What you need to know to know and why to maximize your cylinder head investment.
We, at Dart, asked engine research consultant, university lecturer and world renowned performance tech author (over 4000 magazine articles and 32 books with #’s 33 and 34 in the making) David Vizard to write up for us the results of his port volume tests. Our goal here is to show that bigger is not always better. A fifty year veteran of high performance engine building and dyno testing plus a 4 figure number of race wins using his go fast technology puts David Vizard in a unique position to do this write up from firsthand experience. My suggestion is take the time to read it and reap the substantial benefits that this knowledge will impart.
- Jack McInnis
Dart Machine Ltd.
More airflow usually equates to greater output but in the case of a cylinder heads port sizing the lure of ‘bigger’ ports can be a trap waiting to snare the uninformed.
A 4 cycle engine is far from being a simple air pump. The principle reason turning apparent simplicity into real world complexity is the dynamic ‘stop – start’ nature of the flow through the engine and the fact that air is very much heavier than is often supposed. Rapidly changing rates of pressure and suction bring strongly into play both the momentum and the pressure wave effects that can be used to boost cylinder filling. At the end of the day this means that for a given displacement and rpm band there is a set of ports that are right for the job. Anything more than a few percent bigger or smaller is not. The following tests will demonstrate the difference delivered by a range of port sizes.
The Test Engine.
This was one of my budget builds, a replica of which you can get at Pro Stock engine builder/racer Terry Walters in Roanoke VA. () for about $5400 turn key. The final price though will depend on the exact spec. For this test this Scat 3.75 inch stroker (budget cast steel series 9000 crank teamed with Scats budget 6 inch I beam pro-comp rod. Pistons were forged Silvolite ICON items. These in conjunction with the ‘as-cast’ 72 cc combustion chamber Dart heads tested delivered 9.8/1 CR. Had the 64 cc heads been use the CR would have been bumped to 10.7/1 for use with 93 octane. For what it is worth the torque and horsepower jump by about 15 numbers with the CR increase. For this engine the cam used was a custom 276 hydraulic roller grind done to precisely cater for the cylinder head characteristics, CR and displacement involved. If the saving of about $250 is of interest then it’s worth noting that a hot street hydraulic flat tappet cam with a 280 duration will deliver very comparable results. My book ‘How to Build Max Performance Small Block Chevy’s on a Budget’ (available www.Amazon.com) goes into very precise detail on the cam selection for a given combination. It goes so far as to give the cam for your build to a precision equal or better than testing a half dozen or more likely candidates on the dyno.
Headers used were a set of street Hookers with 1- 5/8 th primaries and a 2-1/2 secondary (collector) 18 inches long. The carb was a entry level 750 cfm AED unit mounted on a Dart two plane intake. It is worth mentioning here that it is, for this test to be valid, important for the intake manifold is capable of servicing the needs of the engine at both low and high speed. This means a manifold that flow well by virtue of with efficient ports not big ones. The Dart two plane item met those needs. At each cylinder head change, which ran small to large, the intake manifold was re- matched to the bigger ports at the manifold face. Ignition was via a Pertronix contactless unit. These are low cost and get the job done well.
The Test Heads.
The intent here is to run four pairs of the Dart Platinum Pro 1 heads having intake port volumes of 180, 200, 215, and 230 cc. Regardless of port volume all these heads flow about the same cfm until some 0.400 inches of valve lift have taken place (see Fig 1). Even at 0.600 were the test engines valve train tops out there are measurable flow differences. Because of these differences are the result of a port size increase they are a legitimate component in our bid to investigate the overall effect on the engines torque/power curve.
Check out the flow curves in Fig 1. and you will see that the majority of the flow increases with increasing port volume occurs at the higher lift value. So much so that any test that did not put enough lift into the valve to access the extra flow at high lift would be totally skewed in favor of the smaller port heads…
We talk port size in cc’c but the reality is that it is the port cross sectional area that is the factor we wish to control. So why is port cross sectional area important? If the area is bigger the flow surely goes up and that’s what we want is it not? Sure the engine wants as much airflow as possible but much of the flow through-put depends on port velocity and the generation of pressure pulses. This means an overly large port can hurt power even though it may, on the flow bench at least, flow better. The question is how does this work out on the dyno?
So you can better see what’s going on here the torque and hp graphs have been separated. The effect any particular head has on low speed output can be more clearly seen by considering the curves shown on the low end of the torque graph. To see the effects on the top end check out the curves from mid to high rpm on the hp graph.
Fig 2. The dyno results tell us that smaller, higher velocity ports, favor low speed output. (black 180, red 200, green 215, blue 230). These results also show that going too big (blue curve of 230 cc port) on the ports, for the intended combination produces worse results almost everywhere in the rpm range.
Inspection of the torque curves in Fig 2 show the 180 cc ports (black) turned in the best numbers up to 3400 rpm with a peak of 482 lbs-ft. The 200 cc port (red ) though marginally lagging initially proved stronger from 3400 rpm up where it ran up with, or close too, the bigger ports.
Considering the torque curves of Fig 2 and the hp curves in Fig 3 shows that for our spec of 383 incher, the 200 cc ports delivered the best curve. The 215 cc (green curves) heads made the highest hp by pumping out 478 horses as apposed to 457 for the 180 cc runners, 472 for the 200 and 475 for the 230’s. The price the 215’s pay over the 200 to achieve this 7 hp advantage is that they give away up to 10 lbs-ft of torque in the rpm range from 2300 to 3200.
As for the 230 cc port runner heads these, on our 383, failed to deliver any advantage anywhere in the rpm range. The smaller 215 cc port heads actually outperformed the 230’s everywhere! If the test engine was capable of more rpm or was of a larger displacement the bigger port heads would have paid off.
So how do you decide what port volume your small block Chevy should have for best results? Check out the chart Fig 4. This will give a good starting point for port volume selection for a 23 degree headed small block Chevy. A word of caution here. Do not overestimate the power you are likely to see. All that will do is lead you into a bigger port than would be optimal. This leads to less power than you had hoped for. At the end of the day a little too small is better than a little too big!
A final point, just in case you are wondering, with a slightly bigger cam and 10.5/1 CR the as-cast 200 cc Platinum Darts, on this engine, allowed it to crank out 500 lbs-ft and a tad over 502 hp.
For more in-depth David Vizard tech on hi-performance engines and cylinder heads in particular go to www.motortecmagazine.net
David Vizard, considered by many to be one of the world’s leading Performance Auto Tech seminar speakers, will be holding a two day seminar on 10th and 11th Sept 2011 at TPI specialties in Chaska Minnesota. If you want to get the benefits of a seven figure dollar R& D budget for the cost of a seminar ticket this is the place to be. As a true performance enthusiast you about owe it to yourself to check out this seminar at www.davidvizardseminars.com.