Yellow Bullet Forums banner

1 - 6 of 6 Posts

·
Registered
Joined
·
3,666 Posts
Discussion Starter #1
can any one tell me what the pro's & cons are when you get a cam thats got 4-7 swaped in the firing order I'm looking to change my cam from last year. looking to get the best cam for my motor (sbc on nitrous)
 

·
Super Moderator
Joined
·
35,256 Posts
Well... I have used the 4-7 swap, the 4-7/2-3 swap, and the std in n/a applications.

I have done back to back on the standard and 4-7-2-3.... and saw nothing in "ET"... that's not saying I had everything in the engine optimum for a firing order swap.

I've had discussions about it taking these issues into account.

Cooling issues - In the conventional firing order there is cylinders 5 and 7 firing consecutively. Well this is in the worst area of the cooling system(rear of the block) and those cylinders can run hotter. So now with the 7-4 swap you have 4 and 2 firing consecutively... in a much better area of the cooling system, right after the water pump. Well, all that being said, there are probably much better ways to address cooling system problems than worrying about which 2 cylinders are going to fire together.

Bearing Loads - Apparently there are some firing orders for certain makes that aided in breaking crankshafts... I think one of them was the old 289 ford firing order??? Not certain. Maybe someone could pop in there. But I think it was significant that 1-5 fired together in the front of the engine. I don't know how significant it all is because there are opposite cylinders firing in any firing order I can think of.

Exhaust coupling. With altering the firing order, it is possible to route the headers, with a 4-2-1 (tri-y) so that no 2 primary tubes fire consecutively, into their respective merge. I'm certain there is some validity to that, but I have not built a set of headers yet, to test this theory for my own benefit.

Intake Pulses. Through the induction system there are intake pulses created from the valve events and if you're savey in designing the valve events, and the intake manifold you can make these pulses work FOR you... but there are conditions in the plenum that can supposedly work against you as well and by altering the firing order you are in effect changing the 'order' of the intake pulses within the plenum... If I spent more time I could probably explain that a lot better but I figure you get the picture.

Anyhoot.... those are the issues involved in changin F.O.

If there are more, I'm not aware, or don't remember.
 

·
Registered
Joined
·
2,276 Posts
One of the pro's for firing swap is the cam company gets to charge a lot more for a cam that in most applications will yield zero results.

There are some who claim a significant torque increase near peak torque RPM with the 4-2-1 header combination, but the headers by themselves can make this happen.

Others claim benefits when the induction system is restricted. Maybe, maybe not. Usually these are 2bbl. situations that don't run on drag strips so teenie-tiny gains are never really appreciated because driving, the car, the track, the other cars, and everything else under the sun are affecting performance.

Crankshaft durability does factor into this, and this is why Ford switched to the 351 firing order on 302 engines years ago. Prior to the change 1 and 5 did fire in sequence and that was causing excessive loading on the front of the newly designed lightweight crank.

Coincidentally, if you run the sequence for a Chevrolet with 4/7 & 2/3 swaps, it comes out to be identical to the 351 Ford.

Also cute is when you compare the stock Chevy firing order. The sequence is identical to a Ford with the 2/8 switch, which is commonly referred to by cam suppliers as the "Pro Stock" firing order.

Then compare the Chevy with just a 4/7 swap and you'll find it's identical in sequence to the stock Ford 289 sequence that proved itself to be detrimental to life on a light-weight crank and therfore was discontinued for production. 460's retained this order though.

Confusing? I think the guys with parts for sale like it to be this way because nobody's ever stepped up & provided any real A-B-A data to support any alleged gains. Instead you'll get a lot of theory shoved at you by the self-proclaimed experts.

Just like the rod length story..........
 

·
Registered
Joined
·
2,276 Posts
LOL!!!!

Here we go with that strange timing again.
 

·
Registered
Joined
·
3,666 Posts
Discussion Starter #6
thanks for the responses guys sounds like i'm gonna stay with the normal firing order for a small block chev. :-D
 
1 - 6 of 6 Posts
Top