Increasing compression

[mtnrat]

I currently have 8:1 compression on my stock 250hp 350. On another forum it is suggested that by going to 1.6:1 rockers with the vortec heads I can get 300hp out of my same setup. I always thought that just changing rockers to increase valvelift was a no no because of spring rates, pushrod geometry etc. That seems too easy. Is this correct info?
Current engine specs are:
crankshaft type - reground/polished cast crank
piston type - Sealed Power cast/dish pistons
camshaft specs -
intake duration is 202 @ 50
exhaust duration is 212 @ 50
intake camlift is 393
exhaust camlift is 408
110 degree lobe seperation
valve specs - 1.94 (intake) + 1.50 (exhaust)
compression ratio - 8:1

 

[bjcsc]

Re: Increasing compression

If you change heads I don't see how the concerns (spring rates, etc. ) would even be relevant. That said, modifying valves won't change your compression at all...it will still be 8:1.

 

[180shabah]

Re: Increasing compression

Don't worry about changine the rocker ratio. Just changing to the Vortec heads, will give you the power you are looking for. Keep in mind that you will also need a different intake manifold and a higer pitch prop( to keep WOT in check)

 

[mtnrat]

Re: Increasing compression

"If you change heads I don't see how the concerns (spring rates, etc. ) would even be relevant. That said, modifying valves won't change your compression at all...it will still be 8:1. "

bjcsc, I was asking if changing rocker arms to 1.6:1 from a 1.5:1 is an ok way to increase valve lift.?? I have heard that it is a no no to do so. I do know that changing to vortec heads will increase the compression ratio because of the smaller chambers, but I want to know about just changing the rocker arm ratio?

180, I do not think that I will get 50hp from the head change alone. It is my understanding that a cam change will also be needed to realize that type of increase, which then requires matching valve springs for it to work properly.
I know about the intake and I already have a prop that the boat can now only turn 4190rpm.

So to restate my question: I currently have 8:1 compression on my stock 250hp 350. On another forum it is suggested that by going to 1.6:1 rockers with the vortec heads I can get 300hp out of my same setup. I always thought that just changing rockers to increase valvelift was a no no because of spring rates, pushrod geometry etc. That seems too easy. Is this correct info?

 

[John_S]

Re: Increasing compression

Standard Vortec heads can not handle the same lift as the older style heads. I have seen conflicting specs on how much lift they can handle. I have seen it listed as low as 0.420 max lift (I don't believe it is that low because standard merc roller cam has higher lift). Also seen 0.450 and 0.470. I also saw an article that suggests higher ratio rockers might require slight machine work on the pushrod holes.

There are a good number of articles on the web. Here is one:
http://www.chevyhiperformance.com/howto/97458/

 

[Buttanic]

Re: Increasing compression

Don't bother with 1.6 rockers, there are no noticable gains to be had there. It will all come from the Vortec heads but I think you will need a roller cam also to get 50 HP. It's not just lift that makes the difference. A roller cam with the same duration and lift as a flat tappet can make more power because it can open the valve at a faster rate.

 

[180shabah]

Re: Increasing compression

The Vortec heads really ae that good. Don't believe me, check out Merc's hp ratings. The easiest one to compare is the 4.3; old heads with a 2bbl, 160hp; vortec heads with a 2bbl 190hp. the 4bbl power increase was just as good. an extra 40 -50 on a V8 is not rediculous.

 

[bjcsc]

Re: Increasing compression

I do know that changing to vortec heads will increase the compression ratio because of the smaller chambers

Can someone please explain the technical side of this statement to me. It very well may be true, but I don't understand how. I don't know how you can increase the compression ratio without either enlarging the bore or lengthening the stroke. If the chambers on the Vortec heads are smaller (lower volume) , wouldn't the volume drawn on the intake will also be lower. How does it result in higher compression? I'm missing something...

 

[180shabah]

Re: Increasing compression

For this we will ignore inneficientcies and losses as air travels throught the carb, intake and valves.

The amount of air drawn into the engine is "fixed" and determined by the swept volume of the piston. On the compression stroke all of that air is squeezed into the chamber in the head. The smaller the chamber, the higher the compression(Same volume of air going into a smaller space)

 

[Reel Poor]

Re: Increasing compression

wouldn't the volume drawn on the intake will also be lower

Combustion chamber volume has nothing to do with intake and head runner volume. The combustion chamber is the pocket on the bottom side of the head that the vavles seat in, if you increase the size of that pocket/combustion chamber you inadvertently
decrease compression, and if you decrease the size of the pocket/combustion chamber you inadvertently
increase compression accordingly. It works the same as using thick or thin head gaskets. If you use a thick head gasket you increase the quinch area which lowers the compression, and the opposite with thin head gaskets, compression increases.

Compression can be increased in several ways, lowering block deck hight, milling/surfacing the head, adding dome top pistons, using heads with smaller combustion chambers, and by using metal shim type head gaskets.

 

[bjcsc]

Re: Increasing compression

Thanks fellas. The "swept area of the piston" is what I was missing in thinking about the intake. Makes perfect sense...and it makes me think of another question, but I'm starting a new thread so as not to hijack this one.