Which power piston should I use?

[Walleye4Days]

I recently bought the spring kit from Cliff (of course) and it came with an Orange, silver/natural, green, and black/brown springs. I just picked up a reman 305 bored .030 over,
with new everything internally (stock'ish parts) except it has a new marine Comp Cam from summit (biggest one they offer for stock'ish 305's) and am now currently rebuilding my 4bbl Qjet (17059298 stock on Volvo Penta marine 305's) and am not sure which power piston spring to use from the kit.

There's no way for me to check vacuum on the engine because it's not installed yet and obviously needs the carb for break-in and etc. So, which spring would you guys use? Also, this reman came with 2 different heads on it, which is something I've never seen before. A 416 head and a 376450 head... I'm assuming that's the same exact heads just different casting numbers(?)

Anyways, I digress. Which spring do you guys recommend in my 17059298 Carb with the following specs:

- 1984 Volvo Penta 305 SBC/gen-1 SBC (marine application)
- Jets: 69
- Metering rods: looks like 403 or 40B (?)
- electric choke
- stock manifold/heads, just port matched
- Comp Cam with the following specs:
--- Adv Duration 262/268
--- Dur @ .050 218/224, Lobe Sep 112
--- Valve Lift .462/.477

Sorry for information overload and rambling, I'm just so confused and figured too much info is better than not enough. Hopefully someone will have some good advice, because I'm lost at this point as I don't understand how exactly the power piston functions/full purpose.

[Cliff Ruggles]

Might want to do some research on those heads, they are NOT the same from what I can remember.  The combustion chambers may be close, the 416's are around 58-59cc the 450's were a little larger, maybe 60-62cc.  I do remember the ports being different, especially the exhaust port so even if the compression were close flow characteristics are much different.

I'd use the orange PP spring from my kit for what you are doing.......Cliff

[Walleye4Days]

Thanks Cliff! Yeah, I was kinda curious about seeing 2 different heads, but after calling the local machine shops and sending them pictures, they eased my mind. They said "you'll be fine.  Sometimes if the core head is bad 416, and there's not any 416's laying around, then they'll grab a 450 smog head (the same as a 416 but not as hogged out) and machine both to match each other on the flow bench & equal out the shrouding amongst the two. So, guess I'm just gonna send it until these new heads arrive, hehe.

Thanks very much for the response, it is greatly appreciated! I'm definitely dropping the orange spring in now as we speak.

[Cliff Ruggles]

Doesn't ease my mind.  The ports aren't the same and the exhaust port on a 450 is deficient compared to a 416.

WAY back when I was into building SBC engines we used to look for 416's instead of early double hump heads to build high compression 350 engines.  They have good flowing ports and small combustion chambers, most were around 58-59cc. 

I'd open them up for larger valves and end up with a really nice flowing set of heads.

The 450's are deficient and similar to the 882's from what I can remember, and the exhaust ports are closed off down around the valve guide bosses.  This was done to choke things up and create some natural EGR for emissions from what I was told back then.

In any case I don't agree with mixing head castings when they are not the same in all areas.......Cliff

[Cliff Ruggles]

As it relates to this topic the engineers at GM were doing a LOT of R & D back in those days to get those older engines to pass emissions.

Starting in 1970 they started making changes to the heads that were NOT for the better anyplace, which included larger combustion chambers to lower compression, then smaller intake and exhaust ports that hurt air flow. 

At about the same time they started doing this dual pattern camshafts started showing up from all the major cam grinders, even though single pattern cams work better for the earlier heads in most cases.  Adding some additional exhaust duration was done to help scavenge the cylinders better due to the poor flowing exhaust ports, crappy "log" exhaust manifolds and restrictive exhaust systems found on the later vehicles.

By 1974 that had pretty much ruined the 350 heads and the 882 castings were developed and used on just about every 350 that left the factory.  They are HORRIBLE heads in every respect and to be avoided for any type of performance use.

Kind of interesting but they applied the same design (restrictive exhaust ports down by the valve guide bosses) to the 305 heads as well, but they didn't do it to all the castings.  The 416's still got the good ports, as did the 081's from 1987-up.

I paid very close attention to these things as I was building a LOT of "budget" 350 engines and we had pretty much exhausted the supply of the excellent 041 and 186 castings from the late 1960's.  Plus the later castings already had hardened exhaust seats vs the "soft" seats from the early heads. 

Anyhow, not trying to put a big black cloud over the engine build but I'll bet that the folks building that engine did NOT cc the combustion chambers to make them identical.  An no amount of "machining" or porting will help the restrictive ports.  The only good news is that most customers would never know that they are not the same as the engine will run fine as the combustion chambers are close in size so compression would be as well. 

The cam really isn't big enough to make great top end power and the Marine application doesn't go there anyhow, so overall the engine will probably be fine and you woln't have any negatives from miss-matched flow characteristics between the heads.

You will also find out that the folks building those engines are going to defend their choice, but it still doesn't make it the right thing to do.  I've been at this deal for about 40 years now and have found that attention to detail ALWAYS produces the best end result vs just using parts that might work OK together even though they are not exactly the same.....FWIW......Cliff