Anyone using cut back valves?
Thread Starter
Burning Brakes
Joined: Nov 2002
Posts: 952
Likes: 44
From: NC
Anyone using cut back valves?
I just got some vortec heads and they are the stock set up right now. They will be going on my 355 with 9.8:1 CR, crane 272 cam, and rpm intake. Should i spend the money to get the valves cut back 30 degrees for better flow? how many horses can I look to free up....or if not horses...how much will my cc flow increase (if that makes sense).
Thanks
James
Thanks
James
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I always thought they were worth it. The tulip stem makes them lighter than stock valves. I used the stainless Manley Race flows years ago they are about 10 bucks each. Now I use the Proflows
How much do you gain is measured in port "Cubic Feet per minute" of air. I don't know what Vortec CFM rating is for the different valve lift points. But lets say 230 CFM @ .500 valve lift. With race flow type valves it might jump up to 245 which might be 6% more air flow. Air flow and HP are two different things. You might get 10 more hp on a 300 RWHP car.
How much do you gain is measured in port "Cubic Feet per minute" of air. I don't know what Vortec CFM rating is for the different valve lift points. But lets say 230 CFM @ .500 valve lift. With race flow type valves it might jump up to 245 which might be 6% more air flow. Air flow and HP are two different things. You might get 10 more hp on a 300 RWHP car.
air flow ,etc. 
Safety Car
Joined: May 2001
Posts: 3,758
Likes: 3
From: Surrey
Not on the Vette (what's the point with smog heads? 
) but I'm assuming you mean valves that have been machined to chamfer the back face from the seat face (it's the old anglo/us language difference)? If so, then yes, I've taken the valves out of my bike, machined a chamfer (about 30*ish) to improve gas flow & stuck them back in. Power gains? Haven't the faintest idea, but the exhaust sounds a bit sweeter & throttle response is crisper (blip the throttle in neutral & revs pick up quicker with more of a "crack" to the exhaust note). 20 years experience of this engine tells me that when the exhaust sounds sweeter & throttle response is better then I'm heading in the right direction.
At risk of hijacking the thread, does anybody know if reduced diameter valve stems improve flow (undercut stems?). I know that on my bike engine they make a difference on the intake, but are pointless on the exhaust as the port size is ample for street use. Does anybody know if reduced diameter/undercut stems improve power in a SBC, does it depend on make & model of head, port sizes, do intake/exhaust have different requirements, etc? Reason I ask is that fitting undercut intake valves made a difference that was more obvious than machining the rear of my valves.
) but I'm assuming you mean valves that have been machined to chamfer the back face from the seat face (it's the old anglo/us language difference)? If so, then yes, I've taken the valves out of my bike, machined a chamfer (about 30*ish) to improve gas flow & stuck them back in. Power gains? Haven't the faintest idea, but the exhaust sounds a bit sweeter & throttle response is crisper (blip the throttle in neutral & revs pick up quicker with more of a "crack" to the exhaust note). 20 years experience of this engine tells me that when the exhaust sounds sweeter & throttle response is better then I'm heading in the right direction.At risk of hijacking the thread, does anybody know if reduced diameter valve stems improve flow (undercut stems?). I know that on my bike engine they make a difference on the intake, but are pointless on the exhaust as the port size is ample for street use. Does anybody know if reduced diameter/undercut stems improve power in a SBC, does it depend on make & model of head, port sizes, do intake/exhaust have different requirements, etc? Reason I ask is that fitting undercut intake valves made a difference that was more obvious than machining the rear of my valves.
Thread Starter
Burning Brakes
Joined: Nov 2002
Posts: 952
Likes: 44
From: NC
Originally Posted by UKPaul
Not on the Vette (what's the point with smog heads? ) but I'm assuming you mean valves that have been machined to chamfer the back face from the seat face (it's the old anglo/us language difference)? If so, then yes, I've taken the valves out of my bike, machined a chamfer (about 30*ish) to improve gas flow & stuck them back in. Power gains? Haven't the faintest idea, but the exhaust sounds a bit sweeter & throttle response is crisper (blip the throttle in neutral & revs pick up quicker with more of a "crack" to the exhaust note). 20 years experience of this engine tells me that when the exhaust sounds sweeter & throttle response is better then I'm heading in the right direction.
At risk of hijacking the thread, does anybody know if reduced diameter valve stems improve flow (undercut stems?). I know that on my bike engine they make a difference on the intake, but are pointless on the exhaust as the port size is ample for street use. Does anybody know if reduced diameter/undercut stems improve power in a SBC, does it depend on make & model of head, port sizes, do intake/exhaust have different requirements, etc? Reason I ask is that fitting undercut intake valves made a difference that was more obvious than machining the rear of my valves.
) but I'm assuming you mean valves that have been machined to chamfer the back face from the seat face (it's the old anglo/us language difference)? If so, then yes, I've taken the valves out of my bike, machined a chamfer (about 30*ish) to improve gas flow & stuck them back in. Power gains? Haven't the faintest idea, but the exhaust sounds a bit sweeter & throttle response is crisper (blip the throttle in neutral & revs pick up quicker with more of a "crack" to the exhaust note). 20 years experience of this engine tells me that when the exhaust sounds sweeter & throttle response is better then I'm heading in the right direction.At risk of hijacking the thread, does anybody know if reduced diameter valve stems improve flow (undercut stems?). I know that on my bike engine they make a difference on the intake, but are pointless on the exhaust as the port size is ample for street use. Does anybody know if reduced diameter/undercut stems improve power in a SBC, does it depend on make & model of head, port sizes, do intake/exhaust have different requirements, etc? Reason I ask is that fitting undercut intake valves made a difference that was more obvious than machining the rear of my valves.
awesome, thanks for the info.
James
