My 3.5L EPIC waste of time!!! Why not!

**magnuman** · 01-05-2021, 11:00 AM

Originally Posted by reefstar

Thanks for the comments. Here’s my thinking behind the exhaust, (BTW I build performance motorcycle systems).

The 3.5 is a 1-2-3-4-5-6 firing order. So the pulses alternate banks evenly. So, I kept the x over installed in the system. So scavenging at the x over should function well (it did before I did the engine work) especially with the new headers. Also I am keeping the mufflers installed (because it’s a 300, and I want a comfortable ride). If I eliminated the mufflers, I think you’d be right on point.

I also believe that the engine is very restricted at the throttle body (they a boring the 3.6L out to 80mm). So with the new 74mm TB and intake plenum updates, I’ll be moving more air than I was before the build.

Also, the exhaust still has the valves behind the mufflers, so if I’m getting too much flow through both pipes, I can adjust the tension on the valves to keep one side closed until it reaches high rpm.

Anyway, thanks for the comments and input, I appreciate it!

I can't see how an 80 mm TB will help on a 3.6 or that you're moving MORE air. Internal combustion engines are just big air pumps and can only process just so much air, based on the displacement. The 5.7's and 6.1's come stock with 80 mm TB's and they're just about right for their displacements. As far as "moving more air" goes, you won't be moving MORE air, but you'll be moving it more efficiently because the Voumetric Efficiency (VE) of your engine will be a little higher than before. Again, with forced induction, everything changes in this regard.

On the 5.7 side of things, at 100% VE, this displacement is capable for processing 658.275 CFM of air. Most "stock" vehicles usually have a VE of somewhere around 70-80% which means less than 450-500 CFM is processed. Bottom line, going bigger is not always better. When everything is set up to match the displacement, more power is generated. To give you an idea, I pulled the heads on both of my early 5.7's to have new valve seats installed (dropped valve seat problems with the early 5.7's) and, while I was at it I had the heads ported and polished and .030 in. milled off them to raise the compression ratio by about a half point. Since the 5.7's heads flow pretty well in stock form (on a relatively stock engine), both of these mods. combined netted only somewhere around 25-30 horsepower. IMO, the gainse were not hardly worth the $$$$ for a daily driver, and I probably wouldn't do the port/polish again...I'd probably still mill them though.

**reefstar** · 01-05-2021, 12:01 PM

Originally Posted by magnuman

I can't see how an 80 mm TB will help on a 3.6 or that you're moving MORE air. Internal combustion engines are just big air pumps and can only process just so much air, based on the displacement. The 5.7's and 6.1's come stock with 80 mm TB's and they're just about right for their displacements. As far as "moving more air" goes, you won't be moving MORE air, but you'll be moving it more efficiently because the Voumetric Efficiency (VE) of your engine will be a little higher than before. Again, with forced induction, everything changes in this regard.

On the 5.7 side of things, at 100% VE, this displacement is capable for processing 658.275 CFM of air. Most "stock" vehicles usually have a VE of somewhere around 70-80% which means less than 450-500 CFM is processed. Bottom line, going bigger is not always better. When everything is set up to match the displacement, more power is generated. To give you an idea, I pulled the heads on both of my early 5.7's to have new valve seats installed (dropped valve seat problems with the early 5.7's) and, while I was at it I had the heads ported and polished and .030 in. milled off them to raise the compression ratio by about a half point. Since the 5.7's heads flow pretty well in stock form (on a relatively stock engine), both of these mods. combined netted only somewhere around 25-30 horsepower. IMO, the gainse were not hardly worth the $$$$ for a daily driver, and I probably wouldn't do the port/polish again...I'd probably still mill them though.

Hold on, there’s a flaw in your theory. Let’s back up.
First I was replying specifically to your comment about using the SRT dual exhaust, and your comment that it was too large for the 3.5L. So let’s keep with that premise.

My response is that the 3.5L is throttled down from the factory (I have specific reasons to state that).
One observable reason is that the 3.6L comes factory equipped with a 74mm. The 3.5L came factory equipped with a 68mm TB. My comment about the 80mm was that people are boring it to that size and at the very least are not recognizing adverse performance. So therefore I feel VERY comfortable (along with the plenum modifications and port matching, etc) going with a 74mm TB. (Not to mention the historical 3.5L dual TB history). Manufactures do not send vehicles out with the ultimate performance tune installed. Actually, the opposite is often the fact due to EPA restrictions, and universal tuning sweetspots.

So, when you say, you cannot move more air, I disagree. If that premise were true scavenging would be a theory, not fact. Scavenging in fact moves more air AND moves it more efficiently. Furthermore, you CAN restrict air flow on an engine, it’s called “throttling”. Otherwise the ONLY way to shut off an engine would be to shut off the fuel source. It’s not. Typically restriction will improve low end performance, and kill top end.

Air flow is dynamic, not static. At lower RPMs a restrictive system will function fine, but at higher RPMs it can/will literally stall the engine. So although your premise of cc’s (volume) not changing (under the premise of absolute no restriction and no valve timing overlap), you must also keep in mind the RPM dynamically changes with/without restriction (air flow).

So with that being said, back to the exhaust, with all the added airflow potential, I don’t think the SRT system will be adversely to large. I also said the dual exhaust still has the actuator gates. So if bottom end suffers (by having too much volume), I can simply restrict half of the system until rpm is reached to justify both pipes to open. As a matter of fact I’ve considered de muffling (what would be) the “high flow” side.

The only way to know is to do tuning on the system once it’s all up and running.

You are 100% correct about the marriage of parts, and the incorrect marriage of parts can definitely be detrimental to power. But I don’t feel I have reached that point with any modification. But I’ll find out when I turn the key!

**Lemniscate_Mike** · 01-13-2021, 10:07 PM

Been a while since the last update, I hope everything's going alright.
Looking forward to seeing the rest of the build!

**reefstar** · 01-13-2021, 10:22 PM

Originally Posted by Lemniscate_Mike

Been a while since the last update, I hope everything's going alright.
Looking forward to seeing the rest of the build!

Yep, everything is fine. Sent the new intake tube and a couple of parts off to be chromed, so I’m waiting for those to come back.

I needed to make an adjustment to the front engine cover, but both sides are done.

I ended up buying a Magnum with a dead 2.7L engine for my next project.

So, once again I’m at hurry up and wait... but there’s snow on the ground so it’s a bit easier.

Hopefully I have some new updates soon. The 2005 has inserts for the fog lamps, I’m thinking I may build a tube from my air box, and draw cold air in from right next to the fog lamp, by cutting those slots open.

**reefstar** · 01-23-2021, 09:41 PM

So parts left from chroming (supposedly) yesterday.

In the mean time I’ve decided to go ahead and fill the headlight gap with the front engine cover at the same time.