Thread: What makes some 4.6 V8s high compression?

the difference is in the pistons - rest should be the same.

HTH

LRH

IIRC the piston are different between the low and hi comp engines. The low comp pistons (obviously) have a deeper dish in the top of the piston than the hi comps.

Can't give you exact figures but i had to reduce the combustion chamber volume on my 3.9head to 28cc (From 32cc's) to bring the comp ratio up to around 9.5:1. The amount of material removed worked out to be almost the exact thickness of a compressed composite gasket.(my 3.9 originally had tin gaskets)

I was fortunate in that my manifold fitted up fined between the heads after machining, i have heard of other having nightmares trying to get the manifold to seal after the heads have been machine "too" much.
You will also need to shim up your rocker pedestals to maintain correct lifter preload

If you could source a set of hi cmp pistons cheap and get any machining done on the cheap i would go that way

Luke

Originally Posted by LRHybrid100

the difference is in the pistons - rest should be the same.

HTH

LRH

So if that is all that is different, would it be worthwhile swapping over the pistons while the engine is out for the Hi-Comp ones. Or would the cost of the work and the pistons, plus the fact that you need to use premium grade petrol, be more than the savings and benefits when I use LPG?

Diana

When I had my 4.6 rebuilt, I opted for HC pistons ($800 including rings and gudgeon pins from Bruce Davis) as I was planning sequential LP gas injection.

It doesn't worry me paying a bit more for high octane petrol as I use so little of it.

Hmm, this is typical. You need a hi comp for gas and I need a low comp to work with my blower. Most rebuilt 4.6s these days seem to be hi comp. You just need the pistons I would not shave the heads as may have issues later on. Just my thoughts. Tony

Hmm, what did I do with the old LC pistons?

Originally Posted by p38arover

...It doesn't worry me paying a bit more for high octane petrol as I use so little of it.

Ron

What I'm trying to do is make this vehicle and engine as simple as possible, so will be using a regular vapour system. I was thinking of trying to do the Canning Stock Route sometime in the future and having LPG delivered to well 23 (or whichever one,) is currently out of the question. The vehicle will have about 200Lt LPG capacity but only about 150Lt petrol. If it has to have premium to run on then that may be an issue, excluding the cost.

So the question remains - is it worthwhile spending $1,000 to convert the 4.6 engine to HiComp if it is to run 95%+ on LPG with the CSR calculated in?

Diana

Originally Posted by Lotz-A-Landies

The vehicle will have about 200Lt LPG capacity but only about 150Lt petrol. If it has to have premium to run on then that may be an issue, excluding the cost.

There is a guy here in Perth who aparently makes LPG jerry cans. I have just contacted my customer who told me about them only yesterday! He has the details somewhere and will call me when he finds them.
As soon as I know, I will post it here and PM you.

Andrew.