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XFLOW K3A CAM (carb set up?)


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I have re-joined the club to gain the collective wisdom of Blatchat.


Story so far:


My Xflow was only putting out 110bhp at the flywheel (Supersprint Spec) so I thought it must be a 1600cc Sprint spec.


On taking the engine apart it had 1300 +90 pistons and a 234 cam, there was an issue with the crank and the bearings 'opening up' to allow the crank to vibrate which did a little bit of damage, now fixed, but a mystery as to 110 bhp.

There may be an issue with oil surge and the sump baffles, these are being looked at, at the moment.


I decided to go for a Holbay K3A cam as I have a Lotus Holbay rocker cover and like the Holbay association with the Seven, I also upgraded almost everything but leaving forged pistons for the next upgrade with an overbore, I want to see that the Ital diff can cope before I go to the next stage.


I get the engine back at the weekend after months of waiting.


So the next job is to set up the carbs, does anyone know the carb set up for a K3A on 40's?


Coltec Holbay cams







Edited by - MarcNS on 7 Jul 2014 20:20:09

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Hi marc,

Cam choice is a bit wild for road use. You'll see very little below 35oo rpm and it tops out at 8000, where you don't want to be on a standard crank. Roger king might be along soon and he's god with xflows. Built properly its got good compression. Id go for a fast road cam. Around 234 degrees. And then a good rolling road set up to get the carbs jetted and find any faults. Cheers ed

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I thought the 1300 pistons where used in the 1700 super sprint engine with the cam you have, (Tony Weales book quotes a cam type Caterham BCD)something to do with piston height and stroke length, to achieve the 1700 cc.


So you probably have a 1700 cc engine down on power, where did the 110 hp come from, a rolling road session somewhere? Rolling roads are notoriously inconsistent one to another, there doesn't seem to be any standard to which they are calibrated.


I would say a K3A cam profile wouldn't be much fun on a public road, have to keep it revving to do anything, with consequential fuel cost and noise, but your choice, your car.



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I know nothing about the K3A cam so can't offer any advice about it I'm afraid.


From what Ed White says, it seems to be a rally/race spec and would certainly not suit Supersprint pistons which struggle to cope even at 6500rpm.


With regard to your power figs, if it's a later Supersprint with the "unleaded" distributor (late 1980's onwards) you'd be very happy to see 120bhp in reality when new. Chuck in some wear and tear (which is usually quite rapid) and the variability of rolling roads and 110bhp doesn't really surprise.


The 1300 +.090" pistons are standard fit for a genuine Supersprint. They are extremely marginal in this application and commonly don't see beyond 20,000 miles without either ring failure or piston breakage. Whatever you do, don't use them on anything more highly tuned. Don't forget that these pistons were designed for use in a 55bhp 1300ccm engine, so it's not really a design fault, more a case of unsuitable application

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Many thanks


I mainly use the car (the Club built) for regional sprint events, auto tests and auto solos and do very little road miles 😳 , so the engine was ready for a rebuild as it is on 11,000 miles and I got it at 3,000 miles.


The engine is now almost put back together, so no going back.


The cast pistons are a stop gap until I can afford forged but hope they last a year at a lower rev limit.


The cam is brand new and one of only two left from Holbay (Coltech).


I have the original spec sheet but can't work out how to put it on here, the engine has been built in accordance with the spec sheet. The cam was designed with the limits of the standard crank in mind and from what I have read pulls below 3000rpm with no issues if car is as light as a Seven.


I will stay on the standard 32 choke size to run in but think the twin 40's will need to be at max chokes sizes when I get to a rolling road.

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The spec sheet you link to shows that the power range is from 3200 to 8000 rpm. Peak power at 7400rpm.


I would be cautious with those revs and the pistons/crank you have. As Roger points out the Supersprint pistons are renowned for ability to Rev.

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If you've got the time then fitting a Megajolt 3D ignition to it will help the tractability at lower revs no end.


Made a startling difference to mine, and that is on a 234 cam. FWIW I'm getting 138bhp at 6000rpm according to the rolling road I use, and it was effective enough when I used to sprint it. It's a Steve Parker Racing unit rather than a Caterham one.



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Marc, that sounds reasonable enough. I built my last one for a rally car and give or take it was 130ish bop. I did about 20,000 road miles while I built/developed the car and then 6 events without taking it out. I've got a block, crank etc sat at work now to build one for a westy I'm building. Me and swmbo are going to spring it as a shared car so not too fussed about huge grunt. More cheap fun. If anyone has any old x flow spares cluttering up the garage please let me know. Cheers, ed
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I would say 110 BHP is to be expected from a tired Sprint spec XFlow. XFlow upgrades are a well trodden route, but the bits are quite expensive these days. My own personal experiences are:


Forged pistons, excellent investment, +10 BHP and with better fuel consumption (can you justify the cost on fuel savings?)


4 into 1 side exit exhaust, also worth a few BHP


Dry sump, worth a few BHP and better fuel consumption and improved ground clearance.


Cam change from 234 to 244 gives more top end but moves the power band up, so think carefully whether that's acceptable. Initially I missed my old 234 cam! It's also important (nay critical, ask me how I know) to make sure the pistons are pocketed for the cam you want to use. The head may also need machining for the spring seat.


3D ignition is great provided you put the effort/money in to get a good map. I managed to widen the power band of my 244 cam, and the engine will take full throttle from 1400 RPM, so the car is definitely quicker.


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Many thanks


The Holbay cam has quite a wide power band and yes it comes in at 3200rpm- I will let you know how it goes at low revs as there is one left on the shelf and there won't be any more made.

I have always planned to do further upgrades and wanted to have a cam that would work with the future upgrades.


It's just money, but feels better spending in in stages.


3D ignition is next to cope with low revs.


Then Forged pistons, but I may as well go to 1760cc at the same time and will limit the revs until then.


There is a route of using a steel crank from a Datsun 180B 😳 and modifying it, not sure if anyone has ventured in to this uncharted territory?


I think the auto tests and auto solos that I have done, have led to the oil surge in the sump - it has been baffled and re-baffled but I don't think it is a complete solution.

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The Datsun crank route is possible, but it needs a lot of machining. I saw one years ago, and it was bloody hard whatever grade it was made from, so expect to pay quite a bit to get the machining done.


Now, finding a Datsun 180 steel crank.....................................



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