Newbie help needed - 310 or 360 or 420

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Newbie help needed - 310 or 360 or 420

Hi all,

42 years after seeing a line of Sevens at a Prescott Hill Climb event & lots of dreaming ever since, I've finally put a down a deposit for a kit - building my own one has been part of the attraction & I've now got a keen son to help me.

I thought making the decision to actually buy one would be the hard bit, but I've now realised finalising the spec (the engine in particular) is the real challenge - although there are much worse problems to solve!

I'm planning to use it mainly on the road for day blats, but also for longer trips from Essex to places like Scotland & Wales. I'll also be doing the odd track day, & I suspect that the number of these will increase over time.

Despite planning to use it for some reasonably long tours I'm going for a S3 (with lowered floors) as it will be living in a narrow home at night, I prefer the classic looks of the S3 & I've done lots of cycle touring with a tent over the years so I'm used to travelling light with as little as possible! I've also decided to go for a R Pack with a windscreen & hood although sure I'll mainly use a half-hood. Other extras I'll go for are a heater, removable Momo wheel and battery isolator. 

I've become a regular visitor to Crawley and have tested a 360R, 360S and a 420R, as well as hiring a 310R for a long weekend. However, despite experiencing the different characteristics of these, and also spending endless reading blogs & forums, I'm really struggling to reach a decision about what engine to spec - this is where I need advice from some time-served owners...

My thoughts so far;

310R - I loved the high-revving nature of the engine, and the pops & farts from the exhaust are quite good too. It's plenty fast enough for my current abilities but conscious that I might benefit from more power on the track and I might start wanting more power in the future.

360R - Driving this after the 310 I really noticed the extra low down torque & I couldn't stop grinning at the instantaneous & savage acceleration. Trying to control the rear end was also fun (the 310R had 8" rear tyres & I felt it almost had too much grip). Compared with the 310R I think that a Duratec would make the motorway sections of tours a bit more bearable (I won't be able to visit Scottish roads & family solely on B roads). Also there are probably more opportunities to further upgrade than a 310 Sigma engine. I was starting to conclude that a 360R was the one for me, but then I started reading tales of woe about the ground clearance of its wet sump. Which led me on to start thinking about the ...

420R - This is definately more expensive than what I want to spend, and probably more powerful than I'll ever have the skill to use,  but I think if I had a 360 the thought of cracking the sump would always be at the back of my mind. CC offer a dry sump conversion on a 360 for £1500, but for (yet) another £1500 you get the 420R. 

I apologise if these questions have been asked on this forum many times before but I want to make the best informed decision possible - although I don't think there are any bad Sevens out there :) 

Cheers, Richard


R400svn's picture
Last seen: 1 hour 40 min ago
Joined: 28/07/2020

Hi i have recently brought a 420r and gone over my original budget a far bit and was struggling to decide. Already though i am so glad i did as i am pretty sure with the 360 i would have just wanted more. Having said that go for the 360 its easy to upgrade to more power but hink probably more costly in the long run especially when you add the dry sum too. You cetainly wont need much more than the 420 can give. 

Baggiebird's picture
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I don't have any of the models you are considering however for the use that you propose the Duratec has to be the way to go.  Cruising longer distances will be more pleasant - however all Sevens are loud :-) You will enjoy spec'ing it and building it.

One thing I think you will notice too (on all of them?) if you have never had a Seven or Seven-like vehicle before, is the amount of transmission and differential related noise/lack of refinement - but most of them have it to a greater or lesser degree.  It is something you don't notice at first, and it depends what else you drive or have driven, but after a while you do.  With a differential bolted to a lightweight chassis it is very difficult to control effectively and part of the character of the cars.

Sorry if I am telling you things that you know already.

Jonathan Kay
Jonathan Kay's picture
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Private Message sent.

Piers300's picture
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Joined: 17/04/2014

If you order a 360 with a dry sump, then you will always yearn for the 420. Personally, I would be more than happy with a 310, but having driven the 360 engine in the Caterham R300 Duratec demonstrator some time ago, it was a great engine. I have not driven a 420.

220 bhp in the 420 is a rapid machine and you need to be pretty handy to exploit it to the full. In the 310 you would be probably having just as much fun while driving it hard. If you do that in a 420, you won’t have your license for long.


mandalman's picture
Last seen: 18 min 33 sec ago
Joined: 29/08/2019

You could save a bit of money towards the 420 by not buying that full hood that will never get used. Hehe

1999 K Series Superlight R, plaque Number 43

Last seen: 1 hour 32 min ago
Joined: 17/04/2014

Get the 420R, bigger engine working less hard especially when doing your touring loaded up with gear, and you will learn the skills to exploit the power when you do track days.


Last seen: 32 min 51 sec ago
Joined: 17/04/2014

Hi Richard 

I am Essex based, and more then willing to help, as most of the Essex bunch are.  Having owned a Sigma 150, 360 and 420, I feel qualified to answer your question.  

My Sigma 150 (similar to the 310 but with throttle bodies) was the car I felt most comfortable with. I felt I extracted the most out of on track. On the road you could stretch it's legs a little just for the enjoyment without things getting to silly.

My 360 lacked something, character I think, I still haven't gelled with duratecs on track, but I just need more track time to do this.  I had a high ride height set to resolve the sump issue.  Performance was ok, but it felt faster, but comparing video lap times there was little in it (but my 360 had a windscreen).  Road use was limited more, possible but in fewer places.

The 420 needs respect, it feels angry, and planting the throttle needs to be a considered action, so you won't find may places on the road to stretch the legs.  I have only had one wet Trackday in my 420, but I still felt that balance thing I have to work on.

For me it would be between the 310 and the 420.  The 310 is easier to build. The 420 is a little OTT.  If you want bonkers mental, get the 420.  If you want balanced all rounder go for the 310.

K7 VCT's picture
Last seen: 55 min 32 sec ago
Joined: 17/04/2014
310 is the idea car having driven many over the years, had mine over 4 years and still smiling

Orange is my new black

Ballistic orange 310s
Last seen: 6 hours 49 min ago
Joined: 17/04/2014

To move on from the engine, have you seen the thread about Limited Slip Diffs? If you go with the 420, this comes with an LSD as I recall. The Titan diffs appears to be made of cheese, so you might want to think about this aspect. 

Tazio's picture
Last seen: 2 hours 10 min ago
Joined: 17/04/2014

Also the dry/wet sump issue is overblown, in practice how many people have damaged their sumps beyond repair?, and a good corner weighting session should sort out you optimum ride heights, to mitigate the risks.

I'd go for the 420, for the extra torque, for those touring days when you want to pootle along, and then on track you can exploit every bhp, whilst remembering the accelerator is not a digital device.......