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Found useful power gains..

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Some of you may find this worth considering in your tuning plans..

Just finished a dyno session with the 45mm long OE intake trumpets replaced with some 16mm stub stacks...

Test was run on '85 S3T with the HC engine using 45DHLA; 9.5psi and 19 degrees total advance. Jetting 185M/240A

Results came back with a 15% improvement in HP and 20% increase in torque over the entire rev range, plus, gained 500-1000 useful rpm!!!

Discovered the OE BOV is pretty much crap at consistency and vents randomly at peak boost/rpm, dumping 40-50hp in the process. A hyperboost CORE DV with an adapter to mount the FPR, looks like a good option.

A cam timing change from OE would aslo help improve the power more.

Jan

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I would be interested to see a torque curve for that as shortening the inlet runner by 29mm shouldn't give that kind of improvement, especially at the low RPMs as the inlet resonances will be pushed further up reducing low end torque.

The inlet runner length on the S3T is something like 350mm in length so loping off 29mm is only a 9% length reduction so I don't get where the huge improvements are coming from????

Was the testing done back to back and did anything else get changed in between fitting the shorter velocity stacks ???

Hilly

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Jan,

I am very interested in your results. How did you arrive @ the decision to reduce the length of the intake trumpets?

I agree w/ Hilly, that the improvement shouldn't necessarily be over the entire RPM range. I can see how the shorter trumpets may reduce the restriction @ high rpm, as they aren't blocking as much flow from the sides. I am also wondering why you only have 19 degrees total advance, or is that w/o initial advance? I'm running 32 degrees total. 12 @ idle, and 20 more @ 3000 rpm. Do you have AFR data from a wideband 02 sensor? I'm curious to see your AFR @ WOT.

Do you have the dyno run slips to compare? That is such a huge increase for so few $$ that one would be stupid not to do the conversion. All else being equal of course.

Thanks for the info!

Lee

Edited by Esprit Aviation

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Interesting results Jan. :thumbsup:

It seems your theory that the carburetor trumpets are too close to the plenum lid (and are being masked) has been proved .

I have some shorter trumpets around here and will be trying them. :veryangry:

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Exactly Wayne! That was my thought, especially w/ the air being 'blown' into the plenum @ such a radical 180 degree angle. Who knows what is really going on inside there! There must be a great deal of turbulence, and as long as it gets straightened a bit by use of the short trumpets, it must reduce the restriction.

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It seems your theory that the carburetor trumpets are too close to the plenum lid (and are being masked) has been proved .

Ohhhhhhh, I hadn't considered that......

I was think about it using my NA airbox which is a fair bit bigger than the Turbo plenum.

If there is a flow restriction due to the velocity stack face being too close to the wall then shorter ones would certainly help everywhere.

Still be interested in the results/curves

Hilly

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The only drawback I could think of would that having the shorter stacks would probably affect low speed (off boost) drivability , slowing down airspeed in the inlet tract and possibly causing fuel to drop out of suspension.

But of course thats probably the reason Lotus went with the longer trumpets in the first place.

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If the proximity of the stacks to the edge of the plenum is causing such high flow restrictions, why didn't Lotus make the plenum deeper ????

Hilly

Edited by hilly

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I dunno mate, maybe making it an inch larger in depth meant it was impossible to change the oil filter or the engine would no longer fit in the Elite/Eclat like they were planning in doing , maybe it was just an oversight ,who knows!

Maybe Brian Angus remembers the reason?

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Certainly would be a boon to all carbed turbo owners! I can't think of another mod that would be so inexpensive and yield such magnificent results.

I do agree w/ Wayne again concerning the precipitation of fuel @ lower engine speeds, but it still seems like it may be worthwhile. Besides how many of us really put around all the time @ low speed? Anything over 2000-2500 RPM should still be Ok AFR wise.

I can't wait for further results!

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If the proximity of the stacks to the edge of the plenum is causing such high flow restrictions, why didn't Lotus make the plenum deeper ????

Hilly

If the stacks are in close proximity to a surface then they MUST NOT have an edge as this severely restricts flow. What they should have is the radius coming completely back around on the outside so that it is at least 180 degrees worth - radius should be at least 6mm for best results.

Phil

98GT3

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Considering how difficult it is to get at the distributor and the oil filter with the standard plenum in place, that may be why a larger one wasn't fitted...then again, I haven't noticed much thought for the maintenance technician in the Esprit design!! It would indeed be interesting to see what's going on with the airflow in the plenum...needs a transparent one on a flow table or somesuch, smoke trails as in windtunnels. If simply fitting shorter trumpets liberates lots of power, why not simply radius the carburetter chokes?

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Any benefits of shorter trumpets on a non turbo...theoretically of couse?

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Any benefits of shorter trumpets on a non turbo...theoretically of couse?

Pete

The answer is "depends"

As with everything in the real world, design of engine components as a compromise.

Shorter runners give better engine output at higher RPM level at the expense of the low end.

It is all to do with how the slug of air moves about in the inlet runner.

Time for a bit of science.

On the inlet stroke, air is sucked into the cylinder until the piston stops moving and the valves slams shut.

The slug of air continues moving down the runner (as it has mass) and compesses, it then pushes it'self back out the runner until a vaccum is formed at the valve and it sucks it'self back in. This "organ piping" happens at a frequency defined by a number of parameters, but mainly due to runner diameter and length. There are number of frequency waves happening at once which reduce in force as they go down in frequency.

At certain engine speeds the frequency resonance will match the up with the valves opening and it will push extra air/fuel into the cylinder, which is good stuff, but it only happens over a narrow range.

The longer the inlet runner, the lower frequencies these resonance occur, but long runners are more restrictive at high RPMs.

As the inlet runner length is fixed (inlet port of the head, the inlet manifold, the carb throat and velocity stack) the engine designer chooses a length to give the best overall performance. Making it longer will improve low-end torque, short makes it rev better.

If interested here is a web calculator Inlet Calc Clicky

It is only a basic result as the actual answer is dependant on a lot of parameters, but it gives you a good idea.

The inlet length on the 9xx engine is about 16" and this gives the most effect @ 8500 rpm (ish), so not much.

To get the benefits at 3000rpm you need a inlet length of 36" (3 feet!!!), hence you can see it is a compromise.

Now the above is pretty simplified, but you get the idea.

As your S2's 907 is a bit short on low-end torque to start with Pete, making the velocity stacks shorter is probably not a good thing. You wouldn't see much high end gain either as it pushes the resonance further up, it just reduces the inlet resistance.

The air box is much bigger on the N/A so potential inlet masking is not an issue like on the Turbo.

In newer engines the inlets are actually variable length. My Audi engine is a complicated affair, which has got a rotating assembly which allows the inlet length to be switch between 2 lengths, long for low grunt and short for top end whiz. I have programmed the ECU to switch it over at 4500 rpm and you can feel it kicking in.

Phew!!!, my fingers ache......

Hilly

Edited by hilly

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In a nutshell,

long trumpets=low RPM torque

short trumpets=high RPM power

D'oh!!! Now why didn't I think of just typing that :veryangry:

Hilly

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Ok Jan, we are going to need details. I too am running an HC w/45s and about 10 psi. Im running 15 before TDC. Are you running an innercooler or methanol/water? Factory ignition? Other? How about actual dyno figures? What size shoe do you wear? These are thing we need to know!

Cheers,

Clay

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Nice answer Hilly - clever stuff. I didn't want to ask - so :veryangry: to Pete for us 907s.

I have seen a few engines with no airbox at all - separate air foam filters (like on bikes). What is your view on this Hilly?

Iain

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Jan/Clay,

What is your total ignition advance, and how much of it is static and how much dynamic? :respect:

C'mon Jan, give up the dyno figures; don't be a tease!! :harhar:

Clay,

I think we need to know the shoe size. If big, then it is heavy, therefore more power is liberated @ a faster rate :w00t:

In a nutshell,

long trumpets=low RPM torque

short trumpets=high RPM power

MK8CEngine.jpg

Wayne, I never considered it, but why would they want to have two different sizes on that engine? @ high RPM WOT there would be less power from 4 of the cylinders and vice-versa :harhar:

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Nice answer Hilly - clever stuff. I didn't want to ask - so :harhar: to Pete for us 907s.

I have seen a few engines with no airbox at all - separate air foam filters (like on bikes). What is your view on this Hilly?

Iain

Iain

The short answer is no, don't do it as it will actually make the engine perform worse.

Right, time for a bit more science.

Everyone knows stuff expands when you heat it up, well the same goes for air.

Cool air is more dense and hence contains more oxygen.

As cool air has more oxygen in it than warm you can add more fuel and get a bigger bang out of it for the same volume.

A VERY basic rule of thumb is that for every 1

Edited by hilly

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Wayne, I never considered it, but why would they want to have two different sizes on that engine? @ high RPM WOT there would be less power from 4 of the cylinders and vice-versa :respect:

I asked a fellow that was running a McLaren Mk.8 Can-Am car with the Crower Injection that same question once, and he told me that they had found when running the big 427cuin ZL-1 engines on the dyno, the airflow into each trumpet was being affected by its neighbouring one, so they liberated more power at the top end (6000+RPM) by staggering them so each stack had its own relitively undisturbed supply of air.

They nicknamed these Crower injector stacks "porcupines" and you can really feel the ground shake when your standing next to one of the BB Chevrolet Can-Am motors when its running :harhar:

m8f.jpg

Edited by WayneB

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Lee,

I'm 15 @ 1k and 9 @ 5k. I'm running a Pertronix II conversion with a Flame Thrower coil.

Cheers,

Clay

Clay,

Is that total or just dynamic?

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