Hopping up the 500 Cadillac
From Crankshaft Coalition Wiki
Many things were learned about what to use, what works and what doesn’t after many dyno pulls.
- The crankshaft can be offset ground 0.300" smaller to the same size as a big block Chevy which yields a 35 cubic inch increase. Coupled with a slight overbore you now have 540 cubic inches, and the ability to use BBC rods. This does not affect longevity but does increase torque. Horsepower does NOT really increase without a change in the heads and cam.
- When this is done you can use 6.800" H-beam Chevy rods which are MUCH stronger and very affordably priced. Cadillac rods will fail above 5000 rpm.
- While 1974-'76 large chamber heads can be used, there is a substantial power increase to be had with 1968-'72 small chamber heads if they are fitted with BB Chevy 2.19" intake and 1.84" exhaust valves. Small chamber heads with big valves and bowl work are worth 25 hp over large chamber heads with big valves and bowl work.
- The BEST heads to use are the new aluminum heads from MTS. When these are Fully Ported by an experienced head porter (like BPE) they are worth about 100 horsepower over a stock head of any type.
- The Edelbrock intake works good on a 500 inch or smaller motor if you don’t exceed 4500 rpm. Larger motors or those with good heads need the new single plane from MTS. Power will NOT drop off like the Edelbrock which is airflow limited. The Edelbrock will make 30-40 ft/lbs more torque under 3500 rpm. The MTS is worth 50 ft/lbs torque and 50 horsepower at 5000 rpm over the Edelbrock.
- The factory Q-jet works good on the Edelbrock. It does NOT seem to work very well on the single plane even with rejetting. Reversion from the huge secondaries on the single plane may be the problem.
- Multiple sizes and types of carburetors were tried. Best torque and horsepower with the large chamber heads was with an out-of-the-box 750 Holley. Best by 20 horsepower. The large chamber heads flowed only 710 cfm MAX measured through the carburetor at 5200 rpm under load on the dyno. Larger carbs up to 1000 cfm were tried and ALL lost horsepower and torque.
- The long stroke Cadillac, whether 4.300" stock or 4.600" stroker, needs plenty of cam. Several cams have been tried. For any type of performance use, a minimum of 234-244 degrees intake duration at 0.050" tappet rise with as much lift as you can get, works well. This cam will peak at 5100 rpm but still have good torque at 3000 rpm. With really good heads, add 10 degrees or 244-254 degrees intake duration. Stock heads and stock valves only need about 220-230 degrees intake duration. Anything bigger will not flow any more air because of head flow limits.
- Engine size does not affect HP as much as torque. Engine size DOES affect torque. Shocking as this sounds, A 350 cubic inch engine with good flowing heads will out horsepower a 550 cubic inch engine with poor flowing heads. The 550 ci engine will out torque the 350 at low to mid RPM. TORQUE is what makes the car go. The Cadillac has poor flowing stock heads so you have to maximize torque and help the horsepower under 5000 rpm. This is why the Cadillac is generally limited to 5000 RPM.
- Stock rocker arm stands are prone to breakage at over 100 lbs of valve spring seat pressure. This light spring pressure limits RPM to about 4600. More radical cams or more rpm require more valve spring pressure. This requires aftermarket rocker arm assemblies. MTS and others have some excellent ones that will take high spring loads. The heavy rocker arms require a minimum of 130 lbs of seat pressure. Beehive conical springs work best to counter the harmonics that cause premature valve float. With a normal non-beehive 130 lb spring we were on the verge of valve float at 5100 rpm. Higher spring pressure can wipe out the cam lobe of the Cadillac cam blank because of its small diameter. Beehive springs can extend this to perhaps 5500-5700 rpm with the same pressure. You better have really good heads to flow enough air at that rpm to still make horsepower at that rpm.
- With aftermarket rocker arms you cannot use stock valve covers. You have to use tall ones for clearance.
- The factory HEI distributor works fine if it is modified slightly. The stock advance curve is about as screwed up as any I have ever seen. For performance engines with over 9:1 compression ratio, the curve that seems to work well is 8 degrees initial and 25 degrees mechanical advance at 3000 rpm. For low compression engines under 9.0:1 CR, use 12-14 degrees of initial advance, and 20-22 degrees mechanical advance all in at 2500 rpm. This will give the motor low speed pep. Plug the vacuum advance. They over-advance and cause pinging. This distributor tip also works well in stock applications.
 Dyno results
With the wimpy large chamber heads and Pontiac valves the best observed and uncorrected pulls were:
- 549 ft/lbs torque @ 3200 rpm, yet still made 502 ft/lbs @ 4600 rpm
- 442 hp @ 4500 rpm, and still showing 438 @ 5100 rpm
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