[Tim]

I have a 360 block and "915" heads that I am considering to be the replacement for the 318 in my '70 hardtop (along with a 727 torqueflight). I'd like to build it as stout as possible but still have it reasonably streetable. What cam, pistons/rods, intake/carb should I consider? How should the heads be done? Is internally balancing the way to go (its a 1975 block with what I think is a shot-peened cast crank). What size/make headers are recommended?

[Tim]

Okay. I guess I've asked too much. Or maybe everyone has big blocks. Is there anyone out there with a stout 360 used on the street but geared more for the track that would like to share their experience (what did you do right/wrong?)to help me with plans.

[dave]

Tim.
I highly recommend the company:

HUGHS ENGINES

Have a read as their web site is full of good information for big and small blocks.

[Brian]

Well, Tim, you've been a little broad about what you want exactly, so it's hard to make specific recommendations. The two most important things we need to know are: 1) What is your budget? and 2) What is your performance goal? If you have a 1/4 mile time in mind, that would really help. Also, what's your definition of reasonably streetable? What quality of fuel is available to you? What rear end ratio do you have? Are you planning on using a high stall torque converter? How much freeway driving do you do? All these things will affect your parts selection. I'd be happy to make some recommendations, but I need more information.

[Tim]

The car has an 8 3/4 with 3.91 gears. Thinking mid 12's in 1/4. Can this be achieved with a broad powerband (1500-6500) with low-medium stall converter (to achieve reasonably streetable as car will be used a great deal around town)? To top it off, needs to run on pump gas, 92 octane premium is best available here.

[Brian]

The good news is that it's fairly easy to build a 12.5 second 360 Challenger. The bad news is that to do so you need to give up a fair amount of street civility. The car would certainly still be streetable, but it would have a definite lope at idle, low idle vacuum (not too good if you have power brakes), and a narrower power band than you're hoping for.

The alternative way of getting what you want would be to build a somewhat milder package, say one designed for 13.5 second quarters, and then to add nitrous oxide to make up the difference. This is more suited for your existing gears, and is also the less expensive approach.

Either way you're going to have to move your power band up a bit. Figure on 2500-6000 RPM for the 13 sec. setup, and 3000 or 3500 to 6500 for the 12 sec. Choose a converter accordingly.

13.5 second package: 650 CFM vacuum secondary carb. Aluminum dual plane manifold, Edelbrock RPM or MP M1. 3-angle valve job on stock size stainless steel valves. 9-9.5:1 compression ratio. 340 HP cam or aftermarket equivalent. 1 5/8" headers. 2 1/2" dual exhaust with crossover. Minimum 2300 RPM stall torque converter.

12.5 second package: 750 CFM vacuum secondary carb. M1 single plane manifold. 2.02" intake valves. 3 angle valve job, pocket ported heads. 10:1 compression ratio. "509" MP cam or equivalent. 1 3/4" or 1 7/8" headers, dual exhaust with crossover. 3000 RPM stall converter. 4.10:1 rear end ratio. XHD leaf springs. Adjustable pinion snubber. D.O.T. approved racing tires.

With either combination, don't skimp on block preparation, especially if you're going to use nitrous. Make sure the deck is flat and square, the crank journal bores are true, the crank is straight and indexed. Have the rods prepped and use ARP bolts. Forged pistons may be a good idea if you think you may want to increase nitrous levels for even more power (the 12.5 second package with a 250 HP nitrous shot could easily break into the 10s).

Make sure the head surfaces are flat. They may need to be milled anyway to bring up compression. Use a double roller timing chain and degree the cam. Use a windage tray. There are lots of other details that are important, get a book or two specific to Mopar engines. The MP engine book is a good place to start.

Good luck, and let us know how it turns out!

[Tim]

Thanks for all the great info, Brian. I'll probably end up going the "13.5 second package" way with a little more head work and a slightly bigger cam. What do you think about the Edelbrock performer RPM package? Both intake and cam/lifters (should provide 400+ hp). Will the compression ratios you listed allow use of pump gas without detonation?

[Brian]

The Edelbrock package is pretty good, but it's closer to the 12.5 sec specs. Keep in mind that the numbers given (417 HP for a 340) are developed with their aluminum heads. It is fairly easy to match the flow characteristics of these heads with your 915's, but not necessarily cheap. But you definitely will need better heads to take advantage of the larger cam and carb.

What I really don't like about the Edelbrock package is the cam. It's a "Chevy grind", meaning that it's the same lobe profile used for their Chevy cams, and that's bad. Not really bad, but not optimal. Mopar lifters are larger in diameter than Ford lifters, which are larger than Chevy lifters. So what? Well, the lifter diameter determines how fast the rate of lift can be without prematurely wearing out the cam. The larger the lifter, the faster the permissible rate of lift. That's one reason why Chrysler was so dominant in drag racing during the factory-sponsored days. You really want a cam that is specially ground for a Chrysler engine. You'll pay more, but it's worth it. Use a Mopar Performance cam if you want to save a little without sacrificing a lot, or go with an Engle for the best performance (these are what Hughes engines sells).

As for compression ratio, a lot depends on the cam profile. The longer duration cams allow (in fact require, to work right) more compression. Notice I specified different compression ratio recommendations for each package. Those are based on the cam selection. You should be able to use premium fuel, with an optimum ignition advance curve and proper carb jetting, without any problems. If you do go with the aluminum heads, you can add at least another half point to those numbers. The aluminum dissipates more heat, and reduces detonation. Of course the 40 pounds or so of weight reduction helps performance, too.

[dave-r]

I can tell you that I am very pleased so far with my Hughes/Engle cam. I think their headwork is pretty good for the price too.

I also think that roller rockers with 1.6:1 ratio are worth having. Just in themselves they will give you more/faster lift without any increase in duration. Add to that the lower friction losses and I think they are worth the outlay. Not many bolt-ons will give you more power without any downside. Well. They have one downside. They are about twice the cost of a cam.

I'ts only money. You can't take it with you!

[Tim]

I'll keep looking around for a cam that I think will give me the right compromise.

The only other downside I see with higher lift rockers is the resulting higher load on the cam/pushrods which will wear them somewhat faster and if the pushrods were marginal, fail/bend them.

BTW, if money were no object, I'd be looking at installing a 426 HEMI.......... :>)

[dave-r]

But a performance cam is doing the same thing as the rockers! Using them means you can use a milder cam without a drop in performance. Power is got from lifting the valve as quickly as possible and holding it open for as long as posible. But the longer the duration the less streetable your engine is. The faster you make the lift the more streetable your car will be.

[Tim]

Absolutely Dave. However, while the overall duration does not change, the effective duration (say at 0.050 lift) increases. I'm not sure if you're point is that there would be more flexibility by running a milder cam, being that with high lift rockers one achieves a "bigger" cam and with standard rockers the "smaller" cam. Wouldn't you say that optimally a cam ground for a specific high performance application would have a superior profile than a milder cam with high lift lifters achieving similar lift and duration?

[dave-r]

Effective duration does increase you are right as the 50 thou lift will occur quicker but it is only by a small amount. What is more important is the rate of lift. The faster the valve opens the better as gas flow will reach peak quicker. Meaning more air and fuel gets in. With 1.6:1 ratio rockers the valve will move faster because it has to travel further in (almost) the same amount of time. Plus it will stay open longer and close faster. For a cam alone to do the same thing you would have to increase duration significantly.

That's how I see it anyway.