Recommended modifications

Always need these. The worst piece on the whole TPI system is the cold start injector or referred to as the 9th injector. Regardless, problematic at best. I recommend you get a block off kit off ebay and remove the thing and tell me when you order a chip so that I can program the chip to run with the other 8 injectors and that will solve a lot of cold start issues on these cars.

Next is that intake manifold, for some reason people seem to like the looks of it. Just reminds of another of GM’s screw-ups and poor design. with a base intake, a plenum and then 4 long runers giving 8 problematic places for vacuum leaks and those long runners just kill any top end power potential. In a jeep or 4×4, great low end torque although still 8 places for vacuum leaks. Changing runers can help some but still perpetuating that long runner torque only design. If you want good horsepower numbers from your car that intake has to go. EVERYONE needs to read  so I might need to move the edelbrock up a notch to my most recommended intake setup for a TPI engine.

The Key reason the the 88-92 L98 350 TPI motors made 245hp and the 93 LT1 made 275hp is that intake. heads flow practically identical numbers and the cam was almost as similar. Best intake for TPI in my opinion is the TPIS miniram.

which is pretty much a LT1 intake designed for TPI. A bit expensive really but nice one piece intake design that cures the issues with TPI intakes. Another similar option is the Edelbrock pro flow xt which is the Ramjet intake GM uses on their Ramjet crate engine. intake is much more affordable than the TPIS intake but it requires a LS1 or LS2 single blade throttle body which I don’t necessarily think is a bad thing just adds to the expense. This intake also comes in a version for vortec heads which opens up another avenue of performance heads for you.

Last intake on the recommend intakes list is the Holley Weiand stealth ram, 2 piece intake setup which does have some possibility for vacuum leak but 2 much better than 8. Intake is great performing intake and the reason its not number one is its too tall. BARELY fits under the Camaro hood and most Firebirds it won’t fit under and is a good inch too tall for under a Corvette hood. Why they couldn’t have made the intake a inch shorter, I don’t know. On a STRONG 383 engine though this intake is HARD to beat. also comes in a standard and vortec version as well.

Note that the Edelbrock and Weiand intakes do not have EGR provision for emissions and its an optional piece on the TPIS intake. Not a fan of EGR myself. If you don’t need it get rid of it but some just have to have it although I have had some customers epoxy one on before and made it look functional but not and get through the emissions test on a good engine build.

Now if your really on a budget and have more time than money, you can modify the stock TPI intake base to shorten the effective intake runner length by cutting out a section of aluminum between the intake runners on that intake base and you will get at least 1/2 the gain if not 3/4 the gain of the aftermarket intake.  Madmax on the website had a very nice post with picture showing that porting.

Once the intake is addressed on your project the next place to modify is the exhaust. exhaust was restrictive on your 200hp cars and will choke the snot out of good modern power level engine. Single biggest issue with the exhaust is where the 2 pipes merge together. GM intentionally made this piece restrictive to build back pressure so that the EGR system would work well. Restrictive on stock motor is bad enough but if your wanting 400hp type power levels to run with modern cars or over 300hp for that matter, that needs CHANGED. Best option is the flowmaster Y250300 collector which currently is less than $50 from your favorite vendor. The next issue is the floor pan of the F-body cars was not designed with exhaust in mind. just look under the thing. You can run long tube headers but your likely to drag them on an acorn but long tubes are always best. My car with sub frame connectors that just was going to drag for sure so I used short headers and had custom 2.5″ pipe bent around the factory routing to my flowmaster collector and then a nice mandrel bent 3″ exhuast. I went overkill on my Camaro project and a good 3″ system is fine but there aren’t many options out there anymore. Hooker is about the only 3″ intermediate pipe system I could find. Everything else listed 2.5″ which is awful. Now if you have really done your exhaust well that single wire o2 sensor isnt going to stay hot enough to give the computer a good consistent accurate reading so I came up with a little kit to convert to a 3 wire heated o2 that first appeared in GM vehicles with the 1992 LT1 Corvettes from what I could find anyway number may change so you can search ebay for o2 conversion and it will pop right up. Plugs into factory wiring and has a black wire for ground and pink wire for key on 12v for the heating element.

Corvette owners have the exhaust much better. Long tube headers fit pretty easily and can run dual 2.5″ pipe right down that car.

Once the intake and exhaust are addressed then its time to talk cam and heads. Everyone typically wants to run way more cam than they should in their engine. You need to decide realistically how much power you want or can afford and how you intend to use the vehicle, daily driver, weekend cruiser, street strip, or drag only as the choices will be different for each. A drag only car will be miserable to drive on the street. A good engine build makes its power with the smallest possible cam. So buy the best flowing heads you can afford but not necessarily the biggest intake runner heads as large intake runner will kill low end torque because it takes too many rpms to get the air velocity up in the heads to where they start making good power. Just one of the reasons LS motors make good high rpm horsepower numbers but not great torque numbers or low rpm horsepower numbers. Cubic inches will affect that intake air velocity as well. A 383 can run a larger intake runner head without sacrificing low end power than the same head on a 350 engine. i am always glad to recommend head and cam combos for customers so if in doubt send me email with what your thinking about and what power you want to make and your budget and I will do my best to help you get there. On a street engine, try to stay under 200cc. Here are the stock TPI head flow numbers along with some other popular GM heads.

416 305 heads
Lift/intake/ exhaust

0.050 31 15
0.100 63 37
0.150 92 61
0.200 116 78
0.250 134 92
0.300 157 101
0.350 170 105
0.400 182 107
0.450 189 109
0.500 195 110

GM 882/441 heads

TPI and LT1 Z28 heads


Vortec heads.
Valve Lift—-Flow———Flow

for camshafts where most people make their mistakes. Unless your drag racing only don’t go over 230 duration @ .050″ on the intake duration and STAY ROLLER CAM. 1985-86 were flat tappet cam engines and so going roller can be expensive and would recommend buying a roller motor like a 1996 vortec truck roller motor that gives better flowing heads and a roller cam. In a nice well geared car with at least a 2000 rpm stall converter on the automatics you can run up to a 224 duration @ .050″ without too much drivability issues. If your running a speed density Map sensor based 1990-92 TPI system then you need to be careful on that lobe separation angle and for less tuning headaches the more lobe separation angle the better. you get down around 110 and your going to be sending me a lot of datalogging data with a laptop trying to get it to run worth anything. With cams more cubic inches typically the more cam that engine will like. A 383 will run similar powerband with that 224 duration cam as a 350 will with a smaller 218 duration cam and a 305 with a 210 duration cam so 305 guys need to be VERY careful what cam you put in those little engines. Again always happy to recommend cams too as its better for me to advise in the begining than try to help you straighten out a mess on the tail end.

I recommend at least changing the heads before even considering an injector change. WAY TOO MANY people contact me with their build for a chip with way too much injector. Injectors are not power adders, they simpy provide the fuel for the amount of air that motor is going to suck in. The stock 19lb injectors can support a 300hp engine, the stock 22lb 350 TPI injectors easily support 375hp. so until your build is looking like its actually going to make more than those numbers don’t go changing the injector flow rate. if you need new injectors then size accordingly. I have been getting my injectors from the ones in my Camaro came from Racetronix which are also good injectors. I have had way too many issues with Accel and Venom injectors to recommend them so use at your own risk. For up to 420hp a set of 24lb injectors will be fine. Up to 475hp 30lb injectors and past that you should know what your doing and decide for yourself. Typically its not going to hurt anything running 1 size injector higher than your motors power level as long as the chip is tuned for that injector size. But putting a 30lb injector in a 300hp motor is just going to cause issues as the duty cycle never gets high enough on the injector to really run the motor well and you will just ruin the gas mileage on your car. Which is why we have so many injector choices and why we don’t all run 100lb/hr injectors.

GM Fuel Injection for the average guy