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Discussion Starter #1
i have a 91 corvette with a 6spd 80,000 miles. the mods vortex ram air,52mm throttle body,ported plenum and ported accel lg tube runners tpis long tube headers and a heated oxygen senser, with muffler eliminators, msd coil, new cap rotor, all tune up stuff is done including new fuel pump. 345 gears, i have a fuel gauge mounted in the car so at wide open throttle my fuel pressure is not dropping, here's the problem when i get to about 3800-4000 rpm's feels like i hit a wall will not go past that my freind has the same car but a auto and had the same problem and he bought a custom chip problem solved, i do have the ez link scanner all shows good. grt back to me asap it's driving me nuts. i do have the stock prom in.
 

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El Teafive
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What year is your friends car? if it is 89-older the Tune in the PROM is more Forgiving then your 91.

I would think that part of the problem is the PROM, but you still have the Stock Base which it a choking point for your set-up

I think a new base and a better Chip may help.
 

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Discussion Starter #5
my friends vette is a 91 to auto with same mods he had tpis burn him a chip and he told them he wanted the rev limiter rasied to 6000 this is what he told them if he power brake d his car as soon as he hit that 3800-4000 rpm that was it like he hit a wall now with the new chip car spins well past 4000.
 

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Official Forklift Driver
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I don't know exactly all the changes done to my chip, I have a cam and rockers as well but I really can't think of what would need to be changed for those mods. These cars run rich from the fatory and doing intake should help lean it out some but I don't think enough to change anything in the chip unless you are cammed at minimum. I could be wrong though, Jeff, can you discredit this?

You said everything is good by what you've seen on a scanner, how about the cat(s) if you run them? Maybe you're cat is bad.
 

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Discussion Starter #7
i just have the one in the middle of the car. i just find it weird that i have the same problem as my friend and the chip fixed his. i don't know i can explain it the car runs so mint till that point
 

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6SPD said:
i just have the one in the middle of the car. i just find it weird that i have the same problem as my friend and the chip fixed his. i don't know i can explain it the car runs so mint till that point
My old engine use to do what you are describing...It was like I hit a rev limiter or somthing....I had weak springs causing Valve float!:whip:
 

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Are there any codes??
How old is your fuel filter???
What is your O2 sensor readings when this happens??
What is your oil pressure doing at those RPM's??
what's your air filter look like??
How about the basics.... plugs, wires, coil output... base timing ok, advance working??? what's your knock sensor doing??
More info needed !!!
 

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Discussion Starter #11
the answer to all your questions is all that stuff is new and there is no knock retard and the o2 is around 850mm it is a heated sensor
 

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Discussion Starter #12
i forgot to mention my timing is set to stock 6 btdc no vacuum leaks fuel pressure is not dropping at wot i will call tpis like i said my friend had the same problem both cars are 91's he bought a custom chip and that fixed his problem his car would die out around 3800-4000 rpms it could be possiable that my stock prom is bad??????
 

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if your still running stock cylinder heads, cam and intake manifold base your severly restricted as to the max airflow into the engine, limiting your power potential.
large tube runners and plenum partially remove part of the restriction but the limiting factor remains the most restrictive part in the induction system, if the large tube runners and plenum can flow 240cfm, it does little good if the stock base intake and heads max out at 210cfm (WHICH IS VERY COMON BTW)
if you want max performance ALL THE PARTS IN THE INDUCTION SYSTEM must flow AT LEAST the minimum airflow needed to reach your engines potential horsepower BEFORE that air flow restriction limit is reached
YOUR CYLINDERHEADS ARE VITALLY IMPORTANT HERE!

http://www.geocities.com/z28esser/headcomp.html

while theres no direct linear relationship between port flow and hp there is a relationship.
the ballpark formula is
.257 x port flow x 8 = potential hp

EXAMPLE
if your intake port flows 250cfm
.257 x 250 x 8 = yealds APPROXIMATELY a 514hp figure before your heads become the limiting factor

this ballpark formula is based on average results FROM WELL TUNED HOT ROD TYPE ENGINEs USEING DYNO FLYWHEEL RESULTS
NOT true race only engines or street engines
also keep in mind that the results youll get will differ and to get those numbers it requires the engine to operate in its best rpm range with a cam and compression levels that match and all other components must also match
if your heads flow 279 cfm at .700 lift but your intake only flows 240cfm and your cam has a max lift of .550 your not going to get the max potential HP results
for the ballpark formula to work you must have a tuned exhaust, a cam that matches the compression ratio and all other parts must flow at least close to as well as the heads at the max figures
also keep in mind that the ports cross sectional area should keep the airflow in those ports in approximately the 200fps-300fps ranges
airflow speeds that very greatly from that 200fps-300fps will not tend to give best results, thats why huge ports that flow exceptionally well don,t work well on smaller displacement engines
port lenth also has a large effect on the rpm range that the ports can effectively pack the cylinders at due to harmonics in the collum of air
PORT SIZE FLOW AND THE RELATION TO CAM DURATION, and your displacement and the tuned rpm that your headers operate best in for scavageing the cylinders also comes into play here!


FIRST, This will not be anything more that a brief glimpse into a subject that takes years to understand and I’m sure there are a few people on the site that can give more exact info! This is meant to apply to the 350-383 sbc engines most of us are useing
My purpose is merely to give an idea as to the relationship between the factors and yes IM ignoring several minor factors to make things easier to understand
But lets look a a few concepts

(1) There are 720 degrees in a 4 cycle engines repetitive cycle of which between about 200degrees to about 250 degrees actually allow air to pass into the cylinder, (the valves open far enough to flow meaningful air flow) and the piston has a maximum ability to draw air into that cylinder based mostly on the engines displacement and the inertia of column of air in both the intake port and the suction (or negative pressure the PROPERLY designed headers provide) this produced a max air flow thru the ports, the greater the volume of fuel/air mix effectively burn per power stroke the greater the engines potential torque production, the faster you spin an engine the greater the NUMBER OF POWER STROKES PER MINUTE, and up to the point where the cylinder filling effectiveness starts falling off due to not enough time available to fill that cylinder the torque increases, above that rpm or peak torque it’s a race between more power stokes and lower power per stroke
(2) look at this diagram
(3)

As air enters an engine it normally travels thru both an intake system and the cylinder heads intake port to eventually pass into the cylinder thru the valve. The valves in a normal small block corvette engine are between 1.94 and 2.08 in diameter, that’s between 2.9sq inches and 3.4 sq inches of area, but because the valves require a seat that at a minimum are about 85%-90% of that flow area we find that the intake port even with out any valve has a max flow of not more than about 90% of the flow thru a port of valve size. Or in this case 2.46 sq inches-2.9 sq inches of port area, Since you gain little if any flow having a port that’s substantially larger than the valves AT NORMAL ATMOSPHERIC pressures and since you can’t substantially increase the valve sizes for several mechanical reasons you must improve efficiency, this is done in two major ways, you can match the intake port length and cross sectional area to the engines most efficient rpm range on the intake side, to build a positive pressure behind the intake valve as it opens and match the exhaust length and diameter on the exhaust side to provide a negative pressure to help draw in more volume this will require the cam timing match that same rpm range of course. By experimentation its been found that air flow port speeds in the 200-320 cubic feet per minute range are about the best for a chevy V-8 now lets say you have a 383. 383/8=47.875 cubic inches per cylinder, the rpm range most used is 1500rpm-6000rpm so that’s where are cam and port size must match, you can do the math , (47.875 x ½ engine rpms = cubic inches, divided by your cams effective flow duration, (use 210-235) as a default for a stock cam) x 720 degrees/1728 (the number of cubic inches in a cubic foot) to get the theoretical max port flow required (I will save you the trouble its 250cfm-275cfm at max rpms and about 2.4-2.9 sq inches of port cross section, depending on where you want the torque peak, or use this handy calculator,

Intake Runner Area = Cylinder Volume X Peak Torque RPM 88200
Or this helpful site
http://www.newcovenant.com/speedcrafter/calculators/intake.htm
Either way you’ll find that you’ll want a port size in the 2.4sq –2.9 sq inch area
Now use this calculator to figure ideal port length, REMEMBER youll need to add the 6” in the cylinder head to the intake runner length to get the total length and you can,t exceed the engines REDLINE RPM which with hydrolic lifters seldom is higher than 6400rpm

http://www.bgsoflex.com/intakeln.html


Ever wonder why your engines torque curve gets higher with the engines rpm level until about 4000rpm-5500rpm(DEPENDING ON YOUR COMBO) but fades above that rpm level?
well it depends on several factors, first as long as the cylinders can fill completely you get a good fuel/air burn so you get a good cylinder pressure curve against the piston each time the cylinder fires, THE ENGINES TORQUE CURVE INCREASES WITH THE NUMBER OF EFFECTIVE POWER STROKES PER SECOND, at very low speeds there’s not enough air velocity to mix the fuel correctly or produce a effective ram tuning effect but as the rpms increase the cylinders fill very efficiently until the rpms reach a point where the cylinders just don’t have the time necessary to flow
enough air through the valves to fill the cylinders , remember a 5000rpm the intake valve out of 720 degs. in each cycle opens for about 250degs of effective flow even with a hot roller cam, now that’s only about 35% of the time and there’s 41.6 intake strokes per second , that’s only 1/60th of a second for air to flow into the cylinder
Its your engines ability to fill the cylinders that increases your power and the more efficiently you do that the higher the rpm level you can accomplish that at the more power your engine makes, remember the formula for hp is (torque x rpm/ 5252=hp) so moving the torque curve higher in the rpm range increases hp but at some point the time available to fill the cylinders becomes so short that efficiency begins to drop off rapidly, the peak of efficiency is reached normally in the 4500rpm-5500rpm range, and as rpms increase its a race between more power strokes per minute trying to raise the power and the increasingly less effective percentage of cylinder filling dropping the power.
Volumetric Efficiency
The volumetric efficiency of a 4-stroke engine is the relationship between the quantity of intake air and the piston displacement. In other words, volumetric efficiency is the ratio between the charge that actually enters the cylinder and the amount that could enter under ideal conditions. Piston displacement is used since it is difficult to measure the amount of charge that would enter the cylinder under ideal conditions. An engine would have 100% volumetric efficiency if, at atmospheric pressure and normal temperature, an amount of air exactly equal to piston displacement could be drawn into the cylinder. This is not possible, except by supercharging, because the passages through which the air must flow offer a resistance, the force pushing the air into the cylinder is only atmospheric, and the air absorbs heat during the process. so, volumetric efficiency is determined by measuring (with an orifice or venturi type meter) the amount of air taken in by the engine, converting the amount to volume, and comparing this volume to the piston displacement.
this increases until the torque peak then falls as the rpms increase. Here is a rough guide to match duration to port flow at different rpm level

if you’ve been following along you’ll find that you’ll need intake ports about 2.3-2.9” sq inches in cross section, and between 12” and 21 “ long (DEPENDS ON WHERE THE ENGINE IS DESIGNED TO MAKE MAX HP) and cam timing in the [email protected] to [email protected] lift range, as the rpms or displacement increase either the port flow or the cams duration must increase or the engines cylinder fill efficiency rpm will drop!
Now this is important, as the port flow efficiency goes up though the use of longer and larger intake ports the cam duration could remain the same or even be lower and you get more efficient cylinder filling as the rpms increase, that’s why high efficiency port designs like on the LS1 can use lower duration cams to flow similar total air flow thru the ports than the lower efficiency ports like the old fuelie heads could but at some point all ports reach max flow and an increase in the time the valves remain open at higher rpms increases the cylinder fill efficiency and that increases the engines ability to make torque at that rpm range


Runners (measured individually)
Stock....................203.17 cfm
ACCEL................242.02 cfm
Extrude/ACCEL...275.83 cfm
Super Ram............289.18 cfm
Intake manifold with 3/8 inch radiused intlet
.............................222.45 cfm
Stock intake manifold with runner
Stock....................198.72 cfm
ACCEL................213.52 cfm
Extrude/ACCEL....217.11 cfm
Super Ram............220.67 cfm

ACCEL Hi-Flow intake manifold with 3/8 inch
radiused inlet.........251.51 cfm

ACCEL Hi-Flow intake manifold with runner
Stock....................215.83 cfm
ACCEL................232.53 cfm
Extrude/ACCEL....243.21 cfm
Super Ram............240.24 cfm

Extrude-Honed ACCEL Hi-Flow intake
manifold with 3/8 inch radiused inlet
............................275.83 cfm

Extrude-Honed ACCEL Hi-Flow intake
manifold with ACCEL runner
............................266.94 cfm

Edelbrock Performer RPM manifold
(Stock)..................286.51 cfm

Edelbrock Victor Jr.
.............................275.24 cfm

http://www.eecis.udel.edu/~davis/z28/winter01/dyno/dyno021105/webpage021105.html

http://www.gmhightechperformance.com/tech/0307htp_holleystealth/
 

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Discussion Starter #14
okay i think i found the problem i took my car out tody and hooked up the ez link my main focus was knock retard and yes 1st and 2nd gear at wot i had knock retard. i went wot a few times and noticed it went as high as 5 some were 4,3 now what???
 
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