Even-Fire engine

[Rolon]

I'm instaling a MSD 6AL in my 1977 911. The instruction said to modified the ignition if the engine is even-fire. How I know?

[jluetjen]

911's are even fire engines. Some of the GM and Ford V6's are odd fire. It has to do with the interval between spark plug firing.

In general, V8's and V6's with 60 degree or 180 degree spacing between the cylinder banks are all even fire. For example the 911 motor has a spark plug fired every 120 degrees. V6's with 90 degree spacing and common crank-shaft journals (often the result of taking a V8 design and lopping off two cylinders), V4's and V-twins are odd-fire. For example the big GM V6's (such as the 4.3 liter engines) with a 90 degree bank angle have a spark plug firing after 90 degrees, and then 15 degrees, 90 degrees and then 15.

Of course there are exceptions, the Ferrari Dino 65 degree V6 was even fire as a result of a crank design with individual journal for each rod, and they were arranged in such a way that a cylinder fired ever 120 degrees.

[mpdevelopment]

Hey John do know how many Ferrari people still think the Dino is a 60deg V? Had a 246 4v race motor in a shop I worked in at one time, it needed a new crank, sent the old one to moldex they made us a new one but had no idea how to balance it. By the way 60deg v6s also have one journal per rod set at 60deg to each-other.

[James Renfroe]

All 90 degree V8 are even firing that use the 90 degree crankshafts. A V6 like the Chevy 4.3 with 90 degrees cylinder angle is also even fire but the crankshaft rod journals are offset to achieve the 120 degree firing rotation. Smaller 60 cylinder angle V6s are balanced using the common rod journal spaced at 120 degrees. Inline sixes, horizontally apposed twins, 4s and 6s cylinder are also balanced; horizontal twins fire every 360 degrees of crank rotation, H4 every 180 degrees, while the H6 fires every 120 degrees. The only odd fire engine that I can think of was the early Oldsmobile/Buick engines that had 90 degree cylinder angles utilizing common crank pin 120 degree crankshafts. Redesigning their v6 engine to 60 degree cylinder angle was probably considered a cost venture in those days. I only wish I knew how they balance the inline 3s and 5s that are about today!

[Bill Verburg]

Even firing is generally assumed to be even crank degrees and alternate bank firing, this later criteria is where 90&deg dual plane V8s are not even firing and what gives them their distinctive note. There are always 2 cylinders firing consecutively
on one of the 2 banks. On a small block chevy 1, 3 and later 2, 6 exhibit this oddity.

A Ferrari V8 using a 180&deg or single plane crank and a Porsche H6 are both true even firing engines

[Bill Verburg]

What are the advantages/disadvantages (if any) of an even-fired or odd-fired engine?

even fire advantages :
exhaust tuning is easier, engine is smoother and takes revs more gracefully

odd fire advantages: packaging and cost, chiefly cost

[mpdevelopment]

James, all 90deg v-6 engines without balance shafts are dynamically unbalanced, they have an unbalanced rotating couple.

[kjb]

An automotive engineer I used to work with said a lot of bad things about the design of the viper V10 engine, and it being odd-fire was one of them. At the time, most if it went over my head. Apparently you can really hear it if you let a viper idle next to a wall: it sounds like it's running rough.

/ J

[mpdevelopment]

The reason we have odd fire engines today is the advent of digital engine control. These computerized systems allow the engine designers to produce v-engines with any bank angle thus any firing order.

[jluetjen]

I think that people are confusing crank design, balanced engines and odd-even fire engines. They are different things and describe different facets of the engine's design.

All 90 degree bank angle engines are even fire no matter if they have 90 degree crank or 180 degree crank. This is because a spark plug somewhere (anywhere) is firing every 90 degrees. Here's an interesting discussion about the GM Odd-fire V6, what it is and how it works.

All 90 degree V8 are even firing that use the 90 degree crankshafts. A V6 like the Chevy 4.3 with 90 degrees cylinder angle is also even fire but the crankshaft rod journals are offset to achieve the 120 degree firing rotation.

Not so. I specifically checked this before I used the example. GM themselves describe the motor as being an "Odd-fire" engine.



As far as a "balanced" versus an "unbalanced" crankshaft design, that has been covered on another thread that I don't have time to look for.

[randywebb]

I agree - they are different. But as i understand it, the ignition and me3chanical concepts are related. An unbalanced design often (always?) has odd-firing. A balanced design (e.g. straight or flat 6) would have even firing - tho some crazy person could probably implement odd-firing on it, there is not reason to do so.

And I mean 1o balance, not 2o, 3o rocking couples or what have you.....

[jluetjen]

Originally posted by randywebb
I agree - they are different. But as i understand it, the ignition and me3chanical concepts are related. An unbalanced design often (always?) has odd-firing. A balanced design (e.g. straight or flat 6) would have even firing - tho some crazy person could probably implement odd-firing on it, there is not reason to do so.

And I mean 1o balance, not 2o, 3o rocking couples or what have you.....

That doesn't explain the 90 V-twin which is a balanced design, but an odd-firing engine..

[Bill Verburg]

V twins

[jluetjen]

Thanks Bill!!!! I was trying to search for that link before and I couldn't find it.

[James Renfroe]

It is well known that racers prefer the "odd fire" (common rod pin) crankshaft for the 4.3L engine because of the strength advantage. But I know that the factory crankshafts for these engines were offset for the even fire 120 degree configuration. My definition of even fire is that there is an ignition of one cylinder in "even degree" increments. In contrast, an odd fire engine would are odd because they fire in "unequal degree" increments.

Even number cylinder engines operate in balanced pairs. The perfect example is the inline 4 and the Ferrari 8 (essentially mirror image inline 4s), both sport the 180 degree cranks. Another advantage of the 180 degree crank V8 is that the crank can be lighter (less need for counter weights and can be made stronger. I think passenger V8s are designed 90 degrees for of low rpm smoothness and torque. Horizontal twins like the BMW R series twins are perfectly balanced because of the even number cylinders and equal 180 degree ignition.

[mpdevelopment]

James, the 180deg or flat crank v-8 has the same 2nd order vibration as an inline 4cyl ( x 2 90deg out of phase). This high frequency vibration can't be balanced with counter weights on the crank. The Ferrari engine is a small displacement unit with light reciprocating parts this keeps the vibration at a tolerable level. The 90deg crankshaft v-8 has no secondary imbalance it only has a primary rocking couple that can be balanced with counterweights on the crankshaft. Horizontal twins also have a primary rocking couple but it can't be balanced with counterweights on the crank you can feel this vibration while riding a BMW twin.

[SC-targa]

I sort of associate even firing with evenly spaced firing impulses. I don't know that alternating planes is part of my definition, though I suppose it does make sense. I consider Chevy V8's even firing, even if they don't use a 180 degree crank.

The early single plane (180 degree) Caddy V8's were real shakers at certain RPM ranges, even though they were even firing. Until they introduced a dual plane crank (early 20's), the Packards were much more refined.

Back in the olden days when Packard, Marmon, Lincoln and Caddy made V-12s (and Caddy also made V-16's), the 12's were all 60 degree designs so they were even firing. All classic Ferraris are 60 degree V-12s too

The current Ferrari V-12s are 65 degree designs, as were the old Dino V6's. If they want to retain the perfect primary and secondary balance, they must by definition be unven firing engines. Did Ferrari do this for packaging reasons, to fit a bulkier induction system or cam drive? Do they assume that with 12 cylinders and modern engine mounts, that the drivers wouldn't notice?

Also, to add to Bill's list of advantages, though I disagree with smoother in the case of a single plane V8, it simplifies intake tuning if you don't use an individual carb or throttle body for each cylinder.

Regards,

Jerry Kroeger

[Rolon]

Thanks to everyone for all the great information. That I found very useful, until next time.