Would like to get some real answers on the question of what is the largest cam i can run in a stock 93 chevrolet pickup with a 5.7 TBI without causing bad driveabilty problems.I have heard that a 268 is the largest one.I also have a friend that put a 270 in a 92 chevy and it run fair with a vaccum canister.All of the cam companies i have called give the same answers "we recommend our tbi cams" which are not very much different than stock.
TBI is very sensitive to camshafts...if you want more power, change the heads. Those truck heads are very restrictive.
I would say the biggest cam I've used is the xe262 computer cam from Comp. But I change the heads, so this helps power alot. I also use a 6" rod to lighten the piston, and KB pistons.
Then do some chamber porting and minor short turn work to the heads and I run 1.6 x 1.5 rockers. Seems to work really well with the bigger throttle body and a spacer.
You can really feel the power of all these mods, and the more you do the better they run. If you only do one or two things, it's not very noticeable.
We do a lot of performance 383 TBI motors, they pass smog and the drivability is great. We do a scat 3.75" stroke/Manley 6.0" rod/Wiseco forged piston/World Sportsman II(EO Number for smog), put the big block injectors in the TBI unit and tune the computer - and I forget the exact cam specs. They are very well running packages, make about 250hp at the tires with the TBI manifold being the bottleneck - if you do a carb conversion its an instant 40hp increase.
The Holley TBI manifold will pick you up some over the factory, but not much.
Pick a cam with a duration of no more than 220 @ .050 with a lobe center as far apart as you can (114 works well). This will give you a good vacuum signal so the MAP sensor won't go nuts. Better heads (like Vortecs) are a good idea. There are few options for intake manifolds, but your motor will be mild enough that a manifold change won't give you phenominal results anyway. There are changes you can do on the TBI unit that will give good results (injector spacers, BBC injectors, profiled inlet vanes). Free up the exhaust, but don't go too big on the tube diameter. Believe it or not, a single 3" works better than two 2.5s with a TBI.
I have a 91 GMC with 406 small block. I'm using a larger Comp cam with 1.6 rockers. As long as you have somebody to re-tune your chip, you can run whatever you want. I run 13.1's at 2500 feet. The truck runs well on the street and is good on fuel too.
Last edited by gmc406 on Mon Nov 19, 2007 12:06 am, edited 1 time in total.
If you are talking about using the stock computer and chip, you don't want to deviate too much. It will compensate for small changes, not large ones. This is why the cam company cams are not much different than the factory cam.
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Is what really throws them out of tune is the map sensor reading all over the place and very low on vacuum correct? Couldn't a guy unhook the map sensor then the cpu will only run from the TPS, RPM and temp, correct?
F1Fever wrote:Is what really throws them out of tune is the map sensor reading all over the place and very low on vacuum correct? Couldn't a guy unhook the map sensor then the cpu will only run from the TPS, RPM and temp, correct?
The map sensor is what tells the CPU when the engine is under load, like when you put it in gear. By installing a larger cam, the vacuum drops, and the CPU will drop in more fuel. Getting your chip reprogrammed will counter-act this.
I find this interesting as a couple weeks ago I played with the idea of a duntov 30-30 cam, highrise single plane and TBI, or possibly a lowrise 2x4 with dual TBI's. Then I came to my senses and got a vortec
Anything more than 204-210 @.050 and tighter than 107 centerlines (at that duration) will be dicey with the stock TBI set up, which also doesn't like the added flow of Vor-Tec heads, either with or without a cam change.
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I don't know what your ultimate goals are but if you get a customized tune for the computer it opens up alot of opportunities. In fact I would just buget some money for a custom dyno tune as part of the upgrade.
That way you can maximize the gas milage, horsepower and drivability. Adding a cam, headers and an improved intake system will need a tune to get the most out of it.
I used a Crane HMV260 retarded 5 degrees to give the same event timing as their computor controlled cam. 3" single Exh, chip, K+N, valve bowl work. Works really well with the EGR unhooked, but has a real lean spot at idle if the EGR is operational. It needs a reprogram to fix that but I have an 03 6 liter now so I won't be doing anything more to it. Really happy with the power though.