Enlarging PRP doesn't always help
- Thread starter Earlie A
- Start date
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In the end of the day your getting 308 cfm with a 2.02 valve, your doing pretty good, doubt many would be unhappy with those results. Do you plan on playing around with with larger valves at some point ?
There needs to be a big asterisk beside that 308 number. As I mentioned early on, I have used all the cheat tactics that I could come up with to get that number. For instance, the combustion chamber has clay covering the spark plug and sometimes covering the whole exhaust valve. This creates a top angle 'cone' around the valve that adds a few cfm. I have tried to optimize my port entry plate for a good smooth radius. The guide boss in the port has clay in front and behind. So honestly it's more like a 295 cfm port. And it's easier to get a port that peaks at 295 at 0.600 lift then drops off (like mine) than it is to get a port that peaks at 295 at 0.900 lift with no stall or back up. It has taken PBR a whole lot of effort to get the ports he does. I'm on flow test 580 something and haven't figured it out yet.
I do plan on trying the larger valves, I'm just not going to give up until I get the dip out of the flow curve of the 2.02 valve.
Brian Hafliger
Well-Known Member
20 years ago I read a post by Darin Morgan, I think on speedtalk where he said he never leaves a belly on his short turn. This really got to me, and at the time I was getting help with some of my porting from Larry Meaux. I would send him photos, air speed data, and area at particular points in the ports and he would lead me in a direction but never tell me much more than that.
One thing Larry did lay out for me was how important the valve job is on a port that is way too fast. So now if I have a problem with the port, I make sure the area is where I want it and go from there.
The ONE most important thing I've learned in my 34 years porting is that you have to correlate flow bench findings with engine power...in other words if you gain on the flow bench (anywhere) you better gain on the dyno. If not, you have to learn what the flow bench is telling you...and never forget we modify heads for POWER not flow numbers. I can tell you a bunch of my sb mopar heads that make good power are considered too fast by some. Yes I've slowed them down, and never picked up any power. It's not a sb chevy!!
One thing Larry did lay out for me was how important the valve job is on a port that is way too fast. So now if I have a problem with the port, I make sure the area is where I want it and go from there.
The ONE most important thing I've learned in my 34 years porting is that you have to correlate flow bench findings with engine power...in other words if you gain on the flow bench (anywhere) you better gain on the dyno. If not, you have to learn what the flow bench is telling you...and never forget we modify heads for POWER not flow numbers. I can tell you a bunch of my sb mopar heads that make good power are considered too fast by some. Yes I've slowed them down, and never picked up any power. It's not a sb chevy!!
Well, I've resorted to this method, if the cured compound & form allow..;
Which allowed Me to extract these..;
I've noticed while watching cylinder head reviews of all kinds with flow charts, that there seems to be a somewhat fixed general range in the lower lifts no matter it's size, total cfm, valve size etc.. if it's a 300 cfm 289/302 sbf head or a 540 cfm big block Chevy head.
Now that is redneck engineering at it’s finest. I love it. For those who have never tried to get a silicone port mold out of the port - it can be tough. Like 15 minutes of hard work tough. I use a Jorgeson woodworking clamp to push it out, but your method is way better.
You are correct and there is a perfect explanation for it. I know you like numbers and math, so do this: Read up on Bernoulli's principles in articles such as this
https://insights.globalspec.com/art...ext=Consider a steady flow of fluid in a pipe
It's dry, it's boring, but I promise a small light bulb will turn on when you (somewhat) understand it.
Here's the key. On the flow bench, the depression (the difference between pressure at the inlet to the port and the pressure in the cylinder itself) is usually set to 28 inches of water, or about 1 psi. So that gives you a certain amount of energy (or driving force) to work with. There is a limit to how much flow you can get. Bernoulli's equation helps define that limit. Dig into it.
I've never heard it stated that way before. Thanks. I've been testing on the same port for months. Make changes, try things, epoxy the throat and seat back up and try again. I've seen 20 cfm swings from one valve job to the next, but never really thought of it in terms of port speed. Good insight.
I really appreciate this point of view. If the flow bench and the dyno disagree, don't throw up your hands and say 'the flow bench is lying to you'. Dig deeper. If you understand WHY the flowbench and the dyno are saying different things you have probably unlocked a very deep level of understanding.
I have realized that there is a limit to what I can learn on the flow bench alone. I can't get my wife to understand that what she really wants is a dyno....
Thanks for jumping into the post Brian. Your advice is appreciated.
Catching up on parts of this interesting post that I read on the run yesterday. I have a little different thinking on helping guys. I’m a self taught head porter that has probably every book out on head porting and never read one. I do have a copy of a Chevy head porting book that is dumbed down a little that I read most of during my Chevy porting days. Even during my two day I bought the recommended calculator that I couldn’t operate and a voice recorder that I recorded everything and never listened to it. But during the 30-40 minutes that we spent on exhaust ports made it all worthwhile because I learned about targeting airspeed more and flow less. There have been days I’ve done 8 flow tests finding tidbits of info but I worried less about manipulating the port but working with what I had. So from going through what I’ve done over the years if I can shorten the learning curve I’ve tried to do it. I’ve been thanked on posts, messages, and even by guys walking up to me at the track. Nobody has all the answers and will continue to debate even the subject on port finish (rough burr finish, double cut finish, or even sanding roll finish). All I can say is make it fun and when you get frustrated take a walk, go shooting, or fishing, or even take a trip to Walmart.
Dan the man
Well-Known Member
I read again the beginning of this thread and I figured out for myself what PRP meant.
Or you could of read post 52 when Earlie A answered you, or is that about the time you figured it out for yourself
Dan the man
Well-Known Member
dusterbing the peace
FABO pyrite member
Genuine question, not posing as argumentative or anything negative-have you (or anyone else on here) tested different finishes?
The follow up, and actual nature of my question is specific to the ribbed CNC finish vs a smoothed surface. Have you ever done before and after of flowing a CNC surface, then smoothing it out to see effects? I would suspect that not many people would risk (or pay for) the chance of losing flow…but needed to ask. I have a set of CNC heads and have wondered what having them hand finished would do.
A BIG thanks to all of you for your continued shares of information!!! I don’t see these kinds of discussion elsewhere.
Good question. Hopefully one of the experts will jump in here.
Newbomb Turk
Well-Known Member
It depends. If the step over is deep enough and spaced far enough apart it will affect airflow and you can’t have that.
If anyone is interested in the Bernoulli principle and how it explains the 350 fps flow bench velocity number and the 146 cfm/square inch theoretical flow limit let me know. I'll start another post.
I'm glad you brought this up. I hit me that I have a single Speedmaster CNC head that I bought on Black Friday just for the purpose of flow testing. I was so disappointed with the head that after one flow test I've put it up and tried to forget about it. This is the perfect opportunity to make it useful. I'll try to clean up a port and compare the before/after.
dusterbing the peace
FABO pyrite member
I wasn’t ignoring your question but I don’t like to be “that guy” and answer everyone’s questions on a post that wasn’t started by me. And it paid off because now he’s going to start a fresh post on this exact question. It’s nice hearing replies from everyone not just 2-3 guys.
This would be a great post too. I for one would be interested.
dusterbing the peace
FABO pyrite member
Here's a link to the new thread by @Earlie A for other's who may stumble into this current thread:
CNC Mill Mark Removal in Intake Port
CNC Mill Mark Removal in Intake Port
4eighteener
Well-Known Member
Great discussion from the A team here on porting, PHR, PBR, Earlie A, Brian H, plus others!
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