Rev range is
1. parly cam related,
2
. partly Length or Rod (L/R)ratio,
Even a 3% improvment from 1.505:1 L/R to 1.55 via a 6.06" rod instead of 5.885", the stock 250 rod, can raise the rev range 500 rpm with other changes. Ford Australia did this with the smaller 200 in 1971 to 1992 , and in all test data except one test in 1983, It always got better acceleration with 25% less capacity. The rod ratio was 33% better, but the engine was 25% smaller. Yet every 3.3 4 or 5 speed combination was better than the later 4.1 4 speed combinations.
I out wore my welcome before being able to imtimate that in the last post (Cost-effective-331-stroker-base-Windsor-5-0-or-Cleveland-351), but I got all the info I needed corroborated by five impecable sources. All this was birthed in
Bill In Indy's Engine Analser posts here on Ford six. And
57Stan's Atlantic TRD Toyota.
We added Port on Port Carbuation and then a longer conrod on a Big Six, and found that the longer rod (On an AirFlow Restricted Engine) always have half the algeric difference in rod ratio extra power. 3% rod ratio improvment, 1.5% extra power. 33% extra rod ratio, 16.5 % more specific power in cubes per hp. It has to be"Air Flow Restricted" where the cylinder block's peak CFM demand exceeds the ablity of the heads and induction system to supply power.
http://vb.foureyedpride.com/showthread. ... ng-rod-351
http://vb.foureyedpride.com/showthread. ... veland-351
https://www.youtube.com/watch?v=ow5cGV7bXCw
in the 2 minutes out of 2hrs with David Vizard A Series from 1 min 47.18 to 1 min 49.24sec
A 4 % Longer rod (6.000" vs 5.750" on a 3.206" long stroke A seriesMini Cooper S 1275/ BMC/ Austin/ Frog Eye) 1.874 vs 1.793 gives a 4.5% algeraic improvemt in L/R, but a 2 hp increase in an engine already giving 100 hp net modified.
A. more intake valve opening (12 thou more intake opening)
B. That intake opening happens when piston speed is reduced, with more dwel time
C. and there is less cylinder wall friction (considerably less on one 1435 vs 1549 Mini Engine, but even on a 1293 "1275" A series engine with just the rod changed for a 4%, 1/4" longer one. )
3. partly Air Restriction related,
Take the Holley Carb CFM calculation,
That also produces a carb venturi area per cubic inch of engine formula. On a 240 Big Six, or a 250 with the 1.29"Veturi Autolite 1101 or 1.3125" YF Carter 1-bbl, it would have a just 180 cubic inches of engine to every 1 sq inch of carb venturi area. Rated at 1.5"Hg, a normally 195 cfm carb was just 137 cfm, yet at even 4500 rpm, the air flow demand was (250 * 4500 /3456)*0.75 for a 75% VE six cylinder engine....The engine could suck 245 cfm, but the carb only made 137 cfm at 1.5"Hg, or 195 cfm at 3.0"Hg. Noramlly, an engine starts to heal over at about 265 feet per second of carb flow, which was at 3600 rpm at 3"Hg on the 240 or 250.
Same applies with the 170. Air flow creating at 4500 rpm is (170*4500/3456)*0.75, or 166 cfm. Stock carb flowed something like 125 or 150 cfm at 3"Hg. So the engine healed over mostly due to carb restriction, but also because of cam duration and port size.
Adding a 32/36 Weber or 5200 Holley Weber, those carbs flow 280 cfm at 3"Hg, so with a nice 256 degree cam, they would rev past 6000 rpm if the heads were ported enough. With a 240 degree cam and the stock C1 170 head, it would die at 4500 rpm.
My C1 170 head on my 200 1981 Mustang was able to rev to 5300 rpm. It had a 202 cfm 1946 Holley with 1.3125" venturi, but just a 1.3" carb hole, and a 256 degree cam. Thats 230 cfm of demand (200*5300/3456)*0.75.
The head went critical air speed of 265 feet per second at just 3800 rpm, where the maximum power came in. It started to heal over at 3800 rpm, where it made 91 hp. From 3800 to 5300 rpm, even with a 256 degree cam with 30 thou lift, it was just theashing gain.
The Heal over point is then determined by how close the cylinder head CFM and BMEP matches it. Intake port flow and size (diameter/cross sectional area), exhaust flow must be 65 to 80% of the intake flow, and then the intake flow must be greater than the deamand of the bore and stoke CFM of six cylinders. If its way higher than the amount the head, carburation, cam, conrod to length ratio and bore to stroke ratio can supply, then you need to upsize the ports, or rephase the cam Lobe Centers and duration specs to compensate.
4. partly Bore to Stroke ratio
Any 144 or 170 will have the very restrictive 240 degree cam on a 109 Lobe Center with just 370 thou lift, so it surely won't rev to 6000 rpm. Its not and never was a 4 bearing crank shaft vibration issue, all that stuff is Bovine Scattology. But when 60's Hydrosurf racers added fuel injection and a 300 degree cam, these engines would come alive and rev to 8500 rpm for almost a whole season, then the crank would have to be changed due to fatigue. The 144 was the honey of the two engines....you could bore it out to 155 cubic inches with a set of solid skirt 3-5/8" 125 thou over cast or forged pistons, and the 2.5 inch stroke would give the ideal 1.94:1 L/R ratio (4.855" divided by 2.5" stroke).
Having Mechanical Hillbourne Injection ensured the hogged out C0DE 6090 "144" heads could breath at 8500 rpm. The log got cut off, and the lovley thick wall casting with more than 240 thou casting thickness could be taken out from tiny 1-1/8" ports to huge
An over square (3.625 to 2.5, 1.45:1 ratio) helped as well.
The 200 had a 256 cam with extra lift, but a 1.5:1 rod ratio (4.715/3.126 for the 200, 5.885/3.91 for the 250) also common to the US 250. Both engines ran out at 4800 (200 cube) to 4500 (250 cube) rpm or so.
The Australian 200 engine ran a 2:1 rod ratio, but log head or 2-bbl, it would always rev another 800 to 1000 rpm on beyond the US 200 or 250. 5500 rpm.
The peak rpm is decided by the four factors above. Past the peak rpm level, the heal over point pont, the % of rpm the engine can over rev, is really defined by the air flow limit of the parts aboce the pistons and bores. And engine is a pump. If you restrict air flow, you ensure it won't over rev.
That was the lesson of the small six and also the 240/300 Big Six during the 1-bbl area....if you restricted air flow, you'd kill the revs, and the engines would awlays live a longer time than if they were doing 8500 rpm. That was the Whizz Kids master stroke of creating the air flow limited in line sixes.
To raise the rev range, you have to add some duration, port area, carb area, cam lift, lobe center, cylinder head flow, or balance the intake flow with more scavenging from the exhaust via a split duration cam or better exhaust system.
The ulimate best way to raise the rev range stop the engine healing over?
Port Fuel Injection or a close to Independnet Runner Port on Port induction system or at the very least, the great American halfway house, the Tri Power staged intake manifold. With no other modifications, swapping from a 1-bbl carb to a Tri Power will raise the Rev range 1000 rpm even with a 240 degree cam.