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Urgent advice request  Rating:  Rating
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 Posted: 07-27-2015 07:58 pm
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NigelK
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Dear JH/GT owners

My GT was on the dyno today to assess the impact of the 907/912 HC hybrid engine upgrade and big bore exhaust system fitted by LotusBits. I was expecting a number slightly above 180bhp but the actual reading was 155bhp ie not significantly greater than a new stock 907 engine. When the stock airbox setup (with new K&N filter) was removed, the figure increased to 175bhp.

Mike Taylor has suggested fitting the Lotus airbox setup (as was I think fitted to T75 series engined later GTs) which he believes will boost power to 180bhp. I'm not sure. I certainly don't want to remove the airbox setup entirely and run with just K&Ns on the carbs, as I'm trying to quieten the car down and don't want a lot of induction roar.

There'll be another opportunity tomorrow UK time for another dyno run, so I want to finalise the airbox options before then. In this context, what experience have you guys had with different airbox setups? Is the Lotus setup significantly less restrictive than the stock Jensen setup? Why did Jensen use the Lotus airbox setup on later GTs? Are there any other options?

Any advice would be greatly appreciated, this evening UK time if possible. Many thanks in advance.

Best wishes,
Nigel

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 Posted: 07-27-2015 09:05 pm
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gmgiltd
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Hi Nigel,
My engine when last dynoed was giving 205 but has had several updates since then including fuel injection, crank fired ignition, éclat air box, Lotusbit's big valves and vernier cam pulleys. It also had a LOUD exhaust.
At the time it was tested it had dellorto 45s and a custom aluminium air box which was very similar dimensionally to the éclat air box. It worked for a while but developed cracks and was binned, replaced initially with K&Ns and then the éclat air box with a Pipercross external filter. I'm afraid I can only say that it felt right as I have not had it tested since. With the setup I have the car idles like a bucket of nails but after 2k goes really well - really need to get it set up on a rolling road back in the UK.
I am pretty sure that the éclat air box is a big improvement on the standard one, but not convinced that the standard panel filter in it is worth having.
Gordon

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 Posted: 07-28-2015 10:39 am
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NigelK
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Thanks Gordon. I've decided to go with the early Excel airbox setup (round hose attachment, clipped not bolted together) with a high performance K&N filter. While this setup is not original, it at least looks like the later GT / Lotus Eclat setup while being more robust (the box tends to crack when the bolts are tightened).

I've read Greg's comment on here that the stock Jensen airbox setup actually offers a small power advantage over K&Ns fitted directly to the carbs. While this sounds counterintuitive from a "free access to air" perspective, perhaps the fact that the stock setup draws slightly cooler air and then funnels it into the carbs actually leads to faster and denser airflow at the carbs compared to pancake filters. I suspect the exhaust system is also a factor - perhaps it makes relatively little difference what airbox setup you use with the stock exhaust system, as it is the latter which is the real constraint on airflow through the head?

In which case the replacement of the stock exhaust system with the LotusBits big bore system on my GT has in effect shifted the airflow constraint from the exhaust to the intake side of the head, which is why there was such a big dyno reading difference yesterday between the Jensen airbox setup and having no airbox at all?

This is all very interesting stuff, and has helped me to understand that engine tuning can sometimes be a bit of a black art!

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 Posted: 07-28-2015 05:44 pm
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gmgiltd
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Hi Nigel,
I was not aware that there are more than one version of the Jensen dellorto air box, live and learn.
One thing I didn't mention is that the head and intake manifold were ported to Sunbeam rally spec.(remember them). When Mike Taylor reconditioned the head and put in his even bigger sport 300 valves he commented that it was a good hand porting job. Side by side with his latest £1200+vat CNC ported head the ports look small but he is getting around 300hp on his 2.5litre rally cross engine at 8k+ rpm - way too radical for me.
Usually with air boxes bigger is better as the airflow is more even to the carbs/injectors- there's a lot more turbulence around pancake filters-that's the theory anyway and if you look at the induction system on any modern bike engine it seems to be correct.
Has your head been ported and what cams are in it?
Gordon

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 Posted: 07-28-2015 10:11 pm
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NigelK
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Hi Gordon

As far as I know, there was only ever one airbox setup (the AC muffler type) for Dellorto carb JHs and GTs. The same set up was also used for all Stromberg carb JHs aside from those with T75 engines (very late JH5s and most GTs) which used the Lotus Eclat airbox setup. Later UK-spec GTs were also fitted with the T75 engine, presumably because that was all that Jensen had in inventory at the time. I suspect the Eclat setup was used on the T75 engines to try to compensate for the negative impact on power and torque of all the emissions measures.

The head on my GT is standard 912 HC spec - Mike has been trying to tempt me with the CNC'd head but I have managed to resist (thus far). The cams are standard 912 HC too - 107 in 104 ex I think.

Best wishes,
Nigel

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 Posted: 07-30-2015 05:38 pm
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NigelK
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Latest figures hot off the dyno. With the LotusBits big bore exhaust, replacing the stock Jensen muffler airbox setup with the early Lotus Excel airbox setup adds nearly 35bhp peak. An amazing difference. It's really hard to see on the dyno plot below, but black is the original 2.0l 907 before the upgrade, blue is the upgraded 907/912HC hybrid with the stock Jensen airbox, and red is the upgraded 907/912HC hybrid with the Lotus Excel airbox.

Last edited on 08-08-2015 06:42 pm by NigelK

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 Posted: 07-30-2015 08:31 pm
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subwoofer
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Just to make sure I understand you correctly, you went from 155 with the stock airbox to 175 with trumpets in open air to 190 with the excel airbox? If those are wheel hps (the only kind that counts), you have a fairly hot setup!

As we all see, one single component can always make a mess of everything if it doesn't match the rest.

--
Joachim

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 Posted: 07-30-2015 08:53 pm
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NigelK
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Hi Joachim

I don't have the raw data yet so I am reading the figures off the dyno plot. But, roughly speaking, peak bhp at the flywheel is 150 with the stock Jensen airbox, 175 with no airbox, and 183.6 with the Lotus Excel airbox. As you say, the Lotus Excel airbox and the LotusBits big bore exhaust would seem to go together quite well!

Best wishes,
Nigel

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 Posted: 08-10-2015 08:13 am
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subwoofer
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Thanks for reuploading the pictures, that made everything clearer.

30 - 35bhp difference going from one airbox to another, that is massive! Do you have the raw data from those dyno runs, or at least the correction factor used to convert wheel hp to flywheel hp? Wheel hp is what counts, flywheel hp is for Top Trumps and bragging. :-)

--
Joachim

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 Posted: 08-11-2015 03:10 pm
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NigelK
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Hi Joachim

Here are the wheel dyno results with various airbox setups. Peak wheel bhp with the 907/912HC and Lotus airbox is 153.5bhp at 6000rpm. With the Jensen airbox, the same figure is 126.8bhp at 5450rpm. With no airbox, the same figure is 146.3bhp at 6040 rpm. With the original 907 engine and Jensen airbox, the figure was 102.4bhp at 5750rpm.



I've added the flywheel dyno results with various airbox setups. Peak flywheel bhp with the 907/912HC and Lotus airbox is 183.6bhp at 6850rpm. With the Jensen airbox, the same figure is 150.6bhp at 6460rpm. With no airbox, the same figure is 173.5bhp at 6680 rpm. With the original 907 engine and Jensen airbox, the figure was 118.6bhp at 6250rpm.



The figures suggest that, with an unrestrictive exhaust system, the Lotus airbox setup offers optimum power and torque.

Best wishes,
Nigel

Last edited on 08-12-2015 12:56 pm by NigelK

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 Posted: 08-11-2015 05:07 pm
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Jensen Healey
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The large gap between 3000 and 4000rpm suggests the Dellortos are not transitioning from the idle to the main circuit effectively. Was a co or O2 meter connected during the Dyno run?
Kurt

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 Posted: 08-11-2015 05:17 pm
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NigelK
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Hi Kurt

I asked Mike Taylor about this. He said it is due to the tuned length of the big bore exhaust system. No O2 or CO2 readings were taken during the dyno runs. Is the idle / main jet progresssion at the same rpm on DHLA45s as DHLA40s?

Best wishes,
Nigel

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 Posted: 09-02-2015 12:41 pm
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NigelK
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Dear Forum Users

I’m posting this chart more to catalyse debate than anything else…



It shows engine torque and lambda readings (I managed to extract these from the tuner) from my GT with different airbox setups. Some of you have suggested that the dip in torque at 3500rpm might be a progression issue. The lambda figures show i) a leaner mixture of 0.9-1.0 at cruising revs (2000-3250rpm) providing optimal fuel economy, with a richer mixture of 0.75-0.85 at higher revs to optimise performance, and ii) that the progression from leaner to richer mixture (which starts at just over 2500rpm) does not coincide with the dip in engine torque (which starts at 3100rpm).

Mike Taylor tells me that the lambda figures are consistent with what one would typically expect from an in-tune Excel 912HC engine. They also suggest that carburation is not a significant contributing factor to the dip in engine torque at 3500rpm. Rather, this dip is driven primarily by the tuned exhaust length combined with the intake length. Mike reckons I might be able to smooth out the dip with velocity stacks – there is room for stacks up to 70mm length in the Excel airbox without removing the panel filter.

But, as I do not notice the dip in everyday driving, it’s not something I feel the need to fix any time soon.

Best wishes,
Nigel

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