U20 EFI Stroker build (Scarlett's new heart)

[Pjackb]

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The Journey to Building a Hot Datsun Engine

Back in 2015, my journey into the world of Datsun Roadsters began quite unexpectedly. It started with a photograph—a single image of a stunning Mike Young built 1967.5 SPL in Spanish Red with Panasports. That moment was transformative. Something about the lines of the car, its timeless aesthetic, and the promise of spirited drives down winding roads captivated me. That very roadster would inspire the car I came to call "Scarlett," my own vision of the perfect Datsun. Little did I know how much that single picture would change the trajectory of my automotive passions.

The First Build and the Lingering Regret

Scarlett has been and continues to be a labor of love from the beginning when she was Mr. Green. First Jim (JT68), then myself, poured time, energy, and resources into her, ensuring she embodied everything I admired about the car. However, there was one decision from that initial build that I have regretted ever since. During those early days, I had the opportunity to have Jim build me a 2.2L stroker engine. We discussed it at length, and while I was tempted, the added cost was simply too much for my budget at the time.
In hindsight, it’s clear that I underestimated the value of that upgrade. The performance gains, the sound, the sheer driving joy—it would have been worth every penny and then some. Compared to the cost of doing a standalone stroker build now, that decision seems like a bargain I let slip away. Back then, though, I didn’t have the understanding or perspective I do now.

The Accumulation of Knowledge and Experience

Over the years, this missed opportunity stayed with me. It became a quiet itch, a challenge waiting to be tackled. Since then, I’ve immersed myself in learning everything I could about building high-performance Datsun engines. I started accumulating parts, scouring forums, and devouring technical manuals. Every lesson, every mistake, every success brought me closer to my goal.
I’ve since gained invaluable hands-on experience building two stroker engines: a 2.3L L4 in my 510 and a 3.0L L6 in my Z. These projects were milestones in my growth as a Datsun owner and deepened my appreciation for Datsun engineering. While there are significant differences between the U-series engine used in the roadsters and the L-series engines, they share the same OHC (Overhead Camshaft) design philosophy. The parallels between the two architectures have allowed me to transfer much of what I’ve learned from the L-series builds to this next endeavor.

Preparing for the Ultimate Build

Now, with years of accumulated knowledge, hands-on experience, and a collection of carefully sourced parts, I am finally ready to revisit the stroker engine I originally dreamed of for Scarlett. This time, I’m going all-in. I’ve learned from past decisions, gained a new perspective, and developed a clear plan for achieving my vision.
Building a U20 stroker engine isn’t just about performance; it’s about completing a journey that began nearly a decade ago. It’s about taking everything I’ve learned—the technical expertise, the patience, the persistence—and channeling it into something tangible. This build isn’t just a project; it’s a reflection of how far I’ve come since that fateful day in 2015 when a single photograph changed everything.

What Comes Next

As I embark on this next phase of Scarlett’s evolution, I feel a mix of excitement and resolve. I know the road ahead won’t be without challenges, but I’m ready. Every setback I’ve faced has taught me something, and every success has fueled my determination. This build is more than just an engine; it’s the culmination of years of passion, learning, and growth.
I will update this thread periodically to share the steps already taken in the build, my objectives, part selection, and the reasoning behind each choice. Stay tuned, because Scarlett’s story is far from over. With a 2.2L stroker at her heart, she’s destined to become everything I dreamed of back in 2015—and so much more.

[Stick]

Sounds like you have stayed true to that vision for quite some time ... Looking forward to watching this transform from Vision to reality. Best of luck in the build and thanks for sharing.

[Gregs672000]

I'm in! Fully understand your heart and passion. I've never ever gotten tired of my car, warts and all! Like you, I fell in love 38 years ago and that feeling has never dimmed. Onwards!

[Alvin]

It started with a photograph—a single image of a stunning Mike Young built 1967.5 SPL in Spanish Red with Panasports.

Do you still have this photo? Are you sure it wasn't on Libras?
Excited to follow the build!

[Pjackb]

Do you still have this photo? Are you sure it wasn't on Libras?!

You know my passion for wheels I wouldn’t mistake that
Here’s the picture, I didn’t know it at the time but that car had a nice KA24DE

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[drieseck]

Pjackb, Very much appreciate the description of your Journey.
Likely with way less technical expertise, my initial 1600, (neglected by previous) swapped with a 2L heart ignited an unforeseen addiction. Adding a KA 510 and 280Z to the mix provided balm. But recent employment & oversight by an ex-Andial builder (of Porsche pursuasion) actually directed me back to the wonderment of original Datsun L's and U's --engineering, performance and their legacy. Surely amateur to your work, I'm hip deep in a stock-ish Solex build, so your upcoming evolution will help educate the needy.
Congratulations; very excited to watch your new phase unfold. I appreciate you sticking with the U (adding, volume and EFI)! Like many here, (who have been remarkably helpful) I'm looking forward to you keeping us posted.

[Daryl Smith]

Very interested in this build.
Don't want to get ahead of your story, but,....is a custom header a part of your build? Either from scratch or modifying an existing one as I did?
Likely as you did, I have spent many hours over the years learning the ins and outs of the drivetrain and how the different parts interact - from the engine thru transmission, rear end and tires. From your posts it would seem the engine is the only change to be made as the rest is pretty well done.
Sounds like you have all the pieces sorted, so will be interesting to watch and see where our ideas of the "ultimate build" differ......

I'm guessing a target of around 200 hp and 175 ft lbs at the crank? A bit high, but, I think doable with this engine for the street......

[Pjackb]

is a custom header a part of your build? Either from scratch or modifying an existing one as I did?

At this time, I will be sticking with the ubiquitous 1.5-inch primary headers for a few reasons:

Performance: 1.5-inch primaries favor low-to-mid RPM torque and perform well for up to 50–60 horsepower per cylinder below 7000 RPM.

Practicality: In my opinion, to see any real benefit for my use—which is mostly between 2500–5500 RPM—equal-length long tubes would be required. However, they are a pain to fit and not worth the hassle for my setup.

Additionally, this setup pairs well with my DRP 1.875-inch exhaust system, complementing the overall balance of performance and drivability

I'm guessing a target of around 200 hp and 175 ft lbs at the crank? A bit high, but, I think doable with this engine for the street......

I would like to say I don’t have a power goal, but that wouldn’t be entirely true.

While I’m aiming for a balanced build that delivers strong drivability and responsiveness, I do have a target in mind. My build combines elements from Lou Mondelo's typical U20 builds with some additional tweaks that I hope will improve on them. Lou’s builds generally peak at around 170–175 bhp at 6500 RPM and 155 lb-ft of torque at 5000 RPM. (at the crank )

My focus is to match or exceed those numbers while enhancing midrange torque and usability in the 2500–5500 RPM range, which is where I spend most of my driving time. With the right combination of parts and careful tuning, I believe this build can strike the perfect balance between performance and reliability.

[Pjackb]

Bottom End

After careful consideration, I went with a larger stroke of 89mm and a slightly larger 88mm bore instead of just increasing the bore alone. The reasoning comes from the Lseries world where we see that while larger bores (88mm or 89mm) perform well, increasing the stroke even slightly (to 81mm or 83mm from 79mm) not only increases output without sacrificing RPM capabilities but also delivers significantly more torque throughout the range for equivalent compression ratio and camshaft specs and I wanted something similar

In the U20 engine, there are fewer shared formulas available compared to the L-series, but strokers have been done successfully. The most common approach I could find involves using a Honda-sized journal and rods to increase the stroke to 89mm. once it was decided that was the way i wanted to go I sent the crankshaft to one of the best shops on the East Coast for offset grinding and balancing to zero.

Lou Mondello offers custom Manley rods, which I opted to use. These rods weigh considerably less than the stock ones at approximately 580g versus 900g for the originals. Each rod was meticulously weighed prior to installation to ensure perfect balance.


For pistons, I chose OEM VG30 units that were milled to the proper height. These pistons are commonly used in builds exceeding 70 horsepower per cylinder, which is more than sufficient for this application. They also weigh slightly less than the original pistons, contributing to the overall reduction in rotational mass.



I wanted a strong, reliable bottom end and strongly considered forged pistons. However, given the power requirements and my driving habits, cast components were deemed more than sufficient. The block was magnafluxed and sonic tested before starting work, which revealed it could have gone all the way to a 90mm bore. However, we chose 88mm to leave room for growth in the future.

With the crankshaft, rods, and pistons ready, the block was assembled and is now complete. It’s a solid foundation for the high-revving, responsive engine I envision for Scarlett.



Clutch and Flywheel Combo

A key component of the bottom end and the way the engine will rev is the clutch and flywheel combo. A lightweight flywheel won’t give you more power, but in my experience, it enables you to put more of it to the ground and reduce losses. It has also been my experience that running a very light flywheel with a regular-weight clutch can prove temperamental, as is the case with Mrs. Blue, which requires finesse to get going from a standstill, especially when cold.

It’s also been my experience that a slightly heavier clutch and pressure plate provides a perfect combination with a light flywheel. I run such a combo in my Z by using a beefier 240mm clutch with a 10lb flywheel, and Scarlett has a 225mm clutch with a 10.5lb flywheel. Both setups have proven very smooth to drive.

For this build, I went with the same DRP-available 10.5lb chromoly flywheel and Exedy clutch. This combo is rated to 190hp, so it should be sufficient. There is a heavier clamping clutch good for 225hp, but this isn’t a race car, so the lighter setup strikes a better balance for my needs.

beautiful piece

[Daryl Smith]

"Lou’s builds generally peak at around 170–175 bhp at 6500 RPM and 155 lb-ft of torque at 5000 RPM. (at the crank ) "
I like those #s. 1.33 hp/ci is very respectable for a street engine, as is 1.17 ft/lbs per ci. Not stressing the engine at all.

"Practicality: In my opinion, to see any real benefit for my use—which is mostly between 2500–5500 RPM—equal-length long tubes would be required. However, they are a pain to fit and not worth the hassle for my setup."
Actually, in my opinion, you would be best served by a 4-2-1 header, equal length would be nice but, not particularly 'nescessay'. An available 1 1/2" primary header can be modified to a 4-2-1 with 3 merge collectors and a 1 5/8" U pipe cut in half, as I did and pictured in the header thread. Fits in the same space, not as expensive as a complete custom header (tho still $$$ - collectors from Burns are over $200 each), will broaden your torque band, AND possibly increase both power and torque. Yes, the downside is the cost.

You mention EFI, but not crank fired ignition. I know the EFI is a lot easier/cheaper to do when keeping the distributor, but the programmable crank fired ignition will be far more noticeable in your power curve.....just saying....

[Gregs672000]

Very much enjoying this conversation and exchange of ideas! I completely agree that ignition timing is key to output and can be very rpm and engine build specific. I've watched engine tuners vary A/F ratios several points with almost no power changes, while a degree or two of ignition timing (and not necessarily advancing the timing) can make significant power output changes, as well as protecting the engine from unnecessary stress where folks add timing that does nothing, hurts power or puts the engine right at detonation (detonation is NOT a good guide for setting timing!)

I'd be interested to hear about the cam. We often work with B or C cams or some variation to lift or duration, but lobe separation and other aspects of cam design could play a big role here, including cam timing... the cam is the "brains" of this operation! There are a few folks here who have posted some unusual cams, and I believe Robello has developed a few so that might be interesting. Whole other box of worms that... and lots of cooks in the kitchen... we all get to learn!

As Daryl knows, I've been thinking about header primary pipe sizing as well. I MAY have a 1 1/2 header on it now, but just purchased the 1 5/8. I will probably make my anti-reversion plate using the 5/8 header, though it may be too large (perhaps the plate will mitigate that?). David Vizard likes to use exhaust port CFM at full lift as a way of determining proper pipe size and has a chart one compares to. I don't know mine (it's modified as well), but if anyone has that data please pass it along. I've not been able to find it on the net.

[Pjackb]

You mention EFI, but not crank fired ignition. I know the EFI is a lot easier/cheaper to do when keeping the distributor, but the programmable crank fired ignition will be far more noticeable in your power curve.....just saying....

I completely agree that ignition timing is key to output and can be very rpm and engine build specific.

I will be crank triggered detail to follow when we get to the section of the build about ignition setup and selection but here's a teaser

I'd be interested to hear about the cam.

All in due time

[Gregs672000]

Saaame bat time, saaaaame bat channel!!!

[Pjackb]

Actually, in my opinion, you would be best served by a 4-2-1 header,

As Daryl knows, I've been thinking about header primary pipe sizing as well. I MAY have a 1 1/2 header on it now, but just purchased the 1 5/8.

from what I've seen lot of the discussion around header design is theoretical and speculative and not backed by hard data comparing apples to apples.

While I agree that a 4-2-1 configuration is generally better for mid-range performance in a street car, It's also my opinion that header design, exhaust system, compression ratio, and camshaft tuning need to be considered holistically. From my personal experience with 4-1 and 6-1 setups, I’ve observed how these components can work together to deliver comparable top-end power on 4-2-1 vs 4-1 and surprisingly similar mid-range performance in a 4-1vs 4-2-1

Case in point: my L28 engine runs a 6-1 system, and my U20 utilizes a 4-1 setup. Both engines exhibit distinct advantages, with torque curves that are impressively flat.

This is the U20 currently in Scarlett and using an A+ cam and Mikuni 40s and 4-1, we did more torque than HP in this config ,44s would have increased torque and HP across the board


This sheet is my L28 stock bottom end and a long duration and low lift cam with 6-1 headers



This topic has been discussed many times on the L-series forum, and after reviewing countless dyno sheets, I’ve consistently found that a well-tuned 6-1 system, when paired with the right camshaft and exhaust design, delivers a punchy and flat torque curve. and I like the way they sound at full throttle

For these reasons, I feel confident in my choice of exhaust system. However, if the power results fall short of my expectations, my next step will be to explore long-tube headers.

[Gregs672000]

Again, I appreciate the discussion and sharing of info. It is hard to compare apples to apples, and that's why I enjoy watching the YouTube guy who runs the Garage 4A-GE site... does baseline runs then adds whatever he's testing and is always clear on whatever adjustments he's done. I've also seen other sites where 4-1 headers produced more torque than the 4-2-1 design they tested (opposite of expected), but most tests are done on V8 engines. I agree fully that we're working on systems, and much of this is beyond my understanding and/or my ability to change AND objectively confirm it helped (let alone hurt)... dyno time is expensive! For example, adjusting cam timing a degree or two... does that change the "timing" of how the headers scavenge, and does that change based on pipe size? Bottom line, one is able to say "this is what worked best on MY engine in the following configuration, your results may vary."

For example, the above dyno run has the engine making peak torque 400rpm below what an A cam SU motor makes per the manual (4400rpm) and peak hp is well below stock (5600 vs 6000rpm). What size choke is in the 40mm carb? I've switched from 45mm carbs with 37-39mm chokes to 40mm TBs, so at first glance my engine should lose torque at lower rpms (and it did with the 39mm, gained top end), however I can feel it has more than ever now and I'm sure the dyno will show that... but that's MY engine... higher compression, bigger valves, port work, ignition timing may be different, B cam duration but a lot of lift... so many factors, and some may help or hurt. When I do the before and after with the anti-reversion plate the dyno results will be on MY engine, and the only way to know if it works as well (or better/worse) on a stock or stockish engine will be to try it, preferably on a dyno! Again, this is why I love those 4A-GE posts... he changes one thing at a time and knows his engine really well.

I thank you Pjackb for your willingness to share your knowledge and experience. It can be hard some times to post opinions, but unlike many other sites I expect no flaming here and instead we encourage and appreciate your efforts!