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Velocity Stacks  Rating:  Rating
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 Posted: 10-02-2014 03:06 pm
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UKJames
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Having ironed out most of the main issues with 17607 I'm now down to tuning it up for the best to match my driving style. I'm looking for a little more low end torque.

I am unlikely to drive the car at anything near max rev's in all likelihood most of the time, so looking for it to be optimal at around 3500-4000rpm

I know that the velocity stack length can have a bearing on the torque/ bhp trade off. I've got K&N filters at the moment with 40mm stacks, on Dellorto 40's.

I know there's a formula to calculate the optimum length for a given RPM, but not sure whether I am in the ballpark, or whether longer stacks will increase low end torque (or is it shorter?)

Any thoughts or experiences gratefully received..

 

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 Posted: 10-02-2014 04:39 pm
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subwoofer
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On stock cams and stock compression? Don't think you'll find much more torque by just swapping velocity stacks. The 907 was known as "the torqueless wonder" back in the day, it needs revs to do anything useful.

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Joachim

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 Posted: 10-02-2014 04:41 pm
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Jensen Healey
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The simple answer is longer stacks for midrange power, shorter stacks for high rpm power.

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 Posted: 10-02-2014 06:44 pm
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UKJames
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I've heard that the JH was known as the Torqueless Wonder, wasn't sure whether that was referring to the Strombergs and different valve timing, or that all models were all a bit weak lower down. In a way that's why I was wondering if the stacks might help a bit. But then my daily driver is a diesel which is all low down torque, so probably not a big deal, just a different drive.

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 Posted: 10-02-2014 08:41 pm
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Jensen Healey
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All the models have the short stroke crank with low compression 8.5:1 pistons. High revving is fun!

If you have stock Dellortos they can be tuned to eliminate the 2000 rpm flat spot.

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 Posted: 10-02-2014 09:01 pm
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subwoofer
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Dropping in a set of E cams from a later Éclat engine will reduce the overlap and shift the torque curve down a bit, but the 8.4:1 compression is stil rediculously low. You need to clear your mind of any diesel residue and let the poor thing work where it does its thing best. Life begins at 4000 rpm!

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 Posted: 10-03-2014 06:34 am
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UKJames
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Yes must just drive it harder, even more fun when all around you are whirring hybrid eco-boxes and you have crazy induction roar... Now I've got it working I need to find some decent open road to get acquainted.

Thanks- Jim

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 Posted: 10-04-2014 05:26 pm
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gmgiltd
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In addition to what Joachim suggested above I would add that binning the K&N's and fitting an Éclat airbox would be a noticeable improvement. You can fit up to 92mm stacks inside the air box and fit a remote filter like a Pipercross to the air box intake. This would enable you to use 4k+ rpm without the undesirable attention of Mr Plod - still sounds sharp but much less volume.

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 Posted: 10-04-2014 08:13 pm
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Esprit2
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Shorter air horns to optimize higher rpm performance,
Longer air horns to tune for low rpm performance.

Given the range of available air horn lengths, and the space available within the airbox (or even in a K&N air filter), the degree of impact that longer air horns will have on low end performance won't be significant. If you're not pleased with the current low end torque with stock air horns, then you won't be pleased with the result with longer air horns. The change isn't that dramatic, so apply your budget where it will have more impact.

Air horns, with their rolled and tapered inlet shape are important (!) to smooth airflow into the carbs, so don't delete them. Or if they're currently missing, install some. Even if they're the wrong length, the smooth entry is worth having even if the length isn't to spec. Something is better than nothing.

On the other hand, the top end performance when you're bouncing off the rev limiter is noticeably better if the correct, tuned length air horns are installed. With a near-stock engine, that would be the stock length air horns.

The "Torqueless Wonder" tag really applied to the Federal emissions spec 907... it really was weak. The compression was low, the cams were timed for emissions (can easily be set back to the correct 110 MOP for both intake and exhaust), the ignition timing was really lame (dial in more static advance, and/or re-curve the centrifugal advance), and the Stromberg carbs were tuned "emissions-lean" (strangled).

The Strombergs could be re-tuned for performance, but most owners just converted to Dellortos. J-H used DHLA 40 carbs on all their engines, while Lotus used DHLA 45E carbs on all of theirs. Different goals.

The J-H Dellorto spec 907 used the same 8.4:1 compression ratio in all markets (emissions & non-emissions); however, Lotus' own Euro/ Dellorto spec engines used 9.5:1 compression. That made a very worthwhile difference. If you're going to invest in new pistons, then I'd recommend 11:1 on pump premium fuel.

With the exception of compression ratio, all those changes can be made to a J-H 907 for a noticeable improvement in performance.

The single largest improvement you can make to any 907 is to convert it to 2.2 liters. It's a small change in stroke, but it makes a big difference in low end torque.

Last edited on 10-04-2014 08:19 pm by Esprit2

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 Posted: 10-04-2014 09:54 pm
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gmgiltd
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Tim - fully agree with what you are saying, but it sounds like Jim's JH is pretty close to standard and the suggestion about the airbox was more to do with toning down the racket that K&N's make.
After a modern turbo dweezel, most 907s are going to feel gutless at low rpm. For example a Landrover 2.2 diesel produces 190 hp and 420 Nm of torque at 1750 rpm and gives 50mpg in a vehicle weighing almost double what a JH weighs.

I went to 2.2 in 1984 with 12.1 pistons, 104 cams, 45 delortos, polished rods , (Sunbeam rally parts)ported head and manifold, HD clutch, uprated cooling, oilstat, electronic ignition +++. A couple of years ago it was four wheel discs, last year fuel injection with 48mm bodies and crank triggered ignition, this year it was bigger valves and an HTD vernier kit. I found that the 92mm velocity stacks were a big improvement on the 40mm ones in terms of driveability - but this could well be because of this particular configuration.

Attachment: Jennymotorsmall.jpg (Downloaded 137 times)

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 Posted: 10-05-2014 03:39 pm
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UKJames
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My engine looks stock, but this car has some paperwork with it that implies an engine swap may have been made in its past. The engine number is different to the normal numbering, it's D907E00508- It looks like a completely stock 907 engine block (later type with newer belt tensioner etc)
Not checked the cams out in detail apart from setting them both to 110
I 'believe' everything to be stock. I think it's probably a combination being too used to the torque of my BMW diesel, too much induction racket making me drive it more 'steady', and some more fettling required on the Dellorto's.

gmgiltd- that's a really good looking engine by the way..

Last edited on 10-05-2014 03:40 pm by UKJames

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 Posted: 10-06-2014 12:18 am
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Esprit2
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The Elite-Eclat airbox used to be available through Lotusbits in the UK. However, I talked with Mike Taylor a while ago, and he indicated they don't have them any more. He's considering making up a mold and producing his own reproduction parts, but that's a ways off in the future.

The airbox is much quieter than the open, unsilenced air cleaners, like K&N. They're also more efficient and the engine produces more power with the airbox than without it. There is a low restriction advantage to the K&N filter, but you can get a K&N flat panel filter that will fit the Lotus airbox.

Last edited on 10-06-2014 12:29 am by Esprit2

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 Posted: 10-06-2014 01:52 pm
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UKJames
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I'll keep a look out to see if any Lotus airboxes come up for sale in the UK.

In the meantime I've got the original JH airbox in a load of stuff that came with the car, all complete just needs cleaning up. Would there be any advantage in installing this, with say a pipercross filter, in performance? I have always assumed the freeflowing characteristics of the K&N's helped the engine to breath more efficiently ergo more power, but maybe there is more to it.

The noise reduction would probably help with my right foot reluctance though (it'd be nice to hear the exhaust over the engine for a change)

 

Cheers

Jim

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 Posted: 10-06-2014 02:08 pm
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gmgiltd
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Hi Jim,
Try Mike Taylor at Lotusbit's he may well have one kicking around. Normally for an air box bigger is better and the standard one is not great.
On an Elite/Éclat air box you have a couple of options - you can fit a standard Lotus or K&N (aftermarket) flat screen type filter inside the airbox but this limits the length of the ram pipes. Alternatively a Pipercross universal (or similar) can be used externally. Will go and take some pics to show what I mean.
Gordon

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 Posted: 10-06-2014 02:40 pm
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gmgiltd
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Pictures showing Pipercross filter and 92mm x 48mm rampipes-the fromt one needs to be trimmed slightly to fit the airbox.
Gordon

Attachment: Pipercross.jpg (Downloaded 113 times)

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 Posted: 10-06-2014 02:41 pm
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gmgiltd
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And the rampipes

Attachment: Rampipes.jpg (Downloaded 115 times)

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 Posted: 10-17-2014 10:44 am
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UKJames
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Quick update- Lotus fibreglass airbox now obtained from Lotusbits, (last one he had, off one of the old rally cars), filled a few holes, sprayed up and fitted. Looks OK.








Noticed when I was refitting my 40mm stacks the bases were warped, I flattened these out before fitting the new box. I guess if the air was being pulled sideways through the gaps at the flange it would almost be like running with no stacks, useless torque.




Short run of silicone hose from the box to a K&N Apollo, then a cold air feed hose out to the corner of engine bay. Tight fit, but happy with the result.

Has made a difference, much quieter, but also smoother and feels like it has better pick up. I'll give it a proper test this weekend after I've rebalanced the carbs and mixtures for the new set up.














-Jim

Last edited on 10-17-2014 10:47 am by UKJames

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