light off at anything above 1 % tps which helps them keep everything nice and clean. very stable idle even without the flapper and only 1 injector firing, no worries. As soon as you get on it all is ready.

I'm in Alex VA if you'd like a ride, and we can definitely help you out with this. Nice numbers on that 368!

Paul Workman

Good question...and in fact in the later development of the gen-III LT5 there were no secondaries or dual runners. What made the difference? (Mostly) it was variable valve architecture.

At the time, the short answer (was) the smaller primary runners improved low rpm torque and gas mileage to optimize (city) driving including the air conditioning. This was primarily the advantage of a DOHC design with facilitated individual primary and secondary lobe parameters.

Fact is, a carefully modded LT5 can develop over 500 clutch HP while retaining the stock cams and smooth idle and essentially stock driving characteristics including low rpm torque and gas mileage, all the while operating with the secondaries removed. THAT is saying a mouthful for any motor!!

Goldcylon

TWICE. One screw per side !

Dominic Sorresso

I am certain the SPT was a setup made to allow the LT5 to pass emissions and to avoid the gas guzzler tax. It also has CAGS and a shift light for economy. So they could get away w only 8 ports operating and generating ~ 240hp much like the base C4.
If we use the Brake Specific Fuel Consumption(BSFC)
we have 8 injectors at 22 = 176 then divide by .5
= 352chp BUT we want about 80% Duty Cycle for the injectors which brings the HP supported by those primary injectors down to 281chp. A little better than the base C4. They needed additional injectors to support hp up to the 375-400 level. The additional injectors were added, IMO , as a way of managing Pulse Width at idle and slow rpm operation. In fact even when u delete the SPT and modify the tune, the primaries are the only injectors operating at idle. After that the calc'd PW is split between the two injectors. When the C6 ZR1 came out they only used 8 injectors because they managed fuel flow based on fuel pressure. IIRC the ZR1 fuel pump was a 3 stage. So the injectors flowed the necessary fuel because the fuel pressure was being modulated. Don't forget that the LT5 ECM was basically a derivative of the L98 which only ran 8 injector drivers. The LT5 secondary injectors are piggybacked onto the primaries for the firing signal only. The injector drivers are the secondary relays that energize the secondary injectors.