3.9 vs Bigger engines

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Allan Robinson
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3.9 vs Bigger engines

Post by Allan Robinson »

I have a 3.9 RV8 Dax Rush weighing 765 kg running on an Edlebrock carb and crane 224 cam. When cruising at 70 mph it will return 25 mpg. I’m very pleased with this car especially after installing a bespoke 13 gallon fuel tank which has really enhanced the touring car experience. Unfortunately I have this irrational and compulsive desire to fit a bigger engine. It is a big boys toy after all. I’m sure many of you guys will have been down this same path and might be able to provide some feedback on the following.

What sort of 70 mph cruising mpg I could expect from a 4.6 or bigger engine?
When changing to a larger engine in a lightweight car, did it enhance your driving experience or perhaps something you regretted?

Any views would be very welcome.



DaveEFI
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Post by DaveEFI »

25 mpg at 70 isn't good. A Vitesse will do more like 35 mpg.
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Post by DEVONMAN »

I expect your engine RPM in 5th when cruising is higher than it needs to be for good MPG figures. If you fit a 4.6 then the Mpg will suffer as in such a light car the 3.9 is not working hard at all and the 4.6 will be using more fuel by virtue of the fact that it is spinning faster than it needs to.

A longer legged diff would help but I expect you are after the buzz and better acceleration than the 3.9 gives.
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sidecar
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Post by sidecar »

There is more to MPG than the revs that the engine is running at, out of the box the Eddy 500 carb is generally not setup too well and will run most RV8's way too rich in all modes of operation. (tickover, cruise, acceleration and WOT). The ignition system also plays a large part in all of this, with the vac advance being the main component to aid good MPG. Unfortunately the Lucas vac canisters don't work too well with the Eddy carb.

My mate runs a stage III 4.6 in an SD1, he also runs an Eddy carb and MSD ignition system, we have spent a long time sorting out the carb and the ignition system which runs a MAP sensor, we run around 46 degrees of advance above 3000 RPM when the manifold vacuum is high, this backs off to 32 at WOT. His car returns over 36 MPG whilst cruising at 80 MPH

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Post by DaveEFI »

That is excellent. My 3.5 SD1 EFI (with MegaSquirt and EDIS) only manages about 32 mpg over a long motorway journey at 70. But it has the ancient GM 3 speed auto, so not as high a top gear as the manual. Wonder if I could get away with that much advance at cruise?
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Post by sidecar »

DaveEFI wrote:That is excellent. My 3.5 SD1 EFI (with MegaSquirt and EDIS) only manages about 32 mpg over a long motorway journey at 70. But it has the ancient GM 3 speed auto, so not as high a top gear as the manual. Wonder if I could get away with that much advance at cruise?
I think that you could get away with that much advance on cruise as long as you do some careful testing, you need to make sure that you only run high advance at very high vac levels, it needs to back off very quickly as the throttle is opened and the vac level falls. From memory anything more than 1/4 throttle will cause my mates system to back off the advance. If you are running a 3.5 engine then I guess you are running around 36 degrees at WOT so 46 is only another 10 degrees, you might be able to push it to 48 degrees.

How much advance are you running now including whatever the vac system adds?

Like you said, an auto is always going to struggle with regards to MPG against a manual box due to slippage. Also I guess your 3.5 is having to work a bit harder too.

The leanest that we can run the carb on cruise is only 14.5-15:1 AFR which is not all that lean in the scheme of things, I guess a big old carb just does not atomize the fuel well enough to go leaner, when we go leaner the engine goes as flat as a fart!

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Post by Allan Robinson »

Wow, and there's me thinking 25 mpg was good! I am using an Edie in my 3.9 RV8 with 65/57 control rods. The second stage of 57 is actually too rich, unfortunately I'm limited to the rods available and this would appear to be the best compromise combination. I have been using blue or pink springs to hold off the second stage. The primary jet is .083. When cruising my Left bank AFR is approx 14.5:1 and the right bank is approx 15.5:1 (left bank has alway been richer than the right for unknown some reason) so AFR pretty reasonable I think. I have an MSD 6AL ignition and run a dizzy with no vacuum and 15 degrees BTDC at 1200 rpm. Cam is a Crane 224 and Diff is 3.62:1

So I'm not sure why my MPG would appear to be low seeing that the AFR is in my view reasonable. Anyway, for me the whole point of this post was to gauge if a 4.6 would be a viable option and based on your comments it would appear to be so with very little if any mpg penalty compared to present.

Thanks to everyone for your input and will now be looking for a suitable short 4.6 engine. As always very grateful for your views.

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Post by sidecar »

I use a spreadsheet to compare all the rod and jet combo's, the Edelbrock charts have errors and don't include every jet and rod combo. It is great to see that you are using a Lambda probe, loads of people just guess when setting up the Eddy carb then moan when their setup does not work!

On the primary side you are 16% leaner than the stock setup which is the same amount leaner that my setup is. On the power step you are 22% leaner which is a hell of a lot, my setup is 0% leaner, on the step my AFR is 12-12.5:1. At WOT I'm at 12.5-12.8:1 using 83 secondary jets.

The MSD system that I use and my mate uses is programable which allows us to adjust how the vac advance is setup, unfortunately you can not do this with the standard MSD 6AL system and getting a vac canister to work with an Eddy carb can be difficult.

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Post by Allan Robinson »

Yes I have Lambda probes in both exhaust system connected to dash board gauges. I have owned this car for 10 years now, most of them happily messing around with the carb settings. I've even resorted to making my own control rods but with limited success.

When putting my foot to the floor (low vac), control rod moves to power mode and AFR drops to about 10. (left bank <10 as off scale). I've never managed to resolve this.

Enlighten me please. What exactly is the definition of WOT with regards to secondary jets. Is it as I've just described above but still under acceleration or is it foot to the floor but the engine has caught up?

I probably won't make any more changes to this set up but when I move to the 4.6 is there an advantage to having a programable vacuum ignition as has been mentioned? Is the benefit wort the cost?

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Post by Ian Anderson »

Have you tried removing the lambda sensors and swapping over to see my f the richer side moves from side to side? Could be your sensors are not perfectly matched.

Or a small air leak on inlet somewhere.
Are all your plugs exactly the same colour, lighter would show an air leak on that inlet

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Allan Robinson
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Post by Allan Robinson »

Yes have swapped over sensors and control units but always the same. From memory plugs have been a reasonably even colour but one or two richer. I'll check again tomorrow and post the result.

Although the Edie has duel ports it does feed into a offenhauser 360 manifold which is basically like a plenum. So in my mind should even things out. I did wonder if it was just a phenomena cause by the RV8 firing order and gas flow. The car does; however, run very well and doesn't appear to result in any noticeable lack of performance.

It will be interesting to see what happens when I replace with 4.6 short engine but won't be until after the summer.

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Post by sidecar »

I would not set up the power step by flooring the throttle as that will bring the secondaries into play. Make some sort of throttle stop so that you can open the primaries as far as possile BEFORE the secondaries start to open (a lump of wood under the throttle pedal will do), this will be 1/2 throttle, the primary butterflies will only be 1/2 open. Drive the car in 3rd or 4th gear flooring the throttle as far is it will go (1/2 throttle). Take a note of the AFR and adjust the rods to suit. You must be sure that the rods actually lifted so fit a vac gauge to the non-timed port on the carb, the vac reading will change as the revs change so you sort of have to take an average reading, lets say that it's 5 inches of mercury, you will need to fit springs that lift at this figure of at a slightly higher vac level but certainly not at a lower vac level because this would mean that they will lift too late, in fact they won't lift at all during the 1/2 throttle test!. (weak springs allow the vac to hold the rods down for longer, stiff springs will allow the carb to go on to the power step even at high vac levels). When setting up the power step you need to be 100% sure that the rods have lifted, you can usually tell just by looking at the AFR gauges. Once all that lots done remove the throttle stop, test the WOT AFR and change the secondary jets to get a reading of 12.5-12.8:1, don't go leaner than 13:1 because you will lose power. In order to change the WOT AFR I have found that you need to make fairly big changes to the secondary jets. I think you have been adjusting the wrong circuit, you have been changing to thin part of the rods (The power step) when the throttle is on the floor, really this is a WOT test and therefore the secondaries are providing the fuel at this stage. Don't get me wrong though even at WOT it is both the primary side of the carb (running in the power step mode) AND the secondaries that are fuelling the engine. My thinking is that all the adjustments that you can do on the primary side of the carb should be done when only the primary side of the carb is in use, then adjust the secondary side of the carb when that is also in use. The larger the engine is that is sitting under the carb the smaller the secondary jets tend to need to be, this is because a large engine creates more vacuum at WOT than a small engine. My tests have indicated that the standard 95 secondaries seem to be about right for a 3.5 engine, they are way too big for a 4.6 engine. As a total guess I reckon you need 83-86 secondaries.

I have also made several sets of rods, it is a total pain in the ass, they have to be accurate across their length within 1 thou. My engine runs 69 on the cruise, the car drives very well, if I go to a 70 thou rod I can't even get the car off my drive. I run a stage III 4.6 in a Cobra replica, my mates SD1 runs a similar engine to mine, his rods are 68 on cruise, if he runs a 69 the car is a flat as a fart!

My definition of WOT is wide open throttle at high-ish revs (over 4k), of course the revs will still be rising and in order to slow the rate at which the rev are rise down you need to be in a high gear, this also means that you are doing 'naughty' speeds so choose your test location carefully. This is where a rolling road would help but in my opinion they are useless when setting up the cruise setting.

I run the programable MSD 6AL, there is a fair amount of wiring required and you need a locked out dizzy, I modified a standard lucas dizzy by removing all the guts from it, I made a new base plate and fitted a an MSD pickup. This system made a huge difference over the modified Lucas system that I was using, it would not make any performance difference over the standard MSD 6AL but with my system I can change the advance curve to anything I like using my laptop, the MAP system is also controlled by software.

One last thing, I have seen loads of these carbs where the air filter lid is way too close to the top of the carb, straight away you can spot the issue when the air filter stud in the middle of the carb is only about 1" long! This really messes up the calibration of the carb, you can help the primary side of the carb breathe a bit better if you lose the choke then cut the whole choke horn off the carb but this won't improve air flow into the secondary side of the carb.
Last edited by sidecar on Thu Apr 27, 2017 8:33 am, edited 4 times in total.

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Post by DEVONMAN »

Allan Robinson wrote:
Although the Edie has duel ports it does feed into a offenhauser 360 manifold which is basically like a plenum. So in my mind should even things out. I did wonder if it was just a phenomena cause by the RV8 firing order and gas flow. The car does; however, run very well and doesn't appear to result in any noticeable lack of performance.
I had a similar situation when I had a 360 Offenhauser fitted but after I changed to a Edelbrock Performer both sides of the engine then gave the same mixture readings.

Like you I put it down to the RV8 firing order , 5&7 firing close to each other in one corner of the plenum.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8.
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 57 years of driving. Same Car, 5 engine upgrades !!!


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sidecar
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Post by sidecar »

DEVONMAN wrote:
Allan Robinson wrote:
Although the Edie has duel ports it does feed into a offenhauser 360 manifold which is basically like a plenum. So in my mind should even things out. I did wonder if it was just a phenomena cause by the RV8 firing order and gas flow. The car does; however, run very well and doesn't appear to result in any noticeable lack of performance.
I had a similar situation when I had a 360 Offenhauser fitted but after I changed to a Edelbrock Performer both sides of the engine then gave the same mixture readings.

Like you I put it down to the RV8 firing order , 5&7 firing close to each other in one corner of the plenum.

Fuel slots and or dams in the bottom of the manifold might help but really you would need to monitor each cylinder before going down that route!

I have wondered whether there is anying to be gained by swapping out my Eddy Performer 180 for a 360 manifold, did you notice any loss in performance when you swapped to the Performer manifold?

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Post by DEVONMAN »

Without testing on a rolling road it's difficult to say. The engine has twin turbos so all hell breaks loose when they kick in. However the low revs are more responsive and there is less lag with the performer manifold as expected but top end performance is probably affected slightly.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8.
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 57 years of driving. Same Car, 5 engine upgrades !!!


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