Enginerd
Registered User
Hello everyone, a little intro to me before I ask my question:
I'm new to the diesel world! A few weeks ago I bought a 1993 F250HD 4x4 Supercab with a newly rebuilt E4od and 318,000 on the odometer. I've been looking for quite a while on what diesel to get and with this truck I got the deal of a lifetime. I wanted this engine specifically because I've gotten comfortable with the design and mindset of gas engines and wanted to transition into diesels. Being N/A, fully mechanical minus some sensors, and IDI, I thought I couldn't find a better transition piece. This truck will be my daily driver once I get the brakes in good working order, get it cleaned, flush the radiator and put in the coolant additive, etc. I'm lucky in that the only thing that looks to have been done to the engine is a straight pipe and turning up the fuel. I turned it back down to clear up all the smoke except near WOT and put a muffler back on for now (boy it sure was loud haha) since I had a good one off a 2013 silverado laying around. I love how simple this engine is!!
So, now (almost) to my question. I absolutely love designing and building exhaust to manage both the flow and sound/tone, but I've only done gas engines personally, and researched turbocharged diesel engine exhaust. Exhaust is where I'll start on my engine improvements. My goal is not to build a hot rod, I already have one, and it's not to build a grocery getter, I already have one. I want something that has good reserve power for towing but is efficient in its task as a daily driver - essentially a do-it-all workhorse of a truck. So with this exhaust, I'll build a set of headers and more than likely merge through a y-pipe into a single exhaust. I believe I have the math worked out on the primary lengths for optimal timing to produce the best scavenging but this is where I've hit a dilemma. This is a big engine compared to my 350 small block Chevy that I have 1 5/8" primary pipes on, however it is a much lower revving engine. I'm looking to build peak torque at 2,000 rpm instead of 4,000 like on my 350 sbc. Once I build a good exhaust, I'll work the intake side to match. I was thinking with this big of a motor I would start with 1 3/4" primary tubes on my headers, but ran into a question I can't seem to find an answer to.
My question is about flow vs. scavenging. On a carburated gas engine like my 350, scavenging is just as important as flow-ability when producing torque so the leftover exhaust gasses are cycled out and a potentially larger volume of air/fuel is packed in the cylinder than the displacement itself. Generally speaking, too large of an exhaust kills the ability to scavenge and therefore hurts torque due to the decrease in exhaust velocity. But on an IDI diesel, how am I supposed to look at the balance of flow and torque? A boosted diesel can benefit from large exhaust due to lower egt's and just that you can't really outflow the turbo's flow... But I want to stay naturally aspirated for now and see what I can do.
That's a terribly long winded piece I know, so in short, can y'all explain to me the effects of scavenging on this N/A 7.3 IDI in relation to the ability to flow a lot of exhaust?
Thanks in advance for your responses. This group has an amazing wealth of knowledge and has already helped me quite a bit.
- Levi
I'm new to the diesel world! A few weeks ago I bought a 1993 F250HD 4x4 Supercab with a newly rebuilt E4od and 318,000 on the odometer. I've been looking for quite a while on what diesel to get and with this truck I got the deal of a lifetime. I wanted this engine specifically because I've gotten comfortable with the design and mindset of gas engines and wanted to transition into diesels. Being N/A, fully mechanical minus some sensors, and IDI, I thought I couldn't find a better transition piece. This truck will be my daily driver once I get the brakes in good working order, get it cleaned, flush the radiator and put in the coolant additive, etc. I'm lucky in that the only thing that looks to have been done to the engine is a straight pipe and turning up the fuel. I turned it back down to clear up all the smoke except near WOT and put a muffler back on for now (boy it sure was loud haha) since I had a good one off a 2013 silverado laying around. I love how simple this engine is!!
So, now (almost) to my question. I absolutely love designing and building exhaust to manage both the flow and sound/tone, but I've only done gas engines personally, and researched turbocharged diesel engine exhaust. Exhaust is where I'll start on my engine improvements. My goal is not to build a hot rod, I already have one, and it's not to build a grocery getter, I already have one. I want something that has good reserve power for towing but is efficient in its task as a daily driver - essentially a do-it-all workhorse of a truck. So with this exhaust, I'll build a set of headers and more than likely merge through a y-pipe into a single exhaust. I believe I have the math worked out on the primary lengths for optimal timing to produce the best scavenging but this is where I've hit a dilemma. This is a big engine compared to my 350 small block Chevy that I have 1 5/8" primary pipes on, however it is a much lower revving engine. I'm looking to build peak torque at 2,000 rpm instead of 4,000 like on my 350 sbc. Once I build a good exhaust, I'll work the intake side to match. I was thinking with this big of a motor I would start with 1 3/4" primary tubes on my headers, but ran into a question I can't seem to find an answer to.
My question is about flow vs. scavenging. On a carburated gas engine like my 350, scavenging is just as important as flow-ability when producing torque so the leftover exhaust gasses are cycled out and a potentially larger volume of air/fuel is packed in the cylinder than the displacement itself. Generally speaking, too large of an exhaust kills the ability to scavenge and therefore hurts torque due to the decrease in exhaust velocity. But on an IDI diesel, how am I supposed to look at the balance of flow and torque? A boosted diesel can benefit from large exhaust due to lower egt's and just that you can't really outflow the turbo's flow... But I want to stay naturally aspirated for now and see what I can do.
That's a terribly long winded piece I know, so in short, can y'all explain to me the effects of scavenging on this N/A 7.3 IDI in relation to the ability to flow a lot of exhaust?
Thanks in advance for your responses. This group has an amazing wealth of knowledge and has already helped me quite a bit.
- Levi