I know that no two engines are the same, and that the goemetry of the valvetrain also affect this...but
On a general basis, how much lift could I get before I bang into the pistons?Also considering, it is on a 305 with -2.8 dome pistons and 65cc combustion chambers...Just an idea is what I am asking for, .45? .5? .55? Just a general figure...
Thanks.

 

Depends more on the valve timing, than on just the peak lift #.

Remember, the valves are at max open, when the piston is somewhere around halfway down. NO CHANCE WHATSOEVER for interference at that point, with the piston an inch and a half or 2 inches away.

The point in the engine's cycle when the problem can occur, is around TDC, but not the firing instance of it; rather, the occurence of TDC at the end of the exhaust stroke and the beginning of the intake stroke. At that time, the piston is close to the top, and the valves are somewhat open.... creating the possibility for interference. The later the exhaust timing, the more likely it is to hit that one; and the earlier the intake timing, the more likely that one is to hit.

But, in general, with typical cams that one might use for more or less routine engine builds, it isn't likely to be a problem until you get above .600" of lift and above 260° of .050" duration. Again, the deatils can vary quite a bit; but if you're not in that range, there's nothing to worry about.

Chamber volume doesn't matter.

Valve size does, a little bit, because the valves aren't parallel to the piston; one edge is closer. So for any given lift and timing, a larger valve will come closer.

 

Fantastic. I see what you are saying...So say a cam with 210/222 duration a .05, with .576 lift with a intake center line of 104 and an exhaust center line of 130, LSA of 117, on a 305 with 2.02/1.6 valves, would that cause it to hit the piston at all?

 

HIGHLY unlikely. Although, it's not impossible; if the edge of the intake valve happens to be outside of the circumference of the valve relief, considering that the relief is probably .150" or so deep there, meaning that if the valve lands outside it, you just lost .150" of the clearance you should have had. Which it could be, depending on exactly where on the block the head gets registered by the dowel pins, where in the head the guide happens to be, and various other production tolerances. Assuming that the valves don't hit the deck first, which is also not impossible.

It's easy to fix though, if it's a problem. All you need, is the tool from Isky that looks like a big hole saw or Forstner bit. They "rent" that fairly cheap.

 

Fantastic!! Thanks for all the info, a great help!

 

The thing you have to watch out for on lift is breaking valve springs if you don't change your stock ones out.

 

Well the heads I am talking about are AFR 195cc Street Heads...they can handle up to .6" lift...so im safe I would say, (from that at least...)....

 

Is using 2.02 valves on a 305 bore gonna cause interference issues with the BORE????

 

No...that is kind of a myth you know... no one likes the 305 so I guess they never tried it, but its been done MANY times before with no problems at all...

 

It usually works, but there's no guarantees. Given the tolerances in everything - the dowel pin locations in the block, the same in the heads, where the guides ended up in the heads, how far off of "nominal" the cylinders are in the block, etc. etc. etc. - there's plenty of "wiggle room" in there for parts to line up, or not. Just because somebody else did it with their pile of parts, doesn't automatically certify that it will work with YOUR pile of parts.

2.02" + .030" between the valves + 1.60" = 3.630"
305 bore = 3.736"

Not a whole lot of tolerance in there, eh?? Stuff doesn't have to be very far different from one motor to the next, for it to work fine in one, and explode in the other.

OTOH, of what value is having huge valves, if the valve is RIGHT NEXT TO the bore itself? If the valve is, let's say, .100" from the bore (plenty of clearance as far as not hitting goes), then how much flow will occur there? Will it matter that the cam gives .400", .500", even .600" of lift, if the material has to all squeeze through a .100" crack?

That's the REAL reason for not putting those big valves in a 305.... they can actually REDUCE flow and HURT performance compared to smaller ones.

It's another example of how just because a thing is possible, doesn't necessarily make it a good idea.

 

i would NOT waste time putting AFR heads on a 305 and not just buy a 350 or larger at the same time.

 

good point... well would .2 still be to little of clearance... i an going to bore it.06 over... if not well... the trickflows flow good as well for a 305... anyway thanks for the info!

 

The LSA is a big factor in this also.
Since the 305 pistons aren't expecting big valves, I would measure the VPC.
It's just not that hard. You don't have to guess.
The procedure is spelled out in this THREAD.

 

Right: LSA is a "timing" feature, i.e. when something happens in relation to something else.

LSA is a measure of how many degrees (of cam rotation) the exhaust opens, prior to the intake. The higher it is, the earlier all the exhaust events occur, with respect to the intake events, which are the basis for expressing timing.

Low LSA = late exhaust opening = brings exh closer to piston
High LSA = early exhaust opening = exhaust opens, and closes, earlier, meaning the piston is sort of "chasing" the exh valve as it comes up.

Similarly,

Low ICL = intake opens early = valve begins opening well before the piston reaches the top
High ICL = intake reaches peak opening later = as piston descends

The thing I would be most concerned about in your situation, besides just the heads being a major mismatch to the rest of the motor, would be that the mfr of the pistons wasn't expecting such large ones; so the reliefs may not be large enough in diameter to accomodate them. Might be entirely deep enough, just not big enough around.

You should look for about .100" clearance to the intake valve, and a bit more, maybe .120", to the exhaust. There's several easy ways to do it; modeling clay and a pair of solid lifters works great; or you could use the solid lifters and checking springs, rotate the crank to 20° BTDC during the exh stroke, and see how far you can push the valves before they hit, then rotate it 5° more and check again, then 5° more, and so on, until you've covered the whole range of while the piston is up there near the top.

 

Right...well the cam i am gonna get done is "kinda" similar to the LS7 cam but not...
.575/.580 lift w/1.6 rockers
262/284 dur @.006
210/222 dur @ .05
104 Intake centerline
130 exhaust centerline
177 LSA
What do you guys think? Good? Bad?
I will check those clearances too, but I wanna get an opinion on the specs I have before I get it done...Thanks!
Oh and I am just considering sticking with 1.94/1.5 valves, buy the trickflows and put manley Race Flo...spare me the a lot of trouble...
Anyways what do you think of it?
Thanks!