Porting School #5 Identifying Primary Restrictions

[olNick]

Originally Posted by FlowSpecialist

I've been digging through my dyno data to see if there's anything in there of relevance to this topic.....

The second is a 2v 1360cc Peugeot TU engine with 39.4mm inlet valve, throttle bodies, rally cam and fully modified head. It puts out 139 bhp or 102.2 bhp per litre and 74 ft lbs per litre.....

Dave

Hi,

Veryfying what Dave is saying here, the newer Pug 207 is using a variant of this engine and is available with a 75ps 1.4/2v or with a 90ps 1.4/4v, around 20% more power mainly due to a head change....

Dave, I'm running your calcs in my current bike engine of interest and am getting

Valve area / cc = 1710 / 680 = 2.514 sq. mm/cc
Valve perimeter / cc = 207.35 / 680 = 0.305 mm/cc

major bummer, some pretty measly figures similar to the bb chevy example you gave, in valve area at least


This engine makes about 46hp/liter in stock form
What does one do when essentially stuck with this inherent configuration, if we are talking a tractable, street engine here?

regards,

nick

[FlowSpecialist]

Same as any other engine. Get as much valve area in there as you can and do the best porting job possible.

Dave

[rookie]

Originally Posted by rookie

One thing I have not seen addressed in this low vs. high lift flow issue is rocker ratio, me being mathematically challenged, maybe some of you math magicians can put actual #’s to this.

Because the discussion is more towards race engines, I will use these Lunati cam specs for my question.
Intake only,
Total duration 309
.050 – 276
.0200 – 194
Lobe lift .446, with a 1.5 rocker 669, 1.65 RR it goes to a .735

As Larry and others have shown on other posts they use 1.8 RR, and some engine builder’s use as high as 2.0 ratios with even more aggressive cams.

Say this cam is installed at a 103 cl and opens at 51.5 before tdc.

How high will the valve be lifted at 55 degrees after tdc with a 1.8 RR vs. 1.5 RR.


So even though the head may not flow good at low lift, the engine sees higher flow #’s sooner in relation to degrees of crank revolution with higher ratio rockers and more aggressive cams.

I think that Nascar and Pro Stocks use of 55degree valve angles to control valve bounce has led people to look at it in terms of flow and believe it is being done to reduce low lift flow #’s and has caused people to over look other possibilities.

The lack of response to this post leads me to believe that I have struck a nerve.

So I am going to post some hypothetical #’s as I don’t have enough different rocker ratios in my shop to do actual measurements.

I will use my Edelbrock head chart for the #’s and will have to fill in the 25, 50 & 75thousands #’s from my own data so these #’s will not be 100% accurate, some #'s I will have to ass-u-me on, but close enough for comparison.

Intake flow, using only lower half of the lift range.

lift.
25.....50…..75…..100…..125…..150…..175…..200…..225 …..250…..275…..300…..400

Cfm @28”
Stnd. 23 degree sbc. 200cc runner 2.08 valve as cast.
18…..33….48…….67…….81…….98…...112…..119…..133…..14 8…..160…..177…..226
23* raised port sbc 238cc runner 2.125 valve CNC
20…..38…..54…...70…….89……109…..128…..148…..166…..1 87..…207….226…..270
18* sbc 236cc runner 2.125 valve CNC
20…..38…..55…...71…….93……115…..137…..159…..175…..1 91…..207…..224…..273

Using the book supplied with my SF flowbench and its magical mathematic conversion charts we can convert the 28” to 10” by multiplying flow x 0.598 and then take the flow at 10” x 0.43 to get power per cylinder, then multiply that x 8 for the # of cylinders and get a theoretical (I know some on GFN love that) power figure.

Using this cam and rocker combo you can see a .066 increase in lift.
Lobe lift .446, with a 1.5 rocker 669, 1.65 RR it goes to a .735

If we only get .025 of that lift increase, at say, a ¼ of the way in to the intake stroke and say the intake valve would normaly only be opened 150 thousands, due to the increased rocker ratio (rr) the engine now sees .175 lift and 14cfm increase based on the stnd. 23* head flow #'s above.

14cfm x 0.598 gives us a corrected flow of 8.372 cfm at 10”, take 8.372 cfm x .43 gives you 3.59 horse power per cylinder x 8 is 28.79 horse power.

If you think about the increase of a 1.8 vs 1.5 rocker and either of the other two head comparisons you can see how fast rocker ratio related LOW LIFT FLOW can add up.

Not to mention that you increase duration at .050 by some 5 degrees.

I hope at least half of this make sense, this is a lot of thinking for a 4th grade edumakation.

[DavidVizard-GFN]

OK guys,
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