Hey Guys,
Let me start by saying you are a wealth of information...what an awesome board! I have had my 87 IROC T-Top Auto since new (my first car) and it has 80000KM on it....I won't be touching it anytime soon as it's all original.

My question is about a car I am looking into buying, it's a 1990 IROC Convertible that has no Rear Discs and 180K Miles on the original 305TPI...but it's a 5spd

Rather than refreshing the 305, I want to do a L98 swap that I can get from a local scrap yard pretty cheap. The thing is that reading this board has got me all excited about doing a 383 Stroker to the L98! The difference in price between the Stroker kit $1K and the rebuild kit $500 sounds well worth it!! Especially since I don't plan on pulling the engine out again later.

I think I may be getting carried away because I want to keep the TPI set up and I can't afford to swap heads right now. My rationalle is that the bottom end with a 383 will be ready for upgrades later on, and heads/Intake can be swapped without removing the engine again.

I know from reading what you guys wrote that the TPI is a "Torque Monster" and I should upgrade the Intake and put a miniram to get better airflow and increase the powerbands RPM range. I understand that I will be stiffling the 383 and the power will be at lower RPM's. But will I be better than a stock 350 and mild cam with a 383 and mild cam? Can I expect more than 300hp (for now ) or am I just doing something stupid?

So I have 2 Questions:

Will the 383 Stroker and Stock TPI set up, with mild cam (it will probably stay that way for a couple years) drive better and have more power than the 350 with my limited mods (just a mild cam upgrade) or should I just rebuild the 350 and call it a day?

Is there anything I can do to the stock heads to generate better performance either way.... I am thinking better components since the price difference seems marginal and I will probably be changing the valvetrain either way, especially to accomodate the 383 anyways.

I would rather put the money into the bottom end for now and do the brake upgrade next summer and probably a paintjob the summer after....so realistically the 383 would stay that way for a while.

Should I just forget the stroker idea all together?

Edit: I will also be doing the Headers and a performance clutch...probably Hooker Shorties.

 

In order to properly answer the question, a little TPI theory is necessary.

TPI works by capturing and using the pressure wave generated by the intake air rushing down the runner and smacking into the backside of the intake valve at the end of the intake stroke. This pressure wave (sound wave), a positive-pressure pulse, travels BACK UP the runner, and DOWN the NEXT runner in the firing order; thereby re-inforcing that cylinder's "fill" event. Then that cylinder, in turn, when its intake valve closes, sends a pulse to the NEXT cylinder; and so forth.

Given that mode of operation, it should be obvious, that there is an "ideal" RPM, at which the pulse from the prior cylinder arrives at "this" cylinder, at just the right moment to produce maximum reinforcement of "this" cylinder's fill. And, it should be equally obvious, that this RPM, is the one at which the pulse travels up the "prior" runner, into the plenum, and back down "this" runner.

Since this is a sound wave, it travels at the speed of sound. Duh. Lots of stuff about engines is based on the speed of sound, so this certainly fits right in. The speed of sound is about 1100 ft/sec, give or take, depending on a number of variables; temperature, air density, and so forth. But, the number is useful, if not absolutely accurate, as an estmation.

In other words, sound travels about one foot in one millisecond.

The length of the runners in TPI is 22". That's almost 2 feet!!! To put that in perspective, if this was a carb intake, let's say like the old Chrysler Cross Ram system, it would be somewhat like putting the carb for the right bank on top of the left strut tower, and the carb for the left bank on the right strut tower. (Not exactly the same of course, since we're talking about dry flow vs wet flow, and one plenum vs two). We'll come back to that whole notion in a minute.

So, the pulse in question travels up one runner for just about 2', goes around the plenum in whatever way, and goes down the next runner; and therefore would arrive at the next cyl, about 4 milliseconds later. Which means, the "tuned" RPM that corresponds to the "Tuned" in "Tuned Port", is when the cylinders fire 4 milliseconds apart.

That "tuned" RPM is 3600, more or less.

Which means, that EVERYTHING ABOUT TPI, is optimized to produce maximum cylinder fill at about 3600 RPM.

As long as you keep that intake, that means, you need to optimize the engine for operation at 3600 RPM.

So: let's divide the things you can do to a motor into 2 kinds; things that change the operating RPM range, and things that make it work better at any RPM.

Things that change the RPM range, if done to a TPI motor, can actually SLOW THE CAR DOWN. Putting the wrong cam in it is the #1 MISTAKE people make with this system. It doesn't work like a carb setup, with little short runners and no (or minimal) tuning, where you have a fairly broad range that an intake will co-operate with various cams. With TPI, the intake WILL DOMINATE the effect of the cam; and all that will happen, is if the cam would provide best operation at some RPM other than 3600, the intake will prevent the cam from doint is job; and likewise, if the cam doesn't produce the correct intake valve event to produce the reinforcement pulse, the cam will defeat the intake design.

The whole system works best if you WORK WITH the design of TPI, not AGAINST it.

Things you can (and should!!) do, that DON'T upset the RPM tuning, include first and foremost, the exhaust. Get a set of good-quality chassis-specific (NOT universal) headers, and a good-quality chassis-specific pre-manufactured (NOT "custom") exhaust. SLP headers are probably the best, Edelbrock TES is a good "budget" choice. Whichever you get, have them ceramic coated. It makes a world of difference to the reliability of the car as a whole, since the underhood temp will be 50 to 100 degrees lower, so the wiring and hoses and all that, will last ALOT longer. Headers can be very destructive to that stuff. Hooker, Dynomax, Flowtech, etc. will "work"; but your overall long-term satisfaction will be far less than with the better headers.

Look at Comp & Crane for cams that work well with TPI. Not too many of the other mfrs have TPI-specific ones. The Comp "304" grind is a good one for a 350 with improved exhaust.

A 5-speed car may already have good gears in it. If not, get a set of 3.42s. 3.73s are usually too much for a car with a stock TPI; they force the engine RPM up past that 3600 RPM ideal, which SLOWS THE CAR DOWN. Whatever you do, you want the car to go faster, not slower; so understand what you have and how it works, and it will be easier to work with instead of against it.

Remember: 3600 RPM.

The same thing applies to a stroked motor. What you'll end up with, if you stroke it for example to a 383, is your peak torque being 383/350 times what it would have been; which is a good thing. Peak HP won't be increased as much, and will be at a somewhat lower RPM. But, torque is what makes a car fun to drive on the street, so there's no reason NOT to stroke it, if the little bit of money isn't an issue.

Because the runners are so long, and so small diameter, there's NO WAY to significantly increase the operating RPM, with the stock system. SOMETHING has to change. Otherwise, those long tiny runners just make a giant restriction at higher RPMs. That's what the MiniRam is for: it replaces 22" runners with 3" ones. It totally does away with the restriction, but also elminiates the "tuning".

 

TPI was originally designed for 305 engs.

Using the stock intake would choke the 383 above abt 4000 rpm, but would be very responsive and torquey on the bottom end.

I'm running a 383 TPI. I built the eng and added Accel high flow long tube runners along with a Super Ram intake manifold all at the same time.

This combo pulls hard from idle to abt 5K rpm. Makes for a good street combo. Like sofakingdom said, you want a cam that was meant for TPI systems. I'm running a Lingenfelter 74211 that was designed as a long runner TPI cam. It works very well. The heads need some work to use this cam since the valve lift is considerably higher.

I had my stock heads fitted with 2.02" and 1.6" valves. The bowls were blended, short side radius cleaned up, and a few other flow enhancements were done. I don't quite have the flow#s of aftermarket heads but they aren't bad heads for the $500 I have into them. Plus they look stock (imagine that).

As for exhaust, I have the Hooker 2055s short tube headers with the ceramic coating. They work very well and I had no fitment issues.

If I was you, I'd build the 383 botom end pretty stout, run the stock TPI for while, then upgrade to a Super Ram, Stealth Ram, LT1, miniram, or ???? later.

BTW, you will need to learn chip burning to get the 383 to run right, especially if you upgrade the cam and/or the injectors.

 

Sofa:
Good explanation, except for one detail: Each cylinder creates its own pulse. Sound pressure pulses are reflected either by hitting something solid, or by the end of the passage. In the case of TPI, it's the end of the passage, or the plenum. So, the cylinders don't "feed" off of other cylinder's pulses (like the exhaust does), just their own.

To expound a little more, the speed of sound in a fluid is very much affected by density. Density is affected by temperature and the fluid type or mix itself. TPI is a "dry" system, meaning only air is going through the majority of the induction system. That's a good thing, because fuel ratio changes in a "wet" system like carb or TBI also affect density changes, changing the speed of sound in the induction system, which would affect the RPM "tuning" of the system. Density consistency is also why the factory put the coolant "heater" behind the throttle body, to keep the air temperature in the system more consistent.

But, I'd have to agree that the most important consideration is to upgrade the flow capability of the system, and alter the port "tuning" with aftermarket runners.

 

Not really true. It was used on both 3rd gen f-bodies and Vettes, and the Vettes only got 350's.

 

From what I can remember from Lingenfelters book on SBCs, GM was going to drop the 350 eng, but at the last min decided to keep it. A FI system had not been designed for a 350 yet, so they used the 305 TPI since that's all they had available at the time.

 

We may never know exactly what was going through their minds. The days of the early 80's were a lot different than what's been going on in the last 15 years from a performance standpoint. A "restrictive" TPI on a 350 would have been a major upgrade from what they were putting out in 1981, so whether or not it was "designed" for a 305 or 350 doesn't add much to the discussion.

Since Lyle27 already said, "I should upgrade the Intake and put a miniram to get better airflow and increase the powerbands RPM range", most of the discussion about TPI basics is moot, anyway.

 

Won't argue that TPI would be better or worse than what was offered in 81. Carb VS FI is a debate I grew tired of many years ago.

The point I was trying to make is the stock TPI will be very restrictive on a 383 and that is what it sounds like he was wanting to do until he's ready to switch to a different intake in a year or so.

IMHO, the eng displacement VS what size eng the intake was designed for is one of the most important issues in regards to eng perf.

 

sofakingdom, ZZ28ZZ and five7kid.....You guys are AWESOME! you just reinforced everything I thought about this board....it is invaluable to 3rd Gen owners!



I can't tell you how glad I am that I decided to ask. Not only have you taught me a lot about a car I have owned for 18yrs, but you have given me a lot to think about.....and ofcourse I don't have the project car "yet". So I still have time to research.

That's next on my list of "New Years" resolutions!! I have already started reading those threads!
You have already got me started on the "understanding how it works part"

Thank you! All of you, very, very much!

 

That is what I was thinking of doing......I may still do it if I can get away with an intake and runners....relatively cheap. I do want to rebuild the valvetrain whichever way I decide to go (and I guess would have to with a new cam), but I didn't want to have to spend the money getting a shop to rework it.

I am not looking for big HP gains....but would like some. But I do want the engine to be New and the best I can afford for now, because chances are it will be that way for a long time.

That being said, I also don't want to lose power or cause driveability issues. The Chip tuning I want to do and the Cam and exhaust were being done anyway, so I am cool with that.

I guess I just need to put a package together that won't screw things up.

I am open for suggestions....
My thoughts right now (because I don't know better) are that...
Do the 383 bottom end...Displacement = Good
Get a TPI tuned Crane Cam
Rebuild the Heads to accomodate (don't know how or what yet)
Do the Exhaust and Headers (ceramic coated)
Get a Chip Burned appropriately tuned.
New Injectors (Not sure how big yet)

And if I am understanding you all, it seems I would have to do an intake mainifold and runners (possibly Edelbrock)

Any idea what sort of power this would put out (best guess)?


Thanks again guys!

 

I was doing some reading and wondering....are you guys talking about a Accel SuperRam Base Intake Manifold ($500) and maybe some runners to solve the intake problem? Keeping the TPI Plenum.... or the complete SuperRam set up with Throttle body etc $4K (out of the question for me)

 

There are few different high flow intake manifolds. TPIS, Accel, Edelbrock to name a few. Same with the runners. The stock plenum can be re-used. There are a couple of things it will need done to it for perf reasons, but nothing you can't do your self with a die grinder in a couple of hrs.
I wouldn't pay $500 for an intake. Keep an eye on Ebay. I see them on there for $250-$300 or so fairly frequently. Same with the runners.

I went with all Accel/Lingenfelter parts (intake, runners, cam) since they were designed to work with each other. I'm sure other brand parts would work pretty well too, but I try to use "proven combos" whenever possible.

I've seen complete Superram set-ups sell for under $1000 used. Just have to keep your eyes open and ask around.

The larger throttle body isn't as important as it may seem at first.
Do some research before spending a ton of money on one.

Some other things to keep in mind:

Chip tuning is a slow and meticulous (sp??) process. You will need a laptop, software to monitor and datalog eng data, a high speed serial adapter is almost mandantory, a chip burner, s/w to burn chips, some chips, modification to your ecm so chips can be easily swapped without damage (ZIF socket installation), a wide band O2 meter to monitor air fuel ratios and probably a few other gizmos I can't remember at the moment.

Chosing crank, rods, and pistons can be a little tricky with a stroker. In my case, I went with a pre-balanced set. I wound up being unable to use the rods cause they hit the cam. Since I went with different rods, I had to use different pistons. Then had to have everything re-balanced again!! Choose your rods carefully.

On the heads, if you use more than .480" of valve lift, the valve guides will need to be cut down. You'll probably need to pin the rocker arm studs or have screw-in studs installed. Pushrod holes in the heads will need enlarging if you use rocker arms that use guide-plates. The list goes on and on. Best to have the heads done at a good shop.

Building a fuel inj stroker is fun and rewarding, but for me,
it really turned out to be more of a long term hobby than a little project.

 

this is something i have ben thinking about for a long time as well, a 383 upgrade, stock intake for a while, my questiuestion is, if he upgraded to a first intake, would ge be able to keep the massive torque? and get more hp? or would it still choke?