I think we have two distinct scenarios of no US involvement:
1) the US gives no lend lease, no loans, no aid to anyone.
2) the support the UK and USSR with lend lease, loans, and other assistance
I think the latter ends in a German defeat assuming nothing wild happens, but takes 2-3 years longer (Operation Torch, Overlord, and Invasion of Italy much slower, and much less bombing occured). Its possible then that the USSR would be completely crippled after WW2 after having taken millions more military and civilian losses because of the drawn out war and having to expend much more resources as well.
The first scenario though goes much differently. Namely without lend lease the USSR is massively deficient on trucks, boots, and trains (also alot less tanks in 43 and 44) forcing less production into tanks, artillery, and planes. Additionally the civilian population is now in a famine by 43 after the Ukraine and West Russia fell. I think given those deficiencies Germany wins 1944, or 1945 because the USSR cannot do the following:
A) feed its civilians
B) supply its army at the front as well
C) conduct offensive operations as well since it has a critical shortage of trucks and trains
Germany also has more resources since the UK alone cannot win North Africa as fast, nor take out Italy as fast, nor invade France as fast. Id estimate 6-24 month delay on those. So torch by mid 1943, invasion of italy by mid to late 1944, and invasion of france by late 1945 or early 1946.
E) Less bombing raids leaching production. I estimate roughly 1/3 of what happened irl.
The delayed schedule arguably makes overlord impossible since the beaches would be even more heavily defended, and the reduction in bombing allows more ammunition and better equipment for the defenders. Furthermore, the allies have only 2/5 of the manpower to draw on and lack air supremacy, though perhaps still have the advantage in the air.
Nikita Krushev also suggests that Stalin privatly said they would have lost the war without US aid.
Lend lease numbers:
~420,000 less trucks and jeeps, ~2,000 less train engines, ~11,000 less train cars, ~2,000,000 less boots, ~12000 less armoured vehicles(including ~7000 tanks), ~11,000 less planes, ~58% of aviation fuel, ~2,500,000 tons less petroleum, ~4,500,000 less tons of food, ~33% of ammunition
Can we let the meme die that the USSR could win ww2 without the allies? They would have been crippled with the USA never joined the war or amended the Neutrality Act.