Any time you press the boundaries of engine output, you recognize there will have to be part dyno testing, and also that"s specifically the case here. We had some borders to occupational within, so the the end an outcome was a sane construct for the typical mid-budget warm rodder. These borders included pump gas, a minimum of 350 lb-ft at 2,500 rpm, and a near lope-free idle of no much more than 850 rpm. As for output, our goal was to check out if 600 hp indigenous a 350 block running on pump gas was even feasible. In the process, we would look at some more recent speed parts, plus part tried-and-tested parts. All this, it need to be said, will be attempted without the assist of a blower or nitrous.

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400 Cubes through A 350 Block! It"s doubtful any type of of us have to be reminded that an ext cubes means a stouter engine, but it"s been part 35 years because GM do the 400-cube version of the small-block. These often tended to have actually an overheating and cracking problem, which has actually resulted in a scarcity the usable ones. The same cannot be claimed of the 350 block. It"s about as reputable as a block deserve to get and there are still millions easily accessible to the hot rodder. Stretching a 350 come 383 inches has been a usual practice for the best part of 20 years. The question here is, have the right to we walk more? In terms of bore diameter, over there is not lot room to go more than 30 over, back a sonic tester can uncover blocks good for as lot as 60 over. The mix to obtain a 383 is to bore the block 30 end (to 4.030 inches) and use a 3.75-inch stroker crank. The stroke boost over stock is 0.27-inch. Crowding this right into a 350 block normally way cutting more rod clearance right into the bottom that the bores and the political parties of the crankcase. The amount necessary to do a 383 occasionally compromises the block, and the clearancing goes right into the water jacket.


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Going to 408 inch in an initiative to accomplish our 600hp goal could look prefer a huge expense, however a stock factory crank in ~ this power level will certainly be fine on the method to that limit. This makes a stout aftermarket crank much more or less essential to the engine"s can be fried survival. If a crank is needed, climate it may also be a stroker crank, as this will add to the output as well as beefing increase the bottom end.


The piston needs to meet the requirements of an engine such together this room a little more an essential than normal. First, to lull the reciprocating lots brought about by a stroke rise of over fifty percent an inch, we must go because that a difficult piston that is likewise light, and also these two determinants are, for the most part, diametrically opposed. Fortunately, Mahle has a piston that fits the invoice really well. Not just is it light, however this high-tech piston features thinner rings the are additionally narrower in plan view. This makes them able to seal up better with less ring drag, and also that"s critical issue ~ above a stroker motor.


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At this point, our 350 update is feather good. We have actually 58 cubes much more than stock, however that will not be of much use if the heads can"t it is provided the waiting to feed those extra cubes.


Cylinder Heads when AFR introduced their 195cc harbor Eliminator heads, it to be without any an excellent fanfare, so fist from ours direction was rather limited. However rumors of good results through these heads (typical street price under $1,500) retained cropping up, so it looked like time to investigate. The was a year ago. Since then, we have actually sat in on dyno tests on four occasions and the output figures completed were strong. Because that an air-hungry 408, some top-notch heads would be needed if outcomes were come reflect the invest of time and also money in the bottom end. These AFR top looked prefer they would certainly fit the bill and also do so in ~ a great price.


The AFR Eliminator heads function 2.05-/1.60-inch valves and also are CNC ported. As a pioneer in the field of both aftermarket cylinder heads and also CNC porting, AFR has over a quarter century of endure to draw on. The Eliminator heads space a culmination of this experience, and represent some significant fine tuning of the 23-degree Chevy heads.


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AFR has always been very strong in the cost department. This is due to the fact that they CNC port these heads to a complete that it is provided the many cost-effective result. If the complete is made finer, the expense escalates dramatically, if power might only change marginally-and it might go one of two people up or down here. If the finish were made coarser, the price would drop just minimally, when the flow and power would drop measurably. Yet at the end of the day, complete is, at best, a second-order effect. The primary aspect toward a useful head is port and also chamber shape. Fail here and also it matters tiny how an excellent the surface finish is.


Along with an excellent flow, the AFR input ports additionally generated swirl numbers that frequently deliver great results. Added probing showed great port velocities and velocity distribution. All this adds up to promising outcomes on the dyno.


As good as every the gas dynamics of these heads were, it to be still short of the amount total. Other alternatives could make a large difference, especially if you intend to use them through a hydraulic roller cam. The valves, springs, and also retainers we used have been selected to boost the rpm ability of the heads without going come a feather as heavy as would generally be used. Because that instance, the valves had an 8mm stem (a tiny under 5/16) v a lightweight head. This chops the weight of the intake valve from commonly 129 grams come 105 grams v a commensurate save taking place on the exhaust. The valvesprings provided for these heads space high-dollar items and likewise present a performance advantage. Because that our 408, we room going to need a many valve lift. To do a secure valvetrain, the feather must have low mass and also a high herbal resonant frequency. The AFR spring has both. That weighs a scant 92 grams (typical is 110) and delivers 160 lbs top top the seat, and 424 lbs in ~ .600-inch valve lift. Because it is a much smaller diameter than most springs, it needs only a small retainer. This weighs a slim 17.5 grams, as opposed to a common retainer mass of 33.5 grams. Every this attention to detail chops a worthwhile amount of mass out of one of the most mass-critical areas of the valvetrain. AFR"s efforts in this direction average the heads room ideally suited to hydraulic roller cams, and also to high-lift heavy street rollers-the last is simply what we are going come use.


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Cam an option Probably the two peak reasons why street engines use a hydraulic roller room the zero-service factor and also the quiet operation. What this has done for countless hot rodders is exclude the hard street roller native consideration. We have run COMP"s solid street rollers for plenty of years; our longest one in service has end 80,000 mile on it, and has however to need a lash adjustment. Also, by make the lash adjustments v care, the valvetrain is far from noisy. The sound is more like a purr, and also is far from objectionable. This is being carried up due to the fact that a solid street roller to be a more appropriate choice; let"s look in ~ this 408"s requirement. First, it has a lot of inches, so it will require to have a high-lift valvetrain. Secondly, there is enough airflow v the heads to do power approximately 7,000 rpm. This method we have to resolve two inconsistent situations: high lift and high rpm. Added to that, we needed a long and also reliable company life. A COMP Xtreme energy street roller takes treatment of that. The profiles favored were 286XSR because that the intake, and also 292XSR for the exhaust. These profiles are 248 degrees and 254 levels for the intake and also exhaust respectively. Through COMP"s 1.65:1 rocker (PN 1006) top top the intake and also 1.5:1 (PN 1004) on the exhaust, network valve lift after lash was .620-inch and .588-inch.


Induction The will is come test 2 intakes. First, we wanted to see how well an Edelbrock Air-Gap show RPM fared on one engine v a projected calculation of end 520 hp. Experience has presented that on engines in the 300 to 500hp range, this manifold yes, really delivers. Several of our previous testing has additionally shown that at about 530 hp, it appears to it is in approaching a limit. Yet testing on our 408 should better define its already well-appreciated features for creating a really solid low finish with what is, because that a two-plane intake, a very creditable height end. Next on the list to test is a Holley strip Dominator. This is a manifold which appears to be short on credit transaction for its great power capabilities. On engines qualified of 500 or much more horsepower, ours previous experience has presented it gets the job done in style. With the hot spec the this 408, it seemed favor a good idea come reacquaint ourselves with the strip Dominator"s power potential. Both manifolds were checked for a port match. Both aligned quite well, so that was chose that we would certainly run both join in out-of-the-box form.


Spark Generation because that ignition, we provided a Pertronix HEI-style distributor v vacuum advance. This was equipped through the lightest springs because that the mechanical advancement and fed the spark to the plugs via ACCEL 8.8mm plug cables. For this reason why the vacuum advance? with this combination to manifold vacuum (not the porting vacuum source) us were may be to acquire the motor much more street friendly. Not just did the vacuum advancement allow us to get a far better and smoother idle, but likewise it consumed less fuel at idle and part throttle cruise.


Our main push was to watch if we could make 600 hp, and also with only 6 hours of dyno time available to us, we did not spend too lot time v the Edelbrock Air-Gap performer RPM tests. The allude was an ext to watch what the "bulk" difference could be in between the two-plane Performer and also the single-plane Holley strip Dominator. Going in, that was already known that this combination would need a large cfm the carburetion. At the exact same time, we did not want to compromise driveability, therefore we determined to operation the Holley 750 equipped with the 850 baseplate. With its measure 900-cfm capability, this looked to it is in a good compromise on what had to it is in a one-shot deal.


The tradition calibration on the carb showed to be appropriate on the money, as any jet change up or down shed output. The outcomes are in the graph on p. 92. Uneven you are acquainted with this sort of result, this numbers space going to take a little explaining because that the true photo to it is in more clearly seen. First, keep in mind that the numbers room all rounded to the nearest entirety lb-ft or hp. The truth is the this engine missed the 550hp mark by just 1.4 hp. That"s a an extremely creditable number because that a two-plane intake. But look at the height torque. The is only about 510 lb-ft. Normally, this would be somewhat higher, but because the optimal torque is at fairly high rpm, the manifold is already pushed close to its border in state of ideal airflow per revolution of the engine. At the peak torque figures, this combo is already well over the 500hp note that Edelbrock envisaged would certainly be about the limit of that is use. What we have actually here is an instance of a subtle parts mismatch. The demand from large cubes and also high rpm is being successfully communicated, via a collection of highly effective heads, come the input manifold and also carb. Yet as the huge demand native the cubes/rpm/heads combo is increasing, the flow ability of the intake/carb combo is near to being maxed out, even at 5,000 rpm. Result: since peak torque wants to happen so much up the rpm range on this big-inch motor, the optimal torque worth is being endangered by lack of enough airflow. Don"t acquire me not correct here, the Edelbrock Air-Gap performer is a an excellent manifold. If, 20 years ago, you had actually asked a agree engine builder even if it is a two-plane intake could be made to support also 500 hp, the price would have been, "not a chance." If we want to view that 5 come 10 lb-ft the increase, climate it"s a situation of using a cam about 10-degrees shorter to relocate the powerband down right into a range more suited come the intake.


So, as an excellent as the Edelbrock two-plane may be, our following tests will display if over there is any kind of substance come our theory that this engine simply needs an ext induction capacity.


Single-Plane check The an initial move to be to swap the end the two-plane intake for the single-plane Holley Dominator. On peak of this go the basic mildly-modified 750 with calibrations deemed vital for the single-plane setup. Although output listed below 4,500 rpm dropped, this 408 come on solid from there on up. Optimal torque went approximately 516 lb-ft, and peak hp to 572. The was good, yet the carb capacity quiet looked a small on the weak side because that this deep-breathing 408. To check and see if this was so, the 900-cfm 750 (with the 850 baseplate) to be installed. Presto! 526 lb-ft and 579 hp. We are beginning to look good here. Currently working on the assumption that if some is good, and much more is better, the 930-cfm (modified 850) can be just right, therefore it was installed. Result: 534 lb-ft and also 587 hp.


At this point, you deserve to see we are closely approaching our target 600 hp. The difficulty is, we have actually now used our greatest carb, but as frequently as not, a 2-inch spacer on one engine the this size have the right to make the carbohydrate look bigger, so the was the next move. This led to a ideal of 541 lb-ft and 595.4 hp. Seeing together our 600hp target to be so close, ours next relocate would have actually been to do a tiny stagger jetting and readjust the oil for a great synthetic. That would have almost certainly provided us the 600 hp we were feather for, however time was versus us.


It"s worth looking in ~ our graph comparing the output with the two-plane against the output with the single-plane. In round terms, the Air-Gap Performer developed 45 lb-ft much more down low, vice versa, the piece Dominator produced 45 hp an ext at the height end. As can be viewed from the graph, the crossover point was a shade over 4,500 rpm. As far as the valvetrain went, this COMP heavy street roller ran choose a 595hp Swiss watch at 6,800 rpm, and valvetrain noise in ~ idle for our diligently lashed system was barely any much more than a common high-performance hydraulic. As for the AFR heads, they verified to be qualified of feeding the 408 inches created by the K1 crank and also rods, and Mahle pistons.

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See all 22 photosOur swirl and flow tests were conducted on this SuperFlow flow bench right after that calibration to be checked and verified.
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