I don't see much difference in building a motor for 400 BHP verses building one for 500 BHP, I wouldn't recommend doing anything differently. Engine speed is the real issue, and what I've described is how to build a tough motor that isn't going to break when used for high performance street and sports car applications, with a red-line in the range of 6500 to 7000 rpm.
If what you are suggesting is hot-rodding the motor with limited preparation, some guys get away with it, and some don't. Some of that depends upon just how hard the motor will actually be run after its hot-rodded. Some of that depends upon what shape the motor is in right now. I don't know how hard you or the other people reading this will run your motors, so I have to be careful and thorough, and make recommendations that won't result in damage to you, your motor, or your car. I have to assume that at the very minimum you'll run the motor on a high rpm blast every once in a while, like accelerating up a freeway on-ramp, or when you drop a gear or two to pass somebody on the freeway. I have to assume you may succumb to temptation on a lonely stretch of road and wind the motor out WFO to see what 150 MPH feels like.
I have "freshened up" 351C's before, even back in the '70s when the motors were still relatively new. But as I "freshen up" the motor I also toughen it up to prepare it for high performance. As it comes from the factory the 351C 4V (M code) isn't prepared for engine speeds much higher than 5000 rpm, the 351 Cobra Jet (Q code) maybe 6000 rpm, but not 6000 rpm for extended periods of time. The 351C 4V is not at all prepared for the kind of hard flogging owners have given it over the years. The ruggedness is there in the major castings, its the small details that need improving; lets put it this way, it needs to be brought up to Boss 351 spec.
To have confidence in the durability of a 351C 4V on a track day, or powering a Pantera or Mustang being flogged on a winding mountain road at high speed; we need to toughen the motor up while we freshen it up. Here's how I would approach freshening up the motor, the choices I would make.
First, here's a link to a thread with another post of mine with tons of very well "edited" and up to date rebuilding information: Cliff Notes Version of Sticky #3
The 351C 4V has 5 weaknesses that stick out in my mind.
(1) Over the last 4 decades many 351C owners have experienced the misfortune of having one of their 4V valves drop its brittle head while the motor is running, resulting in devastating damage to the motor. The valve head breaks off just below where it is welded to the stem. It has happened to completely stock motors, mildly modified motors and extensively modified motors too. The damage can occur while the motor is idling in the driveway, cruising on the road or wicked fully open (WFO). There is no way to predict if or when it will happen. The fact that a motor has run for 4 decades using the original valves is no guarantee it wont drop a valve tomorrow. If your 351C 4V is still equipped with the original Ford valves, it is a ticking time bomb.
The 4V valves are also designed with 4 groove, loose fitting valve spring locks, which are universally shunned for high performance use, higher valve spring force, higher rpm. So that's the second reason for replacing the valves with single groove valves.
My preference is Manley valves:
*Manley Performance severe duty intake valve, 2.08" dia. x 5.24” long (std. lg.); #11762-8, 131 grams
*Manley Performance severe duty 2V exhaust valve, standard length, #11807-8, 102 grams
4V intake valves
* Manley Performance severe duty 4V intake valve, standard length, #11800-8, 139 grams
(8 grams lighter than the factory intake valve)
* Manley Performance race master 4V intake valve, standard length, #11872-8, 129 grams
(18 grams lighter than the factory intake valve)
4V exhaust valve
* Manley Performance severe duty 4V exhaust valve, standard length, #11805-8, 108 grams
(15 grams lighter than the factory exhaust valve)
Consider this, by using the lightweight Manley Race Master intake valves in conjunction with titanium valve spring retainers you can appreciably lighten the intake valve train, improve the valve train dynamics, and raise the rev limit of the motor. You can save money and use chromoly valve spring retainers in conjunction with Manley Severe Duty exhaust valves.
Whatever brand of valves anyone chooses, it is imperative the stainless steel valves have hardened steel tips. If the heads need seat inserts cast iron (or beryllium-copper) valve seats are complimentary to stainless steel valves. To prevent rapid wear of stainless steel valves in the valve seat area the cylinder head’s intake seat width should not be less than 0.060” and the exhaust seat width should not be less than 0.080”; seat run-out should be 0.001” or less. I would equip the cylinder heads with silicon-bronze valve guides to best compliment stainless steel valve stems. I would utilize spring loaded elastomer valve stem seals such as Ford Racing Performance Parts #M-6571-A50 or Manley Performance #24045-8; installation of this type of seal requires machining of the top of the valve guide to 0.530” diameter.
(2) The production connecting rod nuts were the weakest link in the 351C reciprocating assembly. The threads will strip out of the nut when the motor is run hard and the result is major carnage.
An easy upgrade is to simply replace the nuts with parts from ARP, #300-8371. The connecting rod bolt can be re-used. If you replace the bolts the big-ends of the rods will have to be re-sized. Leaving the bolts in place and replacing only the nuts will avoid having to have the rods re-sized. The nuts truly are the problem
(3) The ring of the OEM crankshaft damper is not bonded to the hub, and it is too light for performance use (with the exception of the dampener found on the Boss 351). PLUS the OEM dampeners are decades old today, it is a common malady of all unbounded crankshaft dampers that as they age the ring spins on the hub or slowly creeps forward or backward off the hub, therefore the dampener MUST be replaced. Purchasing a new R code dampener is not possible, so a good choice for a frugal engine project is a 100% steel SFI approved Romac #0203 dampener.
(4) The 351C lubrication system has a propensity for low oil pressure and worn bearings. The basic design flaws of the lubrication system are that there's no control of where the oil is flowing nor is there control of how much oil is flowing. The large port in the walls of the tappet bores is another design flaw. It allows too much oil to flow to waste, it creates tappet compatibility issues, and it allows cavitation resulting from the motion of the tappets to spread within the oil passages. The lubrication system problems impact solid tappet motors and hydraulic tappet motors equally. The symptoms are the same regardless if the rev limit is 5000 rpm, 6000 rpm, 7000 rpm or higher; the symptoms merely worsen as rpm increases. If I am not going to disassemble the motor it shall not be possible to fix the lubrication system right (install tappet bore bushings or cam bearing restrictors).
The bottom line in regards to an unmodified lubrication system, the motor should have more than 50 psi hot oil pressure (the target is 60 psi) from about 2000 rpm all the way to the rev-limit. The oil pressure is low because the volume of the oil pump is over-taxed. The motor will have 60 psi hot oil pressure with the standard volume oil pump and the standard oil pump spring as long as the capacity of the oil pump is not over-taxed. Therefore the proper way to boost hot oil pressure (and my first step) is to limit excessive oil leakage and limit the amount of oil flowing to the cam bearings or valve train. About the only thing we can do without disassembling the motor is to limit the amount of oil flowing to the valve train. Assuming we're installing a hydraulic cam, I would purchase a set of 5/16" push rods with 0.120" thick walls, these have a 0.072" passage over 8" long down the middle. Or 3/8" pushrods, with 0.080" walls, with 0.040" restrictors in the tips. The restricted push rods are a decades old recognized method to control the amount of oil flowing to the valve train with hydraulic tappets.
Limiting the amount of oil flowing to the valve train will help, but it will most likely not be adequate to boost the hot oil pressure to 60 psi, maybe not even above 50 psi. So the question becomes what else should we do to "boost" the pressure. I would not use motor oil thicker than 20W50, that only slows down the already inadequate amount of oil flowing to the reciprocating assembly. I've used the Moroso #22850 high pressure oil pump spring in the past ... but I've changed my mind about that. A standard volume oil pump with a 1/8" to 3/16" thick washer between the oil pump spring and the pin which retains it may raise oil pressure enough. A high volume pump will boost pressure too, and it is probably the best measure to take, but there are drawbacks to that. I'm told the standard volume oil pump cavitates, therefore the HV pump will cavitate worse. I've also seen high volume pumps turn motors with a lot of miles on them into serious oil burners because they throw more oil on the cylinder walls. If neither of those "tricks" will boost hot oil pressure above 50 psi then the motor has an excessive oil leakage problem and it should be disassembled.
(5) Cylinder walls that crack is a weaknesses of the production block.
Part of freshening up a motor often includes a new set of rings. If I decide to replace the rings, there's an opportunity to replace the pistons too, I would purchase a set of forged full round skirt flat top pistons, i.e. endurance racing pistons. Summit Racing sells the Ross #80556 round skirt piston for about $586 a set.
These are twice the price of TRW flat tops, but that is a great price for the Ross pistons which used to cost over $800. The round skirt pistons are a proven way to prevent cracking cylinder walls. If I'm going to buy pistons, I prefer to spend the extra $280 for the durability it provides the motor.
Unplanned things can happen during a "freshening-up". Machine shop work tends to snow-ball.
For instance, any freshening up involves a valve job. In this case, I'm having the valves replaced too. If the heads need seat inserts I'll specify cast iron valve seats which are complimentary to stainless steel valves. To prevent rapid wear of stainless steel valves in the valve seat area I'll specify the intake seat widths should not be less than 0.060” and the exhaust seat widths should not be less than 0.080”; seat run-out should be 0.001” or less. I'll also specify a 3 angle valve job. I would equip the cylinder heads with silicon-bronze valve guides to best compliment stainless steel valve stems and I'll utilize spring loaded elastomer valve stem seals such as Ford Racing Performance Parts #M-6571-A50 or Manley Performance #24045-8; those seals require machining of the top of the valve guide to 0.530” diameter.
Another scenario, I pull the heads & discover the cylinders look pretty bad. The amount of horsepower a motor will be capable of making is very dependent upon how well the rings seal in their bores, so I can't be sure how much horsepower it shall achieve "for the time being" if the cylinders & rings aren't put into good shape. And if I am going to have the cylinders bored, I would have them indexed to the crank during that procedure.
I prefer to index the cylinders to the crankshaft "WHEN" the cylinders are bored because if a cylinder is canted to the left or right it will cause the piston to cock in its bore, placing extra load on the cylinder wall, that would make the cylinder wall more prone to cracking. So just like round skirt pistons, indexing is a way to help the cylinders walls resist cracking. If a cylinder is canted to the front or rear it will cause floating wrist pins to hammer out their locks; so indexing is a prerequisite for floating pins too. If you think you may ever want floating wrist pins, that is another reason to have the cylinders indexed to the crank when you have them bored. The 351C doesn't have enough cylinder wall thickness to bore repeatedly, you can't put-off indexing them until the next time.
The prelude to indexing the cylinders is to have the bearing saddles align honed, and the machine shops will want to level the decks at the same time too. So, if you're gonna touch the cylinder walls with anything more than a ridge reamer and a glaze breaker, you may as well plan on going the full route with block machining, its unavoidable.
If a machinist is going to service the block, I would plan to modify the lubrication system at the same time with the simple installation of 5 cam bearing restrictions and 16 tappet bore bushings (having 0.060" orifices). I can do this myself at home with about $420 worth of parts. This flat out fixes the lubrication system once and for all and makes it perform admirably. People will comment its not needed for a street motor, but after fooling around with these engines as long as I have I disagree. Besides, its so inexpensive to fix the lubrication system, I believe its illogical not to fix it.
The crankshaft must be rebalanced whenever parts (like pistons) that weigh more or less than the original parts are substituted. So, if I'm replacing pistons, the weight of the Ross pistons will have to be compared to the weight of the OEM pistons, and if they are heavier we may be in luck, and can adjust them to weigh the same as the OEM pistons. If not, then the crank will need re-balancing, and the machine shop is going to want to regrind the bearing journals before they balance the crank. The journals of a nodular iron crankshaft must be polished after they've been reground too.
If I arrive at this situation I would go the full route towards achieving durability. I would have the crankshaft ground to these clearances (0.0020" - 0.0025" mains; 0.0025" - 0.0030" rods). Then I would have it tufftrided (surface hardened), straightened, and micro-polished. I would purchase a lightweight steel flywheel (manual trans motors). And finally I would have the entire reciprocating assembly dynamically balanced before reassembly. Dynamic balancing takes a lot of vibration out of the motor, it makes it run smoothly. This accomplishes 3 things: (1) it makes the motor feel much more high quality (2) it makes the motor more inviting to rev at high rpm (3) it makes the motor more durable.
The crankshaft rear main seal is a rope seal, its usual to replace it with a neoprene seal, this requires pulling a small pin from the seal groove in the rear main bearing cap and filling the pin hole with a dab of sealant.
Back to the subject of a freshening up ...
The single most important thing to do to improve the performance of the engine is to raise the static compression ratio to achieve 7.6:1 dynamic compression with whatever camshaft you choose to use. Don't stick a hotter cam in a low compression 351C!
The factory connecting rods are fine for street motors and an occasional blast to 7000 rpm ... as long as the rod nuts are replaced with the ARP rod nuts. Pressed pins are just fine for the street too.
The pin height of the Ross pistons will raise the static compression ratio a about 3/10ths. To compute the dynamic compression ratio I need to decide what cam I'm going to use, so I'll know when the intake valve closes. Then I can juggle the static compression ratio to obtain a suitable dynamic compression ratio (about 7.6:1 dynamic c/r is good for 91 octane pump gas). Once I have a target static compression ratio, I can decide how to achieve it. Milling cylinder heads is my preferred way to adjust compression ratio, rather than taking a bunch of material off the decks of the block.
The factory main and rod bearing clearances are too tight for performance usage, and the factory bearings are too soft. When I drop the oil pan I'll likely find ribbons of babbit laying in the bottom.
If the motor is disassembled far enough to replace pistons and connecting rod nuts, then its also disassembled far enough to replace the main & rod bearings. I use Clevite 77 bearings because they are a tougher bearing. They are also available in "plus" sizes, which can help achieve the right bearing clearances even without having the crankshaft re-ground. Those clearances are 0.0020" - 0.0025" mains; 0.0025" - 0.0030" rods. Although it doubles the price of the main bearings I like to use the uppers from two sets of main bearings so that the main journal bearings are fully grooved, 360 degrees. The Boss 351 had fully grooved main bearings, the purpose is to supply lubrication to the rod bearings through 360 degrees of crankshaft rotation, the same reason other performance motors had cross drilled crankshafts or crankshafts with grooved main bearing journals. This doubles the oil flow to the rod bearings. 351C bearing sets used to come fully grooved, but that was discontinued as the viscosity of motor oil was reduced. Fully grooved mains are no problem for 20W50, 15W40, 10W40 or 10W30.
Use a new standard volume oil pump if the motor has tappet bore bushings. Without tappet bore bushings I would use a high volume oil pump OR shim the standard volume pump's oil pressure control spring. I would use a standard oil pump drive shaft (intermediate shaft) with either pump.
I would purchase a good oil pan, because all the preparation in the world won't help the motor if the suction of the oil pump goes dry.
If the motor is still equipped with a breaker point ignition I would replace it with a breakerless ignition; Ford Duraspark, MSD, etc. 20° centrifugal advance in by 3000 rpm, 16° to 18° initial advance, vacuum advance limited to 10° and connected to ported vacuum.
Finally I'll touch on the valve train.
I would replace the camshaft timing set with a new full roller, steel, multi-index (9 keyway) timing set.
Replacement push rods that will not flex (store energy) are always a part of preparing a motor for performance. Push rods are the weakest link of an OHV valve train. Don't be concerned with the weight of the push rods, flexing and harmonics are the issues. The use of premium metals, large OD, thick walls and tapered walls are the ways to combat flexing and harmonics. I like to use 5/16" diameter chromoly push rods with 0.120" thick walls.
The motor needs valve springs compatible with the cam, the red line rpm, and the weight of the valves. If the motor has a hydraulic flat tappet cam acceptable valve spring pressures should fall into the range of 115 - 130 lbs on the seat and 300 - 330 lbs over the nose (roller cams require more spring force than that). Assuming we're using a cam with 0.540" to 0.570" valve lift, that would call for a spring with 350 lbs/inch spring rate that will allow 0.570" lift without binding. That's not much more over the nose spring pressure than what was stock on a Boss 351, but that much valve spring combined with lightweight valves will enable the motor to rev to 7000 rpm without valve float. It is always better to use too much spring force than not enough; always let the cam grinder be the final word on how much spring to use, not me.
The stock rocker arms are OK to 0.615" valve lift if the 5/16" rocker arm fasteners are upgraded with ARP parts(four packs of #641-1500 bolts and two packs of #200-8587 washers) ... and if the geometry is set correctly.
My preference is Yella Terra rocker arms for any and every 351C that is having the rocker arms upgraded. They are a much better design than push rod guided rocker arms which use studs, guide plates and hardened push rods. There are versions that don't require pedestal machining (YT6015) but they are difficult to find in the US. The oem rocker arm pedestals clamp the rocker fulcrum to the head with a 5/16" cap screw, that cap screw limits the use of the oem rocker mounting system to about 7000 rpm and no more valve spring force than 400 pounds over the nose "IF" high strength 5/16" fasteners are used. For higher rpm and/or higher valve spring force the pedestals should be milled and tapped for 7/16" cap screws (to allow the use of the heavy duty YT6321 rocker arms or T&D Machine rocker arms).
To toughen up the 351C 4V while you freshen it up:
(1) Replace the valves (Manley 4V valves: 11872-8 intake & 11805-8 exhaust). These valves use single groove spring locks.
(2) Iron valve seats, bronze valve guides, 3 angle valve job, spring loaded elastomer seals (FRPP M-6571-A50 or Manley 24045-8)
(3A) Valve springs for flat tappet applications rated at 115/130 seated; 300/330 over the nose (such as Crane’s #99839).
(3B) Valve springs for roller tappet applications rated at 150 seated; 370 over the nose.
(4) Steel valve spring cups, titanium spring retainers for the intake valves, and chromoly spring retainers for the exhaust valves.
(5) Push rods made from seamless chromoly tubing, 5/16 diameter X 0.120" wall, or 3/8 diameter x 0.080" wall
(6A) Factory rocker arms & fulcrums, mounted to the factory slotted pedestals, with adjusted geometry (fulcrum height), fastened with ARP #641-1500 bolts & #200-8587 washers. This is good for hydraulic tappet valve train up to about 0.550" net valve lift (rated by Ford to 0.615" valve lift).
(6B) All other valve train applications (especially those needing adjustability) should choose between Yella Terra YT6321 rockers or T&D Machine #7200 rockers. Both mount solidly to the cylinder head with 7/16" bolts and use push rod cup style adjusters.
(7A) Flat tappet cams: The camshaft should be ground on a best quality iron cam core, it should be ground with a guaranteed 0.002" lobe taper, it should receive the best quality hardening treatment (nitriding), and it should receive the best quality lobe polishing.
(7B) Roller cams: The camshaft should be ground on a core manufactured from a material which is compatible with standard OEM distributor gears or compatible with commercially available steel distributor gears. There are steel gears available which are compatible with the proper steel camshaft cores. Don't use bronze gears for a street engine. Therefore don't use cams requiring bronze gears in a street engine. Bottom line, the distributor gear must be compatible with the material the camshaft is made of.
(8) Raise the static compression ratio to achieve 7.6:1 to 7.7:1 dynamic compression (compatible with 91 octane pump gas).
(9) Breakerless ignition (20° centrifugal @ 3000 rpm, 16° to 18° initial, 10° vacuum adv. using ported vac.)
(10) Good oil pan (9 quart), windage tray, high volume oil pump pick-up, etc
(11) Achieve more than 50 psi hot oil pressure from 2000 rpm (60 psi is the target)
(12A) Limit oil to the valve train; tappet bore bushings w/0.060" orifices is always the preferred method.
(12B) In hydraulic tappet applications oil to the valve train can be limited via push rods with 0.040" restrictions.
(12C) In solid flat tappet applications oil to the valve train can be limited via special oil limiting tappets.
(12D) In solid roller tappet applications it is best to assume that tappet bore bushings are a necessity. Choose solid roller tappets that roll on solid bushings instead of needle bearings, and that employ forced lubrication instead of splash lubrication. Such as Isky #3972-RHEZ.
(13) Replace the crank damper (Romac #0203, ATI #918900, BHJ #FO-EB351C-7). The Romac damper is a nice damper but it is not bonded.
(14) If the flywheel requires replacement use a lightened steel flywheel. The Yella Terra YT9902 flywheel, weighing 26.4 pounds, is the lightest steel flywheel known to me. It is an external balance flywheel drilled for "long style" pressure plates.
(15) Replace the con-rod nuts (ARP #300-8371 nuts)
(16) Forged round skirt pistons (Ross #80556). Caution: Some pistons, like the Ross pistons, have increased wrist pin height, they shall raise the compression ratio. Such pistons are best used with D1AE head castings.
(17) Clevite MS-1010P main bearings, fully grooved (requires 2 sets), 0.0020" - 0.0025" clearance
(18) Clevite CB-927P rod bearings, 0.0025" - 0.0030" clearance
(19) Pull the small pin from the rear crankshaft seal groove of the #5 main bearing cap, seal the pin's hole with a dab of sealant, utilize a 2 piece neoprene seal in place of the OEM rope seal.
If machine work is performed, consider paying the extra expense for durability
(1A) Align bore if a straight crank cannot be turned by hand when cinched down into a fresh set of lubricated main bearings.
(1B) Align bore if the main bearing saddles have excessive taper or run-out.
(2) If the machinist insists on machining the decks, agree to only enough required to level the decks front to back and equalize their height bank to bank. This should not require more than 0.015". The deck height should never be machined less than 9.200". Might as well have the deck surfaces finished fine enough for MLS head gaskets while you're at it.
(3) Index the cylinders to the crankshaft (NOT the decks) during boring. This procedure will net you quite a few additional horsepower and better durability ... and the cylinders will be compatible with pistons having floating wrist pins.
(4) Choose full round skirt flat top pistons, such as the Ross #80556 pistons. The Ross pistons utilize modern thin piston rings and are drilled for wrist pin oiling. The full round skirt pistons exert their thrust forces over a wider area of the cylinder wall, they apply less stress on the "thin" cylinder walls of the production cylinder block. However, caution: The Ross pistons have increased wrist pin height, they raise the compression ratio, they are best used with D1AE head castings.
(5) Install 16 tappet bore bushings & 5 cam bearing restrictors
(6) Regrind the crank for 0.0020" - 0.0025" main clearance; 0.0025" - 0.0030" rod clearance. There should be no taper or run-out on the journals afterwards.
(7) Tufftride & micro-polish the crank
(8) Dynamically balance the reciprocating assembly
Performance (volumetric efficiency)
(1) Dual plane intake manifold (Blue Thunder or the older Shelby version recommended for 4V engines). Full height plenum, do not cut the plenum.
(2) Holley style carburetor, 735/750/780 cfm, center hung fuel bowls, vacuum secondaries, electric choke, street performance calibration, annular booster venturis.
(3) High lift camshaft (i.e. approx. 0.550" net lift) with 276°/286° advertised duration, 112°/117° lobe centers (or 282°/282° advertised duration, 109°/119° lobe centers), 114° LSA (112° minimum), 54° overlap (60° maximum).
(4) Camshaft should open the exhaust valve at 80° BBDC and close the intake valve at 70° ABDC (based on advertised duration).
(5) Headers, preferably tri-Y style, 1-7/8" to 2" diameter primaries for heads with 1.71" exhaust valves, 1-3/4" diameter primaries for all other heads.
(6) 2-1/4" to 2-1/2" tail pips, decent mufflers.