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505 Big Block Chevy Motor - Big-Block Street Heat

Here's 700 Horses And 650-Plus Lbs-Ft From A 505-Inch, Pump Gas Big-Block Chevy That We Closely Watched Costs On.

Head Games
Any performance engine is only as good as the cylinder heads. This applies to a big-block Chevy more than it's little brother. Why? Because big-block heads, even with the largest valves typically used, has a lot less valve area per cube than a small-block. The numbers really bring this home in case you are wondering just how short of valve area a big-block sporting 2.3-inch valves can be. For a 350-inch small-block with the normally used 2.02 intake, the valves have to feed 13.65 cubes per square inch of valve. For a big-block like ours the intake valve sizes used are typically 2.25 or 2.3. For the 505 we are building this results in each square inch of intake having to feed 15.2 cubes. To get the same cubes per square inch our big-block would need intake valves about 2.43-inches diameter. As a result the heads used must flow well for the size of valves involved or output, especially at the top end, will really suffer. So why not just put in bigger valves? The problem here is that much over 2.35-inches diameter the inclined valves can clash during the overlap period if too much cam is used.

Continuing our comparison of valves and flow of the small-block versus big, we find that a well really ported small-block head can flow about 290 cfm at 0.700 lift. To stay on par with this our big-block heads need to flow about 410 to 420 cfm at 0.700 lift. Now before getting bent out of shape should that goal not be reached, there is a point of salvation for the rat that is not available to the mouse. Because a big-block has inclined valves shrouding on the way up to about 0.0600 lift is less than on a small-block and, as a result, the midrange flow of a big-block has the potential to be better, size for size, than a small-block. This means we only have to closely approach our target flow to get comparable small block hp per cube from our 505.

On our bench, the as cast RHS heads, at 0.700 lift, flowed some 380/371 cfm for the good and bad intake ports respectively and a creditable 264 for the exhaust. These are very respectable figures and rank a tad higher than average. Also, if you take into account that we elected to use the small port (320 cc) heads then it can be seen that these heads are good right out of the box. But as good as they were, I have to say that their strong point was the ease with which they ported. Doing little more than skinning up the guide bosses and blending out the short side turns and any other less than perfect areas resulted in some good numbers. The mid range flow was also creditably strong with part of this being due to us substituting the normal used 2.25-inch intake for the small port head with the 2.3-inch intake for the large port head.

Coupled to this LJ, T & L's top head porter, also did a Cup Car style valve job on the intake seats during the install of the larger 2.3-inch valves. All this porting was quickly executed and the result was 405/401 cfm at 0.700 for the good and bad intake and 286 cfm for the exhaust. With these results in hand we began to feel confident that the 650 hp target was going to be an easy deal.

With the heads done and assembled the next job was to install them on to the short block. Here we used Fel Pro high performance gaskets and ARP six point head bolts. These were torqued to the appropriate value and, at this point, we put aside any further moves on the top end and turned our attention to the lubrication department downstairs.

For our bottom end, we used a Moroso oil pump, pick-up and basic street/strip pan. We selected a Professional Products crank damper for torsional control of our stroker crank. When these dampers were first introduced I was a little leery that anyone could build a decent damper for so little money. A few years down the road and a lot of dampers later, with no problems, so it looks like it can be done. If you are doing a build and installing an aftermarket damper you need to be aware that no damper should be installed in any manner other than with a proper damper installing tool. I realize it is an expensive piece for the at home builder, but you don't have to buy one to get the use of one. Just go to your local AutoZone and they will loan you one free.

Valvetrain & Induction
Starting with the rollers, we used Comps Endure-X solid roller lifters. These are designed to put up with whatever abuse a fast opening cam delivers with sufficient strength margin to allow a long street life. In short they are ideal for our application here. For pushrods Comp's Magnum design in 3/8th diameter were selected. The final selection on length was completed after a trial build of the valve train had revealed what the optimum length would be. Getting that pushrod just right on a big-block can often be critical due the angled valvetrain geometry.

For rockers Comp's tried and tested basic aluminum 1.7:1 rockers were used. Partly as a result of the pushrod length selection their rockers delivered a tad over 1.7:1, thus adding a few thousandths to the theoretical valve lift. For valve springs we ran those that came with the heads-after all RHS is a subsidiary of Comp Cams so it should know what works in this department.

In spite of being genetically disposed toward selecting "too much" of anything that looks like more power, I continually see the reverse for a big-block Chevy engine. Unless it's a really small big-block (396 maybe) there is really no place for a 4150-style single four-barrel Holley on 505-inch big-block with a target output of 650-plus hp. To meet our 505's induction requirements a 1050 Holley Dominator was chosen along with an appropriate Weiand Team G single-plane intake. The intake was port matched to the heads and the rest of the runners given a coarse (80 grit) emery roll clean up. This was in preparation for an intake coating test that we planed to do with this motor but subsequently ran out of time on. On the flow bench this clean up made very little difference to the flow and while it may have contributed to the engines output it is doubtful that it did so by much.

As for the carb, I don't know how lucky we were here, but as we were to subsequently discover, the jetting and overall calibrations proved very close to optimal.

Dyno Time
This engine was broken in and dyno tested with fuel from the gas station just down the road. After a post break in oil change and a re-lash of the valves, a full power run was made and we were pleased to see the numbers fly right over our 650 hp target and land smack in the middle of the 690s. A look at the plugs and the dyno air/fuel readout indicated that this 505's carb calibration might be just a mite too lean. This prompted a step up on the jets of two sizes all around. About this time, the owners arrived just in time to see their motor go 697.8 hp and 657 lbs-ft. To say they were ecstatic would be an understatement. On the other hand Lloyd and I were a little put out that the final number did not top the 700 hp mark as that sounds a bunch better than 698.

It was obvious that the original Holley jetting on this carb was really close to optimal as our change to two sizes bigger jets only made a very small difference. From the dyno data we figured that though our 1050 Dominator had performed magnificently we may have chosen one that was slightly too small.

After watching this good looking big-block disappear down the road in the back of a truck, Lloyd and I sat down to take a hard look at the dyno printouts. The first question we asked ourselves was did the RHS heads work as we had expected. Answer: They certainly did, to the extent we planned to re-visit them for a test on a bigger, more powerful engine such as a 540 or even a 572. The street price for these heads is likely to be around $2,200 while a whole T&L built engine just like you see here will set you back $7,749 with a cast crank or $7,995 with a forged one. However you need to be aware that upgrading to a 540 or 572 incher is only about a grand extra and either of these will go well beyond the 700 hp mark.

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