Crossover vs non crossover

Stig Svedberg · 05-15-05, 11:38 AM

My current setup uses a set of non crossover headers from GTD. The previous owner, trying to fit them into an early KVA, beat two pipes beyond recognition. While I am repairing this, I was thinking of mofifying other tubes and converting it to a crossover type exhaust. What is the advantage, if any, this set up? Looking at pictures in my GT40 books it appears that it is not done to equalize the tube lengths. I have heard that the exhaust note will be different.

mlevea · 05-15-05, 05:45 PM

[ QUOTE ]
What is the advantage, if any, this set up? Looking at pictures in my GT40 books it appears that it is not done to equalize the tube lengths. I have heard that the exhaust note will be different.

[/ QUOTE ]

I have heard numbers in the range of a 20 to 40 hp improvement with the crossover design.

It is not done to get more equal lengths to the runners. In fact the crossover design exacerbates the unequal length situation.

You start with an equation based on the engine volume, rpm range, exhaust valve duration and a few more items. This gives you a starting header length and the user is expected to make multiple dyno runs to derive the optimal length. On my engine this length came out to be close to 6 feet in length. This is a bit too long for the GT40 if you also want mufflers.

With the 180 degree crossover you get an effective header length of the first runner AND the runner to the cylinder that's 180 degrees out. So with two 36 inch runners you (nearly) get the proper reverse pulse from a single 72 inch runner.

The crossover design conserves accoustic energy. The effect of this is a quieter engine so you can use less muffler and still keep the dB levels low.

You can hear my car run in this parody I made:

Porsche guys hate this movie [img]/ubbthreads/images/graemlins/mad.gif[/img]

It's 14Meg so it takes a while on a dial up but should give you a good idea of the sound of a crossover system. My mufflers are straight through with ceramic fiber packing.

Hope this helps.

Mark

Howard Jones · 05-16-05, 01:05 PM

Send me a email and I'll send you back my home phone number. I'm right across the bay from you in San Ramon.I just completed a major rework on my GTD headers and can offer some free advice.

By the way the muffler will have a much larger effect on HP than the cross over, or lack of, will.

calle_o · 06-10-05, 08:40 AM

Can anyone explain how to build crossover? Which pipe goes to the other side? Not only for the 302, I'm interested in the principle to.

daveg · 06-10-05, 09:16 AM

While I realise that on the 302 the pipes run clockwise as per the dizzy, crossing over every other cylinder ie: the left collector is 5-2-3-8 ,the right collector is 1-4-7-6 as far as I am aware [img]/ubbthreads/images/graemlins/frown.gif[/img], My question is, would using this system work for a SBC or LBC or even the Rover V8?

Steve Toner · 06-10-05, 11:09 AM

I tried to explain this once, here . See if it makes any sense.

Tony Hunt · 06-10-05, 12:02 PM

[ QUOTE ]
Can anyone explain how to build crossover? Which pipe goes to the other side? Not only for the 302.

[/ QUOTE ]
The two 'middle' cylinders' headers go with the two 'outers' from the other side. So, 2 and 3 share a box with 5 and 8. The other box has 1 and 4, 6 and 7. It can be quite tricky routing the headers to be neat, yet not touch each other.
My crossover system sounds great and is pretty quiet. With the standard GTD silencers, it achieved 99dB at Goodwood last summer, and with a pair of Frank Catt's repackable silencers, it managed 96dB at at track day there last week.
I can confirm that, in each case, there's plenty of power!

calle_o · 06-11-05, 08:58 AM

[ QUOTE ]
While I realise that on the 302 the pipes run clockwise as per the dizzy, crossing over every other cylinder ie: the left collector is 5-2-3-8 ,the right collector is 1-4-7-6 as far as I am aware [img]/ubbthreads/images/graemlins/frown.gif[/img], My question is, would using this system work for a SBC or LBC or even the Rover V8?

[/ QUOTE ]

If You with SBC mean Chevy smallblock, it goes the same as Ford 302: 1467 & 2358. But I don't know about LBC and Rover.
.

AdrianBurton · 06-11-05, 02:03 PM

[ QUOTE ]
While I realise that on the 302 the pipes run clockwise as per the dizzy, crossing over every other cylinder ie: the left collector is 5-2-3-8 ,the right collector is 1-4-7-6 as far as I am aware [img]/ubbthreads/images/graemlins/frown.gif[/img], My question is, would using this system work for a SBC or LBC or even the Rover V8?

[/ QUOTE ]

The principal is still the same tough the firing order is diffrent. 1-8-4-3-6-5-7-2 for the Chevy. Not sure about the Rover. Basically what you are trying to do is to have the cylnders paired such that they fire 180 degs. from each other.

Russ Noble · 06-11-05, 04:46 PM

I'm not super informed about yank V8's, but when comparing firing orders you've got to watch how they number the cylinders. I think Rover and Chev number the left bank 1 3 5 7 , right bank 2 4 6 8 whereas Ford is right bank 1 2 3 4 and left bank 5 6 7 8. That's just from memory, I've lent my reference books to a mate and can't consult them right now.

Regards

David Tickle · 06-12-05, 02:56 AM

Hi

Can I join in please? My KVA came with a set of headers that were in need of replacement years ago, they are rusty, full of dints and hammer marks to make them fit and they have developed pinholes.
They have an unusual crossover arrangement. All the four left bank headers go to the lower four entries of the 4 into 1 collectors and the four headers on the right bank go into the top row of collector entries. (Hope I've explained this clearly).
Was there a reason why the original owner had them made this way?
I obviously need a new set of headers, does anyone know of a manufacturer in the UK?
They are quite expensive, so to get it right, what is the optimum cross over set up? The car is for road use only, so I'm not too interested in equal length headers.
Carbon steel with a coating, or stainless?????

It actually sounds great with this arrangement bye the way!!!

Regards

Dave Tickle

ingvar · 06-18-05, 06:53 PM

Hi
I have tested these crossover exhaust and cannot say that there is any great advantage to good "ordinary" or "shorty" headers. Dyno testing shows increased power with 8-1 crossover exhaust. But at higher rpm than normally used on the road. I have tested all kinds of exhaust (open) on my competition dragster and best result (time and speed) was given by eight individual tubes like Top fuel dragsters have..... But I will have a 8 to 1 crossover exhaust in my Gt40 anyway because of the sound. To make a 8 to 1 crossover system I have got a formula to calculate the lenght of the primary tubes.The volume in each tube should be twice the volume of each cylinder. I use a 302 ford engine, primary tubes are 42 mm inside(1 3/4" outside). Each cylinder have 0,618 Litre volume and this gives a tube lenght of 893 mm.
Making my own exhaust I use the method to fill the tube with sand, heat it up and bend the tube to desired shape.Less work than weld multiple pre - bent tube parts. The method works well on steel tubes, I have not tested with stainless steel but will do it when I make the exhaust on my car.
// ingvar

Kalun D · 06-19-05, 07:45 PM

crossover does one thing primarily. It separates each cylinders pulse from the one next to it in the firing order so that with dual collectors the cylinder pulses are going to alternate collectors. This helps scavenging by keeping each pulse more separate from the others in the collector, therefore allowing more efficiency.

Another thing you can do is set up the tubes to pulse in a circular pattern in each collector. This creates a swirl effect like a water drain again increasing effeciency.

Two other secondary reasons are bling and exhaust note.

Howard Jones · 06-20-05, 03:37 AM

I think you will find that better than 80% of the cost of having headers made will be labor. I really don't think that their will be much difference in total time between the two types really. The material cost between the two should be nearly the same also. Use a good thick flange ported to match your exaust ports, 3/8" thick is best. Also the merge collector is the most inportant part, so buy good ones.

I do not think you will be able to tell any performance difference between the two except maybe the crossovers might not be as loud.

So If I was going to have some made from scratch I guess I would do crossovers for the "GT40 bundel of snakes effect"

If you do mild steel instead of stainless then for sure jethot coat inside and out.

Bill Musarra · 06-20-05, 07:30 AM

Hey guys,
This was thrashed in an earlier thread and it is the answer to almost all your questions. I have saved many threads which I consult for my build, and I came across it the other day researching another problem. Adam Christian, the ultimate engineering student(no more) put all this through the number cruching and has all the graphs as well. Should answer "all" your questions. Check it out.

http://www.gt40s.com/ubbthreads/showflat...o=&fpart=1&vc=1

Bill