On an SBC, would it be possible to modify the camshaft & distributor, using an off-the-shelf crankshaft, so that two cylinders would fire at the same time? Would there be any advantages in torque/hp? Theoretically, what would a conversion like this cost? I would think it would only be the cost of a custom camshaft and a modified or custom distributor, correct?

 

Harley Davidson has been firing both cylinders for years. It's just the way the engine is designed. 45* V twin with a single throw crankshaft doesn't make for a symetrical firing pattern between the 2 cylinders. Switching over to a single fire ignition system makes more power out of the V twin.

In what way would you want to fire 2 cylinders at once in a V8? When #1 is at TDC on the compression stroke and the spark plug fires, #6 is at TDC pushing the exhaust gases out. You want to ignite exhaust gases? There might be some unburnt fuel left over but the energy from the burn would just go out the exhaust.

If you want to ignite unburnt fuel then get an ignition system that does muti spark. MSD, Holley Annihilator etc.

If you want something exotic, pick up an odd fire cam. It changes the firing order around. Instead of 18436572 the firing order becomes 18736542. by switching cylinders 7 and 4 around there is better fuel distribution to the cylinders and more power can be produced.

 

You say 1 & 6 are at TDC at the same time; one has just finished it's exhaust stroke and is about to start it's intake, one has just finished it's compression stroke and about to start it's power. I'm saying I want to switch the events on the camshaft so that their intake & exhaust events happen simultaneously. They both fire at the same time...

 

It takes 720 degrees (2 revolutions) of the crank to complete one cycle. With cylinders at 90 degrees in a SBC V8, a pair of cylinders reach TDC every 90 degrees.

I think you'd have an uneven firing engine:
@ 0* 1 and 6 fire
@ 90* 8 and 5 exhaust
@180*4 and 7 fire
@270* 3 and 2 exhaust
@360* 1 and 6 exhaust
@450* 8 and 5 fire
@540* 4 and 7 exhaust
@630* 3 and 2 fire
@720* 1 and 6 fire
etc.

The loads on the crank would be less than optimum, and two cylinders would be drawing maximum from the intake plenum simultaneously instead of just one.

I don't see any advantages, and some serious disadvantages, but it looks at first glance that you could do as you suggested. Maybe I missed something.

Now, many engine fire the plug every revolution, near the top of the compression stroke of course, and near the top of the exhaust stroke. One example is single cylinder Briggs & Stratton type engines. They use a flywheel magneto, so it produces a spark every rev.

Another example is coil-pack (distributorless) engines with 3 coils on a 6 cyl and 4 coils on an 8. many V-6's were like this. They fire 2 plugs at once, one on the compression cylinder and one on the exhaust.

Does it help or hurt power? Probably neither. It does probably wear out the spark plugs faster!

My $.02

 

why would you want to do this, i always thought that you want all the pistons to fire seperately so that they are all pushing each other and are more smooth, plus as a side not, do you think that causing two pistons to fire at the same time would cause too much pressure on the block?

 

There was a great thread on this on the old board but its probably been lost now.

If I remember right its been done before. Someone mentioned the Lotus Esprit used this technique for a few years. Basically it provides slightly more power but it also caused more vibrations.

 

I know of an air compressor trailer done up real strange. It uses a ford V8. The left side of the engine is a 4 cylinder engine. The right side of the engine is a 4 cylinder air compressor. It runs just fine on 4 cylinders. That's roughly what you're trying to do except you want to double the load on each compression stroke. There would be a lot more involved than just getting a cam ground. Think about the incoming air into the cylinders. 2 cylinders both trying to suck the same air in. The intake manifold would have to be redesigned to allow equal air into both cylinders.

Personally the more cylinders there are, the smoother the power output is since there is a power stroke in less crankshaft rotation. A Jag V12 is very smooth because of all the extra cylinders.

I don't think the 2 cylinders firing at the same time would cause too much pressure on the block but would cause a lot of stress on the crank.

 

For the intake manifold, a simple hilborne stack setup would resolve that problem; no plenum to deal with. As for the firing order, it does seem a bit out of order. I'm trying to come up with an idea for a project that is just totally wicked and makes a ferrari out of a small block. I was thinking maybe a flat-fire motor that revved to 9k rpm that would live in a street car. You gotta be crazy to have cool stuff. I guess pneumatic valves is next on the list, huh?

 

How about taking a pair of old GM 215 cube aluminum V8's and closely couple them together nose-to-tail with 45 degrees between top dead centers? You'd then get a firing pulse every 45 degrees, and by pairing the cylinders correctly, you'd get the "ripping canvas" sound of a Ferrari V12 with 4 extra cylinders! The engines weigh about 320 lbs dressed, so you'd end up with less weight than a aluminum head big block Chevy. Rover still uses the basic engine, so you could build it from 7 to over 9 liters. Shoot, one could be 3.5L and the other 4.5L!

As for 9 grand, Winston Cup cars regularly run their flat lifter, metal-spring engines to 9400 all day long. The technology is here; you don't need pneumatic valves. Except for your claim, who could tell if you had pneumatic, electric or manual valves? Besides, with a 45 degree firing, 4500 will sound like 9000 in the V-16. You can probably get them to run at 6000 or 6500 with enough money, so there's 12,000 to 13,000 of sound. 9000 "flat-fire" is chump change!

Of course, using two little V6's would get you the V-12 but it wouldn't have the cool of a V16, nor anywhere near the power. How about using one fairly large Eaton/Magnason supercharger, like the one used for the LS1, centered over the 16-cylinders? Or use two smaller Eatons (from GM 3800 SC?) nose to nose driven off the center coupler of the engines?

If you are thinking out of the box, which you obviously are, take a bigger step. Anybody can make an odd-firing, no power V8 which just sounds like an engine with a bad miss. Step up to the plate and swing for the seats!

 

I remember reading somewhere that the dual-fire ignition systems produced less emmissions because it burn off any unburned fuel. Also, it may make a little more power because it keeps the spark plug cleaner, and (maybe) gets rid of more of the exhaust so that on the next intake stroke, there will be a better fill.

Just a thought on the last one though?

 

Nissan put a 4-cyl. engine in a truck in the late '80's early '90's that was a DOHC with a a single 8 tower distributor and 8 plug wires going to two banks of four spark plugs on either side of the head (one bank on the intake side and one bank on the exhaust side of the head...two seperate spark plug heat ranges). They did this to increase hp (rated 134hp) and to reduce emissions...

 

Why not just run a flat crank, bank to bank?
Very sensitive to tuning but very tunable. The intake pipe length, cross section area, cam timing, exhaust tuning etc. are all purpose built in an engine like this. Can make excellent power/cid though.
The majority of our work is in these types of engines.

Then again, if you have the engineering skills necessary in building a pneumatic valve system... well it should be cake for you.

Chuck Riddeck
Progressive Race Engine Development

 

Another thought- and a do-able one without massive hassle is to simply run 2 primary header tubes to the opposite header collector and vice-versa.

Why?

Well, a production V8 like the SBC doesn't have even exhaust pulses bank-to bank. It fires:

DPPDPDDP where D is driver side bank, P is passenger using the standard 18436572 firing order.

Each header collector on a normal header sees unevenly spaced exhaust pulses. For instance on the pass. side header collector it would see:

wait-pulse-pulse-wait-pulse-wait-wait-pulse

Basically the same unevenness on the driver's side header

Without changing the firing order of the engine but simply routing the primary header pipes so that each header collector sees...

DPDPDPDP. Or, pulse-wait-pulse-wait-pulse-wait-pulse-wait

Would offer better scavenging on the exhaust due to the more even pulsing, and therefore, the potential for more power. Although I've never done it I have read about it and seen it on a few race cars. Check out the old Ford GT40 from the late 60s- it used this type of header arrangement on it's 427. It was made easy on this car due to it's headers being run over the engine and down the rear of the center of the car- which allowed these cross-over primary tubes with less packaging difficulty.

Just a thought. I'm sure others with actual expereince could chime it.

 

What about having a billet crankshaft built that moved the journals around so that it was a true flat-fire motor? Would this setup make good power? Would it sound totally wicked?

 

Just put the 383 w/centrifugal blower in it and forget this misfire setup.