big valves on a stock head

[denn454]

how big can i go with a stock head? I'm not wanting 10's or anything crazy so i don't think spending $1,000 on heads is worth it, but i am going through the motor so while i'm in it, i mine as well make my life easier and do some head work. i found someone here that has a set of factory heads, bored out i forget how much. if there close to being where i would want to be, then i'll probably buy them, otherwise its probably cheaper to just bore out mine.

i'm guessing this is worth the work, right? and a good port and polish job is probably also worth the money?

all i'm looking for is a 12 flat in full street trim. i really don't want a roll bar thats why. anything to help me get there is worth it.

 

[UNGN]

For a 12.0, stick with the stock valves and have them backcut. Many machine shops will do this for a couple bucks with a 3 angle valve job.

$220 for aftermarket valves + $60 for machining the heads for oversize valves would buy a lot of race gas.

Bigger valves are worth about 3% flow increase over stock valves.

 

[gofstbuick]

Also have them put a 30 degree behind the 45 face of the valves. A three angle valve job is just 3 angles on the valve seats, so you may be able to pick up some additional flow by adding an additional angle on the face of the valve. Sometimes the use of larger valves will cause interference issues between the intake and exhaust valve. My machinist said (after some measurements) that I could not use the 1.775 intake valve, but the 1.745 valve would clear just fine. He said I would probably have to replace all the seats to allow the 1.775 valve to clear. SI valves sells a 1.745 intake valve for a stock TR head and the heads can retain the stock valve seats

 

[REDS HOT AIR]

times in my sig was with stock valves

 

[denn454]

thats pretty impressive. actually thats really impressive.

but i'm going to be going through the engine, and anything i can do to help free up some power i think will be worth it in the end.

 

[UNGN]

Originally posted by denn454
thats pretty impressive. actually thats really impressive.

but i'm going to be going through the engine, and anything i can do to help free up some power i think will be worth it in the end.

By all means get a 3 angle valve job, pocket port and bowl port the heads, smooth and straigten the pushrod pinch and port match to the manifolds. Have the valves backcut and smooth the combustion area of the head.

This will result in a 15 to 25% increase in flow. SInce airflow is roughly proportional to HP, a 15 to 25% increase in HP should also result.

This kind of makes the 3% gain from larger valse seam small.

 

[gnx7]

I've heard time and time again that there is power to be had in cleaning up the bowl areas with these 231 heads and simple port matching with a quality 3 angle valvejob.

Boost is what makes these cars run fast with a healthy sized turbo.

-GNX7

 

[GNVAIR]

People also fail to realize that these 3.8's only have a 3.8" bore so adding a larger valve will only result in an increase in valve shrouding. You could open the combustion chamber up more around the valves, but its a lot of work for such a miniscule gain.

 

[Timberwolf]

Originally posted by denn454
all i'm looking for is a 12 flat in full street trim. i really don't want a roll bar thats why. anything to help me get there is worth it.

NHRA just changed the rules for 2005. 11.49 now before a roll bar. Check it out here:
http://www.nhra.com/2004/news/november/110102.html

 

[denn454]

awesome! i'd still like to go 12 flat straight off the street, maybe put some slicks and race fuel in occasionally to get the 11.5...

 

[CTX-SLPR]

For those of us out there with 4.1's how much can we expect to gain by going from the 1.71" to 1.74" valves with the reduced valve shrouding?

Thanks,

 

[83ttypecooled]

fast

Thanks for the information on cylinder porting and 3 angle valve jobs. Now, if I can find someone local to handle this, I'll be all set.