Im building a 383 for in my car right now that I plan on running 15 lbs of boost on, Im doing the heads right now for it, I was just wondering if the little bigger valves over 2.02/1.6 is going to do anything for me, I have big enough seats in there to put them in we checked all that out sence I machined new seats in there, to much valve for the heads? they fit in there nice, I just think it might be a little to much valve in there for all the more the heads flow, but one of the guys at the machine shop wants me to put them in there, any thoughts on this?

BTW: Im going to be running a custom solid roller blower cam in here thats why hes saying to go with the bigger ones in there.

 

since you are going with quite a bit of boost via a supercharger, i would definitly run the larger exhaust valve if it will fit, but keep to a 2.02 (or even smaller) intake valve. if you end up using a 1.60 exhaust valve, then i would run a 2.00 intake. youll want to try to end up around 85% of the intake flow on the exahaust side, which would be much easier at lower lifts with a better valve size ratio. definitly try for at least 80% I/E flow. the 1.625 exhaust valve is also a good idea because when they design your custom cam, it will allow them to delay the opening of the exhaust valve a little more

even on a naturally aspirated LT1 i would stay away from the 2.055 intake valve. running that on a small bore (4.030) with standard valve spacing will result in quite a bit of shrouding which can actually result in less air flow than with a 2.02 valve in many cases. also the LT1 heads would need quite a bit of work on the chambers to unshroud a valve that size.

 

so you think I should put in 2.02/1.625 in there?

 

i dissagree for boost you want the biggest intake and exhaust valves you can fit. you can have the combustion chambers cut to avoid too much shrouding

 

i personally would run the 2.02/1.625 combo. just my opinion though, im sure others who are more qualified will have more to throw in

 

I would try this over in the Forced Induction forum, they prolly have alot more experience with this kind of stuff than some of us would.

 

The exhaust valve size is prolly not going to get you much going to a 1.6 or 1.625 valve in a LT1 casting, I would stick with a 1.56 valve of the exhaust just for the LT1 casting.

Ask Lloyd or Phil what they think and get something from them would be a good idea.

Bret

 

i can tell you from 8 years of forced induction experience that its a waste of time and money. if it doesnt help the heads flow it wont do ****, and because of the shrouding you would get on a 4.030 bore, it wont help flow. blower or not. bigger valves dont equal bigger power. a 2.02/1.60 combination is as big as id go on anything less then a 4.100 bore. and if you can get a smaller valve head to flow the same, you are better off with the smaller valves; especially when it comes to piston to valve clearence.

combustion chambers cut? you mean cylinders? if so thats what they did in the old days with poor flowing ports to get some airflow with big valves. and the tradeoff to that is weakening the wall and reducing ring seal. anything that was done before blocks were bored/honed with torque plates is probably not the best way to make power.

 

I would try the 2.02 int with the 1.625 EXT

BTW I have NO problems running 2.055s on a N/A stock short block MY HP under the curves give me an edge

 

You don't gain any flow with the 1.600" valve over the 1.560" so why even go that big?

Bret

 

I already bench tested the 1.6s in a diffrent set of heads and I can tell you that the 1.6s made a big diffrance once you open them up some, Im just thinking that the 2.055/1.625 is a little to big sence there getting close to the combustion chamber wall. I dont want to grind my new seats for these valves and put them on the bench to only find out that it hurt them and I have to put new seats in the again sence Id have to grind them out harder.

 

I've seen the same numbers with 1.56" and you don't need new seats for those.

Bret

 

your half right you dont need new seats but the part your wrong about is the valves. I own a well known machine shop and going from 1.56 to 1.60 avarage a 12 cfm gain without porting. with porting all bets are off you can really pick some cfm over the 1.56 then. general guide lines is 1 cfm=1 hp so 12 hp i guess aint that big just for $60-80 i guess to you. i also dissagree about the forced induction with the proper unshrouding you want the biggest valves you can. when forced induction is done it becomes less about velocity and more about air space and the amount you can move.


seeing your from ny you most likely know what shop is mine.

 

12 cfm where? at .600"? Brian Tooley of TEA once told me that make more power youve got to pick up at least 10cfm in the .200", .300", and .400" areas under the curve.

 

across the whole scale