Originally Posted by Rickyellison
I recently built my first motor. With the help of an experienced friend. Of course I made a few bad choices 2 years ago when I began it on a budget but I'm just now cranking it up. It's a 355 with dish pistons(unsure on compression) h beam rods and eagle steel crank on bottom. The problem is mainly in the top. First big mistake and most crucial was pro comp heads. Advertised to have already ironed out its old problems. No trick flo or afr heads but should at least be better than stock right??? They're 210 cc intake, 325lbs spring pressure 1.225 open and 140lbs at 1.8 closed and 600 max lift. The cam is a hydraulic flat tappet 214 intake and 224 exhaust at 050. List is 444. My pushrods are 100 longer than stock and have 1.6 ratio roller rocker arms. Summit street and strip carb and stage 2 intake. Somewhere here I made a bad mistake. This bored and cammed 350 is making less power than my old l65 weak 350 bottom end and heads. I am not sure if the double springs are way too much for these summit hydraulic lifters or if these heads are so big i can't make power with these mid range power cam and lifters. Can't afford another set of heads and only have my old heads to put back on. Should I try and go balls to the wall cam and stall (if this will even fix it) or put my old heads on and settle with a bored and cammed decent 350?
The problem most likely isn't the heads, we’ve used them on a number of budget builds and have no issues with them other than the port are rather large for their flow compared to pricier heads.
The problem is most likely with compression combined with the cam combined with the selection of a 210 cc port head. Starting with the latter a 355 expected to operate no higher than 6000 RPM, a 210 cc port is too big which kills off the mixture velocity needed to overcome the reversion forces of the piston pumping mixture back into the intake when the cam is late to close the intake valve. The smaller port 195 cc head would have been a better choice as it speeds the mixture up at a lower RPM which overcomes the reversion forces sooner thus bringing the power band lower into the RPM range.
To a big extent the cam is one of those old grinds that are so cheap guys on a budget can't resist. They do have an advantage of using a lot of duration for their lift which is certainly easier on the lobes and lifters. The down side is that these cams have very long ramps where the valves, intake in particular, are held slightly off the seat for a long time. Your choice of 1.6 rockers only exacerbates this problem. The very long ramp hangs the valve almost but not quite closed exposing the inducted mixture to the force of the rising piston which blows it back through the intake valve into the ports and manifolding, finally out the carburetor, hence the term reversion. As I said in the previous paragraph, the only cure is more RPM to get the mixture velocity up high enough to overcome the reversion force. The better option here is a more modern lobe design like the Comp Extreme Energy or the Lunati VooDoo. These cams use a much more aggressive lift rate with much shorter ramps so the valves aren't hanging almost but not quite closed for long periods of time. The down side is they are hard on wear between lobe and lifter when not properly broken in and high levels of ZDDP additive are not kept up with in the oil. ZDDP is a consumable it is not wise to stretch oil changes. The longer the oil is in use the lower becomes the ZDDP percentage from where it started in the fresh oil change.
The third major contributor to the problem is compression and the relative of squish and quench. Piston selection can kill the motor if you didn't pay careful attention to what you ordered. Circular dish pistons simply kill squish and quench which are needed for successful ignition and complete combustion as well as their mechanical octane contribution to the fuel offering as much as 4 to 5 octane’s worth of performance above the fuel's octane rating. Flat tops are the best choice piston with a small chambered head for best performance, period! The next best option is the so called D dish, D cup, or step dish pistons, These put a flat surface close to the squish/quench step of the head (you want as close to .040 inch as you can get, a couple thou over or under will work) while using a D or step depression under the head's valve pocket to dial in the compression for the fuel you with to use. Many aluminum headed engines have been built with your heads running 10 to 1 compression while running just fine on 87 to 89 octanes unleaded. You gotta be careful with this design I don't recommend it for the casual engine builder, but it can be done.
The other huge issue for pistons that traps the unsuspecting builder is the presence of what's called rebuilder pistons. These pistons measure 1.54 inch between the pin center and the crown edge, however, the stock replacement 350 piston measures 1.56 inch. The rebuilder piston is made for the mass rebuilders that zero deck all blocks they run through their shop; they restore the original .020-.025 inch crown to deck clearance in the decked block. If these pistons are accidently used in a block that wasn't zero decked (it happens a lot) there is a significant loss of compression. When a late closing cam is used in an engine it results in high reversion. To recover the power lost to the lower density mixture left in the cylinder it is a requirement to greatly add to the compression ratio as a means of restoring the bottom end power lost to reversion. This in large measure is the reason competition engines use such seemingly extraordinarily high compression ratios. There is a thing called the Dynamic Compression Ratio (DCR) which adjusts the Static Compression Ratio (SCR) (SCR being the measure of all the cylinder's volumes divided by the volumes above and including the piston crown when it is at TDC) for the loss of cylinder volume resulting from the rising position of the piston when the intake valve closes in crankshaft degrees. The later the valve closes to larger is the loss of effective stroke. A good calculator for this is at this link <<< United Engine & Machine Co. Incorporated
The DCR minimally needs to be 8 to 1. This would sustain regular fuel in and engine with iron heads and ordinary (smog era) combustion chamber. For a Vortec head this could be pushed to 8.2 or 8.3. You could go further but that would step the fuel grade up to 89-92/93. For an aluminum head like the L98 chamber this can be pushed to 8.5 on regular to 8.7 on premium. A better aluminum head that mimics the Vortec combustion chamber which your Pro Comps do can be pushed to 8.7 on regular and 9.0 on premium.
So unfortunately you have gotten yourself into a position of not knowing what the all important compression ratios (SCR and DCR) are for your engine nor probably whether you have a rebuilder piston in there or one with the OEM pin to crown height. Without that knowledge you really don't know where to turn. First thing is to get the heads off and identify what the volumes of the parts are that establish the SCR. From there you can use the linked DCR calculator to figure the DCR. You will also need to know the connecting rod length, standard SBC is 5.7 inches for the 350's 3.48 inch stroke. IT looks like you have the cam timing, The problem with the DCR calculator is the assumption of 15 degrees added to the .050 inch closing point, but for these older design cams the degrees between .050 measures and .006 (considered by the SAE as valve closed timing) can easily be 5 to 10 more degrees than KB's average of 15, this makes a huge difference. Keep in mind how serious this is compare a 28 inches of water depression on a flow bench is only 1 PSI while the rising piston is at this point producing from 20 to 50 PSI, so the backward force on the flow past the valve in CFM is a lot more than would be registered at that same low lift on a flow bench. In terms of bottom end power loss this results a huge number.
So the combination of large ports with a long duration cam having 1.6 rockers and most likely low compression is really crippling this motor. I'd recommend starting with getting control of the compression as you just can't get performance with out it. If you get that correct, even the 210 cc ports and long cam can be sorted out with tuning to a very good degree.