3.9 vs Bigger engines

General Chat About Engine Build

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Allan Robinson
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Post by Allan Robinson »

Thanks for the very comprehensive advice. So as suggested I inhibited the secondary throttle bodies and went for a test run.

Cruise vacuum was approx. 12”hg with an AFR between 14.5 and 15.5. I’m happy with that and assume Vac is within expected parameters?

With a reasonable brisk acceleration vacuum went to 8”hg all the way down to about 2”hg depending on how aggressive I put the pedal to the floor (1/2 way throttle for this test). From this I feel that an orange spring of 5hg would be a reasonable selection. AFR went down to 10 on left bank and 11.5 on the right. OK this is rich but ran well. Step up control rod being 57 should in the theory already be weak!

WOT: I will have to delay playing with this until roads and traffic allow. I have replaced the 98 secondary jet with a 89 for now.

Set up at present is 83 primary jet with 65/57 control rod. Orange 5”hg spring. 89 secondary.

With regards to the perceived low MPG and the rich power mode the only thing that is niggling in the back of my mind is ‘have I previously cocked up the cam timing’ (11000 miles previous). I do recall that the Coyles cam gear had 3 key ways and I selected the Triangle one (4 degrees retarded) for improved lower end power. I also have Rhodes lifters. I will need to recheck for piece of mind and will reset to the 0 degrees, factory setting. Unfortunately this will involve dismantling the front end of the car so not a quick job. I will also pay attention to spark plug condition at this time.

Interesting feedback re the Offenhauser 360 manifold and rich left bank. From memory the 360 gave better max power with the Edelbrock better for torque. I watch with interest.



Allan Robinson
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Post by Allan Robinson »

Ooops correction! The triangle timing mark is Advanced for higher rev HP.

DEVONMAN
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Post by DEVONMAN »

Allan Robinson wrote:Ooops correction! The triangle timing mark is Advanced for higher rev HP.
I believe advancing the cam will give better low end torque performance.
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 »

It really does seem that your carb does need to be setup a fair bit leaner than standard when it on the power step. As I'm sure you know 57 is the biggest diameter for all the rods on the power step therefore your only choice is to go down on the primaries but at the same time you don't want to lean off the cruise setting anymore than what it is set to now. (15% leaner than base)

I have worked out a couple of settings BUT you would need to modify some standard rods which is a pain in the bum! I made a 'rod holder' which I spin in my lathe, I then use fine wet and dry to thin a rod down, I sometimes use an old feeler gauge as the backing for the wet and dry paper. You need a decent mic to measure the rods, I find that a vernier is not good enough.

Your current setup (83 -65/57) is 15% and 22% leaner than base, you need to be even leaner than 22% on the power step.

80 - 61/55 is 15% 28%
80 - 61/57 is 15% 32%

12"hg is not all that high for cruise, from memory mine is 18" hg but then again your engine is smaller. It depends on loads of things, not least the cam, I guess it is what it is and its not too bad.

Getting all this stuff right is important but I can't help thinking that having loads of ignition advance at high vac levels is one of the main things required for good MPG.
Last edited by sidecar on Fri Apr 28, 2017 8:21 am, edited 1 time in total.

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Post by DEVONMAN »

Notwithstanding the need to get the mixtures and timing correct, I'd be interested to know what the RPM of the engine is at say 70MPH in top gear.
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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Allan Robinson
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Post by Allan Robinson »

I am half way through dismantling the front of the car at present to get at the timing cover. I just want to check that I haven't done something stupid like a tooth out. I will reassemble with '0' (factory setting) on the timing sprocket. It does seem strange that my carb settings are so weak yet running result are rich. All plugs look good with right bank slightly weaker than left which confirms readings from lambda sensors. I did wonder if the Rhodes lifter might be an added complication as only fully pumped up after 3500 rpm.

I did read your post on modified control rods. Something I might consider again if no joy.

I'm pretty sure that 70 mph equates to 3000 rpm or just over but will confirm during next week when all back together.

Allan Robinson
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Post by Allan Robinson »

Many thanks for working out the % rod / jet settings and will test out your suggestion below if no joy from my cam timing investigation.

(Your current setup (83 -65/57) is 15% and 22% leaner than base, you need to be even leaner than 22% on the power step.

80 - 61/55 is 15% 28%
80 - 61/57 is 15% 32% )

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Post by DEVONMAN »

Allan Robinson wrote:
I'm pretty sure that 70 mph equates to 3000 rpm or just over but will confirm during next week when all back together.
Those numbers equate to what a typical 2000cc 4 pot engine would be running.

It must be pretty lively even in top gear. No doubt you like that but unless you change the diff ratio you won't get much better MPG .
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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Allan Robinson
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Post by Allan Robinson »

Good point. Unfortunately I just had the diff completely refurbished last year including new gears. An opportunity missed perhaps. I believe the viscous LSD diff was originally from a Sierra Cosworth which would fall in line with your assessment. It does however make for a very lively car even at 70 mph.

I also installed a bespoke 13 gallon fuel tank as constantly looking for fuel stations was becoming a pain, especially when touring with other cars. I'm still looking to upsize to a 4.6 and if I can get at least 20 mpg I can live with that.

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Post by SuperV8 »

For my 2p worth: In my Rush I went from:
3.5 to supercharged 3.5, to supercharged 4.6.
I haven't run just a normally aspirated 4.6 - except when my supercharger failed and I took the belt off to drive home.
I was looking at improving my supercharged 3.5 with a serpentine front end as I was getting supercharger belt slip but ended up buying a 4.6 of ebay! Ended up having to tophat liner the block which I did expect, but now have a very strong engine.
Some people say the 4.6 doesn't rev as well as the 3.9 but mine pulls very well to 6000+ and gets stronger past 5000 which no doubt is down to the supercharger over coming the heads flow restrictions.

I get arround 20mpg with the 4.6 supercharged V8 (mappable spark and injection ECU and coil packs) but that is with lots of 'overtaking' and 'spirited driving'
The main reason to go to a larger capacity is to increase torque lower down the rev range. If your 3.9 spins the wheels then the 4.6 will be no quicker until you get better tyres which I had to! It will also mean your gearing will feel even lower so 3rd feels more like 2nd and makes 1st even more redundant.
I also fitted a taller diff from the cosy 3.62 to 3.3 and my new tyres are wider and taller which is an easy way of helping make the gearing taller.
Don't forget the Rush is one of the most un-aerodynamic type of cars on the road
https://en.wikipedia.org/wiki/Automobil ... oefficient
Worse drag coefficient than a hummer! so it will never give great MPG at 70+.

Don't forget to budget for clutch and gearbox upgrades? I went to Tremec T5 and a Haldex clutch. My HD SD1 clutch had witness marks on the end stops from my 3.5 supercharged (with slipping supercharger belt and poor tyres!)

Tom.
Dax Rush 4.6 supercharged V8 MSII

Allan Robinson
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Post by Allan Robinson »

Cam gear reset to standard timing and now back on the road. Confirmed that 70 mph equates to 2800 RPM.

Side Car, are the 61/55 jets you suggested an off the shelf item? Do you know of a uk supplier who might hold a comprehensive stock of Edelbrock metering rods?

Tom (Super V8). Many thanks for the steer regarding the additional considerations when installing a larger engine. Other have also shared the same conclusion.

Tell me please, is the SD1 gear box not strong enough? Was swapping the SD1 gear box for the Tremec T5 a straight forward job in the Dax? Has gear changing improved? (the SD1 is a bit ponderous) and was there any issues with getting the gear stick in the same location as the SD1?

It also looks like I would need to fit a 3.3 ratio in the diff to preserve a reasonable MPG.

What size tyres/wheels did you fit? I have Toyo 245 x 45 x16. I'm not sure that a larger tyre would fit without hitting the rear arches.

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Post by SuperV8 »

Tell me please, is the SD1 gear box not strong enough? Was swapping the SD1 gear box for the Tremec T5 a straight forward job in the Dax? Has gear changing improved? (the SD1 is a bit ponderous) and was there any issues with getting the gear stick in the same location as the SD1?
The T5 does have a higher torque rating than the SD1 LT77, really depends on usage, waffling along county roads i'm sure the LT77 would be fine but may not cope so well with drag racing?
Also the T5 is a fair bit lighter than the LT77 box, and also shifts much better, my LT77 didn't like being rushed! The cosy T5 brings the gearlever out in the correct place but I used a TVR T5 which meant I had to make a lever extension of something like 100mm. You can buy the TVR bellhousing of the shelf to mate the RV8 to T5 (if your T5 doesn't come with one.
You'll need a new prop shaft, and probably new clutch and I the T5 is wider than the LT77 so I had to cut a hole in the drivers side fibreglass footwell panel.
It also looks like I would need to fit a 3.3 ratio in the diff to preserve a reasonable MPG.
3.1 would be even better
What size tyres/wheels did you fit? I have Toyo 245 x 45 x16. I'm not sure that a larger tyre would fit without hitting the rear arches.
My new tyres are 255 40 17. I'm using the narrow rear arches and have had to cut some away due to rubbing.

Tom.
Dax Rush 4.6 supercharged V8 MSII

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Post by sidecar »

Allan Robinson wrote:
Side Car, are the 61/55 jets you suggested an off the shelf item? Do you know of a uk supplier who might hold a comprehensive stock of Edelbrock metering rods?
I think I stated that you would have to modify some standard rods in order to make the rods that I quoted. My mate does have an extensive collection of rods and jets. The problem with modify rods is that it takes ages and then if it does not work it's a real bummer!

BTW I run a Cosworth T5 in my Cobra replica but with a tall first gear (Quaife gears), the gears and shafts are shot peened and polished, it's much nicer than an LT77 or R380 box, from memory I think that I could hit 50 MPH in first if I revved the engine to 6k. In practice I don't do this but it is much nicer than the standard first gear ratio which ends up revving the crap out of the engine whilst you are doing 10 MPH!

Allan Robinson
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Post by Allan Robinson »

Apologies Side car, yes you did say that standard jest would need modifying.

I made a tentative enquiry into having my diff gears changed. Apparently new 3.1 gears are now hard to come by. A 3.3 would save me 250 rpm @ 70 mph and 225 x 40 x17 rear wheels and tyres another another 150.

I first thought that buying a 4.6 short engine was a reasonably inexpensive project; however, doing the job properly with gear box, prop shaft, clutch, diff upgrade and perhaps larger rear wheels has given me lots to think about. I am however much more informed than I was at the start of this post. Lots to think about and thanks to everyone for your input. I'm now going to look into gear boxes and bell housings which I know nothing about. Yet!

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Post by Rossco »

What diff are you running.

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