Sorry about the formula thingy but I thought this was pretty cool:

1. B18C 1797 cc displacement = 109.7 Cu. in. x 0.5 = 54.85 cu. in. per revolution (1/2 of your displacement is used in 180 crankshaft degrees or 1 crankshaft revolution)

2. 54.85 (cu. in./rev) / 1728 (cu ft/ cu in.) = 0.0317 cu. ft./rev

Now a stock ITR has an 8400 rpm redline

3. 0.0317 cu ft./rev x 8400 rev/min. = 266 cu. ft. /min. (cfm)

What if I got Toda B's and raised the redline to 9300 rpm?

4. 0.0317 x 9300 = 295 cfm

So you would need a port size to support a 295 cfm (or an 11% increase in flow rate) both in and out of the head to ensure volumetric efficiency.

Assuming the porter has some sort of table or formula that relates port size to required flow he could then have an idea of how much to port and not overdo it (killing your midrange power).

Is this what people do? or is it, here's my cylinder head and port it please?




[Modified by Michael Delaney, 10:45 AM 8/20/2001]

 

we can always count on Tuan to come up with these crazy posts...

 

okay so it was another hair brained scheme :

But you get these guys with swapping heads and it's nice to know how much a stock head can flow to support 100% efficiency.

b16a head (1587 cc, 8000 rpm redline) 224 cfm

b18c1 head (1797cc, 8100 rpm redline) 257 cfm

b18c5 head (1797 cc, 8400 rpm redline) 266 cfm

b18b head (1834 cc, 7200 rpm redline) 233 cfm

Hey Frank Lin had that piston speed post up awhile ago and no-one batted an eyelid. I thought they were pretty cool.

 

BTW all the headporters tell you it's a black art. In this day and age of computer simulation and CAD/CAM designing you have to know that competitors are looking at headwork done and making casts of their rivals ports. They can flowbench them and then relate flow to crossectional area along the length of the modeled port from the castings.

So there is a science that relates port size to flow and how much flow you need to keep a hefty midrange and still peak nicely in power. They just want you and I as customers to remain in the dark ages.

Duhhh...pass the nuts and beer bubba.....doh!

 

thanks for the informative post. Most people do assume that their head is flowed right when they go to get a ported head. I'm sure not most people know about verifying the results with a flowmeter. The head is the most important power maker in the engine. A head can hurt or help you if you don't do your research.

 

Ok. Now figure out how much space a specific higher compression piston takes up. do you increase the bowl area or not

easy answer...think about it. just toying around

 

b18c5 head volume 45.5 cc

b18c1 head volume 41.6 cc

b16a head volume 42.7 cc


ITR

Assuming a 0.236 in. (6mm) thick head gasket , standard 3.189 in. (81mm) bore, 3.43 in. (87.2mm stroke), and head volume 2.78 cu in. in an ITR .

1. Volume at BDC =

Cylinder displacement + head gasket volume + head volume

[pi x bore x bore x stroke] + [pi x bore x bore x gasket thickness ] + head volume

109.7 cu in. + 7.5 cu in. + 2.78 cu in. = 119.98 cu in.

(sorry about the valve reliefs volume but this has got to be tiny in relative terms)

2. Volume of chamber at TDC

Compression ratio 10.6 / 1 = Volume BDC / Volume TDC

Volume TDC = Volume BDC/10.6

Volume TDC = 119.98 / 10.6 = 11.32 cu in.

In one revolution this would be 5.66 cu in.

3. If we go to say a 12.4:1 piston

Volume TDC = 119.98 / 12.4 = 9.68 cu in.

In one revolution this would be 4.84 cu in.

So there is a 14.5% decrease in combustion space going from a stock compression to 12.4 CR (say CTR pistons)

Where are we going with the bowl area?

 

versus


hint, hint

 

My DPR clove-leafed head looks somewhat very very similar to the headwork in the upper photo.

 

Bowl area is obviously increased to allow for unshrouding and to permit for better burn. Common sense. Or as Tuan pointed out mathematically

Now, my question - HOW MUCH do you increase bowl area? Do we assume that the stock 5.66 in^3 is ideal? My guess is probably not quite.

Tuan, you posted stock head flows for various models, but have you seen flowbench numbers for a stock head, with before and after porting? Maybe by knowing how much the head needs to be opened beforehand, we can know how much more (% increase) it would need after, or for a certain application. This relates to a post i had a while back - you need to tune your specific combination, not just throw in cams. Tuan, thanks for providing the math to prove this! Also, I made an analogy in my earlier post.. lets take it one step further.

Assume we've got a stock ITR setup. We know compresssion, lift, duration, etc. and we assume that they all work perfect together to allow for max power given this setup. Correct? Lets assume porting provides no gains and it is for now.

Now, can we take these numbers, estimate a ratio between itr cams and a given 'performance' cam, and calculate the percentage increase in flow (which you did, thank you!) as well as increase in compression? (I fooled around with some numbers, just curious to see your results as well.)

Thanks for the continuing thought provoking posts, both here and on TOO.

 

I remember that sgt mentioned that he saw a DPR stage VI head only flow 255 cfm.

I guess my point is if you have a certain cam and redline in mind, it does help to guide the head porter (among other things of course). I don't know what you guys and gals do when you go to get headwork done but no-one ever taught me to think about these things and the articles about headporting were usually by the porters themselves ...providing little useful info for the consumer for research on factors to think about.

Back when I had my head done, you just handed over the head and said I want "porting" done without any midrange loss and to make make it go zoom zoom...oops that's another manufacturer...doh! No wonder you didn't get what you "expected".

are clover leaf chambers out of fashion? Is the current trend to overbore? I saw that 2.6 L H22A of Lookofsky's and the cylinderwalls are soooo thin. The trend to use 84-85 mm bores sleeved is a hot ticket item...so deshrouding valves and spotfacing quench areas are also "hot".

Leo95Se: I think you have a great idea and it would work well on an excel spreadsheet...too many variables all at once to state in a post. You would need a table. I remember those 3D graphs for ECU maps where you have timing, injector duty cycle, and engine load plotted simultaneously....made my noggin' hurt.

cheers





[Modified by Michael Delaney, 12:36 PM 8/21/2001]

 

The hotter cam should have more duration and lift. So even if the head only flows at a max of 266, with more duration, it flows at 266 for longer, which I think would compensate for the higher flow generated by the higher rpm. The math to figure it out is way above my head though.