Real Fiero Tech

This is a cut and paste post from an Aurora board I went to looking for info on the 4.0


What is the actual compression ratio on the Aurora?
I know on the 4.6 it is 10.3-1 and it has a 53cc combustion chamber. Does any one know the cc of the Aurora head?

The 4.6 head also flows better than the 4.0 head.
Is this due to a difference in the head or do they have differing valve sizes? I am having a problem finding engine specs for the 4.0. I have them for the 4.6.

Everything I find seems to indicate the 4.6 heads will work on the 4.0
I will have one of each this week so I can get an actual comparison.

The end result I am looking for is better flow along with a reduction of the compression ratio.

I am thinking to have the same compression ratio, it must have smaller chambers unless it has domed pistons.
After all, you are pushing 13% less displacement and if the piston dishes are the same and they changed the CR through the combustion chamber size, you should end up with a CR of 9.2-1 with the 4.6 heads.
This would take a combustion chamber size of 46cc to maintain the CR at 10.3-1 in a 4.0.

I guess it will be more clear once I get both heads here side by side on Tuesday.

As far as the piston top dimensions, I have a 4.6 here with the heads off, does anyone have any pictures of a 4.0 with the heads off and the piston all the way up?

Gene


After getting the 4.0 and 4.6 heads side by side today the differences are obvious.

The intake valves are 8% larger but the exhaust are the same.

The intake and exhaust ports are the same size.

The chambers are 45cc as opposed to 53 cc for the 4.6.

This does work out to a ~9.2-1 cr for the Aurora with the Northstar heads.

The 4.6 heads must be off a 275hp engine as the cams have the same dimensions as the 4.0.

I have a set of the 300hp cams but need to research into which ones will work better with the supercharger.

Gene



Here is a visual comparison of the two heads. If you look close, you can see the difference in the intake valve size. The Aurora head is on top in eaxh photo.




In these photos, the mic shows the difference in chamber size. The depth is the same.



Gene[/img]

Nice info and informative pics. Got a link to the original thread?

I was aware that the Aurora heads had smaller chambers. I also knew that they flowed less. I had been told that was because the smaller chamber shrouded the valve more, but if they have smaller valves, that explains the flow difference better.

Someone was selling a pair of Aurora heads on ebay once touting that they'd raise a 4.6 compression ratio to 11:1. Too bad they cut flow so much that you'd probably lose power.

I wonder if the 4.0 head could be fitted with the 4.6 intake valves or if that would be more trouble than it would be worth.

I'd like to be able to hit 12:1 compression without having to use domed pistons.

Here is the link, but it will be of no help. It seems to be the land of CAIs and big exhaust. Not much else in performance info.

I do not think the 4.6 valves will fit as the valves on the 4.0 are near the chamber wall. Maybe with a little clearancing they would fit. They are mounted on the same centers.

I will have to fire up the CR Calculator, but I bet the compression would be more like 11.5-1

http://aurorah.proboards47.com/index.cg ... 597&page=1

Gene

Shelby sold the Aurora 4.0s with superchargers in the Shelby Series-1. I think they were rated at like 400hp or so. Maybe you could find some info on those for reference or something.