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Tue Jul 29, 2025 1:53 pm

Hey everyone,

I'm planning my first build, primarily wanting a better gas economy engine, power is secondary which is why I'm in this quandary.

My vehicle currently has a 1600DP and 3.88 geared freeway flyer, but she has the infamous H5 crankcase. I'm wanting to go with a 1745 (76x85.5 build based off the HotVW mileage motor) then a 1956 (76x90.5) later. My concern with the 1745 is the continuous RPM/Load required to maintain highway speed (75+) even with a freeway flyer, hence the 1956.

Also: is it possible to reuse all stock items from a 1600 into a 1745?(aside from the crankshaft, push rods, shims, bearings etc)

The initial plan is to build the 1745 in a better case (don't want to clearance an H5 due to the inferior reputation of the metal). Then next rebuild (when I can save for the heads). bore out the case for 1956 sense it's already cleared for the 76mm crank.

Should I stay with the 1745 plan or wait and build a 1904 CB gas saver style. I'm concerned with a continuous high RPM that might be required with the 1745.

Or am I just being a muppet and overthinking this save the money and send it with 1956 unless the 1904 is a better idea. Not sure if the extra 50cc will be noticeable.

If it matters here's some extra details for the builds (aside from the different piston/crank combo)
-It will be for a VW Trike so vehicle weight might be a factor.
-Street use only
-Refurbished Case
-Engle Gas Saver Cam
-Stock rockers
-Stock Flywheel
-reuse whatever I can to keep costs down.

From where I'm at I see that the most cost effective upgrade is the 1745 due to just needing a new Crankshaft and reuse what I can, especially if it will be a suitable engine for today's speeds.

Thank you for your time, and look forward to the insight.

Tue Jul 29, 2025 2:08 pm

Lets talk $$$ here. Not including heads the price difference on the 2 motors you're talking about is minimal. You're already buying pistons/cylinders for the small motor, so why not buy the 90.5 because they're very similar in price. You're already going to have the case at the machine shop to inspect & line bore, having it opened up for the 90.5 will only cost $100(maybe a little more depending on your machinist).

You definitely don't want to re-use bearings. You won't know about pushrods until you start measuring the valve train geometry.

Tue Jul 29, 2025 3:22 pm

Is there any significant difference between the 1904 and 1956? Don't know if it's more marketing on their gas saver with 1904 vs 1956 due to the extra bit of crank for fuel burn.

Also wondering if it's better to buy a 40mm intake head or bore out a stock 35mm intake. I'm thinking of the same thing Hot VW thing did with their mileage motor with the 85.5 piston and undersized 32mm intake. Just scaled to 90.5piston

Tue Jul 29, 2025 4:00 pm

There is not a magical displacement that will get you better MPG. The biggest factors are how you drive it, the tuning, and your attention to details when picking the combo.

Those old Hot VW's articles were more influenced by the advertisers selling parts than the actual MPG research.

If it were me, I would just build a stock crank 1776cc with a CB2280 cam, .040" deck, 8.8:1, kadrons, and some really good, well ported 35x32 heads. It will be a little boring, but it will run and drive fantastic. If you want a bit more fun power, use a cb2232 cam.

The best money you will spend is on a wideband O2 sensor, and learning how to use it. Spending the money on big crank will take forever to recoup in the very minimal gain in MPG.

Tune it right, and keep your foot out of it, and it will get great MPG.

Brian

Tue Jul 29, 2025 8:46 pm

This is a subject that interests me a lot! I will start by saying that I have not built an engine specifically for fuel mileage and to this point have not found anyone on this site who has either. This is the "performance" section and most of the advice here comes from those that build for maximum possible HP in the higher RPMs. There has however been some suggestions that were useful.

The engine I built started as a mild street performer and I tried to morph it into a mileage engine. I have had some limited success but it has fallen short of expectations for sure. My approach I now feel was wrong. I should have done like you, build a mileage engine and added some performance to it later if it needed it after it has proved itself.

If I were to start over (and I just may do that) I would do as you are, keep the bore small and add stroke if I wanted more displacement. I have come to believe that a small bore is the key to best flame travel and the most complete combustion. I cannot think of any engine I have had, worked on or studied the specifications on that had a large bore and got outstanding fuel economy. This may be because large bore engines normally have a large overall displacement but I think there is more to it than that! One of the clues is that it seems to be a known fact that larger bores benefit from dual plugs and small bores do not seem to.

Smaller bores also seem to tolerate more spark advance and lower grade fuels so tuning lean and adding spark advance seems less risky.

Looking at what the car manufacturers are doing today is an eye opener too. They are going to ever smaller displacements, micro managing spark and fuel to the N'th degree and adding a hair dryer to get the power back when needed. They are producing engines now with fuel economy and power levels that could only be dreamed of just a few years ago. These guys obviously have figured out where the bear poops in the buckbrush!

Yes it all depends on how you drive it but that is pretty common knowelage already and goes without saying for most who have reach adulthood with of course some exceptions...

As for dual carbs getting better mileage... better than what? A badly tuned, worn out, gummed up, 50 year old single center mounted carb with a plugged heat riser and a mismatched distributor? Yes! You bet! This is exactly the base condition that 99% of air cooled engines are found in today and yes a good set of well tuned dual carbs makes all the difference in the world. In my experience, limited as it is to Kadrons and Webers I have yet to match the fuel economy that my bone stock '71 Super Beetle got back when it was in good nick. Close but not there yet.

In my view the ultimate in fuel economy is achieved with fuel injection. Just ask the present crop of automotive engineers.

Sorry about the long rant but I am keenly interested and will be following your progress!

Wed Jul 30, 2025 1:56 am

I don´t think the OP intends to go ballistic on the fuel efficiency matter. just "fair"
That said.:
3,88 freeway flyer(?) do you have a 0,89 or even 0,82 4rth?
Mini strokers are NOT easy to build engines. "Nothing" fits as is.
Stepping stone engines are THE WORST, because somewhere you will either spend your money twice, or you will have to cut a heel and chop a toe to make it fit. Set your goal on a size, and build that.
Driveability, fuel efficiency etc. depands on the entire package, and it is not just pick the best deals at jegs.
A trike will never be really good on fuel efficiency because like f.i. a buggy, it picks up a lot of air. But you can make it fair.

WRT chasing fuel efficiency. There are so many things which can be done, but about none of it is main stream, and 30% of it, at least, is misunderstood. The rest of it drowns by the fact that 99,9% think that it is either stock or hot rod. Building a fuel efficient engine requires a specific way of building, then the power will be as it comes. But, if you actually use and implement todays knowldge you can come a long way on both accounts.

Wed Jul 30, 2025 4:49 am

Alstrup, at one point in my quest for 35 mpg you said that a 1911 is the wrong choice for a mileage engine. Could you explain why please?

Also if mileage over power were your goal (end result being drivable on today's roads) and you were starting with a clean slate, what parts would you choose for a build?

I have been giving it thought and at this point a 1300 or 1500 is what I would start with and add a 74mm crank. Not sure about heads or cam choice. Possibly stay with late single port heads.

Wed Jul 30, 2025 5:29 am

I think all ya'll made good points , and I dont build anything with fuel mileage in mind so I got little to add to this with the exception of 2 things.

1 a smaller engine is typically easier on the gas tank , but a bigger engine doesnt have to work as hard either so your foot isnt as far into it and you dont need to down shift as much to keep rpms up. but as I always say and was previously mentioned , tune. none of how an engine is built makes a shit of difference of you can't / dont tune it right.

2 I wouldn't be afraid of the H case, ive built quite a few of those into 1915's.

Wed Jul 30, 2025 6:25 am

oprn wrote: I have come to believe that a small bore is the key to best flame travel and the most complete combustion.

Exactly the reason Chevy built the 305 engine which had the same stroke as the 350 and they were fairly successful.

Wed Jul 30, 2025 6:59 am

oprn wrote: Alstrup, at one point in my quest for 35 mpg you said that a 1911 is the wrong choice for a mileage engine. Could you explain why please?

Also if mileage over power were your goal (end result being drivable on today's roads) and you were starting with a clean slate, what parts would you choose for a build?

I have been giving it thought and at this point a 1300 or 1500 is what I would start with and add a 74mm crank. Not sure about heads or cam choice. Possibly stay with late single port heads.
Thats a topic for its own thread. Lets try and stay reasonable in relation to the OP´s questions and wishes.

Wed Jul 30, 2025 9:56 am

A lot of this engine theory is beyond me.
All I can say is that I’m running a 2276 with a Web 86a, CB Mini Wedge Ports and Dual 45 Dells mated to a 3.88 with an.82 4th.
I’m getting 28 mpg. I’m pretty happy with that. My wife’s Honda gets 32 so I figured I’m in the ballpark.

Wed Jul 30, 2025 10:12 am

I wouldnt worry about weight savings or mpg. But i would budget for some wheelie bars in a trike

Wed Jul 30, 2025 10:24 am

chrisflstf wrote: I wouldn't worry about weight savings or mpg. But i would budget for some wheelie bars in a trike

I agree.

Most every time someone goes and tries to build a MPG specific engine, they make a bunch of sacrifices, and end up with a boring engine that might get 2-3mpg more than something way more fun.

The OP needs to be realistic here. How many miles are you planning to carefully ride a trike in a year just to maximize MPG? How many years or 10,000's of miles will it take to make it worthwhile?

I have never owned one, but I would imagine people own them to ride for fun.

Build a simple 1776 with basic parts. Nothing fancy, and use a proven combo that DOES NOT involve "big valve heads" or a w110 cam. Keep it simple, and spend your time learning to tune it.

Careful tuning, a good combo, careful driving, and a MAP programmable ignition like a 123 dist., will make for really good MPG. regardless of the displacement.

Brian

Thu Jul 31, 2025 4:14 am

Alstrup wrote: Thats a topic for its own thread. Lets try and stay reasonable in relation to the OP´s questions and wishes.
Fair enough.

Thu Jul 31, 2025 4:17 am

Brian_e wrote: Most every time someone goes and tries to build a MPG specific engine, they make a bunch of sacrifices, and end up with a boring engine that might get 2-3mpg more than something way more fun. Brian
True enough but does it have to be that way or are we just inexperienced at that game?

Thu Jul 31, 2025 10:34 am

Isn’t about 80% of fuel economy found under your right foot? (Or in this case, twisting of the right wrist?)

If mpg is what you’re after, you can always get a Prius, head straight for the #1 lane, and drive 15 under the limit. Works great for some people…

Thu Jul 31, 2025 11:07 am

oprn wrote: Brian_e wrote: Most every time someone goes and tries to build a MPG specific engine, they make a bunch of sacrifices, and end up with a boring engine that might get 2-3mpg more than something way more fun. Brian
True enough but does it have to be that way or are we just inexperienced at that game?

It doesn't need to be, but from my experience, people either refuse to accept modern tech, such as a 123dist., they refuse to learn how to tune it within a gnats ass, or they just plain give up.

There is a crazy amount of learning and engine theory that needs to be done to squeak that last 3-4mpg out. Even if a guy does build an engine that will get a real 34+mpg, carefully driving, and never exceeding 55mph, how many miles is he actually going to drive the thing? I think there are very few people around the aircooled world that put on a real 800-1000miles per month year round. At 3000 miles per year.... why even bother? Bragging rights just so you can compare your car to a prius??

I drive my aircooled cars 1000+ miles per month almost year round. I could car less if it gets good MPG. They are toys, I drive them like a 16 year old, and I am not unhappy about the extra $5 per month it costs me at the pump. I just want them to put a smile on my face on the way to work each morning.

Brian

Thu Jul 31, 2025 12:13 pm

You are right. For the most part the VW crowd are a bunch of hobbyists that don't give two farts about throwing money at the pumps for the reasons you stated. There are a few of us of more limited means that have to make each dollar count but still want and enjoy these old relics.

We unfortunately are a pain to those with deep pockets because we keep asking questions that the usual crowd have no answers for aside from "It's all in how you drive it." which in reality is code for "I don't know and don't really care".

I find it interesting that no expense/effort is too much to find 2 or 3 HP but 2 or 3 mpg... not worth it.

Fair enough. We don't all have the same interests and priorities. No offence meant, just an observation.

Thu Jul 31, 2025 1:20 pm

I don't think the OP has said what MPG they are looking to realistically get?
15 to 20 years ago Hot VW's had a mileage motor series (advert for CB) where they got mid to high 30's with a 1745 in a bug.
I would guess that to get much more than that with a bug would require a newer engine design than one that was conceived 90 years ago.

If mileage is the ultimate goal, a 1L Suzuki 3 cyl from a Geo Metro in a trike should net 60+ MPG with 50-60HP and that is with 30+ yr old tech.

Personally, I don't give two farts about mileage.
But I am one of those that haven't ever since my Dad & Uncles street raced and they sold (gave) me my first car for $50 which was a retired SC police interceptor with a 427 and helped me rebuild it.

Thu Jul 31, 2025 1:45 pm

hey Oprn, not sure how Canada works as far as this goes, but from how it seems your buggy could be classified as farm use or farm equipment and the gas used in it an operating expense. in which case you'd have to be less concerned about the fuel mileage and those extra dollars counting.