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Author Topic:   Exhaust Valves
I-B-RACIN
Dirt Maniac

Total posts: 132
posted March 13, 2005 06:28 PM  
It is for a 350 sbc with cast four barrel intake, stock cast heads with a 500 holley carb crossover headers with two into one collecter, flat top pistons limited to 9.5 to 1 comp. on pump gas. My ? is is it worth the extra money to get hi flow exhaust valves when working with restricted intake I plan on running good valve on the inake side but will there be a gain to be had on the exh. no valve size specifide for rules but not allowed to do any porting or unshrouding so I"v beeen told to stay with 1.94s on the intake they will flow just as well as 2.02 because of the no unshrouding. or do i just leave 1.94 on the intake and cut exh.to 1.60 instead of 1.5 any advice would be appreciated thanks

leapinlizard
Dirt Freak

Total posts: 402
posted March 14, 2005 10:55 AM  
I'd say a 1.6 valve will help. It's cheaper and easier if you can run a split pattern cam. That's what they are designed for.

dirtbuster
Dirt Forum Champ
Total posts: 2007
posted March 14, 2005 11:20 AM  
stock heads are pretty restrictive all around. I think the exhaust is more restricitive thus the reason split patter cams work well with them.

When you say hi flow do you mean bigger exhaust valves or the necked down stems?
The necked down stems might help more than the bigger valve, but overall the port itself is prob the biggest restriction.

I-B-RACIN
Dirt Maniac

Total posts: 132
posted March 14, 2005 04:19 PM  
When you guys say split pattern cam are you talking split duration or somthing else??And yes by hi flow I ment undercut stems...

dirtbuster
Dirt Forum Champ
Total posts: 2007
posted March 14, 2005 04:58 PM  
The undercut stems definetly wont hurt. Again the vavle itself prob outflows the port so without beaing able to do work to the port you might not see a big gain. While the bigger valve (1.6) makes the valve opening bigger the port is still smaller so your not going to get much more airflow because of the small port. Wheras the undercut stem is taking back part of the area of the port that was occupied by the stem. Now if you could massage them a little and open the ports up and take advantage of the bigger valve area well thats a different story.

Yes split pattern and duration are the same thing. The extra duration on the exhaust side helps make up for a poor flowing port by allowing the combustion gases to escape sooner in the cycle, as well as give them a longer time frame to escape to help eliminate some of the pumping losses due to the piston having to push them out instead of flowing out. Shortly after the piston fires and well before it reaches BDC there is still some combustion taking place but the most useful energy has been harnessed. By opening the exhaust valve sooner you allow some of the residual pressure to escape which means there is less exhaust gases to force out after the piston passes BDC. Technically you give up a little bit of torque out the header because of the exhaust opening sooner but you gain in not having to use the power from the other cylinders to force the exhaust out It is necessary to help balance the airflow to get better power. That is why you will hear about single pattern cams that will make more torque than dual pattern cams, but you also have to have the right exhaust port so that it doesnt become a restriction. In some cases if the exhaust flows too well you might even have to use a smaller pattern on the exhaust to get the most out of the engine.

Sorry thats prob more than you wanted hear.....

[This message has been edited by dirtbuster (edited March 14, 2005).]

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