If I use a 390 crank in a 360 engine, without using 390 rods and pistons, what would the compression be? Is this wise anyway?

The lowest octane fuel here is 95, so a higher compression wouldn't be a problem?

Paul

 

I'm afraid in this case that even jet fuel wont help. If you use 360 rods and pistons with a 390 crank, the pistons will definetely protrude past the deck. I guess if you get creative with a dremel tool and make your own "pop-ups" it might be possible, but I doubt it. You wont even be able to bolt on a head. They may even come up so far that the top ring may pop out of the bore.

 

How about a 390 crank and rods, but with 360 pistons?

My pistons have the big football-shaped dish in the middle of them, if that helps.

 

Okay, thats a clear answer to my question. But here's my new question:

If i'm going to upgrade my 360 to a 390, and want some increase in compression-ratio (is 9.5 good with 95 octane?).

Which setup would you guys recommend?

Paul

 

If you use the federal mogul H304p you should end up close to 9.5:1 depending on your heads, gasket thickness, ect.. 9.5:1 usually requires 91 octane or better depending on elevation and timing.

 

My 360 was assembled by mistake with a 390 crank. Upon starting It made a slight tapping noise. I checked the compression and it was about 300 psi. The engine builder took it back and made it into a proper 390.

 

Does anybody have any commentary on my idea of using a 390 crank and rods, but using my 360 pistons? The pistons have the football-shaped dish in them; they're aftermarket from a previous rebuild.

 

I would say that the only way you could use 360 pistons in a 390 would be to make your 360 pistons into 390 pistons by cutting them down to the 390 height, wich is expensive. If you are doing a rebuild the pistons are not an expensive item, and there are a lot of different 390 pistons to chose from to come up with your desired compression.

 

You could use 360 rods (long rods) if you used the 390 crank and 390 pistons. You may already have 390 pistons, as some 360's came factory with 390 slipperier skirt pistons. If you use 360 rods, 390 crank, and 390 pistons, you will have .054" deck clearance. This will give better compression. Dont plan on hot rodding it too much though, as the 360 rods aren't quite as strong as the 390 rods. Have fun!!

Edit: Dont forget to balance it.

 

I would never suggest to use 360 rods. Like was said the 360 rods are a weak point, and is not a good idea to have the rods as a weak point and this would also create a problem with the internall ballancing of the engine. Besides what are you going to save not money. And why take the weakest point of the 360 and try to make a <a href="http://motorhaven.autoanything.com/">390 </a>with them?

 

I just did this to my engine.

I had a rebuilt 360 and a spare new <a href="http://motorhaven.autoanything.com/">390 </a>crank w/bearings. I took off the heads and found the pistons about .1 down the hole, so I installed the crank and the pistons where about 15~20 thou out the hole.

I trimmed the pistons down to even with the block and used the rest (360 rods).

I found out that some of the early <a href="http://motorhaven.autoanything.com/">390 </a>actually used 360 rods. It really depends on what pistons you have and how you want to use the truck.

For low RPM use, and zero decked pistons, I'd say you'll be fine. For Higher RPM use, I'd opt for the <a href="http://motorhaven.autoanything.com/">390 </a>rods with new ARP bolts.

Again, it really depends on the usage.

 

karljay wrote

I trimmed the pistons down to even with the block and used the rest (360 rods).

How did you trim the pistons down?

 

I should point out that the numbers used for octane ratings are different in Europe than in the US. There are Research Octane Numbers (RON) and Motor Octane Number (MON) and then AKI for (I think) Anti Knock Index.

We (in the US) use the average of RON and MON i.e. (RON+MON) / 2 and simply refer to is as "octane". They are usually about 10 points apart.

I believe that AKI and RON are equivalent and so 95 AKI is approximately equal to US 89 octane, and 98 (Super) is our premium 92-93. Can anyone else confirm this?

Anyway the other posters are right on target in that your deck height would be excessive...

-Charles

 

Well, I didn't know about the differences in octane ratings.

Yes the 95 I did mention earlier are RON ratings. Does anybody know if this will work with 1 : 9.5 compression ratio?

I actually don't need a rebuild for my 360, but as I'm going to restore my effie right after the summer I considered the coversion to a 390. If the costs are acceptable, I wouldn't mind the extra torque and HP

 

When I tore down my 1969 360 it had <a href="http://motorhaven.autoanything.com/">390 </a>4v flattop pistons in it. I suppose this was to increase the compression ratio on the 360. They were factory pistons and the block miked at stock bore so I know that someone did not add these later. Just a suggestion if you are set on using the 360 crank and rods.