[Saville]

Hi all,

I'm in the market for a -4 or a -8. There are a lot of variables in my mind and I'm trying to sort them out/eliminate them by getting answers to questions. This particular question is a big one:

There is a "Build or buy?" thread in the -4 forum. The consensus seems to be that if you are going to buy, find a good airframe and then customize it the way you want.


If I buy an a/c with an O-320 FP, and I want to upgrade to an O-360 C/S, the questions are:

1) Is there any difference in the amount of work and/or expense doing this in a -4 vs a -8?

2) In either case, would I need a new engine mount? New cowling? New anything else?

3) Aside from routing the prop control to the cockpit and modifying the throttle quadrant, is this sort of engine upgrade a difficult proposition if you have not built an airplane?

Thanks for any insight.

 

[southtarnation]

That change on a -4 would require:
New gear legs matched to new motor mount
New cowl and obviously prop and spinner
The governor would need a recess cut into the firewall.
Relocation of the battery and any other tricks to move the c of g back away from the nose.

 

[Low Pass]

That change on a -4 would require:
New gear legs matched to new motor mount
New cowl and obviously prop and spinner
The governor would need a recess cut into the firewall.
Relocation of the battery and any other tricks to move the c of g back away from the nose.

You sure the mount would change?? I just changed my -8 from -320 to -360 (parallel, Dynafocal to Dynafocal). Only substantial change was the baffling. I did keep the same CS prop. Just put the cowl back on and go.

The dimensional change was the width of the engine. This would likely be a slightly greater concern with a -4 (stuffing everything into the smaller cowling space).

Whether or not one has built a kit plane isn't really the measure, IMO. Whether or not you're familiar with maintaining/installing/modifying things forward of the firewall in a small plane is. And a swap as you describe puts you back in the complexity of original firewall forward kit construction.

 

[Saville]

Low Pass,

From what you write, you make it sound as if it's easier in a -8 than a -4.

And yes there is also the question as to what an O 360 really buys you in a -4. According to Van's performance charts, we're talking a little over 7 mph difference.

Thanks!

 

[Guy Prevost]

In addition to the comments above:

There is very little room between the engine and the firewall on an RV-4.

There is a lot of room between the engine and the firewall on an RV-8. The 8 doesn't need a recess for the governor--In fact there is probably 6-8" between my governor and the firewall.

Best,
Guy

 

[Mel]

The -4 also used a different cowling for F/P vs C/S.
F/P cowlings were designed for a 4" extension. Later the cowlings were changed to use a shorter extension so the same cowling could be used for both.

 

[rbibb]

The -4 also used a different cowling for F/P vs C/S.
F/P cowlings were designed for a 4" extension. Later the cowlings were changed to use a shorter extension so the same cowling could be used for both.

Not sure when this change was made but I expect the majority of kits for sale out there have the latest cowl that uses the 2.25" extension for the FP prop.

I expect the change would be easier on an -8 given it has more room but there are a ton of -4s fling with an O-360 up front so it ain't rocket science to cam one in there.

 

[southtarnation]

You sure the mount would change?? I just changed my -8 from -320 to -360 (parallel, Dynafocal to Dynafocal). Only substantial change was the baffling. I did keep the same CS prop. Just put the cowl back on and go.

The dimensional change was the width of the engine. This would likely be a slightly greater concern with a -4 (stuffing everything into the smaller cowling space).

Whether or not one has built a kit plane isn't really the measure, IMO. Whether or not you're familiar with maintaining/installing/modifying things forward of the firewall in a small plane is. And a swap as you describe puts you back in the complexity of original firewall forward kit construction.

Yes you are right, I'm not thinking clearly. Motor mount and therefor legs could be reused, sorry bout that.