Oversquare Motors Build, (Shorter Stroke, Bigger Piston Build) Options

[tjtryon]

Good Morning!

I'm looking at a rebuild of my 1973 2.0 Motor. When I raced motorcycles, I was constantly rebuilding my motors, chasing the speeds I want, and putting the power where I wanted it in my motor RPM. One thing I (and most that I raced with) did was rebuilding with an over-square motor build. The 2.0 914 Type 4 motor has a stock stroke & bore of 94mm (bore-cylinder width) and 71mm (stroke - piston travel). By moving to a shorter stroke, such as 68mm, the piston would move less in the cylinders, which would allow me to run a bigger piston with less stress on the piston (rings move less causing less friction and less heat) allowing me to run a bigger piston, and would move max power higher up in the RPM range, and allow for a higher max RPM. Less movement for the piston would cause less movement on the rods and crank as well.


My thoughts are to run 103mm pistons, and a crank of 68mm, allowing for a displacement of a 2.3l (2266 cc). I've been looking into a Falicon forged crank, Carrillo Titanium Rods and JE forged pistons. I think this could allow for a much stronger engine than any other I would have.

Does anyone here have any experience with a short stroke, big bore motor? How about those engine builders reading through this? Any thoughts or experiences?

[Superhawk996]

Heat rejection and deformation of cylinders is a big problem with cylinders larger than about 96mm.

Look into Nickies for any big bore projects.

[VaccaRabite]

You can do it. You can't do it cheaply and expect it to live.

There is a reason most of us stick to 96mm pistons and then stroke for more displacement. I've built the "goes fast, runs hot" motor. Its not terribly fun.

That said, it will probably rip. Make around 150 hp. Good AX engine. Not a good cruising engine unless you put effort into defeating heat. Good cylinders. Internal coatings. Reasonable compression.

Zach

[tjtryon]

Heat rejection and deformation of cylinders is a big problem with cylinders larger than about 96mm.

Look into Nickies for any big bore projects.

Short stroke motors have less heat issues, less piston travel = much less heat, irrespective of the piston size.

[tjtryon]

You can do it. You can't do it cheaply and expcet it to live.

There is a reason most of us stick to 96mm pistons and then stroke for more displacement. I've built the "goes fast, runs hot" motor. Its not terribly fun.

That said, it will probably rip. Make around 150 hp. Good AX engine. Not a good cruising engine unless you put effort into defeating heat. Good cylinders. Internal coatings. Reasonable compression.

Zach

If the motor is a short stroke motor, it will not create as much heat as a higher stroked motor. Heat should be a much less idea. I agree, somewhat more expensive, but the engine will be much stronger as a result, and, with the less heat, and the shorter stroke, the motor will experience much less stress.

[930cabman]

What is your goal, a street cruiser? weekend track beast? ....

Do your research here and around. I think you find 96 is a good maximum for $ and performance

"speed costs money, how fast do you want to go" not my words, but very true

[mepstein]

Heads, cams, induction and exhaust all come into play. Displacement and piston size is just part of the consideration.

[914werke]

what you are leaving on the table is torque. A longer rod will make more torque with the same piston force, & since it's less angular than a shorter rod, will reduces sidewall loading and decrease component friction. All of this adds up to more power, the reason most popular T4 eng. build combos are strokers. Though if engineering a motor from scratch yes all the concepts you note would aid in the "combo"

[tjtryon]

Heads, cams, induction and exhaust all come into play. Displacement and piston size is just part of the consideration.

Yep, I've already been planning for the entire build.

68mm x 103 mm
Falicon Forged Crank ($1500)
Carillo Titanium Rods - length based on stroke ($1500)
Web Camshafts - https://aapistons.com/products/type-4-porsc...s-1-base-circle - $800
JE Forged Pistons & Liners - $ 2,163.90 - https://aapistons.com/collections/porsche-p...ds-to-be-edited
Type 4 Stage 3 DTM Coling Kit (LN Engineering - https://lnengineering.com/type-4-store/dtm-...3-dtm-kit.html) - $1,850.75
AMC Heads 103mm - https://aapistons.com/products/vw-type-4-po...eads-44x36-pair - $ 2,163.90

[tjtryon]

What is your goal, a street cruiser? weekend track beast? ....

Do your research here and around. I think you find 96 is a good maximum for $ and performance

"speed costs money, how fast do you want to go" not my words, but very true

Yep, speed always costs money - this car is already a great track day car stock. I'm hopeful that, with the built motor, it will be competitive against the /6 cars at SVRA races. I'm currently in the process of selling my Maserati, my all-original Triumph Spitfire (I bought it 2 years ago with 4250 miles - parked since 1986 - it was a true barn find), and my VW Passat W8 Variant - 600 horsepower big turbo drag (bracket) car and am looking to sell either my VW GTI track/street/daily car or my VW Cabrio street car (GTI Motor Swap).

I'm looking to spend about $6k-$7k on this build for my 1973 and will be building it over the next 4-5 months. I will also be having someone rebuild my 914/6 motor for my 1972 - though mostly stock. I'm 50, retired, have the money, and always wanted to rebuild both of the motors. The 72 is great for our local Cars & Coffee and our Saturday Evening on the Square. The 73 will be my track day car or the car I take out to our "weekend tour rides" with our local PCA group which are a bit more "spirited" driving.

[tjtryon]

what you are leaving on the table is torque. A longer rod will make more torque with the same piston force, & since it's less angular than a shorter rod, will reduces sidewall loading and decrease component friction. All of this adds up to more power, the reason most popular T4 eng. build combos are strokers. Though if engineering a motor from scratch yes all the concepts you note would aid in the "combo"

Yes, the Carillo rods will be based on the stroke length. With a 68mm stroke, I could lengthen the rods by about 2mm to increase the compression ratio to about 9:1. Since the stroke is close to the original length (-3mm), I could still run the stock length which lowers the compression ratio only marginally. The 103mm pistons have valve pockets cut, which is more than enough to add +2mm to the rod length. I think I could go as high as +4mm to the rod length, but the compression ratio would be about 11:1, which would cause things to run MUCH hotter. I don't want that much extra heat, as that goes against my plans to remove as much extra heat as possible.

[Superhawk996]

If the motor is a short stroke motor, it will not create as much heat as a higher stroked motor.

Not quite that simplistic. Internal friction is only a small percentage of heat generation in an engine.

The fact is you're increasing displacement (i.e. burning more fuel) and you're doing so in cylinders that are thinner and more prone to thermal distortion.

If we are going to be simplistic; Horsepower = Heat

You wont be the first to build a motor with 103's, and of course they can work (for a while) all depending on your expectations of "durability".

Wishing you all the best with your build.

[BillJ]

Have gone the 103 route and built by a shop that knows how to build them. It was a short lived experiment

[brant]

I just think Nickies are a requirement at that bore.
Or nearly anything over 96

[914werke]

(IMG:style_emoticons/default/agree.gif)
+ other efforts consistent with heat management & combustion efficiency.
I as well have a large bore dual plug project languishing in crates awaiting the time to
build & experiment. (IMG:style_emoticons/default/rolleyes.gif)

[burton73]

Mark and I just rebuilt a 2270 built by Jake Raby in 2005. HAM RAT heads, Keith Black Pistons, CR rods special cam and well on and on. $3,000 plus in machine work and new parts from George at European Motorworks. 78 Stroke, 96 MM Pistons

Best Bob B


Old posting from March 2009


Jake Raby
Mar 11 2009, 03:24 PM
It's been so long since I started a new post here that I almost forgot how to do it... I have refrained from "Chest beating" on the 914 forums for a good while, but I figured the results we experienced with this engine was worth the BS that the post would probably create, so I decided to post it and take my chances.

I cross posted this from my forums.aircooledtechnology.com site
------------------

We built a 2270 for one of our race customers 7 years ago. He wanted huge torque and not a ton of revs, so I created him a combo based from these desires.

The engine was, at the time one of our higher HP models of the 2270, running around 10.5:1 CR, Web split duration cam based on the 119/163 base grinds and 2.0 heads with 48X38 valves coupled to 48 IDF carbs.

The engine ran for 7 race seasons flawlessly with only routine maintenance, it still had the same carb jetting that it left our dyno with! I don't think it was adjusted much at all as it looked the same as when it left!

So the graphs below tell the story.. 64HP came from the same exact engine with only a head, CR and camshaft change made!! Look at the torque, the 09 design made more at the lowest test RPM than the 02 model did at peak!!!

This illustrates how performance really evolves and I don't think many people realize the difference even one year can make here!

The 09 design has throttle response like a rocket and the power is very crisp to say the least- the owner is already buying new tires!

I won't go into detail here about what was done since this is a competition engine, but I will say the engine still runs on the same fuel that it did before, has a RAT 9700 cam (non roller) and a set of cylinder heads based off the LE 200 port profile that are not yet released to the public. (they still flow 200 CFM @ .500!)
The engine still uses the same dizzy, same set of carbs and same venturi size as in 2002, but it wants 8 degrees less total advance and has lower BSFC numbers!

There is only one place to buy our level of development and this proves what we have accomplished. I'll bet it lasts another 7 seasons.

Here ya go, the difference in yesterday and today...
The graph from 2002 was still in the customer's file.. There is a huge benefit to keeping records!



http://www.914world.com/bbs2/lofiversion/i...php?t93548.html


Mark and I just rebuilt a 2270 built by Jake Raby in 2005. HAM RAT heads, Keith Black Pistons, CR rods special cam and well on and on. $3,000 plus in machine work and new parts from George at European Motorworks. 78 Stroke, 96 MM Pistons

Best Bob B
THESE ARE JAKES DYNO NUMBERS

[MikeK]

Isn't every combo with a 914 over-square?

[technicalninja]

I am an old school engine builder as well.

I've always been envious of the bike engine builders as their parts are so much smaller/lighter than automotive engines.

The issue of time for flame front travel across the cylinder bore is usually not an issue for a small diameter bore.

My suggestion is to search on "Jake Raby" posts on all of the forums you can find that he has frequented.

His tips and suggestions changed my outlook on the type 4 engine.

A single tip that helps you attain your goals will be worth the time spent.

One tidbit that he shared was bores above 96mm really required dual plug heads to make more power.

Another is "more money spent on the heads than the lower end."

He stated that he takes the new castings (AA or AMC) and removes EVERYTHING including the seats and guides. ON a single plug head, he welds the plug holes up, recuts to 12mm and changes the plug orientations.
This is a BUTTLOAD of work to make his power figures.

I did a bunch of research, and it altered my outlook.

Controlling heat is the single most important factor in making one of these work properly...

Over 400 degrees is a head killer, and that number is damn easy to hit. Just miss-tuning one of these (a STOCK one) and you're there...

[mepstein]

I am an old school engine builder as well.

I've always been envious of the bike engine builders as their parts are so much smaller/lighter than automotive engines.

The issue of time for flame front travel across the cylinder bore is usually not an issue for a small diameter bore.

My suggestion is to search on "Jake Raby" posts on all of the forums you can find that he has frequented.

His tips and suggestions changed my outlook on the type 4 engine.

A single tip that helps you attain your goals will be worth the time spent.

One tidbit that he shared was bores above 96mm really required dual plug heads to make more power.

Another is "more money spent on the heads than the lower end."

He stated that he takes the new castings (AA or AMC) and removes EVERYTHING including the seats and guides. ON a single plug head, he welds the plug holes up, recuts to 12mm and changes the plug orientations.
This is a BUTTLOAD of work to make his power figures.

I did a bunch of research, and it altered my outlook.

Controlling heat is the single most important factor in making one of these work properly...

Over 400 degrees is a head killer, and that number is damn easy to hit. Just miss-tuning one of these (a STOCK one) and you're there...

(IMG:style_emoticons/default/agree.gif) Jake was a big contributor and if you read through all his old posts and the ones on other sites, you could get a really good education on these engines.

[Superhawk996]

Most importantly - Raby has a dyno which most T4 “engine builders” do not. Dyno’s allow for development, tuning, and durability testing.

A man with a dyno is not held hostage to another man’s opinion.