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Old 10-31-2012, 09:25 AM   #2
Devs93
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How do I adjust my TPS?
On TBI engines, the TPS is not adjustable. Just leave it alone. You can test it for proper operation using a volt meter, but there is no need to adjust it. Here’s a link that will help you to test your TPS using a digital volt meter: http://autorepair.about.com/cs/faqs/l/aa042603a_1.htm. It also includes instructions on how to test other sensors for proper operation and may help to fix common engine problems.


Will using higher octane fuel add power?
No. If your 350 TBI is stock and running the stock base timing setting of 0* advanced, then there is no need to run higher than 87. Even with 0* you could possibly still hear knock. This could be caused by excess carbon deposits that are creating detonation usually found in high mileage motors. Seafoam top engine cleaner or equivalent (GM top engine cleaner) can be used to remove these deposits and prevent detonation. Higher octane fuel may be required for engines that are running high compression ratios or advanced timing curves from a custom chip or tune. Running higher octane fuel than is needed for your application will not clean your engine or add power to it. In some cases it may actually decrease power because of an incomplete combustion.


Now that my engine has a tune up, what areas should I start to mod/upgrade?
An extremely popular first mod is removing the stock air intake snorkel assembly and replacing it with an open air element or cold air intake (CAI) kit from K&N, AirRaid, etc. An open element will sit flat on top of your TBI unit and allow you to remove the spacer ring that surrounds the perimeter of the top of your TBI unit. Before you look into serious mods like heads and a cam you need to address the other weak points of the truck, such as the exhaust system and fuel system. A complete exhaust upgrade from the exhaust ports back should be installed as the first major mod to take the full advantage of future upgrades. Also, the stock fuel pump will not be able to effectively support the power your motor will make with a new cam and heads. Installing a larger fuel pump will not affect the fuel pressure of your current setup since the pressure is modulated by the fuel pressure regulator, not the pump. An adjustable fuel pressure regulator and a fuel gauge should be added to adjust fuel to match your power upgrades as you go along. Lastly, you will want a performance chip/tune. The best approach is burn your own chips, but there is a steep learning curve. The next best option is to have tuners such as Wester’s Garage, Wait4Me, TBI Chips, etc, burn you a custom PROM for your application, particularly if you’ve changed the displacement, heads, and/or cam on your engine. On a mostly stock engine, chips and tunes from Jet Performance, Hypertech, etc. will work fine.


How do I do the ultimate TBI mods and are they worth it?
The ultimate TBI mods are a set of modifications that improve the airflow and fuel into your engine. They include: removing the ridge on the injector side of the TB unit, shaving the throttle shafts, installing an injector pod spacer. There are two great tech articles on how to do this below and they are pretty straight forward once you have a grinder and TBI unit in your hands. It will improve airflow by about 5%, but more importantly unshrouds the injectors for a cleaner air entry + fuel atomization. This added flow will help at higher RPMs. Companies like CFMTech and XtremeFI also manufacturer TB with larger than stock 46mm holes (48mm and 50mm). These units already include the ultimate TBI mods mentioned performed on the TB.

Ultimate TBI mods instructions: http://www.thirdgen.org/ultimatetbi and http://www.thirdgen.org/ultimatetbi2
Pics of the ultimate TBI mods: http://www.thirdgen.org/techboard/tb...owing-tbi.html


Will a larger TB make more power?
IMO this should be your last upgrade. I’ll say it one more time: most people overestimate the CFM requirements of their engine. For a more accurate estimate: use the Wallace Racing CFM calculator: http://www.wallaceracing.com/intakecfm.php. I like it because it also takes into account the type of intake manifold. Engine CFM requirements highly depend on the Volumetric Efficiency (VE) of your engine at red line. It automatically calculates the compensatory 120% to 150% of the engine's base requirement using the dual plane intake. If you were to use a single plane intake (yes one exists - the Edelbrock MPFI unit is a single plane design), the percentage over the engine's base CFM requirement is between 110%-130%. Most factory engines are between 65-75% VE at redline. On a mildly upgraded 350 engine (i.e. headers, exhaust, chip) with stock heads and cam, the VE hits about 72%. If you were to change cams, you're looking at about 78% VE. Change heads and cam, about 85-88% VE.

Stock TB cfm on a 350 engine is 490 cfm at 1.5inHg. Redline on the stock engine (or mine at least) is 4500-4750 RPM. Like I said before, the stock engine's VE is about 66%. Input the numbers into the calculator and what do you get: 381 to 476 cfm. So that's why the engineers at GM chose the size of TB that they did. Now let's now say you've changed the cam plus all the other performance goodies, redline now at 5000 RPM, stock heads, VE now at 78%: 474 to 592 cfm required. You could still use the stock TB easily with no major bottleneck in power. Ultimate TB mods would give you 520 cfm. Even with a cam change and other performance goodies, the stock TB is still pretty decent. You could upgrade to the CFM-Tech unit rated at 590-620 cfm if you wanted to.

What if you changed heads/cam now and redline at 5600 RPM, VE now at 88%: You'd need 613 to 766 cfm. The CFM-Tech TB would work, but the larger 50mm TB would be better. It's rated at 690-750cfm, depending if you believe the upper figure quoted by XtremeFI or not. The 454 TB (also 50mm) is rated at 670 cfm at 1.5inHg w/o the injector pod according to airdeano's tests. I'd tend to believe airdeano's numbers more because they're pretty bang on: http://www.fullsizechevy.com/forums/...d.php?t=145234.

A 383, stock heads, with cam, Redline at 5000 RPM, VE again at 78%: 519 to 648 cfm. Either the CFM-Tech or XtremeFI TB would work. The stock TB even with the ultimate TBI mods (520 cfm) would be too small IMO.

The bottom line is: calculate the CFM requirements of your particular engine. Most likely, you won’t need to buy a larger TB unless you’ve made extensive upgrades to your engine. The ultimate TBI mods are relatively easy to do yourself and will increase your TB airflow to about 520 cfm. More than enough to support your performance upgrades. (Note that the required cfm of your engine has nothing to do with hp!!)


What is a TBI spacer and does it add power?
A TBI spacer is just what it sounds like. It is a piece of plastic, wood, phenolic resin, or aluminum that bolts in between the TBI unit and intake manifold. It helps increase the intake manifold plenum size and is designed to add power. It also acts as a thermal insulator between the TBI unit and intake manifold. Resin and plastic materials may be more effective than aluminum and since they will not conduct as much heat into the TBI unit. There are two paper gaskets between the TB and the intake manifold so you be the judge. Most people claim that they have improved throttle response and mid-range torque after installing their spacer. A TBI spacer usually requires a little tweaking of the throttle brackets and TV cables (for pre-93 trannys) to keep it from pulling the throttle open. Make sure you properly adjust your TV cable. Failure to do so could result in transmission failure. Any service manual has an instruct set on how to properly adjust your cable. An on-line service manual is available from Autozone’s website: http://autozone.com/servlet/UiBroker...Parameters=005

Also, keep in mind that if you upgrade to a larger TB, it is a good idea to have your throttle body spacer and intake manifold machined to match your intake tract.


What is an injector pod spacer and do they add power?
An injector pod spacer is a plastic spacer that raises your injector pod up 1/4" to unshroud the injectors for better air and fuel delivery. They can be found on ebay and are also sold by Jegs and Summit Racing. This mod is considered a component of the ultimate TBI upgrade. Do they add power? No dyno tests have ever proved it but when combined with the Ultimate TBI mods, your truck engine will gain both fuel economy and power: http://www.cfm-tech.com/trucksarticle.htm


How much power can my stock fuel pump support?
The answer is not much. On the typical 350 TBI engine with an AFPR, up to 240 hp. Past this, and you need larger injectors, more fuel pressure, and/or a larger fuel pump. A high flow unit will supply enough power to your TBI system no matter what mods you go with. Popular choices include, stock GM TPI pumps, Walbro 190lph and 255lph, and the Holley 255lph. Each of these pumps can support gobs of power but will work safely with your TBI set-up because your fuel pressure regulator will keep the fuel entering into your TBI unit where it needs to be. For Vortec engines, fuel injectors are rated at 19#/hr at 42.5 psi, but the stock fuel pressure is actually at 60 psi (22-23 #/hr). Max hp is about 330 hp because of the limitations of the fuel injectors. There is some discussion that some Vortec injectors make be underrated and may flow up to 28#/hr at 60 psi. This would support around 400 hp. Unfortunately, there are no larger replacement fuel injectors for Vortec engines or direct-fit AFPR that can boost fuel pressures to support more hp. Some Vortec owners have gone so far as to replacement their Vortec induction with the marine version, which uses 24#/hr fuel injectors. This way, the induction system won’t be the bottleneck in making big power.
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