Lotus Elan

Mixing B and Q420 cams?

PostPost by: groucho » Thu Aug 15, 2019 4:58 pm

So after way too long, my engine is slowly making its way back into one piece. My engine builder mainly deals with Alfa Romeo engines but races Kent-engined cars (in additional to his three Sevens) and and certainly has some Twin Cam experience, so I'm confident in his abilities but for questions like this, we'd like to get some ideas from the community.

My engine has the stock B cams currently. The exhaust-side cam is pretty beat up but the intake side is fine. One thought is to move the intake cam to the exhaust side and get a new Q420 cam for the intake side. I'm wondering how much I'd lose doing that versus getting a pair of Q420s? I'm OK with getting two but I'd rather spend the money elsewhere if it's not going to make much of a real difference.

The block had new QED forged pistons and we're going for 10.5:1 compression so that shouldn't be an issue; we're still determining if we'd need to upgrade the springs, but if it's going to ever be done, now's the time to do it, and it sounds like the Q420 is a good fit for a fast road engine.
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PostPost by: rgh0 » Thu Aug 15, 2019 8:53 pm

With an un-ported standard head you will not get the full benefit of the 420 cams anyhow. Restriction in the exhaust cam lift and timing will mainly show itself at high revs so for a road car not that critical and the loss versus two Q420 cams will be small in normal road use. You will get a significant mid range torque and power lift with the proposed setup so it is worthwhile doing.

You will need new springs (QED supply their Q55 spring sets) to go with the cam and you will need to check valve location to ensure you can get the correct installed height of the springs. You may need longer valves and thinner followers depending on other dimensional details of your particular head to correctly fit the springs

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PostPost by: groucho » Thu Aug 15, 2019 9:21 pm

Thanks. The head has been gone through a few times through it's life but I don't think it's ever been really ported for performance. It does have the big valves fitted of course, but probably wasn't taking advantage of them. I'm not sure I'm ready to do that at this point. It's sucking up money quickly!

The car felt pretty snappy even with valves out of adjustment and carbs out of whack so I'm expecting that with a Q420 cam and everything set up properly finally, it should feel properly quick!
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PostPost by: 2cams70 » Fri Aug 16, 2019 10:00 am

As has been said you shouldn't fit QED 420 cams without the proper spring kit that goes with them. You can't buy just half a spring kit for one cam. If you are going to fit QED 420 cams you may as well fit a pair and be done with it. Talk to QED about your proposal and see what they think.
If I was in your situation with limited budget I'd just fit a pair of Lotus Sprint spec. cams. No valve train modifications required. Only if you are considering a full head rebuild is it worth considering QED 420 cams.
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PostPost by: collins_dan » Fri Aug 16, 2019 1:36 pm

I went from L2 to QED 360, or their version of sprint cams and have to say the difference was minimal. If I was to do it again, I would go full on QED 420 and follow Rohan's advice. If you are going to do it, do it. Dan
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PostPost by: 2cams70 » Fri Aug 16, 2019 2:12 pm

collins_dan wrote:I went from L2 to QED 360, or their version of sprint cams and have to say the difference was minimal. If I was to do it again, I would go full on QED 420 and follow Rohan's advice. If you are going to do it, do it. Dan


By going from lowly Lotus "B" spec. cams to Lotus "D" (sprint) spec. cams you most certainly will notice the difference.
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PostPost by: groucho » Fri Aug 16, 2019 2:26 pm

I'm sure I'd notice a difference going to the Q360 but with the amount of work being done on the engine at the moment (it's already got new pistons, getting at least one new cam, adjustable cam sprockets, baffled oil pan, etc), I'd rather go with the Q420 at this point rather than go for a Q360 and later on decide that I want a Q420.

I am not interested in spending more than I have to but I'm willing to dig a little deeper to get everything as good as it can be.

I have asked QED what they think; I'll report back with their opinion.
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PostPost by: groucho » Fri Aug 16, 2019 3:43 pm

QED says:
"We always use cams in pairs but other people don't, so i see no problem with his proposal." (using Q420 on intake and B on exhaust). "Also you should be fine using 1/2 a set of springs."
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PostPost by: 2cams70 » Fri Aug 16, 2019 11:40 pm

groucho wrote:"We always use cams in pairs but other people don't, so i see no problem with his proposal."


Sounds a bit like the kind of answer that isn't an answer!! Of course it's physically possible to do it but without a power/torque curve there's no evidence of the result. Yes if cost isn't a barrier just fit a pair of Q420's. That's what I did. Best to use a pair of new rather than reprofiled cams and if you are intending on working the engine hard you may wish to consider the long sprocket bolt mod. if using QED cast rather than steel cams.

Long bolts on Ebay here:

https://www.ebay.com/itm/Lotus-TwinCam- ... SwrIlav-P7
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