Intake port volume

[jbeech]

My questions revolve more around the difference in runner volume. I see where "race" applications have increasingly larger volumes over "street" applications. Perhaps Grumpyvette can chime in on this. At what point do you reach diminishing returns for a street application? Is it at 195 cc, or 215 cc, or??? When, why, and how much do the heat risers matter?

 

My current SBC incarnation has camel hump 461-casting heads along with a Quadrajet carb in dire need of a rebuild. For decreased weight and increased performance I'd like to swap the heads out for aluminum units and the AFR 195's head the list.

 

I'm also thinking of an FI setup to replace the carb (though that's by no means certain as I really like the look of a multi Weber setup). And yes, the TWM Induction FI-kit based on Webers is running through my mind - but first I gotta see what kind of year I'm going to have before approaching my wife . . . but I digress. Grumpy, if you're following this thread, what say you about port volume? Any thoughts on the TWM kits?

[Mike C]

Look at Pete's chart in his question about heads. The AFR 190's seem a better choice.

 

As far as runner volume vs. diminshed rate of return, it depends on the rest of the combination.

 

I'm a cheapskate by nature, so if you want to do FI, seems like the BEST way to do this is to buy a complete aluminum head LT1. You get FI, aluminum heads AND a roller cam. All for about the price of just the AFR heads.

[grumpyvette]

BTW FOR THOSE OF YOU WHO DON,T KNOW WHAT WE ARE TALKING ABOUT!

ok after looking into this for several years heres what Ive found <b> ITS NOT AN EXACT SCIENCE but..YOU CAN GET CLOSE ON YOUR EDUCATED ESTIMATES and YES IM LEAVING A BUNCH OF STUFF LIKE FLAME FRONTS,IGNITION TIMEING,PRESSURE PEAKS, ROD LENGTH,ETC> out of this discussion</b>

 

(1) volumetric efficiency filling the cylinders and the resulting pressure pulse pushing the piston down into the cylinder, if graphed follows the engines torque curve graph extremely closely, or put another way the efficiency of the cylinders filling and scavaging increases with the rpm level untill a point where theres just not enough <b>TIME to effectively fill the cylinders,</b once that rpm level is reached the TORQUE peaks and altho the total HP may continue to climb for awhile because the NUMBER OF LESS EFFICIENT power stroke increases per second the effectiveness of each individual power stroke effectively falls in power as the cylinder filling time and cylinder burn time gets lower

 

(2) the pressure produced in the cylinder that depends on the cam timing and RPM level exserts pressure for only about 20-24 degrees of the 720 degrees in the 4-stroke engines repeating cycle and the EFFECTIVE cam timing (VALVES FLOWING AIR)filling the cylinders is limited to about 250 degrees with even a hot cam (less with a stock cam)

 

(3) displacement and cam timeing plus compression ratio and rod to stroke ratio have an effect on where and when in the intake stroke the max flow rate happends in the port.

 

 

(4) now lets look at port size and engine displacement, lets say you have a 350 chevy with a cam that effective flow is in the 230 degree range, (that would be about a 250 degree cam) look here, we see the average 250 degree cam is most efficient at about 5200rpm

 

 

so taking a 350 displacement/8= 43.75cid per cylinder, at a max piston speed of 4000 fpm .at 5200 rpm thats 43.3 intake strokes per second, at the probable max engine speed of 6857 rpm thats 57 intake strokes a second thats 1899cid of air at 5200rpm and potential 2493 cid of air at 6857 rpm but remember the valves only effectively open 230 degrees or 32% of the time so the port needs to potentially flow between 5935cid and 7790 cid of air per second, thats between 5935cid and 7790cid flowing past the valve through a port, the calculator say a 2.6sq inch port is the correct size at that displacement and rpm level, now 5935 cid of air flowing past a port 2.6 sq inchs in size is moving at 190cfps (cubic feet per second), at 6875 its moveing at 325 cfps but theres not enough time to fill the cylinders

<b>so as a crude guide your looking too find a port size that keeps the air flow velocity at between 190fps and 325 fps</b>

 

now lets look at my 383, its about 10% larger so its quite logical to figure the engines port speeds will be 10% higher with same ports or youll need a port <b> that flows 10% more, NOT a 10% BIGGER PORT</b> at that cam timeing and torque peak.

Now in your application with a 190fps-325fps air flow as a target plug your own engines info using the same math and see what port size you get, example lets say we want to build a 468bbc (468 bbc have a 4 "stroke so we have a 6000rpm max (STOCK)engine speed that,s 58.5cid x 8 cylinder displacement ,and with that cam timing about a 4000rpm torque peak, that,s 6088cid per second and 211 cfpm at 4000rpm and 317cfpm at 6000rpm , that suggests 2.66 sq inches-4.0 sq inches at 6000rpm for that 468 bbc

 

http://www.rbracing-rsr.com/runnertorquecalc.html

 

if youll look and compare youll see why the 468 engines tend to run better in the mid range rpms with oval port heads

 

http://www.newcovenant.com/speedcrafter/calculators/runnerarea.htm

 

<b>so the bottom line here is your looking for a port that flows about 2.0 sq inchs and 210cfm at 5000rpm and 2.68 sq inches 270cfm at 6857rpm average the two and youll be looking for a port of about 2.3-2.6 square inchs that flows between about 250cfm-270cfm at your cams peak lift because remember the port cant flow enough due to time restrictions at the peak rpm range</b>

 

 

 

BTW thats most closely matched by a 195cc AFR head below 5500rpm and ABOUT 235@.050 lift duration and a 210cc head above 5500rpm and 240@.050 lift duration on a 400cid engine

the short answer here is that its NOT PORT VOLUME ,ITS THE AVERAGE PORT CROSS SECIONAL AREA your looking for! a #1205 gasket size port is about correct for hot street and a 1206 port is about correct for an engine thats mostly used on the track

look at it this way a 195cc port thats a #1205 size cross section tends to flow about the same and have the same average air flow speeds as a 210cc port thats also a #1205 cross section. these about 16.38 CCs in a cubic inch, a port that measures about 2.5 sq inches like a #1205 port needs to be only about .67" longer to remain at the same cross sectional average, raiseing the port floor and roof and changing the entrance angles can easily account for a great percentage of that volume

[pparaska]

Thanks Grumpyvette - great post!

 

I guess this answers my question on whether the 190 or 195cc heads would be preferable on my 406 - the 195s would be. Although I don't know where I'd ever notice a difference - probably only at the very top of the rpm band.

[Mudge]

Alot of people try to stick to modest port volume for street apps, however there are people with hogged out heads making awesome torque and HP with small cams. Example would be 15º SBC on a late model LT1, cam is about a 234/24x and makes about 650 HP @ 6600 RPM. I forget the torque figure but it was much more than I expected and at I think a peak of 2800 RPM or so, the intake runner was around 220. The setup was created by what I'd call a master engine builder, so even with big heads it still stays within a pretty modest RPM band and makes freakish TQ, something the LT1s are not really known for since they are sort of a compromise/bridge between the L98/LS1.

[jbeech]

Whew Grumpy, that was one exhaustive answer - thanks!

 

I'm currently on vacation, but stuck in a hotel room in Atlanta. Why? Pretty simple really. They overbooked the flight from Atalanta to Panama and were offering $600 vouchers plus hotel and food. Lynn and I figured, "what the heck" and stepped up. Not often we can make 1200 bucks for a day of non-work! In return, we spent the night at the Ramada near the airport, and gained a day alone (i.e. sans daughter) which is nearly as good. Why the discourse? 'Cuz I'm not really in a position to study all of what you said until I return home a week from Sunday, but thanks to one and all who took time to respond.