Some 8 months ago we did a kitchen remodel to include replacement of all the appliances. My Sears ceramic kitchen range is fed from the breaker box with aluminum wiring. In-order for me to make the new range fit I had to splice a short copper pigtail onto the aluminum wire and then lower the outlet to allow the range to slide back into place. I utilized split bolt connectors at this junction and believe all connections are tight, etc. It seems that the range takes quite a long time to either boil water or even to pre-heat the oven to a given temperature. I am wondering if I have a problem with the range itself, or if I may have a wiring problem. Everything works fine but it is just slow to heat anything. I do not want to have to re-wire this appliance unless absolutely necessary as attic accessibility to minimal. If I did have to re-wire with copper wire what size would be needed on a 40 amp circuit? Any advice or suggestions would be appreciated.

OK I work for the company that makes these in a call center and we answer theses questions all day I rely don't think its your wiring it's the stove it self we have had a lot of this if I was you and it is under a year old I would call and have them come out to fix this under the warranty if you don't have the 1 year on it still then I would call and get a extended warranty on this just don't tell them that it is not working well or it will not be covered for a existing problem so if it is over a year or out out warranty then I would buy a extended warranty wich you can do at anytime and then wate a few weeks give them a call and have them come out and fix this its either your control panel or the wiring in your control panel and they can easily fix this but if you don't have a warranty on this it can get costly like 500.00 for a control panel alone and then have to pay to have someone put it in hope this helps

Others may say different, but I say run a new 4 wire (#8-3 *cu with grounding conductor) circuit. I prefer copper for all my cooking appliances, yet you can go with a # 6-3 Alum with ground---if you want to save money.
*cu=copper

To add:
Your splice is in violation
also

I'm not going to get into splicing alum to alum, or copper to alum. As I said in the begin, I would run a new circuit <copper>.

Wait for others to comment on their preference.

Get rid of the aluminum altogether and go with a new copper circuit, I don't like aliminum wiring myself, and I would never try to splice aluminum to copper. From what I have seen over the years, any loose connection in the aluminum wiring will cause problems for you and may even start a fire. Don't chance it.