hey, going to get my heads done to compliment a cc306/1.6rr's, comp r's, etc.....the whole top end and fuel system's getting re-done, and i was just wondering what the biggest valves are that i can use for this cam with the stock bottom end? is it ok to go with something like 2.05/1.60's?? or is that too big??
any opinions(with explanations) would be great. thanks.

 

To go over a 2.00" valve you need new valve seats...

After that a 2.05" will work and to go any bigger you would have to cut a lot out of the chamber so you would probably lose some power to the chambers being larger and I doubt you can get a proper throat area in the head going much bigger too.

Bret

 

The new seats would be steel and would not be safe to run considering you have aluminum heads so just go with 1.55/2.00 as your biggest on stock heads without new seats.

 

besides that, on a 4.00" bore a big intake valve like that is just going to be shrouded and hurt flow.

 

BTW Valve seats are usually Iron, they are seat inserts if they were aluminum they would never last.

Theoretically you could run a 2.120" valve on a 4.00" bore, it would take a lot of work. I have no problem doing a 2.08" on a 4.00" bore and even bigger on a overbore.

Bret

 

so even if i went to 2.02's i'd need new seats? then what would be the difference between 2.02 & 2.05? would it make that much difference in the flow?

 

Yeah if you go with new seats which the 2.02's should have under them you might as well go with a 2.055 valve.

Bret

 

When you machine the stock seats in an LT1 head as large as possible, a 2.00/1.56 valve fits perfect. If you use a 2.02/1.60 valve, there is no more material to remove from the seat so you are just placing a larger valve in front of the same size valve job and making the air go around a larger valve before entering/exiting the cylinder. It does not help flow ANY and actually will cost a few CFM. This will still flow better than stock valves though.

When going with larger valves, the larger venturi (smallest diameter of valve job) is what helps out. Some use a % of valve size (85, 88, 90%, etc) and some go a certain measurement smaller than the valve size but just to make it simple, we will just use an example of .200 smaller than valve diameter.

A 2.00 valve allows a 1.800 venturi and still have the valve job angles help the air transition from the 90* venturi to the 45* and around the valve.

A 2.05 allows for a 1.850 venturi.

A 2.08 allows for a 1.880 venturi.

Anyone that has done much flow testing will tell you that opening the venturi uop .050 will allow a much larger bowl and a pretty good CFM increase.

The venturi diameter is important but you need to make sure and have the valve job angles to get it past the valve too.

Lloyd Elliott
972-617-5671
Eportworks.com