M1tch

Hey guys i recently bought an 1989 IROC Z 468 bbc/th400/9" and last night i took it to the drag strip. If you want some more info on the car you can read the specs here https://www.thirdgen.org/forums/theo...c-468-bbc.html. Last night i took it to the drag strip and it runs through first and second fast and then I go into third gear and its almost like it falls on its face and after the pass it runs like crap until i shut it off for a little while. My best trap was 94 1/8 and 116 1/4 but i should be trapping around 125 or higher.

Me and some of the guys at the track are thinking i don't have a fuel vent and that its causing the fuel pump to cavitate because i've also noticed my fuel pressure gauge off the regulator go from 7 to 12 at certain times which seems really strange. it has a remote fuel pump under the tank that pumps 12-14 psi and a mallory regulator.

Another thought is that it seems like timing is pulled at that rpm since it doesnt fall on its face and stays the same it could be pulled back for some 11.50 index class since that seems to be what its wanting to run. I don't know everything about the car yet and dont know if there is something in the msd 6al box or somewhere else.

The car doesnt break up and sounds fine down the track but something is definately going on in 3rd gear. As soon as my pass is done it barely wants to rev or do anything until i shut it off. Oil pressure and temps are fine and there is no apparent engine noise or problems.

Me and my friend plan on running the fuel pressure gauge up behind the air lid and make a bracket to see what the pressures at running in high gear. Were also going to try to drive it without a gas cap. The car also has a little shake at high speeds so were going to try to swap tires and check for bad u-joints.

If anyone has ever had this issue and has any insight I would really appreciate some help. Thanks

FRMULA88

If this is a carb, which I assume it is 14 psi is way too much fuel pressure.
You need no more 7psi at idle and it should stay between 4-7 psi during the pass.


The rev limiter in the MSD box starts pulling out ignition a couple hundred RPM lower than the pill or dial setting.

So a 7000 RPM setting would start at around 6500, when you hit 7000 is when you actually hear it "hitting the limiter".

Bracket / index racers use this to slow down car, it's a way of sandbagging to slow the car down at the big end of the track since they can't use a throttle stop.


Check to see what the setting is and raise it 500 RPM,

M1tch

Hey thanks for the reply It has a 7000 rpm chip in it and the timing is locked in at only 32 degrees. Yes it has a holley 850 double pumper. Like i said it runs up to 6200 rpms just fine in 1st and 2nd and its only in 3rd gear where i see this issue and runs rough after the pass. Its chirping every gear and wrinkling the tires with MT 28x10.5r15s so its got plenty of power up until then. I agree with you about the fuel pressure issue

FRMULA88

By the time you hit 3rd gear you could be running the float bowls low and the excessive fuel pressure is over powering the needles and seats.

I would start by dropping fuel pressure down to correct range and check the float heights.

Here is article that helped me understand this:

http://www.quickfueltechnology.com/t...ents-fuel-flow

M1tch

Yea the weird thing is though is that its normally running around 7 psi but ive seen it intermittenly at 12 psi. What you're saying makes sense though. I think I need to run it without a gas cap and run the gauge up behind the lid to verify when its acting up. This car should be running alot faster than it is

M1tch

I'm going to adjust the regulator today down to 6.5 psi and watch the gauge and drive her around on the highway and see how we do

FRMULA88

I am not sure what RPM you are spinning when you hit 3rd gear,
but with a 7000 chip it could start pulling out ignition at 6400.



I run a 7800 chip in mine, so I can rev to 6800-7400 with no surprises.


Last time out I was shifting at 6800.. but this is not the sweet spot, next time out I will try 7000. to see if I pick up a tenth or two.

FRMULA88

what size tire with that 4.11 gear & are you sure it's a 4.11rear?

when you bought this car was is setup for 1/8 or 1/4 mile?

M1tch

its a 28x10.5r15 tire and i was told it was a 4.11 but i did the calculator today and it said with a 3.73 with 1:1 and 14" radius tire i would be at 51xx rpm at 114-116 so im starting to think thats what it is. It seems like to me its an 1/8 mile car

M1tch

Its really bugging me though because it pulls 6200 rpms in 1st and 2nd with no problem. You can tell this thing has high compression and a pretty good sized cam too. I forgot to turn on the ignition switch and it was compression igniting 93, it almost fired up lol.

FRMULA88

Something is amiss because I would expect a 468 BBC to MPH much better than that.

I would not use straight 93 in it unless you were told it is a pump gas motor.. in 3rd gear you are really loading the engine and it may not be enough octane.

Have you read the spark plugs to make sure it's not detonating?

An 1/8 mile car would have taller gears than 3.73s because the track is half the length.

If you are only spinning 5 grand thru the traps, I would suspect this is not an 1/8 mile car.

FRMULA88

Adjust the carb (fuel pressures) then make damn sure about the fuel you are supposed to run.

Maybe you can get away with 93 on the street, but at the track you may want to cut it 50% with 110.

Also do you have a return line or a dead head regulator? a return is always better since it bleeds fuel off at idle and helps eliminate vapor lock.

You should also check to make sure the fuel tank is vented.. if the charcoal canister was removed the PO may have plugged the vent line... you could always loosen the gas cap to vent the tank.. but that means never forgetting to do that.

if the car sat in storage for long time before you bought it there could be debris in the fuel lines, pump, filter, fuel tank & float bowls... I would check all that to be sure..

M1tch

Yea i mixed 110 50/50 with 93

I also have a dead head mallory regulator my fuel pump is the only thing im aware of with a bypass.

It was raining yesterday so i was unable to do some highway pulls to see if we fixed the problem but we ran the gauge to where you can see it behind the air lid while driving. were going to do that and vent the gas tank. I'm also going to change the fuel filters and then if all that doesnt fix it check the carb. the float bowls are fine every time ive inspected them bc they have the sight glass. I'll do all this and get back to you

*** I found a microswitch on the transmission linkage where the shifter is. My car has a start/retard control box and i wonder if that switch could be cutting back timing in 3rd gear for a nitrous setup that used to be installed? I'm going to do some more investigating but it was just a thought **

FRMULA88

That microswitch at the shifter is most likely the neutral safety switch and should be spliced to the ignition circuit. my car has that and it's needed to pass tech because without it you would be able to start the car in any gear...

you also cannot inspect the float bowls when you are making a pass, unless you have an underhood camera having a fuel pressure gage you can see while driving is a good idea.

M1tch

This is true lol i'll update you when i get some things figured out, thanks for your insight. I also found out yesterday that whoever built this thing spent alot of money on some comp ultra gold roller rockers, so that tells me he didnt go cheap anywhere else. So i'm hoping i get this issue figured out and start trapping 120s like it should! its good to be back into third gens!

FRMULA88

well hopefully it is something that a little maintenance can remedy.

especially if the car has been sitting for awhile.

I have a funny feeling once you get the fuel pressure adjusted you will discover that pressure is dropping below 4 psi..

if that is the case I think you know what the problem is & the fuel system& carb needs to be cleaned.

keep us posted

FRMULA88

I read your other post, and saw the pictures..

that exhaust system looks too small for a BBC..

M1tch

Hey we have the same rear end and brakes i almost caught mine on fire after a pass lol went to get my timeslip and the guy yelled FIRE and they were just smoking. freaked me the f out. ceramic pads dont like long burnouts lol

FRMULA88

I have 3.73 gears with 26 tall tires so almost same as using 4.10 with 28" tall tires. Has a Detroit locker.. I used to drive this on the street so did not want a spool.

same trans too..
I have a reverse pattern manual valve body with trans brake and Cheetah Shifter.

btw when I switched to the Turbo Action Cheetah Shifter it was like night and day compared to the B&M ratchet shifter. It shifts like butter! the B&M always felt stiff and clumsy.. what a difference best change I ever made.

anyway did you make any progress on the flat lining?

M1tch

Working on it when i get home. gonna put some street tires on her and make a highway pull lmao

FRMULA88

good luck,

I don't see your carb specs. edit: I see it has a 850 holley dp..

A motor this big should have at least a 950 CFM carb.

(my 421 SBC has a 950 & makes 780 HP but 18 degrees heads,, totaly different animal)

too small a carb would choke a big cube motor on the big end

but it could be with the heads, intake, cam, & rev limit you have a 850 cfm is fine

Holley has a carb sizing program on their website you could tinker with that to see..

at the track you could play with jetting to see if it wants more fuel with the 850.

tune for best MPH, NOT ET (MPH is HP) if the car goes faster with more jetting throw a little more.. keep doing this until MPH slows down your best MPH / jet combo is your best WOT tune for max HP. You may have too much jetting so go the other direction (smaller jets) until the you get best MPH or MPH slows down. change 2-3 jet sizes at a time be consistant and read the plugs..

you can do the same with timing.. 2 degrees adv. or retard at a time..
be sure to read the spark plugs to make sure you are safe, better to run a little rich and have a safe tune than lean it out too much and then hurt it..

FRMULA88

http://www.holley.com/applications/C...bSelection.asp

I plugged in 468 CID & 6500 RPM ... double pumper, no choke

The smallest carb you should have is a 950 CFM 4150 series carb.

You could even put a small 1050 CFM 4500 series dominator on that thing !

If adjusting the fuel pressure and other tuning does not help it, your carb may just be too small for drag racing use.
the PO might have though raising fuel pressure would increase volume but the CFM rating of the carb is based on the venturi size.
A 850 CFM carb only flows 850 CFM worth of air, if the motor needs more air / fuel and the carb is undersized larger jets may not help.

M1tch

Yea thats starting to make alot of sense. Too much fuel pressure along with not enough cfm and low timing locked at 32 degrees its killing my top end in third gear where its the most noticable. You asked earlier if i had a trans cooler and it has a remote cooler in front of the radiator. Also here are some pictures of the cars exhaust you were asking about

FRMULA88

I see, the headers take a different path and route the exhaust pipes away from the transmission.

They went under the axle but that's ok since they don't hang lower that the lift bars.

That looks great ! but are you sure that the pipes are 3"? they look smaller to me.


Regardless someone spent some money on that car to do things right, despite they undersized the carb for track use.


You stole if for 9 grand ! Enjoy it even if you need a new carb, that is small change.

FRMULA88

32 degrees total timing is not a bad starting point.. better to start there and give yourself room to tune, too much timing can hurt parts (pistons, rings, etc)

A BBC should not need more than 34-36 total, but that depends on the heads & cam.
Fast burn heads need less timing. I have seen some cars run best with 28-32 degrees total.

I don't know much about Edelbrock BBC heads but if I were to guess I think they are copies of the OEM oval ports, but they flow better than stock. I suppose you can play with timing a bit to see if it helps that 850 carb... but the real problem is probably the carb is too small

Good Luck and keep us posted !

M1tch

Well I don't have any updates i'm still waiting on my friends shop to have some space availible to pull the trans out. But here is an awesome picture they took of my car at the drag strip last Wednesday! Nothing like seeing tires wrinkle at 1600 rpms lol

RBob

iTrader: (1)

With this from the first post it isn't a too small of a carburetor.

It is a fuel delivery issue. If you can't run it without the gas cap, remove it right after a run and see if the engine will rev.

If that doesn't make any difference, shut it off and immediately restart the engine, did that fix it?

It may also be that the fuel pump picked up a lot of air during the pass.

RBob.

FRMULA88

Please refer to post #12... OP is checking into that also. BTW he adjusted the fuel pressure and the car ran much better but he hurt the transmission so has to fix that before he can continue test & tune.

M1tch

Thanks for your insight however i wont be able to do anything until I get my trans fixed. Might take a couple weeks and i'll also be getting a new stall. Do you guys recommend any certain stall?

I've been looking at this one
http://transmission-specialties.com/...products_id=52

I'm also not sure if i should go with or without a trans brake and how much it would help my overall et.

On a side note look at my launch picture from post #26, looks pretty cool!

FRMULA88

The best way to size the convertor is to give them all the info you can on the car. in particular the cam specs, rear end ratio, tire size.
This will get you in the correct range.



A trans-brake will make launches more consistent versus foot braking.
60's times will also be quicker so yes ET will improve.

While the car is on the lift getting the trans fixed you can also have them check to see if the fuel tank is vented.

Good luck and keep us posted !

FRMULA88

The starting line pic looks great by the way !

enjoy it !

Hopefully the trans repair won't be too painful, and the tech goes thru it to make sure it's all good.

Try to get the hardened sprag upgrade and make sure the accumulators are installed.

Some builders / DIYers remove the accumulators to get firmer shifts but this mod hurts the sprag (and other parts)

A TransGo kit will get that sorted out.

M1tch

Sounds good i'll bring it up when i speak with him today when i drop my car off

FRMULA88

bump for relevance

M1tch

Well I found out I had a Coan 2600-2800 stall and a destroyed tranmission. Broken Case lugs and 2nd gear was destroyed among a few other parts I cant remember. I ordered a FTI 4500-4700 stall and i'm getting a whole new transmission. My tranny guy thinks the sprag was bad in the converter too. Heres a pic of my trans. Should have her up and going next week. Could explain the shake and low trap

FRMULA88

could also explain the 'flat lines in 3rd gear issue'