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Old 03-27-2013, 09:38 AM
oldbogie oldbogie is offline
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Originally Posted by oldbogie View Post
The TBI engine is an uphill battle to extract more power and hits an upper limit of about 260 HP unless you're prepared to spend some serious money. With serious money you can stare down about 330, if you can afford to push in to Holley 4 barrel TBI territory this can be pushed into 360 territory. Keep in mind that anything above 260 probably can't smog if you gotta get past one of those tests.

There are 3 big limiters other than the TBI itself which is number 4:

1) Pistons, GM uses a deep dish piston regardless of the head you have 191 or 193 which does not optimize squish and quench so in the end mid range power will suffer from the inefficient squish, and top end power will be limited by the detonation threshold of the lacking quench. Flattop or D dish pistons help both squish and quench which would let you push the compression ratio some to take advantage of a larger timed cam.

2) The cam GM puts in these engines has very short durations and low lifts. Replacing this is a big improvement but you can't go too far because TBI is very sensitive to the relationship of manifold vacuum to throttle blade position and RPM. A bigger cam overall drops manifold vacuum which throws the relationship to throttle blade position to RPM all out of whack so the map number for fuel and ignition is calculated incorrectly by the computer which than looks to the wrong place on the maps for the fuel and ignition data for the actual working condition of the engine. Additionally the whole system pretty much fails when manifold pressure at idle falls below 10 inches of vacuum. So there is a limit and any change to the cam and the rocker ratio will require a chip and possibly a fuel pressure change. If you don't need to smog test a fuel pressure change can overcome some of this problem but it shortens injector life. About as far as the cam can be pushed is into timing roughly equal to the old 327 cid, 300 horse, so called 929 cam (GM 3896929). Edlebrock makes a cam specific to pump up the speed density TBI and TPI engines it is part number 3702; see here <<< Performance Camshafts, Lifter Kits and Matching Valvetrain from Edelbrock - Cam Card Locator >>>. Edlebrock offers a custom programmed chip with this cam which you will need. As I said about 260 hp is the limit even with this cam because even with 90 pound per hour injectors, a performance intake like a Performer RPM and Vortec heads, the 2 barrel TBI just can't reliably deliver enough fuel.

3) The heads depending on how heavy duty your 95 engine is it will either have ------191 76cc low compression head or the ------193 74cc higher compression head. Both of these are so called Swirl Port heads that have vane from and adjacent to the valve guide in the valve pocket. These vanes greatly restrict RPMs in stock form they just will not breathe enough air to allow power above 4400-4600 RPM the Swirl Port heads just level off power wise on a 350 from those RPMs up to about 5000 where the engine will pull no further. This is a head function even if you put a great intake like the Performer RPM and bad 750 CFM carburetor, what you got at 4400 is about all you ever gonna get with those heads. The obvious is to change the heads, the L31 Vortecs immediately burn across one's mind and those with a GMPP or a decent aftermarket with TBI adapter plate intake manifold will work. Another choice in about the same cost category is the aluminum 190 cc port Pro Comp, these are import heads at import prices one place to see them is at Skip White performance <<< White Performance Detail Description >>>. We've used these on some lower cost street performance engines and I think they acquit themselves pretty well especially when one considers the port is lower than the Vortec and will accept your TBI intake. That lets you escape the cost of a Vortec to TBI intake.

4) The fourth item at the introduction is the TBI unit itself. The stock unit is good for something less than 450 CFM so you'll never get enough air through it for 300-330 hp anyway. That takes a 640 to 670 CFM unit; the 640 being the 454 GM piece with 1-7/8ths throttle bores or for 670cfm the big Holley 2 inch bore unit. These with 90 pound per hour injectors and a bit of a boost on line pressure can get into the 300-330 hp zone. You must figure this out as to what you want/can afford to do before the purchase of the cam as this will impact the program on the chip. Chip programming is costly so you only want to do this once where Edlebrock or whom-ever you select picks up the cost.
---------------------------------------------------------------------------------------------------------------------------Breath

I need to blow out of the shop early tonight so cogitate on this data dump and I'm be back on line early tomorrow we can pick this up then if like.

Bogie
I miss typed the volume of the 193 head while engaging in last night's indoor speed typing contest with myself; the chamber volume of the 193 head is 64 ccs not 74 ccs.

Bogie
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