Rover V8 Boxer carb installation blog

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Riley RMF
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Post by Riley RMF »

Dave - I think this has to be the way to go. As mentioned before, from the calculations I've been doing the required needles don't exist.

I've been measuring up the BDU needles that came out of my Boxer carbs this morning, they have been substantially modified.

In the Des Hammill book "The SU Carburettor High-Performance Manual" he describes how to alter needles.

Looks straightforward enough

What could go wrong?

;-)



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Post by unstable load »

Riley RMF wrote:Looks straightforward enough

What could go wrong?

;-)
Brave man, watching with interest.
Cheers,
John

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Post by Riley RMF »

So the remaining big thing I haven't talked about yet is the pulsating effect from the firing of cylinders 5 and 7.

As a simple diagram a standard 2-carb setup across 2 rotations of the engine looks like this:

Image

Note that LH and RH on this diagram refer to the ports being supplied by the carbs, not their physical location. And of course I've ignored the effect of exact firing point for simplicity.

Everything is happy, nice and regular.

The Boxer 4 carb looks like this:

Image

And there's the 5 and 7 issue - everybody else has a more regular pattern.

Just for interest, and never mind about how it might be engineered, here's what it looks like if we were to have 4 carbs connected transversely:

Image

Now we have 3 carbs behaving like 5 and 7 were in the last case, and only 7 and 8 behaving what we'd call more reasonably.


Back to the Boxer. So this is why we need a sizeable balance pipe, and I would have thought connected to all 4 carbs to maintain the suction on 5 and 7, and a heavier oil in the dashpots to stop the 5 and 7 carbs jittering about.

If I have this right, because the plunger/needle with heavier oil has a greater lag response following an increase in the throttle disc, the transient airflow across the bridge is increased relative to what it would have been with a lighter oil. This works like a pump jet in a fixed jet carb and improves the pick up of the engine. If you drive with a heavy right foot you'll see the effect in terms of increased fuel consumption.

The thought also occurs that we generally think of the dashpot oil from the point of view of it's primary job in smoothing the movement of the plunger/needle in response to throttle change. But it must have a secondary job. In real-world terms the springs are fairly weak. The movement of the plunger is a bit like having a weight on the end of a length of knicker elastic. So if you're driving at a constant speed across rough ground the plunger is going to want to bounce around and will do weird things to the engine response. The dashpot oil minimises this. I would guess this effect will be well known to the off-road guys?

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Post by ChrisJC »

Unless I've misunderstood your diagram of a standard setup, you haven't factored the plumbing of the manifold into it.
One carb does cylinders 1,4,6,7 and the other one does 2,3,5,8. Those sequences have perfectly regular suction pulses when you relate them to the firing order, which is why there is no need for a balance pipe on the standard manifold.

Chris.
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Post by Riley RMF »

OK! I hadn't twigged that. I'll certainly check it out when I'm next at my workshop.

Thanks

Phill

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Post by Riley RMF »

Chris of course is quite right. So in the way I represented the firing order before, the diagram should look like this:

Image

I've been attending to other bits of my project car for a while now, but back on the issue of carbs. More stuff to follow.

Phill

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Post by ChrisJC »

Looks good. In fact it looks like how a sane manifold designed would deal with the dual-plane crankshaft problem.

It is a shame there isn't such an easy fix for the exhaust!

Chris.
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Post by Ian Anderson »

Need something similar on the inlet to the exhaust on a GT40 which they call a 180 degree crossover.
Basically take the center two cylinders from each side and cross them over to the other

If you can find a way to do that I believe you have your problem solved

Ian
Owner of an "On the Road" GT40 Replica by DAX powered by 3.9Hotwre Efi, worked over by DJ Motors. EFi Working but still does some kangaroo at low revs (Damn the speed limits) In to paint shop 18/03/08.

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Post by ChrisJC »

Yes, it's OK on a mid engined car as you have space above the transmission to sort out the rats nest of exhaust pipes!
Not so easy on a normal car. I have seen it done underneath the sump, but that had very long primary pipes.

Chris.
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Post by Riley RMF »

Thought before I go any further it would be worth stock taking on what we have so far.

Sources are my quote 12/7/17 from kiwicar on this thread, Eliot's posting from the same day, Fast Car magazine March 1994 (included below), and some sound sensible input from ChrisJC.

Assumption is use of HIF6/HIF44 carbs - holler if I have anything wrong.

1) Balance pipes: The writeup from Fast Car shows two different configurations, but in both cases all four are connected.

2) Springs: Only source on this is kiwicar who refers to lighter springs, which make sense as has been discussed. There is only one spring that answers this description which is Red at 4.5oz, standard spec being Yellow 8oz. Fine tuning by cutting down, stretching etc.

3) Needles: The Fast car article states BBG for "most engines".

4) Dashpot oil: A thicker oil required to deal with the pulsation effects


Other points:

5) Linkages need to be in good shape to keep all 4 carbs in tune

6) Radius front edge of the carb pistons

7) Fit parallel trumpets instead of the Rover inlet castings

8) Measure CO at the exhaust-manifolds individually


A fair bit to do on my car before I'm ready to start the engine, so there's time to get out my calculator and run some calcs...

Phill

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Post by Riley RMF »

Work on the Boxer inlet manifold and carbs is progressing, I'll post some more on that in January. Hopefully.

Here's a taster in the meantime

Image

But here's a thought to chew over during the gorging season...

The 35DLM8 distributor advance mechanism is going to see only half the inlet air flow rate. How's that going to affect things?

Cheers all

Phill

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Post by ChrisJC »

Is that right? I'm not sure in my head I can figure that one out. For a given load, you've got 4 carbs instead of two, so each carb is delivering half the fuel of the standard setup. Does that mean it's got half the vacuum?

I suspect that it's a bit of a halfway house between the standard setup and one that flows twice as well. Probably a need to have the distributor curve tweaked.

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Post by Eliot »

Good to see the fast car magazine article - that presumably was the one he referenced in his notes on my manifold archive.
Eliot Mansfield
5.7 Dakar 4x4, 4.6 P38 & L322 TDV8
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Post by Riley RMF »

Well it's that good old Bernoulli equation at work. And there are I guess two factors to consider, the amount of air flowing through the carb, and achieving the correct air/fuel mixture. The distributor only cares about vacuum so for that we can focus attention on the airflow alone.

Loads of references you can find on the equations for a carburettor, but a relatively gentle one here:

http://what-when-how.com/automobile/car ... utomobile/

If you scroll down to "Air Flow in Carburettor" you'll find the relationship between arflow speed V (which for quad-carb is halved relative to the standard twin-carb) and the pressure drop p1-p2. So it's a V squared relationship.

I suppose what was behind my question was whether anybody had any experience of the effect on the distributor in the Boxer, or any other multi-carb setup. I've no idea, for example, how those Eidlebrockky etc devices work and whether they have any effect on the distributor.


And Eliot - you're very welcome. Just trying to work out what's known about the Boxer setup.

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Re: Rover V8 Boxer carb installation blog

Post by Riley RMF »

Hi all,

Been working on the throttle linkage

Image

There's a write up on my blog page here:

https://www.facebook.com/Riley-RMF-V8-T ... 285650879/

Phill

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