: 275hp to 300hp head swap question

Aurora By Olds
11-25-03, 11:59 AM
I have an Aurora 4.0. I'm going to be doing the headgaskets in about a week, and I have access to a set of 4.6 vin 9 heads that have half the mileage. With the higher lift in the cams, I will be able to get a little more power, but I was wondering how easily and readily the PCM will be to adjust the fuel curves and whatnot to compensate for the different lift?
I was going to do a 4.6 swap, however, I have decided on the heads, if they will work. I will also be doing mine without a time-sert kit, but only if the block threads are still good.

11-26-03, 09:02 AM
You should compare combustion chamber size. I don't know if this is something you could find easily ( not like a SB Chevy ) but you wouldn't want a drop in compression.

12-05-03, 04:03 PM
any new info on this? just curious

and it was the piston size that changed, not the travel, so the combustion chamber would be bigger, or are you only talking about swapping cams?

the setup of the cams in the 4.0 is exactly between the 275hp and 300hp motor i thought. IF it was a 4.6, it would be the best setup, having more tq than the 300hp, more HP than the 275, a good mix. (but less hp than the 300, less tq than the 275 too, but probably the best mix)

12-05-03, 10:49 PM
The combustion chamber size might be the same or could be reconfigured to accomodate the smaller size . I don't know but they need to be compared before they are installed.
The diameter of the combustion chamber and the capacity (CC's) need to be compared.

Aurora By Olds
12-05-03, 11:53 PM
From what information I have found, the bore is larger in the 4.6, obviously, so the combustion chamber must be larger in the 4.6 heads. The only facts I have found to back this up, is that the compression between the 4.6's and the 4.0 is identical, therefore, the combustion chamber must be larger in the 4.6 to keep the compression the same. I will be swapping out the cams into the 4.0 if/when it goes back together.
I have 4+ stripped head bolts, so I am contemplating finding a good used vin "9" 4.6 to swap into my Aurora.
If I do go through with the 4.6 swap, I will also be doing true dual 2" stainless exhaust out the rear, as well as adding my cone filter to the engine. Out of this, I guesstimate about 320 - 330hp, which would be pretty cool. I also, according to many deadend GMForums discussions, will have the only Aurora with a 4.6, more than 300hp, and true dual exhaust. Will it all happen? Who knows, but it would be a really fun project, especially once it is all together.
The last and final question I have, is between the 4.6 vin "Y" and vin "9" engines.
I was given specs off of Mitchell for both, and the HP/TQ ratings are almost identical.
I always understood that the vin "Y" had 275hp, whereas the vin "9" had 300hp. Can someone clarify this for sure?
Thank you all for your input so far as well.
P.S. if someone could find me the CC displacement for a 4.6, I have the 4.0 specs from my GM G-Body shop manual. Thanks.

Aurora By Olds
12-06-03, 03:03 AM
Ok, my question has been answered. I no longer need the displacement specs for the combustion chambers. 4.6 heads WILL NOT work on a 4.0. The overall design of the combustion chamber is larger. The 4.6 valves are placed at the edge of the combustion chamber, so they would hit the edge of the 4.0's cylinder walls. The overall diagonal measurement of the combustion chamber is about 1/8" different. So much so, that if the 4.0 gasket it placed on the 4.6 heads, there is no way that the valves would clear it.
So THEREFORE, I will be swapping the camshafts from the 4.6 heads into my current 4.0 heads, if the camshaft lobes are the same width apart. Also, it depends if I continue to use the 4.0, rather than try to find a decently priced 4.6.

12-07-03, 01:59 AM
just keep us updated on if you decide to try a 4.6 swap, i thought i read somewhere that it would end up sitting higher in the cradle than the 4.0, because of the way it mounts or somthing??? can't remember, nothing a custom cowl hood couldn't fix.

you'd be the first, but if it works, take pics! do a write up!

Aurora By Olds
12-07-03, 02:05 AM
I will most definitly take pics and do a write up.
I finished undercoating the entire car and engine bay today. Still continuing the search for a decent vin "9" at a decent price.
I will keep you guys updated though.