Calderone

i have noticed this...if im rolling ...any speed ...and floor it, the car accelerates..but progressive ....
If i do the same at 3/4 throttle,i got a kick in the but and its like some boost comes in ....what's causing this ?
I have a SuperRam,219 cam,Vigilante3200,58mmTB,355 engine.
Is my timing too retarded ? im still running at 6°
Thanks !

TWISTERUP

Throttle body sounds a little large for a 355--but JMO

tehcarguy

Make sure your TPS is adjusted properly.

Calderone

Yes its properly adjusted....i have been thinking in this
on my 58mm Holley TB i have two options to attach the accelerator cable
on the inside or on the outside , i was not able to wot because i had a little bumper
but i cut the sleeve and now im able to do wot,but ....the sweet spot is
at 3/4, not at wot...im sure !

Where did you guys hooked your accel cable ? on the inside or outside ?

Mustang_Eater

My 89 has the same feel to it. I thought it was just the way these cars were designed. I will say this, just replaced my Pre-Cats and seems like there is more pull at WOT.

How old are your Pre-Cats and Main Cat?

Calderone

Hi eater, im totally catless
but don't get me wrong, the car pulls very good at WOT and overdrive
but the big kick that i get t 3/4 is harder than WOT if i apply it when rolling at 40 mph
for example, now from a dead stop,if i build rpm and mash it , there is good power as well
but seems like the 3/4 has some sweet spot there ...i have been researching and seems like other cars has this same "problem" , not sure if its a problem ...

tequilaboy

3/4 throttle is close to the %tps PE (power enrichment) threshold (typically 70% tps).

Outside of PE, your AFR target will be the stoich ratio in the bin (approx. 14.73:1). Once in PE mode, your AFR target will be much richer depending upon rpm and coolant temperature (typically between approx. 11.5 - 12.5:1).

It could be making more power due to the leaner AFR target, but could be on the verge of a meltdown. On the other side, your PE tuning may be too rich.

With a large throttle body, its a good idea to enter PE mode sooner to avoid running dangerously lean at high flow with 60-70% throttle opening.

Also the additional PE spark advance (if present) may result in too much timing for your combination for best power once in PE mode.

blackbear bob

Just a thought and please correct me if I am wrong, but the older systems also went into open loop at 70% throttle? Could be a clue to your sensation.

87 vette 81 big girl

I have noticed the same scenario with my 87 Vert Sammy at times.

Its not the transmission or torque converter.

For me in Illinois it is E10 93 octane gasoline or whatever the crap gasohol mix really is.

I have driven around for days & countless hours with my Snap On Scanner hooked up.
ENGINE KNOCK COUNTS & IGNITION TIMING COUNTS AUTOMATICALLY RETARTED.
Once our L98 ECM's detect a single knock signal from the knock sensor,
timing is retarded not for a fraction of a second but for up to 15 seconds till you let up off the gas some.

Let off the gas slight back to 7/8 or 3/4 throttle position,
engine working load is less and combustion chamber temps lower,
knock counts are gone,

I feel that sweet spot too.

Its much more apparent on very hot ambient air temp days of 90 degrees F or more & relative humidity of 60% or greater outside.

My Vette runs best when its 30 to 65 degrees F outside.

I can eliminate all knock counts by adding 1 can or 1 quart of TORCO MACH SERIES OCTANE BOOSTER per 20 gallons or 1 tank full of SHELL OR BP GASOLINE regardless of outside weather conditions.

WOT then with TORCO Octane booster in gas tank,
I see "0" knock counts from WOT from off the line to 150mph.
Engine sees full programmed in ignition timing tables.
No sweet spot in the gas pedal anymore.
Just flat broad power band till the TPI runs out of air & soft camshaft timing in place.

May help if you watch knock counts to diagnose the Sweet spot problem you have noticed too.

Brian

5abivt

I would bet you are leaning out thats why you are feeling that. AT 70% throttle your TB is able to provide more than enough air. that is why engineers design throttle bodies for oem cars based on their needs. Having a smaller throttle body controls the amount of air that goes in so that it corresponds to the throttle pedal.

Larger throttle bodies are such a waste of money unless you really need more than they provide. Going bigger on a stock motor is like putting an on/off switch on the throttle.

87 vette 81 big girl

Check for Knock counts with your AUTO-X-RAY or similar Sammy.

You have to diagnose for yourself.

I don't believe in too large of throttle bodies or carburetors.

Bigger the better if properly engineered & made.
My thoughts & experience.
CARBS mainly in the past.

If anything,

C4 available exhaust systems suck badly in my book & teeeny weeny tiny to me.

Even the Ford 5.0 guys here in Illinois with 306 ci engines run bigger primary tube headers & 3.0" inch or larger dual exhaust systems.

Brian

Calderone

Thanks for the replies,when i did the tune with PCMForless i believe they de-sensitivized the Knock sensor but im not 100% sure.
I'm still running at 6° , i want to dial it in soon. i will check for octane booster,here in my country we use "97 octane" that's the best
gasoline (we have 3 types, 93-95 and 97) but that's in another scale im not sure what's the USA equivalent.

i still don't understand the on/off switch comparison that 5avibt (and others have said) is referencing too.
I would love to try my car with a smaller TB and see its effects.

I can connect my laptop and check what's going on there. i have all the datalog devices to do it.
I Onlu bought the 58mm holley tb because i wanted a brand new tb with my setup,i would like to compare to know
what's the on/off thing,i don't have any C4 near me to do it.

Thanks !

5abivt

Think of a garden hose with a valve the same size as the hose on the end. Open the valve half way.. you get half flow. open it 3/4 and you get 3/4 flow. A LArger throttle body is like putting a Valve that is much bigger than the hose (ie a 2 inch Valve on a 1 inch hose) . open the valve 3/4 and all the water from the hose is going to spill out without restriction and opening it fully makes no difference at all.

Larger throttle bodies ONLY make a hp difference if the flow of the TB at WOT is exceeded by the demands of the engine.

dynocar

As some have suggested, but I'll explain it in a different way, I think you have an 'open/closed loop' problem. Either your TPS sensor is miscalibrated or defective and not detecting wide open throttle (70%+) that is supposed to cause you to go from closed loop to a more rich open loop. Or when you do go to WOT your open loop, power enrichment fuel maps are incorrect. In either case, if you are to lean at WOT, you will have less power and it can also cause knock retard causing even less power.

An on/off switch throttle is one that acts like a switch when going from near closed throttle to wide open throttle rather then a smooth 0-100% transition. Example, if your engine receives all of the air it can ingest at 50% throttle with a large throttle body, your throttle foot will think you have more power because a one inch movement will provide a much larger % of air volume to the engine. Now go back to my above paragraph and realize that your TPS sensor is only at 50%, when it should be higher and we are not in open loop for a richer, more powerful and reliable mixture. Always error on the to small rather then the to large when it comes to cams, carbs and throttle bodys.

C409

..... Its your tune ... put a wide band on it and check the Air Fuel ratios ... PCM 4 less typically uses the ARAP bin on 165 computer apps and the stock WOT settings may be way off ..... you might want to tune it yourself .........

383vett

Wide band is the only way to figure out your tune.

Calderone

i'll get it soon,any recommendations ? links ?
i have most of it, moates aldl cable i can datalog with tunerpro rt and datamaster
what i have to buy ? is the wideband always in there or is a device that i need to use sometimes only ?

Rodney_Gold

You need to weld in a bung in the pipes and buy a wide band sensor.

As soon as I have any major issues , I'm going to ditch my FI on my 86
Being in South Africa , like you in South America , there are few and far vette experts or parts availabilty is not that cheap and easy, although this forum is fantastic and the people are extremely helpful , it's still sometimes very difficult to diagnose issues. I have a few pals with l98 vettes who have dumped the FI , used a nice intake , a 770 carb and a decent fuel pump and distributor and got great results with the stock cam and l98 heads and even better results when the engine has been modded.

Calderone

Thanks Rod, i will look into it later since im focusing on other areas for now
i have always wanted to re-tune it and dial it in,but i will do it when i get the D44
i don't feel safe playing with it on drag radials but i will totally do it....btw there
is no way i will go back to carb

C409

..... I use the Innovate LM-2 ... check their website at www.tuneyourengine.com