I know lots of abbreviations. But bear with me. I'm really stoked about the car I found. It's a '91 TPI/auto Trans Am convertible.

I hear the stock rear end is pretty wimpy, what gear ratio is it? If it makes any difference, the car is a WS6 with 245's. I heard some conv. has 215 touring tires. Would that mean the one I'm looking at might have 3.23's?

How well does the stock intake flow?

What 1/4 times are we looking at here? If I get a full tune up...etc? Mid 15's?

I'm looking for reliable low 14's. Would a nice cam, headers, cutout, and gears (3.73 or 3.45's) get me there?

Thanks!

 

If it is a 350 TPI (which would mean 3.23's if nothing has been altered), then a good exhaust system (headers, hi-flow cat, 3" cat back) combined with a good state of tune will get you low 14's...

 

if its a 91 convert. its not gonna be a 350 its got to be the 305 tpi.

 

Right as GM was squeamish about putting 350's in T-Top cars let alone convertables. Allen

 

First, and best mod: headers. Delete the AIR system if you can. Around here they don't visually inspect under the hood, and even if they did they wouldn't know what they were looking at anyway. I've taken that junk off of everything I've owned and never had a problem passign the sniffer.

Next: high flow cat or cats if you've got duals.

Third: cat back exhaust & a K&N filter

Those parts alone, even on a 305 shold get you into the 15s.

Modifying the TPI further requires spending considerably more. To really reap the benefits of a cam you need heads and a better intake. You'll also need a new PROM to make the EFI work right.

 

I have to dissagree. On a peanut cammed car such as the one mentioned, alot could be gained from going to a larger cam (go slightly larger than the stock L98 cam)

Also if low 14s are all you are after.... a ported stock TPI is more than fine for a 305.

Some other things to think about after the exhaust and porting the intake and cam that could make a big difference would be a bigger stall converter, and doing some light porting to the stock heads.

as long as nothing is extreme, nothing mentioned above should hurt reliabilty, I would think low 14s would be more than possible with the proper tuning (forgot to mention PROM tuning)


hmmmmm..... MESA AZ?! You better be going to see the Cubs during spring training!

 

hmmmmm super charge it kinda has nice ring to it

 

My vert came stock with 2.77(2.73? Can't remember which; I just know it sucked). So sometimes you'll want to ditch those and get some 3.23 or higher gears. I'd just do basic header/full exhaust for now and see how that works out for you. If the car is a play thing then think about swapping out the slushbox for a 6-speed. It's fun to row gears with the top down.

 

Sure, within reason you might gain some power by swapping the cam for a slightly hotter one. I was talking about bang for the buck. A cam swap IMO doesn't give you much bang for the buck. The mods I mention will make serious improvements in torque and HP across the board, without putting too bad a dent in your wallet.

If you then want more your best bet is going to be to upgrade the cam, heads, and intake all in one shot. You might pick up 10 HP from a cam swap, but for the work involved and the cost outlay that return on investment is pretty weak. Consider that to do a cam swap you'll already have the heads and intake off the motor anyway. At that point it would be easy to port the heads, base, and plenum, or just swap to better aftermarket parts. Upgrading the entire package could net you 50 HP, which is a lot better for the amount of work involved.

I agree that gears are a great bang for your buck. Going from a 2.73 to a 3.73 can be as rewarding as adding a 100HP shot of nitrous to the engine, but without the risks.

Adding a shift kit and a couple of other parts to the trans isn't a bad idea either (Vette servo, .500 boost valve, ect.), and upping the converter stall within reason is also a good way to improve ET.

 

TKOPerformance Said:
The difference between the "peanut cam" and the L98 cam used in manual tranny equipped 305's is worth 20-25 horsepower by itself, with no other changes. So, buying a good used LT1 cam for 30 bucks or so would also be a good investment. Granted, it is a harder install than a cat back, but not that much harder than headers.

So, SMURFN' Z28's advice on the cam change is pretty good.

 

No, its not. As noted by a few other members here ALL 90-92 LB9's got the L98 cam, the tech data on this site is wrong in that respect. The horsepower differences on these cars (90-92) were achieved through the exhaust only ( small outlet manifolds & 2 1/4 inch exhaust vs. large outlets & 2 3/4 inch). I will verify this once again shortley when I pull the stone stock LB9 out of my 92...... I'll check the cam # AND valve timing. I believe it was ED MAHER that changed the manifolds and exhaust on his 92 to a stock L98/LB9 duel cat system and picked up .4 et and 3-4 mph in the quarter. You would'nt get that kind of change with the peanut. So once again, the peanut was available in 86 in flat tappet form and 87-89 in roller form.

 

A cam is a lot hader to install than headers. Headers are just bolt on parts that don't require a ton of dissasembly. There are a lot of things you can do wrong when putting the top half of the engine back together after a cam swap. Especially if you don't have a lot of experience working on engines. You will need to set valve lash, and reinstall the distributor. If those two things alone aren't done properly you can ruin the new cam, and possibly the rest of the engine. Headers will easily net you 15-20 HP and an equal amount of torque. The cost is less than or equal to all the parts required in a cam swap (cam, gaskets, ect).

If he's got a WS6 car shouldn't he already have an L98? I know the stick cars got a more agressive cam than the autos, but I can't believe the difference is that much (from L98 to L98).

I wouldn't recommend an LT1 cam swap either. An LT1 makes a lot less torque than a TPI motor, but an LT1 makes better top end power because it can easily rev to 6,000 RPM. You'll never get that kind of RPM out of a stock TPI engine. You'll be better off sticking to a cam designed for the TPI engine, unless you want to swap to better flowing heads (or have yours ported) and change to an LT1 or other shorter runner intake (HSR, ect).

The goal when building or modifying any engine that will spend the majority of its time on the street is not to just look at peak power numbers. You want to shift the entire curve upward. Headers and exhaust will do that. Doing that with just a cam swap is going to be a lot harder. You may get 20-30 more PEAK horsepower, but the low end will suffer, and low end is king on the street. The car may run faster at the track, but if you never go to the track, or only occassionally go to the track the car won't be as much fun, because day to day on the street it will actually feel slower.

To really shift the entire curve upwards you will need to change the entire combination of the heads, cam, and intake. You can't effectively alter the entire character of the way air and fuel enter an engine without addressing everything in the airflow path. The cam may be capable of producing X-amount of flow, but the intake and head ports are now acting like bottlenecks, effectively limiting the potential of the cam.

To make the engine really work well you need to have matched components throughout the engine. The exhaust side of the SBC is weak, far weaker than the intake in fact, which is why modifications here are your best first course of action. Once the exhaust is opened up you are ready to delve into the intake system, which is now the restriction to airflow. You want to take the heads, cam, and intake right to the edge of where the bottom end starts to suffer, maybe even just a hair past it. Then you'll have extracted the most you can from the engine without increasing the displacement, adding nitrous, or adding a supercharger or turbocharger. This is the approach taken by all of the big name tuners like Lingenfelter, SLP, and the like. If you read a lot of the source material that is out there and turn wrenches and race long enough you'll really see the wisdom of the approach that these tuners take. The end result is a car that is a blast to drive day in and day out, and still rips up the track when the bug gets you to see what kind of numbers she'll turn.

 

Assuming he has a 305, I think that it would be best to check out the magazine article series on the 305 buildup. The magazine is either Chevy High Performance or GM High Tech. I get both mags, so I will look it uo. The series had mods and ETs.

For those who are interested, go on line to each magazine, you should be able to find the 305 articles.

 

Years ago Chevy High Performance did a 305 buildup for the Silver Streak Camaro, which ended upbeing a giveaway car. It used the 335 stroker kit from Powerhouse and a host of other parts. I made decent power for a wee-o-five.

Personally, I'd drop a 350 in before spending any loot on the 305. Yes, I know you can make power with the 305, and yes I know that plenty of people run good times with them. Please don't feel the need to post in just ot be contradictory.

The 350 has an inherent advantage over the 305, because it has a bigger bore. The small 305 bore makes it hard to unshroud the valves, and difficult to run 2.02/1.60 valves (you need aftermarket heads with revised valve angles). The bigger bore also provides more piston area to push down on, generating more torque. All other things being equal a 350 is a better starting point for modifications than a 305. When it comes to street engines bigger is almost certainly better, at least when it comes to displacement.

 

The 305TPI Firebird buildup is in GM High Tech.. The project car and article series is called "Blue Thunder". There is a extension series of articles called "The Mini Mouse of Might" which takes Blue Thunder deeper into mods.

The first series (Blue Thunder) has the 305TPI in the 13s.

 

well... my cam comment was based on thinking he had a peanut cam. My mistake. Even if that were the case, I would still sugest headers and a cat back 1st along with taking the die grinder to the TPI.

 

Well, the car is a 5.0, but I'm not looking at it anymore. I just bought a '91 Trans Am. It has the 5.0tpi/auto. It also has the G80 code, but I didn't get the axle code. (I bought the car, just don't have it in my possession yet.)

Do you guys think it has 3.23's? I asked him if it had the gu6, and he said no. So no factory 3.45's for me. Oh well. At least it has the LSD.

So the newer 5.0's had the L98 cam? Good stuff. Can I do 1.6 RR's ? Won't that help things out?

So here's a revamp mod list:

-full tune-up
-KN filter no airbox
-port lower intake manifold & runners a bit
-afpr
-headers
-cutout
-I believe the car has the flowmaster muffler on it, so all I want are the dual 3" cannon slashed tips.
-Shift kit
-vette servo
-TC with nice stall? 3200 to high?
-if gears are less than 3.23, swap them out for 3.23's or 3.42's.

I'm staying with the 5.0 or now. It has 75k original miles and I'd like to restore this car.

Besides, my just wrecked '97 GTP had over 400lbs of torque, 300lbs is enough for the street.

I just want low 14's, maybe dip into the high 13's and just relax in this car.

Am I on track with this thing?

 

Leave the airbox and install a K&N. Running na open element K&N in the engine compartment isn't a good idea for a couple reasons. One, you are now sucking hot engine air rather than cool outside air. Two, the filter cna pick up eddies int he airflow from the cooling fans that will disrupt the airflow and can cause idle problems (not severe problems, but they are noticeable).

You can't really port the factory runners much. If you're going to port anything port the base and plenum, then just add larger runners.

3,200 stall is too high. Stay around 2,600.

AFPR is always a good mod. Lower the pressure about 5 psi from stock. It'll pick up power, trust me.

If you're gonna do gears I'd go right to 3.73s. You've got a good OD inthe 700R4 at 0.70:1.

 

About the KN, I was gonna leave the duct, just not use the box. I should've worded it better.

As for the runners, aren't the stock ones ok for the 5.0, but a restricion on the 5.7? I can port the plenum and intake base with a grinder right?

As for the gears, again this is what I've heard, that it's overkill right now to have 3.73's. Also, these car's aren't gas sippers by any means, but I want to get as good of MPG as possible. Won't 3.73's just guzzle it down worse? Also, what about exhaust drone with 3.73's. I like a good sounding V8, but not to drive me crazy.

One more thing, about the AFPR, won't the computer reclibrate it back to a stock setting afterwards? I was always confused by this. And if I have to get a ecm to change the fuel pressure table, why not just have the chip do it, and wouldn't a AFPR become redundant? I know hundreds of guys use them and like them alot, I just need help grasping the concept of how it stays at the choosed pressure, that's all.

thanks.

 

The AFPR is what adjusts the fuel pressure, not the ECM, and and you can't buy a chip that will raise fuel pressure. So no the ECM will not re-calibrate it back.

Also 3.73 gears will not make much difference in the fuel consumption, however your foot will. Gears offer great bang for the buck, and my car was originally a 305 TPI but w/ a 5 speed. I had 3.73 gears installed and still knocked off around 20mpg on the highway, and as for the exhaust sound due to the raised RPM's of 3.73's over your current setup pribably won't even be noticable.

 

I understand what you're saying about the K&N. That's exactly what I'm talking about. You're better off using the stock airbox and ducting cool outside air into the box. The belief that the airbox is some kind of huge restriction in the intake system is a myth. I've seen and heard guys for years talk about taking out the sound baffles in the airbox on a variety of different cars, and how this improves power. You won't find a single solitary bit of evidence that proves that this works. You will get a little more noise from the intake, but that's about it. In truth the hot air from the engine compartment probably cancels any gain from reduced restriction. Intake air temperature has a dramatic affect on power. That's one of the reasons that nitrous is so effective. In addition to bringing a lot more oxygen to the party it drops the intake temp by about 100 degrees. There's a formula out there that states that every 10 degree drop in intake temperature is worth about a 1% increase in horsepower. That's definately true. Ever notice how a car seems to have much more power when it's really cold outside? That's why.

To some extent the ECM will adjust the injector pulsewidth in an attempt to get the air/fuel ratio back to the parameters under which the chip was programmed. It can't affect fuel pressure, because that is set mechanically by the regulator. It can only tailor injector pulsewidth (on time) to try and get the indicated air/fuel ratio (based on input from the O2 sensor) back into spec.

However, the ECM only reads the O2 sensor at part throttle. At WOT throttle it ignores the O2 sensor input (the refresh rate on the sensor isn't fast enough to provide an accurate indication of the mixture anyway, so it's best that it goes on a preprogrammed fuel table for WOT). Therefor, at WOT the only things that affect fueling are the pulsewidth table in the ECM and the fuel pressure. If you decrease the fuel pressure a couple fo pounds it will lean out the micture. The factory calibration is a bit rich to protect the engine. Hot rodders always say that "Lean is mean", and they're right. Leaning out the WOT micture will usually pick up a tenth or more at the strip.

There is one down side to this though. At lower fuel pressure the fuel is more likely to fall out of suspension, and atomization isn't as good. This translates into a slight reduction in efficiency, and therefor power. The best of both worlds would be to revise the fueling tables in the ECM to lower the pulsewidth values somewhat, and then raise the fuel pressure. This gets the mixture where you want it across the board, but also improves WOT mixture and atmoization, and therefor power. This will provide the best possible increase in power, plus you can tailor the spark tables in the ECM too. This will get your engine right on the edge of detonation and damage, but this is where the most power lies. If you aren't comfortable with this, then just use an AFPR and drop the pressure a few pounds.

I agree that the difference between a 3.42 and a 3.73 final drive ratio is negligible on the street. The difference is cruise RPM won't even be 200 RPM. Your foot means a lot more than your rear gear.

 

Thanks TKO!!!

I feel silly calling you TKO, what's your real name?

 

Yeah, no big deal. It took me years of reading and hands on experience in the field, plus talking to a lot of knowledgable folks in the industry to learn all this stuff. I like to share the inforamtion as much as possible. A lot of people shy away from EFI because they don't understand it. Most of theose same people don't really understand how a car works either though, but it's easy to swap a couple parts, do a little trial and error at the strip and presto it runs like a million bucks again. EFI doesn't work like that. You've got to have a good foundation in electronics to really understand it, once you do I think it's much easier to tune than a carbureted engine.

My real name is Tom, but everyone here calls me TKO anyway!