Pros & Cons of different exhaust systems (4 to 1 vs 4-2)

[Sparrowhwk]

Although I have an RV-8, I don't necessarily think this question is specific to that model, although the reason for asking may be specific to my installation.
I have a 4 into 1 exhaust system (not Vetterman - not exactly sure of the manufacturer, but possibly Aerospace Welding who are now owned by Hartzell), where the 4 pipes are arranged much like the 4 pips on a game dice. 2 above and 2 below in a box formation. These are connected to a 4 into 1 collector arrangement. I am finding that the lower pipes are touching the inside of the lower cowl (standard cowl, not Sam James).
I have checked there is no significant engine sag and I believe this has been the case since the original builder fitted the exhaust, although it seems to have got worse over time. This may be due to wearing of the heat protection tape on the inside of the cowl.
I cannot see a way to create space between the lower pipes and the cowling by any kind of adjustment, so it seems I am left with either building a new cowl (not something I am willing/able to do), or changing the exhaust geometry so it clears the inside of the cowl.
Visually, it seems a Vetterman 2 into 2 system might work, having a shallower profile under the engine.

So ultimately, I am wondering if there are advantages to the 4 to 1, or 4 to 2 configurations. I'm not sure if there was a really good reason for the original builder to go with a 4-1 system or if it's more of a personal/aesthetic choice.
It seems the Vetterman 4 into 2 crossover is a common system and they don't even make a 4-1, which makes me wonder if there is any reason to try to make a 4-1 system work.

What would be the smoothest and quietest configuration??

Thanks.

 

[FLY6]

Although I have an RV-8, I don't necessarily think this question is specific to that model, although the reason for asking may be specific to my installation.
I have a 4 into 1 exhaust system (not Vetterman - not exactly sure of the manufacturer, but possibly Aerospace Welding who are now owned by Hartzell), where the 4 pipes are arranged much like the 4 pips on a game dice. 2 above and 2 below in a box formation. These are connected to a 4 into 1 collector arrangement. I am finding that the lower pipes are touching the inside of the lower cowl (standard cowl, not Sam James).
I have checked there is no significant engine sag and I believe this has been the case since the original builder fitted the exhaust, although it seems to have got worse over time. This may be due to wearing of the heat protection tape on the inside of the cowl.
I cannot see a way to create space between the lower pipes and the cowling by any kind of adjustment, so it seems I am left with either building a new cowl (not something I am willing/able to do), or changing the exhaust geometry so it clears the inside of the cowl.
Visually, it seems a Vetterman 2 into 2 system might work, having a shallower profile under the engine.

So ultimately, I am wondering if there are advantages to the 4 to 1, or 4 to 2 configurations. I'm not sure if there was a really good reason for the original builder to go with a 4-1 system or if it's more of a personal/aesthetic choice.
It seems the Vetterman 4 into 2 crossover is a common system and they don't even make a 4-1, which makes me wonder if there is any reason to try to make a 4-1 system work.

What would be the smoothest and quietest configuration??

Thanks.

I can’t speak as far as the performance but a buddy has a 4 with 4 pipes, seems quite a bit louder than those with the 4 into 2 set up

 

[DanH]

As originally delivered around 2008, the Aerospace Welding 4 into 1 did indeed hang too low:



I made some measurements, and sent it back for a straightforward mod; lop off four cylinder flanges, shorten the headpipes, weld on new flanges. That raised the whole assembly and inch or two. I do not remember the exact dimension.

I've been very happy with the pipe. It's now at about 1300 hours.

The 4 into 1 may or may not make more power than other choices. Longtime readers know I'm more interested in drag reduction as a better path to good cruise speed. The key advantage in that regard is a single tailpipe is easier to work with in terms of exit velocity and good outer surface shape. This was the ultimate goal:



Early build photo. I also appreciate one very practical benefit...unlike a crossover, the 4 into 1 does not bake the throttle body.

 

[brad walton]

Mr. Vetterman did a write up here on Van’s years ago comparing 4 pipe, 4 into 1, and crossover system. Search Vetterman Exhaust Inc. here or on the web. Good info.

 

[Ralph Inkster]

A new lower cowl won’t give you any more exhaust pipe clearance. Look at DanH’s lower cowl, he has changed the shape (in his case as a speed/cooling mod) to mold the fiberglass shape around his pipes.
Many posts on VAF on folks doing custom fiberglass pieces on their planes & how they did it. This would be the cheapest fix for your situation & YOU GET TO LEARN A NEW SKILL!

 

[Carl Froehlich]

Following the recommendation I got from Clint at Vetterman, I’m running four straight pipes in my RV-8. I’m very happy with how it turned out, and how well the engine cools with the setup. The attached photo implies too many pipes in the cowl exhaust area. Not true. I never have a high CHT or oil temp problem. I’m running a stock cowl and a 8.5 compression IO-360-M1B.

Carl

 

[Low Pass]

Following the recommendation I got from Clint at Vetterman, I’m running four straight pipes in my RV-8. I’m very happy with how it turned out, and how well the engine cools with the setup. The attached photo implies too many pipes in the cowl exhaust area. Not true. I never have a high CHT or oil temp problem. I’m running a stock cowl and a 8.5 compression IO-360-M1B.

Carl

Same. 9:1. Quiet was not a driver in my selection.

 

[Sparrowhwk]

Thanks for the replies. I spoke with AWI today - it is one of their exhausts. They suggest a first course of trying to move the entire exhaust system at the ball joints. With some time and mouse milk I have managed to get a little movement and next week I will try measuring to see if that will be sufficient. The next step would be the same as Dan above and send it in for shortening at the flange. This will certainly lift everything up a bit, but there will be some undesired consequences with clearance below the engine for a few things, so not a simple cut and re-install.

 

[mgroleau2125]

I think everyone one should post a flyby clip of there exhaust noise. That’s what I’m more interested in.

 

[DanH]

A new lower cowl won’t give you any more exhaust pipe clearance.

With slightly shortened headpipes it will easily fit the stock cowl.

Look at DanH’s lower cowl, he has changed the shape (in his case as a speed/cooling mod) to mold the fiberglass shape around his pipes.

..to make the cowl outlet smaller, as well as removing some frontal area. A standard RV-8 outlet is huge. No one should have a cooling issue with the stock outlet

 

[penguin]

Bumping the cowl is not difficult, find some one locally with composite skills.