A single 3" tube flows close to 2 x 2.5" tubes , so that would be fine for c 500 bhp.
Manta Montage T First Build
- Thread starter Eric Hardy
- Start date
Welcome back Eric.
I would strongly encourage you to have the engine dyno tested.
You will learn volumes from that testing in regard to its capabilities and needs.
Most dyno operators have a pretty good selection of various header types and sizes which will tell you what the engine likes.
At the same time,you'll be able to sort out any issues that crop up in the process such as oil leaks with much greater ease.
I would strongly encourage you to have the engine dyno tested.
You will learn volumes from that testing in regard to its capabilities and needs.
Most dyno operators have a pretty good selection of various header types and sizes which will tell you what the engine likes.
At the same time,you'll be able to sort out any issues that crop up in the process such as oil leaks with much greater ease.
Thanks randy, I agree that a dyno test would be advantageous however it still wont tell me how large the merge size will need to be on the 8 to 1. Also, I won't be going larger than 1-7/8 headers and 1-3/4 seems to fit the current setup better from the limited research I've done along with suggestions from those much more qualified than I.
Colin,
How can flow be calculated on the exhaust end? Is there a formula to determine primary header diameters to merge size?
Colin,
How can flow be calculated on the exhaust end? Is there a formula to determine primary header diameters to merge size?
Terry Oxandale
Skinny Man
I would agree with the <1.75" for this displacement and RPM. There are a lot of examples of folks going too large on the primaries in respect to the motor needs. Every calculator I've ever seen assumes dual collectors, so that may be something that will be hard to determine for a single collector. I've seen some stuff that indicates the collector diameter should be larger than intuition, and primaries small than intuition.
My initial intuition for collector was 3" With that insight should I go 3.5 or 4" on the collector?
I apologize but Im not sure I follow. Can you elaborate on this with a bit more detail?
Re post #107... I did a quick calc & your 383 would have a primary pipe length of 36". Your tailpipe from end of primaries to tailpipe end or expansion box should be the same- 36". I had to make some assumptions as I dont know your cam timing.
An eight into one single tailpipe should have a volume of eight cylinders..in your case 383 cu-in/ ~ 6300cc/6.3 liters.
Once you try to convert paper to pipe you will find this is not doable as you have to subtract the exh port length from that 36" which will then be 33" and you probably wont be able to get all eight pipes into one large collector without getting some longer primary pipes.
A few posts back you mentioned swapping crank dampers/pulleys to a smaller poly/ vee type for space reasons... as a 383 is based on a 400 crank have you checked to see if the engine is still externally balanced or converted to internal in the engine build.
An eight into one single tailpipe should have a volume of eight cylinders..in your case 383 cu-in/ ~ 6300cc/6.3 liters.
Once you try to convert paper to pipe you will find this is not doable as you have to subtract the exh port length from that 36" which will then be 33" and you probably wont be able to get all eight pipes into one large collector without getting some longer primary pipes.
A few posts back you mentioned swapping crank dampers/pulleys to a smaller poly/ vee type for space reasons... as a 383 is based on a 400 crank have you checked to see if the engine is still externally balanced or converted to internal in the engine build.
Jac Mac
You are obviously more versed in exhaust design than myself.
Why does the tailpipe need to match the 36" primaries? couldn't i theoretically increase diameter and shorten length to maintain the same volume if in fact volume is the reason for the length?
My engine is in fact internally balanced. I no longer need to swap the HB for lack of space.
Most of the offroad trucks/buggys using 8to1 have very short tailpipes. here is a picture of an ultima GTR that I am looking to base my exhaust design on.
You are obviously more versed in exhaust design than myself.
Why does the tailpipe need to match the 36" primaries? couldn't i theoretically increase diameter and shorten length to maintain the same volume if in fact volume is the reason for the length?
My engine is in fact internally balanced. I no longer need to swap the HB for lack of space.
Most of the offroad trucks/buggys using 8to1 have very short tailpipes. here is a picture of an ultima GTR that I am looking to base my exhaust design on.
Attachments
No, you calculate length first and use that length in the calculation for pipe diameter.
The tailpipe/muffler in pic is essentially an 8/1 collector straight onto a megaphone which ends in a small expansion box/muffler. Megaphones are somewhat difficult to calc for and I would suggest a single straight pipe would be optimum, but it is likely a case of what fits.
The tailpipe/muffler in pic is essentially an 8/1 collector straight onto a megaphone which ends in a small expansion box/muffler. Megaphones are somewhat difficult to calc for and I would suggest a single straight pipe would be optimum, but it is likely a case of what fits.
So Im about to order materials and the collector, in your post above are you recommending a 3.75" exhaust diameter or were you perhaps referring to the 1.75" primaries? I was planning on running a 3" exhaust pipe. I can go larger but prefer not to if I dont need it.
You might find it better to fabricate your own collector, the one shown in your photo looks pretty, but due to that it does not give a definite end/length to the primary pipe.
If you choose to go 3" tailpipe it might need to be longer and could kill a bit of top end power... depending on what your rev limit is.
I can go larger with a 3.5"-4" exhaust if it will shorten the exhaust. The company building it for me will fabricate any straight or transitioned collector and I believe I can also specify either 13° or 20° angles on the collector tubes.
Merge Collectors
Under best case scenario, what exhaust diameter would you recommend? 3" 3.5" 4" or larger?
Terry Oxandale
Skinny Man
Pardon perhaps a uneducated question: If an 8-to-1 collector is used, wouldn't the exhaust timing pulses be evenly spaced as each passes through the collector (assuming equal length primaries), and thus wouldn't the collector then become an extension of any single tube of the primary system, and only need to be of similar sizing or larger by one or two steps? I would assume the scavenging benefits would be pronounced as well.
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I agree with the logic behind this however the same could be said about the crossover bundle of snakes design. I have never seen a crossover system maintain the same or close to the same diameter exhaust tube. This would lead me to believe there is reason behind a larger exhaust diameter. I would personally be concerned that my primaries were not precisely equal length and therefore could result in multiple primaries firing through the collector at the exact same time which could be problematic for an exhaust with a too small diameter. I do still think a minimally sized exhaust is all that would be necessary, i.e. 3" or perhaps eve 2.5" to 3" transition horn (but i really dont know the reason why people transition after a collector). I could be completely wrong though... it happens more often than I like to admit.
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Here is a quick article with a chart calculating rough diameter sizing based on HP.
How To Calculate Muffler Size and Exhaust Pipe Diameter - Exhaust Videos | Exhaust Videos
My car with its estimated HP of 490 should be running a single exhaust diameter of 3.5"-4" based on the chart. However this does not take into account the sequential firing of the primaries. For this reason my thought was to use a maximum 3.5" but would most likely be fine using a 3".
How To Calculate Muffler Size and Exhaust Pipe Diameter - Exhaust Videos | Exhaust Videos
My car with its estimated HP of 490 should be running a single exhaust diameter of 3.5"-4" based on the chart. However this does not take into account the sequential firing of the primaries. For this reason my thought was to use a maximum 3.5" but would most likely be fine using a 3".
Will the old headers that came with the car fit your 383 heads, if so why not simply join those two collectors into one large pipe, rather than waste a lot of time and effort building a complete new system based on a lot of unknown factors for the 383 engine. Even if the head bolt pattern is different you could chop the old plates off and fit correct ones. Then if you like the sound build a correct system later.
The old engine in the car was a buick v6. Retrofitting the previous headers is not a super viable option imo.
So earlier today I spoke with Greg Holman from Racing Engineering Fabrication Unlimited out of Arizona. He fabricated the exhaust on the ultima shown above. He seemed like a really cool guy and was kind enough to spend some time with me over the phone so Im thankful to him for that.
For my specific engine he suggested the following design (based on the specs I provided).
1-5/8" stepped @ 10-12" to 1-3/4"(to maintain/increase velocity). 1-3/4' to the 8 into 1 collector and out into a 3"-in 3.5"-out megaphone (again to increase velocity). He mentioned that the megaphone would need to be a min of 4" in length but could obviously run longer.
I feel confident that this design will be more than sufficient for what Im doing so Im going to get the material on order.
For my specific engine he suggested the following design (based on the specs I provided).
1-5/8" stepped @ 10-12" to 1-3/4"(to maintain/increase velocity). 1-3/4' to the 8 into 1 collector and out into a 3"-in 3.5"-out megaphone (again to increase velocity). He mentioned that the megaphone would need to be a min of 4" in length but could obviously run longer.
I feel confident that this design will be more than sufficient for what Im doing so Im going to get the material on order.