Originally Posted by Timberghost357
I'm thinking about building a chevy 302 for my dirt track camaro. Before I get into this I want to say yes I do know building a 350 will be cheaper and give me more hp and tourqe blah blah.....and building a 383 for a race engine will give me the best bang for my buck. Anyways I want to build a 302 to be different.
So I'm looking for some advice, I have a 1963 forged steel 283 crank, is there anyway I can use my large journal 010 350 4 bolt block? Can I get the Machine shop make the 283 crank into the 350 block like they do with 350 cranks in 400's? If I get that far the rules for my race class are fairly strict in some areas, if I spin the motor over 6500-7000 rpm would I need to get forged steel H beam rods?
The rules for my class state no rules on cubic inch, stock cast iron intake manifold with Rochester 2g or Holley 4412 2bbl carb will be used. stock production cast iron heads/ no vortec heads, no porting/ polishing, milling the heads aren't allowed, stock steel rocker arms with 1.50 ratio will be used. Stock production cast iron exhaust manifolds will be used.
Here's what I picture in my head if I can get the crank to fit the block:
Bowtie cast iron factory hi rise intake with 2bbl carb adapter
Stock ram horn exhaust manifolds
186 camel hump heads or 882 1.94 truck heads
Holley 4412 500cfm 2bbl carb
Scat forged steel 4340 6" h beam rods.
Mahle forged alum flattop pistons with 2 valve reliefs bore tbd
also floating wrist pins
Arp main and rod bolts
High volume oil pump
Double roller timing chain
Circle track hydraulic cam I've had great success with in my 350's
240/250 @ .050", 290/294 advertised,.506"/.510" lift(1.5:1 rocker ratio),107 lobe sep
Comp cam hydraulic flat tappet lifters
The bearing size will be determined if I can get the crank to work
The biggest thing I need advice on is the heads, I want to be able to safely spin the motor 7500 rpms if not more. I'm also not sure if I want to yank my 186 camel hump heads off my truck or use and rebuild the stock 882 heads that came off my truck. Both heads are 1.94's, the camel humps have the small combustion chamber but I'm very hesitant on running them just in case if the motor comes apart, where if I have the 882 heads they're still a dime a dozen.
What can I do to the heads to make the motor survive running 7000 rpms or more?
The smaller displacement will need small chamber heads to get the compression ratio up where it's effective. You need to get into an online Dynamic Compression Ratio calculator (DCR) this will tell you what Static Compression Ratio (SCR) you will need for the cam you're using that will achieve a DCR from 8 or 9 to 1. If you can't run the camel humps can you run somebody's stock replacement head like those from World or Dart? Compression will be everything with this motor, lack of it will be a larger hit than with a bigger displacement engine. If you can't mill the heads then deck the block.
You've got to turn this thing about 1000 to 2000 RPM higher than a 350 or 383 to make comparable power. This is simply a mass flow equation of RPM times displacement equals power. The engineering of how to make this happen is more difficult than the equation supposes.
At the crank the g force on the rod's big end is a function of distance from the crank center line to the rod's big end. Since the smaller stroke reduces the distance this provides some space for the higher RPMs before the critical speed for rod strength is met. So this in terms of dynamics on the crank pin and big end of the rod is probably about the same as the slower tuning longer stroke engine. Unfortunatly the same can't be said for the counterweights which will remain about the same mass but will be subjected to greater speed which results in greater apparent weight. Since you can't beef things up with a girdle it makes a very good balance job and better damper extremely important to take as much loading as possible off the mains. This is a classic candidate for overbalancing the crank 1 or 2 percent.
I don't see induction as much of a problem if the set up can feed a 350 at 6000 RPM it has the capability to feed a 302 at 7000 RPM. The problem is the cam and valve train since as flow demand is going up as the time to deliver that flow 6000 vs 7000 RPM is going down and the camshaft timing and lift is limited the question is how to get at least the same flow in 16 percent less time. Pretty much all you can do is work the edges with the rules you've got. Stiffer springs to prevent valve bounce upon closing, dual winds with a damper should do this. A high seat pressure to keep a tight seal on the closed valve. But these are things that work against the push rod and certainly the plunger stability of a hydraulic lifter. You're going to have to beef up the pushrods so they don't bend but you may have to play with them as getting some loft of the valve over the top of the lobe may be helpful so a push rod that initally bends a little then snaps straight to shove the valve past the cam's max lift value may boost the top end RPM's a little. The trade is lost initial lift against more lift over the top, but given your port limited that may not have any pay dirt, that's to say the port may limit the max flow not how open the valve is. It's a tuning tool to try. The lifters, there's two ways to go with hydraulics these things suffer two big problems one is the tendency to leak down against really stiff springs and the other is to pump up when a lash happens as when a valve is lofted or floated the former by intent the latter by accident, the result is the same the lifter holds the valve off its seat. You can remove the plunger and groove it so it leaks down as fast as it pumps up then collapse the adjustment at the rocker, this takes a .1 to .15 inch extra long push rod to keep the rocker alignment to the stem correct. This gets rid of pump up and leak down. The other is to get low leakdown lifters like stock types then remove the wire plunger retainer to be replaced with a circlip (square edges up) so the plunger can be run at zero lash or a bit more to where it cannot pump up, this does not elimiante leak down so it does not work on anti-pump up lifters.
I would run a longer rod if I could as the shorter stroke gives you some space in which to do that. This reduces the g loads over TDC making 7 grand of RPMs a lot easier on the rod cap, bolts and piston rings. The rod cap and bolts are put under tension and bending loads on the intake stroke which is the major contributor to rod failure. I know it usually looks like spun bearings and failure of the lower shank, but those failures start here, so easing the load build up on the intake stroke is a good thing. If you can modify the crank to run the Honda 1.88 rod bearing and buy like wise sized Chevy rods I'd do so. Especially if you're running a forged crank as the pin to main overlap has plenty of strength to take some reduction in favor of lower journal to bearing velocities. The rings also snap over at TDC. They make not only a seal with the cylinder wall but also a thin contact seal with the ring groove that changes from top to bottom of the groove with piston direction. Do this fast enough and the ring can't keep up and flutters. A thin ring is better at staying up with these direction changes than a thick ring. Drilling several small holes in the space between the oil control ring and the second ring can help keep the oil trapped in this area reduced which takes some work of the second ring and helps prevent charge contamination of oil getting past the second and then first ring.
You didn't sight anything from the rules about crankcase windage, if you can I'd put a full lenght windage tray and crank scraper in there. Oil flying off the crank is always a problem. It's a gift that just gets worse as the revs get higher and the use of a high volume and or pressure pump just adds to the problem of getting flying oil away from the crank and back into the sump. While this is always sold as a means to more power that isn't my concern, getting oil back to the sump and minimizing how much is being blown up on the cylinder walls making it harder to keep out of the combustion chamber is my big concern. The engine quickly becomes toast if the oil can't get back into the sump as fast as it's pumped out or if the rings let it by into the combustion space to cause detonation. Also, if you can pull a vacuum on the crankcase within the rules I'd do that as that will help with ring sealing.
You can run a 302 against 350s and 383s but it takes a lot of attention to the details so you can push the RPMs up, this is especially a problem when limited to the same cam even if all other things were equal you just don't get more time and time is what you need so you've got to figure ways to trick around that problem.
Gearing; the power, torque and horse, are going up the rev range, you'll need stiffer gears to chase after it.