Originally Posted by lowhirod
1995 TBI 350 5.7 10.4 to 1 comp. ratio . will this run on pump gas 93 octane?
Assuming this has the original Swirl Port cast iron heads and of course is cam timing limited by the vacuum needs of the MAF measured TBI, this will prove to be too much compression for efficient operation. If the OEM TBI is functioning, there is a detonation sensor that will pull timing out if it hears pinging. This will operate the engine with less than optimal timing, but it will most likely run without you hearing any pinging. However, tests have shown that good all around performance that's power and mileage as well as emissions degrade faster and deeper when the timing is less than optimal opposed to operation with less than optimal compression.
This engine has from the OEM an extremely mild cam so it isn't going to be compatible with very high compression. The TBI likes a lot of manifold vacuum which is a characteristic of mild cams. The OEM TBI will only tolerate cams with a .050 inch measured duration of 190-210 degrees, beyond that the manifold vacuum (Manifold Absolute Pressure, MAP is what you'll see this as a reference) becomes to low for the system to function properly. Even with such a cam as I reference, you'll most likely need a custom chip that rebuilds the fuel and ignition maps to the new relationships of thorttle position to RPMs to manifold vacuum the integration of which is used to compute mass air flow through the motor which is used to select the appropiate fuel quantity and ignition timing.
PS, the OEM cam uses .050 lift duration of only around 170 degrees with less than .4 inch lift at the valve, so even the "big" cam of 190-210 compatible with TBI is a considerable leap in duration and lift while still being a pretty mild cam itself. As duration gets longer the intake valve closes later this lets the cylinder bleed mixture till about the torque peak. The use of high compression ratios is intended to recover the lost bottom end torque that comes with bigger cams that cause bottom end mixture density loss. This is the Dynamic Compression Ratio (DCR) calculation, you can find these calculators on the web, you need to know when the intake valve closes in crankshaft degrees to use them. For a cast iron head you're looking to a DCR of 8 to 1 for regular unleaded to about 8.5 for premium 92-93 octane. For an aluminum head add .5 to the ratio. This is where Vortec type combustion chambers are involved. For older heads including the Swirl Port this can be backed down .2 or more, especailly where bores exceed 4 inches and the spark plug is located well distanced from the bore enter and not angled toward the exhaust valve. Full dish pistons and piston to cylinder head deck clearance is greater than at least .040 inch and especially if this clearance exceeeds .060 inch for iron heads.