I just bought a 1986 Iroc with the LB9 305 tpi. and want to get as much power out of it as i can with keeping the 305 in it. Does and one know the max lift numbers i can safely run with out valve damage. what is the max lift if i run a 2.02 intake valve and max lift for a 1.94 intake valve. I was told with the 305 the valves will hit the cylinder wall with to much lift because of the smaller bore size is this true and also what are some good heads for a 305 tpi

 

Lift doesn't matter about the valve size vs bore size thing,

Imagine a coffee can, let's say 4" in diameter. Imagine a cover on it with valve in it. Let's say, we can open the valve ½".

Pause for a moment and let the mental image soak in.

Imagine that the valve is 2" in diameter, and right in the middle. Imagine that you're the air coming in through the valve. You've got plenty of room to go into the can, all around all sides of the valve. Eh?

OK now move the valve over to the side; let's leave it 2", and put it 1¼" from the edge. Now, imagine you're the air. The valve is open ½", but where it's next to the can, there's only a ¼" gap. Capische?

Pause for a minute if you're unsure.

Now that you have a complete understanding of what just happened, increase the valve to 2½", minus .030". NOW, the edge of the valve is only .015" off of the side of the coffee can. What just happened to the air trying to come through there?

THAT'S why you don't put huge valves in a 305"; it's because flow goes DOWN as the valve gets bigger.

Now...

Imagine a block with a small bore, and a head with 2 large valves above that bore. Remember that the valves are almost even with the top of the block, when they're at rest; and when they open, they have to go down into the bore. Got it?

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Imagine that the valves are large enough that they won't fit into the bore at all. The edge of the valve is actually above the flat top of the block, poised about a sheet of paper above the plane of the deck. Picture firmly fixed in your mind?

What do you suppose is going to happen when you try to open that valve? Which part do you suppose is going to break? (valve, rocker, push rod, lifter, cam?) Which would you prefer to break? I can guarantee it won't be that one; instead, it will be the one that costs the most, takes longest to get, and requires the most work to fix.

That's yet another reason why you don't put large valves in a 305.

Let's see...

It's a 305 to begin with, so it's never really going to make any power. If you increase the valve size, you reach a point where it actually starts to HURT performance. And if you increase it too much, you break parts.

In what universe would this be "A Good Idea"?

Any questions?

 

actually, the valve can hit the bore in a 305 bore. Using the 3.76" bore or so, valve size should be limited to around 1.94" intake, 1.50" exhaust. No reason for more because it will hit and will be shrouded by the cylinder wall as stated above.

Check this thread
https://www.thirdgen.org/forums/tech...5cc-heads.html

trickflow 175cc heads which have a 1.94/1.50 valve and are good for smaller bores will hit the cylinder wall at .771" lift on intake .728" exhaust. Thats plenty for a serious 305

2.02" valves in same head hit at .433" lift on intake! .507 on exhaust.


GM 601 305 heads with 1.94/1.50's hit the wall at .689" and 1.173" for intake/exhaust. Same heads with 2.02/1.6 valves hit at .505 approx with intake valve but chamber was reconfigured to seat the 2.02 valve so that figure may not be accurate. Exhaust hit at .881"

So it depends on chamber design and valve spacing. Trick flow opened up the valve spacing to allow more clearance with the cylinder wall with 1.94 valves than what you see in GM heads, but this takes away exhaust side clearance. 2.02" in a GM head require serious chamber reworking to make them seat and unshroud the flow path.

Valve seat depth cut into the heads could also increase lift clearance, but there really isnt a need for larger than 2" on a 305. Its hard to find a head with a large enough cross sectional area to pass the airflow required for big hp numbers with the given valve shrouding issues. Air speed thru the port has a limitation with 23 deg heads and small valves just cant pass big flow. With a 1.9-2.1 sq inch cross section, that usually relates to a 190-200 cc head and would be great for a high hp high revving 305 if you can develop a chamber/port for the 1.94-2" valve and still get decent lift out of it. All the ones I seen have 2.02-2.08" valves tho

You can do trick things like offset drill the valve seats or mounting dowels and bolt holes to allow a shift in the valve position to help unshroud. Possibly a 2.00-2.02" valve with a 1.50 exhaust could work and give good intake lift, pass good flow and make hp.

If I get time I want to have a custom Profiler 195 port with 1.94 or 2.00" LS valves installed and build a revving 305. Hope the seats can be cut deep and offset to unshroud the valve.

 

thanks guys Orr89RocZ good information and link. I am just trying to get a good understanding of what works and what does not before i start buying my parts. is that gm 601 my stock head casting number.

 

Not sure. You will have to take a cover off and look. Think 601's were older castings

 

i have a set of 113 heads i was going to use. I dont know if i should have port work done to these heads or go with the trick flow heads you mentioned. the intake i want to buy is a first fuel injection but the are still currently not making them.

 

Depends. If casting is in good shape likely can have someone port them pretty good for less than trickflows new. But after quality port work machine work and valvetrain, it could run you close to 750-1000. Hard call to make.