Increasing. Compression ratio

[v8riot]

Hi all,

I have a 1995 LDV 400V8 which is fitted with the 8.13:1 3.5 litre SU carb engine.
I have been looking to increase the C/R as I intend to convert it to run on lpg
and was wondering what sort of increase would be achieved by just fitting the 4.6 litre cylinder heads? The engine is low mileage and in good order so I thought this may be a simpler solution than changing the pistons.
This is the the later block with the bosses for cross bolting the main bearings but has not been machined to do so.

Many thanks Joe.

[Triumph-V8]

The later heads are machined a little bit to
compensate the bigger composite gasket.

You can mill the original heads but the surface
for the inlet manifold must be machined, too.

Bigger rise in compression ratio must be achieved
with other pistons with less dish.
As the engine did some miles it would be no good idea
to simply swap to those pistons.

[ChrisJC]

You won't get a worthwhile enough change in compression by just fitting later heads. You will make the tappet preload all wrong though and have to faff about with shims under the pedestals...

I would swap the pistons, and also use the later heads with composite gaskets. Get 10.5:1 pistons if you can.

Chris.

[JP.]

...... 8.13:1 3.5 litre SU carb engine.
I have been looking to increase the C/R as I intend to convert it to run on lpg
.

Why bother, it will run good on LPG with just 8:1 cr.

Most 70's & 80's era V8 powered US cars are low compression and most of them in Holland are running on LPG cause its cheap ( about 0.46 GBP Cents/Litre). and they are all running fine.
Running a 8.4Cr Ford 400 V8 (6.6litre) in my daily F250 truck myself on LPG for years. Still goes like stink even with a loaded car trailer behind it.
Running an Impco 300a model E vaporizer with a 50/70 mixer.

[garrycol]

I have a 3.5 running on gas and petrol - ignition set at 9 degrees BTDC.

Any power difference is certainly not noticeable from the drivers seat but it does use 20% more on gas.

If it were me - I would put in a good cam, get the ignition sorted right out and look at your exhaust.

Failing that, then some SDI higher comp pistons but you will not get much from 4.6 heads.

[v8riot]

Thanks for the input I may leave as is first then and see how it runs. I was thinking being an ex police riot van and almost 2.75 tons unladen Plus an auto it might end up gutless on lpg.
If I do feel I need more grunt which cam would you recommend for a heavy automatic ? I am using it as a camper so need good cruising and hill climbing ability so torque mainly not out and out power.

If I decided to fit 10.5:1 Pistons are these compatible, rods etc with my engine it being one of the odd ball 3.5 's manufactured in the mid 90's so has late type block.

Thanks again.

[JP.]

Thanks for the input I may leave as is first then and see how it runs. I was thinking being an ex police riot van and almost 2.75 tons unladen Plus an auto it might end up gutless on lpg.
Thanks again.

'73 F250 Camperspecial. Meaning not the regular lightweight F250, all Camperspecials have the heavy rugged F350 chassis and suspension.
Its not a lightweight.
When I floor it, it does surprise a whole lot of car drivers and this rig doesn't give a sh.t if the trailer is behind it or not. Does a 100+ mph at ease. still on the 8.4cr. 400cubic inches, Edelbrock Performer intake, Hooker headers, MSD6 ignition, non working Holley 600 ( as a base for the Impco mixer ) and just one Impco 300a model E vaporizer.

Economy... whats that, in all circumstances.. 2 miles to the litre but wasn't much better on fuel.

Low Cr gives torque. Torque is what you want in heavy stuff.
You could always ad a turbo to raise dynamic compression an get more benefit from the high octane LPG.

[ChrisJC]

The 10.5:1 pistons will fit your existing rods. It is just the dish in the top that is different.

I would think that 8.13:1 would be absolutely sh*t running on LPG to be honest. I ran 10.5:1 on LPG and it was pretty good.

Chris.

[garrycol]

I would think that 8.13:1 would be absolutely sh*t running on LPG to be honest.

Its not - as I indicated actually goes very well but ignition does need to be advanced to as much as petrol can handle - with LPG only you can go more advance.

[ChrisJC]

I would think that 8.13:1 would be absolutely sh*t running on LPG to be honest.

Its not - as I indicated actually goes very well but ignition does need to be advanced to as much as petrol can handle - with LPG only you can go more advance.

That surprises me to be honest. As increasing CR normally increases BHP, the step from 8.13 to 10.5 is about 60BHP if I recall the standard BHP outputs correctly. I seem to think that the 8.13:1 is about 120BHP and the 10.5:1 Rover P6 about 180BHP.

Chris.

[garrycol]

For sure you get more power with higher comp ratio - but you said that the lower comp ratio runs like poop - well my point is simply it runs like a low comp engine - runs as good as a low comp petrol engine - just as a hi comp petrol engine will go better than a low comp petrol engine.

A low comp RV8 does not run like poop - it just runs like a low comp engine.

[DaveEFI]

I would think that 8.13:1 would be absolutely sh*t running on LPG to be honest.

Its not - as I indicated actually goes very well but ignition does need to be advanced to as much as petrol can handle - with LPG only you can go more advance.

That surprises me to be honest. As increasing CR normally increases BHP, the step from 8.13 to 10.5 is about 60BHP if I recall the standard BHP outputs correctly. I seem to think that the 8.13:1 is about 120BHP and the 10.5:1 Rover P6 about 180BHP.

Chris.

But were those factory figures measured in the same way? IIRC, the way 'factory' BHP figures were calculated changed from when the RV8 first arrived.

[SimpleSimon]

Never in a million years would a 2 point increase in CR on a RV8 deliver another 30 let alone 60 bhp but most of you know that

[DaveEFI]

I'd love to see some actual figures - all from the same test method - on all the varieties of standard RV8s.

On the P6, they dropped the CR from 10.* to 8.* when 5 Star petrol was banned, and there was a very noticeable different in grunt. The factory claimed they had made other mods to the engine etc so there wasn't. But it was well documented in the press of the time, and their performance figures confirmed it.

The Vitesse engine has a claimed 190 bhp with 9.75:1 CR. Some say this is rather optimistic. Seems unlikely the carb P6 3.5 made anywhere near 180 bhp.

[JP.]

I would think that 8.13:1 would be absolutely sh*t running on LPG to be honest.
Chris.

Thinking and experience differs a lot.
What you would think and what is real can differ a lot to.

Had a Ford 2.0 OHC Pinto engine, with 8:1 cr ( Transit and P100 & US Spec had low cr ) it had 90bhp on the rollers and lots of torque but it died after 4500rpm while a normal 9.2:1cr OHC pinto engine is rated at 101bhp and has no issues till 5500 rpm.( Ford Spec ).
The LC engine was quicker out of the box by the extra amount of torque but lost it after 4500rpm by its higher cr variant.

Both engines ( in a Capri ) had the same specs, cam , heads, valves, intake, exhaust, carb, etcetera. Only difference was pistons and so on cr. 8:1 vs 9.2:1.
( USA spec Ford Capri vs European Spec Ford Capri ).