firing order swap on 4&7

[bunner engines]

has anybody done any valid research on swaping 4&7 ? i know its very common on 500 inch pro stock engines but has anyone seen good results in circle track and restricted engines ?

[speedtalk]

I'd be interested in seeing that myself. A can't believe there's much there though.

[rmbuilder]

Here's the best information we have at this time.
1)The advantage is in the fuel distribution onto cylinder #2. In the conventional firing order 5 and 7 pull fuel to the back of the engine and then 2 pulls to the front at the opposite end. This became a problem in BBC because the number 2 and 7 cylinder ports are long. 5 helps 7 pull to the back then 2 tries to pull forward and gets lean. You do not see this on 1 and 8 because they are short runners. By revising order (swap 7 and 4), 8 helps 7 and 4 helps 2. Thus, your fuel distribution is more balanced between cylinders.
2)Helps equalize EGT across the board.
3)Works best with 500+cid/7000rpm+ packages. +/- 1% on the dyno.
4)Reduces crankshaft deflection (jurys out on that one)
5)Has been used in Cup/Busch/Craftsman but unconfirmed information has it that it has been outlawed, stock firing order only now. Don, perhaps your contacts can shed some light on that.
6)I have information that some restricted intake class builders have had very good success with the 4/7 cams. That seems to go against the grain but I have heard it from more than one source.
7) Flatter torque curve.
A few points for discussion. Fire away.
Bob

[speedtalk]

In the conventional firing order 5 and 7 pull fuel to the back of the engine and then 2 pulls to the front at the opposite end. This became a problem in BBC because the number 2 and 7 cylinder ports are long. 5 helps 7 pull to the back then 2 tries to pull forward and gets lean. You do not see this on 1 and 8 because they are short runners. By revising order (swap 7 and 4), 8 helps 7 and 4 helps 2. Thus, your fuel distribution is more balanced between cylinders.

I really love this subject, but I've never tested it myself or seen any data. I've got some good interviews coming up, I'll try to fit this question in. Bob, how do we talk you into doing a test?

[learner]

I have wondered about that in the past. The fuel distribution makes sense. People have mentioned it for the purpose of exhaust timing but if that were the case all changing 4/7 does is place 4/2 together. I can see that it may help fuel distribution with a single plane manifold because #4 will help #2 and #2 will help #1 during acceleration. As far as exhaust timing i've played around with firing orders in the past(on paper)but there is no firing order that will will not leave at least one pair firing next to each other unless your using a 180 degree crank.

[speedtalk]

there is no firing order that will not leave at least one pair firing next to each other

Hi Learner, Are there people saying that exhausts firing next to each other is a negative?

[learner]

I've heard those discussions for a long time by a lot of people, whether they actually knew anything or not who knows. Without unequal length primaries i can't see it being anything but a disadvantage. As you know more than i, i would love to hear what you know about it.


Thanks

[speedtalk]

When you saying firing next to each other, you mean 90' apart I assume. Your right, the 4/7 swap can't fix that. The only way to get 180' firing is through the crank or 180 headers. Like I've always said, if the collector sees firing angles varying from 90-270, there's no point in equal length headers.

[bunner engines]

hey guys , thanks don ,bob ,learner for your input to this thread ! i will be posting back to back dyno results using the same cam profile with the only change being the different firing order . i will also be using the (optimizer) , its a stand alone data acquisition that measures cylinder pressures in relation to crank degrees . its opened alot of doors to engine research for me let me tell you ! it was created by dr. burt northam , have you heard about it don ? this engine is a dirt late model , two barrel , with a slider plate under the carb , ( most carbs are slid to the back of the manifold , or close to the back ) so with that in mind ! this testing should be very interesting what do you think guys ? ... i will try to post results when i get them but im just a one man show here , so please give me a little time .

[speedtalk]

unconfirmed information has it that it has been outlawed, stock firing order only now.

Your right, not legal. Also, Ford testing has not shown much.

[learner]

The only thing i can see is if the 4/7 swap is designed to help distribution under acceleration a static dyno won't tell you anything. I guess there are some dyno's that will simulate g's though i imagine they are pretty spendy. Do you have one?

[speedtalk]

The only thing i can see is if the 4/7 swap is designed to help distribution under acceleration a static dyno won't tell you anything.

Exactly!!!!!!!!!!!, I've tried to tell people the same thing about oil pans - you can't test them on the dyno.

[SW]

Here's a firing order that puts the pulses 90* apart, you should hear it run.
1,5,7,3,6,8,4,2

shawn

[speedtalk]

Here's a firing order that puts the pulses 90* apart, you should hear it run.
1,5,7,3,6,8,4,2
shawn

Hey Shawn, thanks for posting. Actually you're 75% right, it would be
90-90-90-450 which means #6 and #1 will get no low pressure help. Somebody actually built this? Tell me more...

[SW]

My info. on it is a bit sketchy, but i believe it originated in f1 and indy car stuff. I know it was tried by a few teams in pro-stock, which are the ones is heard run.It was amazing, if you had hearing protection. lol. It had a very high pitched scream to it that sounded like an IRL motor on steroids. I thought it was a 180* crank in them, and of course they let everyone else think that, too.From what i understand, the motors accelerated better than anything else, but the power band was very narrow.Hitting the induction/exhaust tuning was very difficult and all the teams i know of quite running it. I would still like to try it in something, just to give it a shot, but don't really have a good application for it yet.
Shawn