Becoming A Diesel Engine!

When training for any swimming event you need to make sure the work you are doing in the pool is the right sort of training for the distance you are racing. If you are a triathlete, open water swimmer or pool-based distance swimmer, this means training your ability to hold a strong pace for a long time. At Swim Smooth we call this "becoming a diesel engine".

If you come from a gym or team sport background then you are likely to be quite fast twitch and anaerobic. You might hold a very fast pace for 50, 100 or 200m but then quickly drop off over longer distances. This is akin to a high revving petrol engine, which is great if you are racing over those shorter distances but far from ideal for 800, 1500 or 3800m events.

Your genetics can bias you naturally towards distance events or shorter sprints but with the right sort of training you can shift your fitness in the direction you want to go and for most swimmers reading this blog, that means training to become more of a diesel engine. We'd all love to have the best of both worlds and be great sprinters and great distance swimmers too but that's not possible, otherwise the best sprinters in the world would also win the gold medals over distance events.

Going Diesel

We recommend CSS training as the mainstay of your fitness training to develop that diesel engine and become a great distance swimmer. You can find out more about training using CSS here, in our book (chapters 24 to 27) and follow CSS sessions in our waterproof training plans.

Even if you consider yourself quite a slow swimmer this sort of training will be very beneficial to you as it's likely your diesel engine is underdeveloped and improving it is one of the keys to you becoming faster and feeling more relaxed in the water.

When performing CSS sessions always bear in mind that pacing things out well is critical. If you start fast and then blow-up you're just training the petrol engine again!

Aim For Less Than 4% Drop-Off

To check the drop off in your pace as distances get longer you can use the 200m and 400m timetrials from the CSS test. If you have a large drop off in speed during the 400m versus the 200m then this trend will continue and over longer races you'll be much slower again.

A good rule of thumb to see how diesel you are becoming is to aim for a drop off of 4% or less in speed between the 200m and 400m timetrials. Here's two examples:
Rory proudly wears our Diesel Swimming
Engine shirt after swimming the
19.7km Rottnest Channel Swim

- If you swim 8:10 for 400m and 3:50 for 200m, you slowed down by 6.5% over the 400m. This says you've got a lot to gain from CSS type training and it should be a real priority to improve your swimming.

- If you swim 6:00 for 400m and 2:55 for 200m, you slowed down by 3% over the 400m. CSS training is still important to you but your diesel engine is coming on nicely - well done!

If you can reduce that drop-off then you're almost guaranteed to be improve your CSS pace and be quicker over any distance swimming event.

We created a quick calculator here to save you doing the maths yourself, if this proves popular we'll integrate it into our site:

Quick tips on performing the 200m and 400m timetrials:

- Do the 400m first, it will affect the 200m less than the other way around.

- Make sure you pace them out well, if you have a friend or coach to help then ask them to take your splits every 50m to check your pacing.

- Don't use your PBs, test your current fitness and swim each as fast as you can. :)

Swim Smooth!


Anonymous said...

Just worked my 'drop off' at 3.8%.
Have been loking for a way to compare my pace at other distances e.g. at 50m up to 2500m. Would you suggest 4% difference in speed is a fair assesment each time the distance doubles?


Odannyboy said...

Nice calculator. I suprised myself with a 2.6% drop off, which is heartening and hopefully shows the work I'm doing is pulling me in the right direction.

I'd be interested also in suggested drop offs across the longer distances (similar to the example of Pauls channel swim used in the SS book p175)

Adam Young said...

Hi John and Odannyboy,

We like to use the calculated CSS pace as the reference and then from there you might get:

400m : CSS pace -2 to -4 sec per 100m

1500m : CSS pace

5km : CSS pace + 2 to 4 sec per 100m

10km : CSS pace + 6 to 10 sec per 100m

Hope that helps! (from page 178 of the book)


Anonymous said...

Is it possible to guesstimate CSS by perceived effort? For someone who doesn't have the sophistication of a stopwatch! I use perceived effort in my running training to do replications at just under race pace. Maybe CSS would be something like 7/10 effort?

Adam Young said...

Hi Anonymous,

Yes, with experience it is. The key thing is pace judgement which is harder to develop for swimming than running but otherwise you should be able to get it just like running.

Does your pool not have a pace clock? You're going to find it difficult with any reference. I tempo trainer pro is highly recommended to help develop this.


Wyatt said...

What kind of rest or work should you do in between each test? I tried it out last night and had a 3.7% drop. I had about 7 minutes in between tests with just 50m of light kicking and 50m of light breast stroke. I definetely didn't feel as fresh on the 200m piece.

Adam Young said...

Hi Wyatt,

That sounds about right, approx 10 minutes of light swimming and recovery.

For sure you won't feel fresh during the 200m but that shouldn't affect your performance too much - many swimmers do PBs at the end of swim sets after all. So much of fatigue is in the mind!

Cheers, Adam

Peter Barnes said...

Hi, when using a wetronome to do CSS training I have it set to 32sec per length but don't want/have time to re-set it at the end of each set which means that my rest interval also has to be 32sec - any thoughts?

Adam Young said...

Hi Peter,

Yes, that's right - unfortunately you're a bit stuck with that with a Wetronome. The Tempo Trainer Pro has a solution as it has a reset button on it, which resets the timer straight away and immediately sets you off on another beep.

Have you tried setting it to beep every 16 seconds? It'll give you a beep half way down the pool (if you have a reference marker there it's useful in itself) and also halve your rest interval.



Peter Barnes said...

Thanks Adam, such a simple solution it's almost genius. Cheers, Peter

Unknown said...

Could you share the formula for the calculator?

Adam Young said...

Hi Eric,

For each distance, divide the distance in meters by the time in seconds work out the speed in m/s.

Then calculate the percentage difference e.g.

Hope that helps!


Anonymous said...

We want to see a 4%, or less, drop off in speed between the 200 and the 400, right?

What about the longer distances? How should one's speed hold up between the 400 to 800 and the 800 to 1500?

Ultimate question - how does/should a 1500m time (24:00 say) convert to 800 or 400?



Anonymous said...

Sorry Adam, you've already answered my question in a previous comment


Nicolas said...


Do you have scientific references that suggest a 4% drop off is optimal?

Adam Young said...

Hi Nicolas,

We can do better than that, when you test the best distance swimmers in the world, they all have drop-offs less than 4%. Really this has to be the case otherwise the best sprinters and middle-distance swimmers would also be the best distance swimmers (and vice versa) but that simply isn't the case.



Helen said...

I've a 1.1% so pleased :-)
But I have a question about my tempo trainer, my CSS is currently 29secs, but I feel like I have become confident at this speed and want to start pushing it down a little, but 28 seconds is just a little too fast! It is ok for 100m, and 200m sets, but not 400m. Is there a way of using my tempo trainer to do say 28.5? My setting 2, only seems to have whole seconds as a choice?

Adam Young said...

Hi Helen,

Yes if you put it in mode 1 you should be able to slow it right down and set it to 28.50 - that's how we do it!



Helen said...

Brilliant, thank you, I knew there must be a way!

Anonymous said...

Hi guys,
Just completed a first css test 400 and 200 as suggested along with a number of other swimmers and I'm intrigued by the 5% drop of "adjustment", which looks to throw up some rogue-ish results.

I was surprised to see that you use a 5% drop off when it's widely understood across multiple sports including swimming that to calculate a good estimate for swim times when doubling distance, you double the time and add 10%. If you look at swim world record times, set by the same athlete at around the same time, you also consistently see that these approximate to doubling plus 10% when the distance doubles.

My question is then why adjust based on 5% being the norm for pacing from say 200m to 400m when the norm is widely regarded to be 10%? A number of the swimmers who has previously done the test seemed to be well aware that to get a " good" CSS time you get a better time by swimming a 'slow' second 200, essentially because of this adjustment being 5% not a 10% ' adjustment'?

Welcome your thoughts on this.


Adam Young said...

Hi anonymous,

First up you've got to be careful comparing swimming to other sports because speed isn't proportional to power output (like it is for running) because drag increases with the square of speed and much more than that when the wake really develops at high speed. So it's going to vary a lot depending on the swimming speed. For instance the slow down between the 200 and 400m world records is 7.8% but between 800 and 1500m only 2.7%! So the 10% rule certainly doesn't hold true for swimming.

Secondly by looking at all the world records you're trying to create the perfect swimmer who's a world class sprinter, middle distance swimmer and distance swimmer - an impossibility! The point here being that with CSS training we are trying to create a great distance swimmer, not a great sprinter. If we took a world class distance swimmer they can't sprint 50 or 100 that well so their rate of drop off is going to be less that a sprinter - anything else would indicate they are doing too much anaerobic training and need more threshold (CSS) work to reach their distance swimming potential. 5% between the 200 and 400 times is our guideline to indicate someone training is about right and their physiology is heading in the right direction.

The 7.8% drop off between Biedermann's 200 and 400 world records backs this up - he was "optimised" as a middle-distance swimmer with a big anaerobic engine. No doubt if he wanted to become a between distance swimmer (say 1500 and beyond) he'd have to forgo some of that Vo2max work in his training to develop a bigger aerobic "diesel" engine over longer distances. If he switched to distance swimming I have little doubt the change in his physiology this would cause would mean his 200 and 400 drop off would fall towards the 5% region.

Lastly, we talked about deliberately gaming the CSS test here. A pointless exercise that tells you nothing about your swimming or how to improve it:

I hope that helps!


Anonymous said...

Hi Adam,

Thanks for your detailed reply. I agree wholeheartedly that gaming the tests clearly makes no sense at all, and thanks for you thoughts on drop off percentages.


Anonymous said...

Hi Adam,
I've been swimming consistently 2x a week for the last 3 months (built up to 2.5k per session).
Tonight I performed a 400m/200m time trial and got times of 7:40 and 3:40 respectively. This would give me a CSS of 2:00 according to the calculator.
Last week I swam 2500m in 48:00* which is a sustained pace of 1:55, my question would be what is your advice on where should I set my tempo trainer to improve my 2500m time (my ultimate aim is an iron distance race) and therefore my 3.8k?
*all times taken on clock on wall of pool

Paul said...

Hi Huw

Any idea of your first 100m split of that 400? It looks pretty well paced as it is but if you went off too quick that'd slow your 400 time and result in a bigger drop off between the 400 and 200 which might explain why your CSS is showing as 2:00 but you can actually hold 1:55 for 2.5km. My suggestion is that you'd be fine to set your CSS at 1:52 per 100m and see how you go with that - using the CSS Tweaker to tweak up and down as necessary. Hope this helps!


Anonymous said...

Hi Paul,

Thanks for your response - I wasnt paying attention to the 100m splits as this was my first attempt at the CSS test so wasnt really sure of how it'd go.
I'll give 1:52 a bash and see how it goes, thanks again.

Jose said...

Dear Paul,
I have just done my CSS timetrial, a couple of hours ago. My drop off is 0%. 400m = 6.30 and 200m = 3.15.
Doest it means that the CSS training is not what you'd recommend for me?

Cyndy said...

Hi Jose,
A few questions to ask about your CSS test. How did you feel after your 200m? Did you feel you went as fast as you could have? Did you happen to get your time for each 100? If your pacing was good to start and yes you feel you went as fast as you possibly could have than you are a super diesel engine. Keep doing the CSS training but you may also want to add a sprint session to your weekly training to help with your top end speed.

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