I think I am qualified to try and answer this question as I have 2 GT6 cars and am building a diesel electric hybrid based kitcar from the Triumph Spitfire/GT6 chassis
I am going to try and be objective here as I am biased to diesel!
First it depends on your goals and objectives. If its gonna be a one off build only for you and if it is to simply get better fuel economy than you can not ignore a small displacement diesel such as what you may come to find in the VW 1.2L Lupo or 1.4L Polo another option is the 1.3L CDTI found in the Opel Speedster cars and the famous Eco Speedster! The Smart car diesel will not work as I have all ready spent the time exploring that option so no need to waste your time with that!
If you live in the states it will be tough to source these engines but I do have a friend who imports these engines and can help but I will say right now they are not cheap they are new crate engines from VW. So expect to pay north of 4 grand for them.
If you went the used diesel engine route you could get older VW 1.6L TD engines super cheap under 1 grand and they will give you 60mpg or better all day in a Spitfire with stock transmission. If you wanted to upgrade the transmission to something more modern than I highly recommend a T5 transmission. They can be found cheap and have literally tons of custom options available to the DIY builder. This would net you even better MPG and performance if modded correctly out of any engine you decide to go with!
Now for the objective part I guess that means gas options LOL
Some have mentioned all ready Geo and Honda. The Geo is light and cheap. The Honda is lighter and well its Honda you get reliability for an increased cost over the similar displacement Geo Metro engine. If it was me I might try and hold out for the Ford 1.0L Ecoboost engine as it is a gassers dream engine in my opinion! You can mod that little thing to over 200hp and still get more than 50mpg!
Here is a link to an article that talks about the engine being offered in the Fiesta for the US market =
Ford Details 1.0L EcoBoost 3-Cyl Engine for U.S.-Spec 2014 Fiesta | Car and Driver Blog
Here is a must see video of it in action
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GH
