Mueller

I have a 89 formula TPI 305/5.0 Vin F with auto trans. Recently did a fair amount of work and am very disappointed!

Had cylinder heads off and gone through

New fuel tank, pump, sending unit and filter.

New spark plugs etc, new tie-rods also
Timing is set to 6* BTDC

New exhaust beyond factory manifolds. Mac Y-pipe - MAC48692, DynoMax intermediate pipe 2.5", Flowmaster muffler with 2.5" Ports, New 2.5" tail pipes.
-Installed a test pipe instead of the cat.

MY PROBLEM -
After all of this work and much aggravation I feel the car has LOST power. Lack of pick up, For instance we have a strech of road that seems to be a very go place to let it fly. Before and after results from my labor is basically a loss of 20 MPH!

Any ideas as to what the issue is would be great. Thanks

lunaticinaZ

I'm sure people will argue but........ with no cat. It will lower your back pressure and cause a loss in torque. That may explain the lack of seat of your pants go go go feeling. Some say that you will gain a little hp with no cat, but In my opinion I think the loss of torque will be more noticeable than the hp gain

formula forever

your right about the removel of the cats losing low end. I have 2 third gens on with no cats on with cats. same mods on both cars and the one with the cats is alittle more powerful on the low end.

TTOP350

iTrader: (8)

Oh here goes the back pressure thing again...

ASE doc

To say back pressure increases torque is to misunderstand how exhaust functions. Back pressure doesn't help anything and if a given exhaust system did produce backpressure, it would be more prevelant at higher engine speeds where the engine is moving more air. What makes a smaller exhaust tube, such as smaller header primaries or even stock manifolds produce more low end torque is evacuation or scavenging of exhaust gases.

The stock manifolds are designed to work as part of the original system to provide scavenging though the engine's torque curve. The stock exhaust, while a cheaply made compromise, is designed to work with the manifolds to acheive this purpose. Small primary headers will gengerally work better than stock manifolds, even at lower engine speeds, by providing a smoother path. The smaller tubes still provide good scavenging by producing velocity and thus a negative pressure zone at the exhaust valve through the engine's torque curve.

Back pressure has nothing to do with any of this and will only rob power by causing exhaust reversion. Any positive pressure at the exhaust port will cause exhaust gases to remain in the cylinder and to revert into the intake, where they mix with incoming air fuel mixture, during valve overlap.

KrisW

iTrader: (5)

I'm gonna start by saying the professor above is right about back pressure. More does not equal low torque, etc... Altering the stock configuration WILL change things and it could be positive or negative depending on what happened.

Let us examine what you did, and maybe where things went sideways...

First, the exhaust. I am going to maybe go against some on this board, but doing nothing else, I have done all of the mods that you have done on my third gens over the years and have been nothing but happy with the results. I don't think the exhaust is the problem.

For future reference, I really do recommend doing these mods one at a time if its your first time doing them so that you can gauge what happens at each step. After you've been successful a few times, you can do them all at once because you know what is going to happen.

Spark plugs and tie rods, probably not... Those should be fine.

Timing at 6 degrees? Start at Zero (I believe this is the factory ECM calibrated setting) or whatever your emissions tag says. Then slowly increase by 1 degree steps, taking a test drive after each change, and then record your results. With factory ECM and chip, normally 4 degrees is the max you will get gains from, and sometimes 4 is too much.

New fuel pump, huh? Hook up the pressure gauge and make sure that you are maintaining more than 41 psi. Make sure while you are driving, too. I have a long hose on my gauge, and I can gently have it sticking out of the back of the hood against the windshield while I am driving. Be careful and don't get hurt or hurt your car. Make sure you have adequate fuel pressure just in case someone accidentally put the wrong pump in or that the new one is defective. Maybe its cutting out, or maybe it just can't deliver the full volume needed at heavy loads.

The cylinder heads. This can really be a killer. You need to define "gone through." If someone did heavy intake porting on there, it won't match well with your TPI intake and you'll lose a lot of bottom end. For a stock TPI intake, I gasket match to the intake only, just a little smoothing but not mirror, and then port the heck out of the exhaust side. I have not lost bottom end doing this. I have seen lots of guys lose bottom end with big porting on the intake, though. Your cam and intake just aren't configured (not to mention the ECM) to compensate for that area.

Let us know the answers to this!! It helps everyone here when we figure these things out.

Good luck!

Gallileo60

I too saw a drop in my "Throttle Response" When I first put my 3 in exhaust system on, altho, it did increase hi RPM power somewhat..I got most of it back when the system was finished...Throttle response is very important to me, my TBI car is much more responsive, but my TPI car is both quicker, and faster 14's versus 13's...You want to maintain as much velocity thru out your intake, and exhaust system as possible..

Mueller

Low end torque does not seem to be my issue. Seems to fall on its face after 3000rpm.

As to the question directed to my head work. All that was done was touching up the valve seats and guides, replacing seals, etc. No porting done.

I am beginning to lean towards the fuel pump myself due to the pump beginning to become noticeably louder.

Planning on checking pressure and volume at next opportunity. Be a shame if the new pump is my issue due to the terrible experience it is to remove the exhaust and pry the new tank back out.

Thanks again

Mueller

Also timing noted on emissions decal notes - 6* checked in drive(for auto trans) w/ timing lead disconnected

mr305

iTrader: (1)

Im not gonna say i know how back pressure and all that works all i can say is that i put hedman headers with 3" collectors 2-3/4" y pipe no cat to a 3" I pipe and a 40 series 3"-IN/2--2.5"OUT on my 305 TBI and felt a BIG difference it picked up alot faster and this is on a TBI motor im sure if you lost power it was something else that went wrong when the motor was put back together.....after the heads were done did you assemble it yourself?

KrisW

iTrader: (5)

If you are hearing fuel pump noise....well...yep

Anyway, this time do it right. Many do not know that the stock replacement pumps are often terrible right out of the box. Get yourself a nice aftermarket like the holley or walbro. If you're on a budget (like I always am) then use the vortech pump.

If you are at the parts store, have them get you the pump for the 89 Turbo Trans Am, vin code 7. This is also the pump for the late 90's pickup with the vortech 350. Its a great pump, and even if it starts to fail, it still supplies more than we need. That pump is designed to provide 60 psi plus, and I have not had one fail in a 3rd gen yet. I have done 4 so far. The TBI pumps are a definite no-go, and the stock new TPI pumps are hit and miss; you just don't know what you're gonna get.

Also, if you put your car high enough up on jackstands, there shouldn't be any prying on the tank to get it out. Driveshaft and wheels off, sway bar disconnected at the ends, shocks disconnected, brake line tee unbolted from the axle (but not disconnnected) and brake lines freed from their hoops, the rear end falls down, the springs come out, and the tank comes out.

Mueller

KrisW - Thanks for the help. You are a true asset to the site. Sounds like your suggesting the pump that i should have went with to begin with.

As to tank removal; Hoist and all with car 6ft up the filler neck still is a tight fit to pry into place.

Thanks again

KrisW

iTrader: (5)

Thank you, but really there are lots of smarter guys than me here!

My advice most often comes from the budget perspective; I am ALWAYS on a severely tight budget so over the years I have found what works for my cars and what doesn't. That fuel pump is perfect for every GM fuel injection car I fix. Up to 400 HP or so, I never have a problem. I like the high ouput, 255lph pumps, if I can get a deal from eBay!

As to the tank filler neck, maybe your neck is bent a little. If you take off the fuel door, and take out that plastic surround for the spout, there is lots of room. If its not fitting right then either your neck is bent, or the car may have been hit in the rear and fixed and the hole is not exactly where it was at the factory.

Either way, as long as you can get it without too much trouble then its no big deal.

Good luck with your project.

Pab's

iTrader: (2)

I actually experienced somewhat of the same ordeal, when I first got my iroc it ran good and a couple of month's ago it would sputter when I would give it some throttle (especially the 2k-3krpm) I got a bbk pump (255lph) and its going in this weds so Ill let you know if that did it

BadSS

Double check the flow direction on the filter, that the fuel lines up front are not swapped around, and check for kinked or pinched fuel lines at and around the tank. If the pump is screaming,, it could be bad, but I'd check the easy stuff first. I'd check the voltage sweep on the TPS also. If you extend a fuel pressure gauge so a passenger can read it while you pull some RPMs in 2nd gear, that would help determine what's going on- or better yet pull a scan.

It is not the exhuast. People that slow down after an exhaust change is because the extra flow causes the engine to run "lean" (which could be compounding your problem). They just don't understand engine tuning 101.

4U2ENVY

Hello all,

I first would like to introduce myself here as the "newbie" but by no means am i a rookie in the automotive field. I recently got my first IROC it's an 85 with the LB9 and a 700R4 transmission and I am looking to add some HP with easy bolt ons and get a better rumble out of the exhaust so here is my questions to all you fellow IROC owners. It has stock equivilent exhaust (bolted not welded) and it's in new condition but not loud enough and i would like some better power when i step on it so I guess im asking should i put 3" flowmaster exhaust with a cat-delete pipe or keep stock 2.5" pipe and just do a Flowmaster Muffler and a High Flow cat? My other question would be what are some other non invasive bolt on mods i can do which will yield the best results? I am going to put in AC Delco Rapidfire plugs, 180 degree thermostat, K&N filters, 8mm Plug wires and maybe a Thermomaster Chip (I have heard alot of good and bad) ANY IDEAS???

Gallileo60

Leanness aside, Exhaust, and intake tracks can be tuned for changes in Torque production at different RPM's..This is done by changing both the length of the Exhaust, and the pipe diameter..you can defiantly loose low RPM torque by choosing too large of diameter headers, and pipes...While it is true that generally when you get rid of a crappy exhaust, you need to enrich the fuel system somewhat, it is still best to use an exhaust size tuned to whatever RPM you are wanting to produce max torque at..You can change your power band somewhat just by changing the diameter of your headers, and exhaust..Race Car 101...BTW, im not saying this is his problem....

BadSS

Sorry if you took my Tuning 101 statement personal. However,,, if you slowed down or lost any noticeable low end torque after increasing the EXHAUST pipe diameter,,, your tune WAS/IS off. A "tune" is more than adding fuel.

That aside,,, what you posted is more like Race Car 50.5 since you’ve confused the relation,,, or you are confused on the relation between header pipes and exhaust pipes. Plus,,, revisiting Reading Comprehension 101 might also be in store since the original poster does not have headers. Anyway,, the “half truths” in your post deals with header primary tube diameter and length and collector "tuning" – not complete EXHAUST systems.

Gallileo60

you are 100 percent correct on primary header tube size, but it is also true that exhaust size, can speed up, or slow down exhaust gases...My 305 was running 14's, but it was not as "responsive " as it was before, the tune was close, also the ob1 should be able to correct small amounts of fuel mixture (within a limited range) yes tunes on these are more than just a fuel curve..Im on third gen number 6 right now..most of my experience comes from carbed bracket cars..I am not very knowledgeable on computer systems..As far as reading, I never mentioned headers, just said pipe size..