Cylinder Head Exhaust Flow

jrstock · 03-13-2015, 08:28 PM

I've recently come across a set of new set of cylinder heads. These heads flow the same on the intake as my old set but significantly more on the exhaust.

I've dyno'ed the new heads and seen a significant hp drop.

Any ideas on what modifications are required to make cylinder heads with better exhaust flow work?

Anyone have experience with this problem? What changes should I be looking at? Cam? Headers? Something else?

The combo is sbc with 520 cylinder heads.

Thanks in advance for your advice.

Alan Roehrich · 03-13-2015, 08:35 PM

You are probably now over scavenging and pulling the intake charge out the exhaust. You probably need to take 4 degrees or so out of the exhaust lobe. You might also consider opening the lobe separation 2 degrees.

SSDiv6 · 03-13-2015, 09:35 PM

Quote:

Originally Posted by Alan Roehrich

You are probably now over scavenging and pulling the intake charge out the exhaust. You probably need to take 4 degrees or so out of the exhaust lobe. You might also consider opening the lobe separation 2 degrees.

What Alan said. Every time you have such a big change in Intake/Exhaust flow ratio, you need to revisit the cam events.

Alan Roehrich · 03-13-2015, 10:29 PM

You need a pretty serious cam change. What you have done is radically altered the intake flow to exhaust flow bias, which is what your cam lobe bias is based on.

My suggestion is to take flow bench numbers and dyno numbers for each head, and your cam card, and email them to Tim Cole at Comp, and have Tim tell you what you need to do.

I do that regularly, as well as sending him results from running lash loops on the dyno.

Then we discuss the next step in cam changes.

Mike Taylor 3601 · 03-18-2015, 08:45 AM

Loosen lash on exhaust .005 rockers see if that helps, that will remove some duration to show you if that is what it is wanting,also can restrict exhaust and see if helps.
Also if have flow #'s if you haven't done so, take intake flow #'s from all lifts and average them, if new heads have same or lower average# may not make better power.
Mike Taylor 3601

R. Thorne · 03-18-2015, 01:37 PM

Like Mike said, it's total flow that counts. Also, velocity is a real consideration. If they flow the same cfm on intake but there is turbulence on one head and not the other, then power will be down. Ron.

SSDiv6 · 03-18-2015, 02:48 PM

Airflow is not linear.

Velocity and behavior of the port is more important than flow numbers.
When flowing heads, a quiet port will make more power than a loud port.
There will always be some turbulence in the port, and many times, necessary.

In addition to the flow numbers, the performance and power is dictated by the camshaft events. If possible, flow the intake and cylinder head as a system.

Like Alan and I recommended earlier, contact a reputable cam manufacturer and share all the numbers including the depression that it was tested at. they should be able to grind a camshaft to maximize the flow curve.

03-13-2015, 08:28 PM	#1
jrstock Junior Member Join Date: Feb 2015 Posts: 16 Likes: 0 Liked 0 Times in 0 Posts	Cylinder Head Exhaust Flow I've recently come across a set of new set of cylinder heads. These heads flow the same on the intake as my old set but significantly more on the exhaust. I've dyno'ed the new heads and seen a significant hp drop. Any ideas on what modifications are required to make cylinder heads with better exhaust flow work? Anyone have experience with this problem? What changes should I be looking at? Cam? Headers? Something else? The combo is sbc with 520 cylinder heads. Thanks in advance for your advice.

03-13-2015, 08:35 PM	#2
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,066 Likes: 1,486 Liked 1,630 Times in 378 Posts	Re: Cylinder Head Exhaust Flow You are probably now over scavenging and pulling the intake charge out the exhaust. You probably need to take 4 degrees or so out of the exhaust lobe. You might also consider opening the lobe separation 2 degrees. __________________ Alan Roehrich 212A G/S

03-13-2015, 10:29 PM	#4
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,066 Likes: 1,486 Liked 1,630 Times in 378 Posts	Re: Cylinder Head Exhaust Flow You need a pretty serious cam change. What you have done is radically altered the intake flow to exhaust flow bias, which is what your cam lobe bias is based on. My suggestion is to take flow bench numbers and dyno numbers for each head, and your cam card, and email them to Tim Cole at Comp, and have Tim tell you what you need to do. I do that regularly, as well as sending him results from running lash loops on the dyno. Then we discuss the next step in cam changes. __________________ Alan Roehrich 212A G/S

03-18-2015, 08:45 AM	#5
Mike Taylor 3601 VIP Member Join Date: Oct 2008 Location: Somerset,Ky Posts: 1,348 Likes: 296 Liked 248 Times in 87 Posts	Re: Cylinder Head Exhaust Flow Loosen lash on exhaust .005 rockers see if that helps, that will remove some duration to show you if that is what it is wanting,also can restrict exhaust and see if helps. Also if have flow #'s if you haven't done so, take intake flow #'s from all lifts and average them, if new heads have same or lower average# may not make better power. Mike Taylor 3601

03-18-2015, 01:37 PM	#6
R. Thorne Member Join Date: May 2009 Posts: 180 Likes: 51 Liked 99 Times in 32 Posts	Re: Cylinder Head Exhaust Flow Like Mike said, it's total flow that counts. Also, velocity is a real consideration. If they flow the same cfm on intake but there is turbulence on one head and not the other, then power will be down. Ron.

03-18-2015, 02:48 PM	#7
SSDiv6 VIP Member Join Date: Feb 2003 Location: Glendale, Arizona Posts: 3,023 Likes: 706 Liked 1,514 Times in 563 Posts	Re: Cylinder Head Exhaust Flow Airflow is not linear. Velocity and behavior of the port is more important than flow numbers. When flowing heads, a quiet port will make more power than a loud port. There will always be some turbulence in the port, and many times, necessary. In addition to the flow numbers, the performance and power is dictated by the camshaft events. If possible, flow the intake and cylinder head as a system. Like Alan and I recommended earlier, contact a reputable cam manufacturer and share all the numbers including the depression that it was tested at. they should be able to grind a camshaft to maximize the flow curve.