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Author
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Topic: Airflow and horsepower
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Rustang
Gearhead Posts: 452
From: Clarion PA
Registered: Nov 2000
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posted 12-24-2001 08:49 AM
   
This is a question about intake port airflow compared to exhaust port airflow. I think there is a rule of thumb that the exhaust should flow 80% of the intake for a well balanced head. Also, I have seen formulas for calculating potential horsepower based on intake airflow. So when you take a head like my Boss 302 head that has a 200cfm exhaust port and a 320cfm intake port (for a intake/exhaust flow ratio of 62%)I doubt this formula would be accurate?
So my questions are:
Would this formula be valid with a large intake/exhaust flow ratio?
Is there a calculation that exists for predicting horsepower potential based on exhaust port flow?
Is exhaust port flow not as important as intake port flow? ------------------
'68 mustang 351 clevor- 10.92@124
'67 Stang, 351W -11.18@118
'69 351C Torino-14.90@100
'78 Pickup 351W-15.56@88
'79 Pickup 460 ET=??
http://www.rustang-racing.cityslide.com/contents/contents.cfm/712231
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SteveLaRiviere
Administrator Posts: 32961
From: Saco, Maine
Registered: May 99
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posted 12-26-2001 08:07 PM
I'll send a copy of this to the Racing forum.  ------------------
'70 Mustang Mach 1 M code 351C 4V/FMX/3.25 open
'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Lok
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
'97 Probe GTS 2.5L DOHC
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S/Q 2204
Journeyman Posts: 93
From: Ozark, AL(again after a year of being deployed)
Registered: Feb 2001
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posted 12-28-2001 12:26 AM
It seems there is a happy medium to the ratio; both are important to volumetric efficiency. Don't know about a single formula. You have to look at it as a portion of the incoming charge being converted to power for pushing the piston back down the hole. This means only a portion will exit the exhaust port. The exhaust port must allow this to exit at a certain velocity to help in scavenging the chamber. We normally look for 72-80%. Anything less & the incoming charge is polluted with excess exhaust gasses, any more& you loose velocity. If you are running boost it will force the chamber clean. You probably need to run your info through one of the more complete engine programs; one that you have to put flow numbers in.
Later,
Mark J.
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Rustang
Gearhead Posts: 452
From: Clarion PA
Registered: Nov 2000
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posted 12-28-2001 05:11 AM
Thanks for the reply Mark. I don't have it in front of me right now, but there is a popular formula I've seen tossed around that uses intake port airflow to approximate horsepower potential of a cylinder head. I guess my engine computer program is the reason I posted this question. I'm using KB Virtual Dyno, that to-date has seemed pretty accurate, but the trend I see in the program doesn't reflect what I thought it would.
It's telling me, based on my flow numbers, (inputs from .100 to .700 lift)that when I start playing around with cam specs, the changes in hp/tq are not near as dramatic when I change the exhaust lobe when compared to similar changes in the intake lobe. I get similar results when moving the flow numbers up or down for either port.
Does this seem reasonable?
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S/Q 2204
Journeyman Posts: 93
From: Ozark, AL(again after a year of being deployed)
Registered: Feb 2001
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posted 12-28-2001 10:26 PM
I would think exhaust is as critical as intake so maybe the formula assumes the exhaust port you have will compliment the intake. If you can't get it out what good does it do to get it in. The A460 heads I started with were flat in flow after .500 of lift. After correcting this problem the car picked up about .14 in the 1/8th. There are going to be some things the programs cannot simulate as well as dyno's ( but they should be able to get you within inches) , the only true test will be the E.T. at a track.
Later,
Mark J. [This message has been edited by S/Q 2204 (edited 12-28-2001).]
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cpmaverick
Moderator Posts: 1564
From: Auburn, AL.
Registered: Jan 2000
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posted 12-29-2001 02:42 AM
It's true, from what I have learned exhuast flow is about half as important or less to total horsepower. It's all about getting air into the cylinder. Scavenging, and efficient exhaust definitely makes a difference but not nearly as much as intake flow.
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chips67
Gearhead Posts: 651
From: louisville, ky, usa
Registered: Jul 2001
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posted 12-29-2001 01:23 PM
wow, what a broad subject! let me say "it depends" a million times just to start. rpm range? boosted or power adder? i dont know about the ratio thing, but i bet a flowbench guy would. i think port velocity and quench are important. i think the exaust side ratio changes when you throw in a power adder....you definitly need less restiction when you spray a motor. scavenging the exaust is very critical on a n/a motor that you are building for max power. i will tell you from experience....a cam with a wide lsa, little overlap sucks on a naturally aspirated motor. thus my current combination. dont ever do what i did....assume. lift/duration/lift/duration/lift/duration!!!! oh crap, screwed by lsa! ------------------
67 coupe, 650dp and rpm intake on 5.0 with afr 165 heads, 4 speed, 4.11's.....best so far is [email protected] in 1/8 mile with 1.79 60ft. time.
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Mpcoluv
Gearhead Posts: 928
From: Charlotte NC usa
Registered: Apr 2001
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posted 12-30-2001 10:06 AM
The 75% (I think) ratio is considered ideal, but the 62% can be adjusted with a split pattern cam. I would use the boss heads and be happy. Or you could sell them to me.
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Rustang
Gearhead Posts: 452
From: Clarion PA
Registered: Nov 2000
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posted 12-31-2001 08:34 AM
I agree, MP, but there's got to be a point where no matter how much more duration you run on the exhaust, the limitation of flow becomes the "cork" in the system. Or at least too much duration probably affects other events in the cycle? (Let me know if you're interested in the heads!)
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kid vishus
Gearhead Posts: 4357
From: middle of NC
Registered: Oct 2000
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posted 12-31-2001 08:40 AM
I had a farily well known engine builder tell me one time, that a good camshaft can make up for a bad exhuast port. But there is no way around a bad intake port cuss thats where all the power is. And he's a former NHRA record holder in the old Mod class running a cleveland, builds several 5.0 shootout motors, and seems to know his stuff.
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Rustang
Gearhead Posts: 452
From: Clarion PA
Registered: Nov 2000
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posted 12-31-2001 08:54 AM
Kid, your reply, along with cpmaverick's, agrees with what I'm seeing on my computer program. The exhaust port just doesn't have as dramatic an effect on overall power as the intake. Thanks and Happy New Year! ------------------
'68 mustang 351 clevor- 10.92@124
'67 Stang, 351W -11.18@118
'69 351C Torino-14.90@100
'78 Pickup 351W-15.56@88
'79 Pickup 460 ET=??
http://www.rustang-racing.cityslide.com/contents/contents.cfm/712231
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Mpcoluv
Gearhead Posts: 928
From: Charlotte NC usa
Registered: Apr 2001
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posted 12-31-2001 09:55 AM
quote:
Originally posted by Rustang:
I agree, MP, but there's got to be a point where no matter how much more duration you run on the exhaust, the limitation of flow becomes the "cork" in the system. Or at least too much duration probably affects other events in the cycle? (Let me know if you're interested in the heads!)
I would guess less than 55% exhaust to intake ratio would be pretty hard to overcome. Now if you are racing for big bucks I would get heads with the magic ratio, but for non pro-stock or Nascar, 62% would work fine with the proper cam.
BTW how much for the heads and "W" intake?
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Rustang
Gearhead Posts: 452
From: Clarion PA
Registered: Nov 2000
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posted 01-01-2002 04:49 PM
MP,
E-mail me for details on the heads. The NMCA dropped the class I built the motor for, so I might entertain selling them. The thing is, I have some "unfinished business" with this combo. I know there was at least another 1/2 second in the combination that I'd like to find, but there is a new class requiring a different motor combo I'd like to try too. ------------------
'68 mustang 351 clevor- 10.92@124
'67 Stang, 351W -11.18@118
'69 351C Torino-14.90@100
'78 Pickup 351W-15.56@88
'79 Pickup 460 ET=??
http://www.rustang-racing.cityslide.com/contents/contents.cfm/712231
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Mpcoluv
Gearhead Posts: 928
From: Charlotte NC usa
Registered: Apr 2001
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posted 01-02-2002 07:31 AM
quote:
Originally posted by Rustang:
MP,
E-mail me for details on the heads.
Your profile contains no E-mail address.
What is your E-mail address?
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SundanceKid
Gearhead Posts: 979
From: UT
Registered: Feb 2001
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posted 01-02-2002 07:07 PM
I like to use split cam designs but companies like Isky Cams seem to hold a differant point of view. That if the heads are properly designed the cam should have the same lift and duration values. There is a whole article about it in their tech tips area. It may have the info you are looking for?
http://www.iskycams.com/techtips.html[This message has been edited by SundanceKid (edited 01-03-2002).]
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Mpcoluv
Gearhead Posts: 928
From: Charlotte NC usa
Registered: Apr 2001
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posted 01-03-2002 08:17 AM
quote:
Originally posted by SundanceKid:
I like to use split cam designs but companies like Isky Cams seem to hold a differant point of view. That if the heads are properly designed the cam should have the same lift and duration values. There is a whole article about it in their tech tips area. It may have the info you are looking for?
http://www.iskycams.com/techtips.html[This message has been edited by SundanceKid (edited 01-03-2002).]
Funny, Rod at Isky reccomended a split pattern cam for my 372C. 224/232 duration at .050 108 LC .545/.565 lift.
BTW the 108LC make it idle much rougher than you would expect. Behaves nicely in traffic though.
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Money Maker
Administrator Posts: 63
From: Lyons IL, USA
Registered: Jan 2002
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posted 01-03-2002 12:04 PM
If there is one thing that I have learned over the years, there is no set formula for cylinder head port flow relationship.
Forever and ever we used to flog the exhaust side of the WIndsor heads because everyone said it was too small. We made them round, "D" ported then, raised them, welded them, you name it we did it. All of this supposition was based on small block chebbie heads.
Guess what? We were wrong! The more we began to reduce the exhaust port size and concentrate on the bowl and short turn radius, the smaller the exhaust port was required. We needed to increase velocity via port shape and camshaft events, not size. We now flow over 20% LESS on the exhaust than we used to. Now the intake, that's another issue. You really do not have to worry about getting it out if you can't get it in.  ------------------
Co-Administrator and Moderator
NHRA, IHRA, and SRA member
NHRA and IHRA SS/LA National Record Holder
2000, 2001, and 2002
US Class Nationals co-organizer
Owner of a fleet of FoMoCo's including 88 ASC McLaren Mustang #28
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Rustang
Gearhead Posts: 452
From: Clarion PA
Registered: Nov 2000
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posted 01-03-2002 11:01 PM
It's always good to here it from "the man" (alex)  !! By the way, that Isky site that Sundance mentioned is pretty cool. Too bad I can't take this computer into the reading room!
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SteveLaRiviere
Administrator Posts: 32961
From: Saco, Maine
Registered: May 99
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posted 01-04-2002 07:57 AM
Get a laptop... ------------------
'70 Mustang Mach 1 M code 351C 4V/FMX/3.25 open
'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Lok
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
'97 Probe GTS 2.5L DOHC
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