Fuel efficiency on an RV8

sowen · Post by **sowen** » Wed Sep 10, 2014 8:13 pm

Just thinking out loud, has anyone got any thoughts on cheap(ish) cost effective ways to improve the efficiency and economy on an RV8? The assumption being to ignore the fuel and ignition systems as they are very well documented, but what would people do to the bare engine itself?

I have a 3.5 in an SD1 with hotwire and was wondering about having the heads skimmed to get the compression closer to 10:1 (I know cr is done by pistons and the manifold port alignment and rocker clearances would be out), just wondered if there were any other practical ideas out there?

It's just that I don't want to feel so guilty on my wallet about going flat out as often, and maybe pick up a few extra mpg on the cruise!

gelmonkey · Post by **gelmonkey** » Wed Sep 10, 2014 8:51 pm

I run a 4.3 with a webber 500 and spent a long time getting the engine built just right.
With my hand on heart I get 30 + to the gallon on a run which is brilliant even if I do say so myself
Muscle Manta is going to sort out the final tune on the carb in the not too distant future and that should produce an even smoother and more powerful motor for me.
Build it right and 30+ should be achievable.
I can give it a bootful whenever I like and still feel comfortable that its not hurting my wallet too much.

CR is 9.53.1
Bottom end lightened balanced and blueprinted to the best of my ability.
Ali flywheel and double clutch

TVR heads and a Typhoon cam from RS
Custom built 1/58 headers with ceramic coating into 2/14 secondaries

cheers
p

Darkspeed · Post by **Darkspeed** » Thu Sep 11, 2014 10:37 am

Do what the mainstream are doing - make all the internals lighter - Teflon coated/Slipper pistons, alloy rods, alloy flywheel, free breathing heads. And then its all down to the fuel and ignition - fully programmable - Then its the car light and as slippery as possible.

VVT may be a touch ambitious with a single cam

so we'll not worry about that.

Oh - "cheapish" - Eeerrrrr - vacuum gauge

DaveEFI · Post by **DaveEFI** » Thu Sep 11, 2014 12:41 pm

The most worthwhile mod you could do to improve the cruise MPG is raise the gearing. Fit an EFI auto rear axle.

ChrisJC · Post by **ChrisJC** » Thu Sep 11, 2014 5:47 pm

http://en.wikipedia.org/wiki/Engine_efficiency

Chris.

mgbv8 · Post by **mgbv8** » Thu Sep 11, 2014 8:48 pm

I cant comment on this one I'm afraid

Quagmire · Post by **Quagmire** » Fri Sep 12, 2014 9:34 am

Vehicle shape has a big impact, also transmission losses have a large effect too!

Both my vehicles have almost the same engine (3.5's):

My LR 90 gets about 18mpg with EFI and EDIS (MS1Extra)
9.35:1 compression, 10 bolt heads. Standard Cast 4-2-1 exhaust manifolds.

My Rover P6 gets about 25mpg with SU's and EDIS (MS2Extra).
9.25:1 compression, also 10 bolt heads. Standard P6 exhaust manifolds.

sowen · Post by **sowen** » Fri Sep 12, 2014 10:03 pm

Well, I've not been shot down for thinking about skimming the heads so may take a spare set round to a local engineers and see what they can do!

A little bit more 'pep' wouldn't go amiss, I just begrudge pouring fuel in knowing that I could get my hands dirty and get a little bit more bang going on inside my engine

unstable load · Post by **unstable load** » Sat Sep 13, 2014 8:10 am

Skimming the heads should achieve your aims a lot cheaper than the pistons route.
Which gaskets you use will also make a difference, so an already skimmed head with tin gaskets could possibly achieve the same effect.

If you skim the heads then the inlet manifold will be a tighter fit, so a smidge off the inlet flange faces at the same time should rectify that. I'm sure you could calculate the "smidge" more accurately than my estimate, though...

sowen · Post by **sowen** » Sat Sep 13, 2014 9:18 am

Exactly, worst case if it all goes horribly wrong would be to fit composite gaskets in or refit the old heads. The original plan when buying the car was to run it off lpg, and I'm sure it's stated lpg likes compression.

Getting hold of a spare injection manifold shouldn't be too hard, just finding one that's dirt cheap enough to butcher.

The engine I have is good, runs smooth, no oil loss and good oil pressure so I'm reluctant to touch the bottom end, and should I decide to go and fit a 3.9, I can just swap the heads over

stevieturbo · Post by **stevieturbo** » Sat Sep 13, 2014 10:40 pm

Really this depends on exactly what you have, what ability to fine tune it you have...How much you're prepared to spend and for what realistic gain ? etc

Is it worth spending £5k to gain 2-3mpg ?

A lot would also boil down to how well you are able to tune fuel/sparks to make it more economical. This would be the best way without even going near the engine

After that it's good efficient heads, no large ports, nice torquey camshaft, higher compression as you suggest.

yes you could go nuts with lightweight internals...but again at what cost ?

The standard tuning principals of head/cam would at least also yield noticeable power and torque gains which would help as less throttle would be needed to maintain progress.

Likewise proper efficient inlet setup, and exhaust.

And as others said, you can play with gearing too. But again...at what cost ?

DaveEFI · Post by **DaveEFI** » Sat Sep 13, 2014 10:44 pm

There is an SD1 final drive available with taller gearing. They are quite rare, but don't feltch the sort of money serious engine mods cost.

sowen · Post by **sowen** » Sun Sep 14, 2014 9:58 am

I only ask about skimming the heads for greater compression as I don't remember coming across it a specific modification as opposed to replacing the pistons or fitting tin headgaskets. Changing the cam would be an idea, but that would realistically only happen if I was to build a new engine. Most of my driving is around 1000-2200rpm, so top end isn't a great concern, and delving deep into the bottom end to lighten the rotating assembly again isn't something I want to do.

DaveEFI wrote:There is an SD1 final drive available with taller gearing. They are quite rare, but don't feltch the sort of money serious engine mods cost.

I haven't actively started looking for one yet, but it is high up on the list of things to acquire

stevieturbo · Post by **stevieturbo** » Sun Sep 14, 2014 10:02 am

sowen wrote:I only ask about skimming the heads for greater compression as I don't remember coming across it a specific modification as opposed to replacing the pistons or fitting tin headgaskets. Changing the cam would be an idea, but that would realistically only happen if I was to build a new engine. Most of my driving is around 1000-2200rpm, so top end isn't a great concern, and delving deep into the bottom end to lighten the rotating assembly again isn't something I want to do.

DaveEFI wrote:There is an SD1 final drive available with taller gearing. They are quite rare, but don't feltch the sort of money serious engine mods cost.
I haven't actively started looking for one yet, but it is high up on the list of things to acquire

Changing cams is not all about top end power.

Really you're most cost effective and viable means of improving fuel economy in daily use is taking proper control over fuel/sparks to allow tuning to achieve better economy.

sowen · Post by **sowen** » Sun Sep 14, 2014 12:44 pm

stevieturbo wrote:
sowen wrote:I only ask about skimming the heads for greater compression as I don't remember coming across it a specific modification as opposed to replacing the pistons or fitting tin headgaskets. Changing the cam would be an idea, but that would realistically only happen if I was to build a new engine. Most of my driving is around 1000-2200rpm, so top end isn't a great concern, and delving deep into the bottom end to lighten the rotating assembly again isn't something I want to do.

DaveEFI wrote:There is an SD1 final drive available with taller gearing. They are quite rare, but don't feltch the sort of money serious engine mods cost.
I haven't actively started looking for one yet, but it is high up on the list of things to acquire
Changing cams is not all about top end power.

Really you're most cost effective and viable means of improving fuel economy in daily use is taking proper control over fuel/sparks to allow tuning to achieve better economy.

I've got Megajolt which I was beginning to fit before I swapped over to efi, and a wideband o2 gauge sat in my other car. The other plan was to run it off lpg so the hotwire efi would be just a means of starting and occasional use when required.