Compression Ratio - 4.6

[debaron]

Got my nice shiny rebuilt short engine back from V8D with flanged liners, pocketed pistons, new cam bearings and the bottom end all built up.

Question is - should I up the compression ratio on it? It's not had the decks skimmed so compression is currently at 9.35:1 for a 4.6 using standard RR gaskets but I guess I've lost a little with the pocketing.

[spend]

Rob does a thin gasket IIRC, I'd rather use a slim gasket than skim the heads (you can only do it so often & they can warp..) You also have to take care if you want a high lift cam, as well as misaligning the inlet manifold across the valley....

If you high compression you should have just asked for 4.0 pistons to be fitted when it was built!

[debaron]

What's the benefits and pitfalls of higher compression in the 4.6 V8?

What compression did the TVR 450 run?

[haircutmike]

spend said:

Rob does a thin gasket IIRC, I'd rather use a slim gasket than skim the heads (you can only do it so often & they can warp..) You also have to take care if you want a high lift cam, as well as misaligning the inlet manifold across the valley....

Approx, how much clearance is there inbetween the valves and fly cut pistons
Using a cam with a lift of 0.50" if the heads haven't been skimmed?

[spend]

haircutmike said:

Approx, how much clearance is there inbetween the valves and fly cut pistons
Using a cam with a lift of 0.50" if the heads haven't been skimmed?

That's a bit of a silly question BTW. Full lift affects clearance at top of valve guide & springs, piston/valve clearance issues are at partial opening (near TDC) so depend on the cam profile (and its timing) not full lift. You are looking for a minimum of over 1.5mm preferably 2mm IMHO when checking the clearance over rotations ~ and if checking do additional checks with cam set +/-2degs for your own data as well.

I'd guess most of the 'off the shelf' cams are inherently designed on the safe side?

[Goaty Bill]

haircutmike said:

Approx, how much clearance is there inbetween the valves and fly cut pistons
Using a cam with a lift of 0.50" if the heads haven't been skimmed?

This is why God invented Play Dough...

[dbv8]

[Goaty Bill]

...and micrometers / callipers...

Edited by Goaty Bill on Friday 21st October 08:43

[spend]

From my POV those big chunks of plasticine like that make it very hard to measure afterwards, either peeling them off will distort or a depth gauge in-situ is hard to align due to the spread..

TBH these days, I prefer to map the cam lift then install rocker arm springs (cut down + washer) and measure how far the valve can be depressed at 5deg increments (check 1/2degs at important 'close' points). Then you can mathematically link the 2 together and easily tweak for changes in cam timing.

[debaron]

What's the benefits and pitfalls of higher compression in the 4.6 V8?

What compression did the TVR 450 run?

[v8 racing]

debaron said:

What's the benefits and pitfalls of higher compression in the 4.6 V8?

What compression did the TVR 450 run?

The 450 ran at the stock range rover 9.35-1, because you have had around 5-10thou of the block face, you will be up to 9.5, if you put an aftermarket gasket on as opposed to a oem part you will go to about 9.7, if you want to skim the heads to give 10, personally unless you are running wild cams and or mapped ignition, 10 would be my limit for an engine with todays fuels.

[Alexdaredevilz]

Any guesses what compression ratio im running Rob?

[v8 racing]

Alexdaredevilz said:

Any guesses what compression ratio im running Rob?

i would say somewhere around 10.3/5

[plushuit]

debaron said:

Question is - should I up the compression ratio on it? It's not had the decks skimmed so compression is currently at 9.35:1 for a 4.6 using standard RR gaskets but I guess I've lost a little with the pocketing.

I am a little confused here. (nothing unusual( But I have answers.

If you are using a 4.6 block with 4.6 heads, you will be happy to know that the stock 4.6 heads are all pre-skimmed. You see, to fit the thicker composite head gaskets in the early 1990s, LR had to skim the heads to keep up the compression. So, if you use the earlier metal gaskets instead of composite, then you increase compression from 9.35 to 10.1.

The advantage is you have no need to skim the 4.6 heads, gaskets and a new set of head bolts are cheap, and you can expect an extra 20-25 bhp.

The disadvantage, is theat you need higher quality petrol, the head gaskets will blow more often and you will be putting an extra strain on the whole lower end..with a block that is notorious for weeping, cracking and slipping liners. On the other hand, you are not pulling a Range Rover's weight, and that will mitigate the risks somewhat.

Your question for yourself is whether you wish to prioritize bhp over reliability and how far you are willing to risk here. Frankly, if a new gaskets and head bolts don't bother you, go for it.

P.

[debaron]

v8 racing said:

The 450 ran at the stock range rover 9.35-1, because you have had around 5-10thou of the block face, you will be up to 9.5

Thanks Rob - so my block did get skimmed then?
That's fine if so - I didn't assume you did it as part of the relinering service.

So if I put a thin gasket i'll be up to 9.7 ish?

Head wise I plan to run some stage 3 heads (most likely yours Rob!) so I guess there will be a bit of skimming gone on there too. So looking at 10 realistically.

Will that all be ok with a Stealth / MC1 ?

[debaron]

plushuit said:

If you are using a 4.6 block with 4.6 heads, you will be happy to know that the stock 4.6 heads are all pre-skimmed.

Your question for yourself is whether you wish to prioritize bhp over reliability and how far you are willing to risk here. Frankly, if a new gaskets and head bolts don't bother you, go for it.

P.

Thanks - I'll be using stage 3 heads (probably skimmed already) and I don't really want to be changing head gaskets on a regular basis so might be best sticking with the composite. It's not intended to be a track car and will be getting regular daily use so reliability is key - big numbers less so (although 310 would be nice!)

[plushuit]

debaron said:

Thanks - I'll be using stage 3 heads (probably skimmed already) and I don't really want to be changing head gaskets on a regular basis so might be best sticking with the composite. It's not intended to be a track car and will be getting regular daily use so reliability is key - big numbers less so (although 310 would be nice!)

Check that skimming out thoroughly. You must be sure. If one use other than 4.0/4.6 heads, the liklihood is that were NOT skimmed similarly. (To fit the composite without losing compression meant that the 4.0/4.6 heads were skimmed to the point where they decreased the combustion chamber.)

If you use earlier heads types (staged or not) with a composite gasket you will achieve exactly the opposite effect from what I described above. pre-4.0/46 heads composite gaskets LOWER the compression. A 9.35 drops to 8.66:1 (from memory) (big ugh!) That will certainly make the car more durable but sluggish as well.

P.

[spend]

plushuit said:

Check that skimming out thoroughly. You must be sure. If one use other than 4.0/4.6 heads, the liklihood is that were NOT skimmed similarly. (To fit the composite without losing compression meant that the 4.0/4.6 heads were skimmed to the point where they decreased the combustion chamber.)

If you use earlier heads types (staged or not) with a composite gasket you will achieve exactly the opposite effect from what I described above. pre-4.0/46 heads composite gaskets LOWER the compression. A 9.35 drops to 8.66:1 (from memory) (big ugh!) That will certainly make the car more durable but sluggish as well.

P.

This is a TVR forum, all Chimaera's & Griff's came with composite head gaskets as std, whether the 3.9/4.2 based 400/430/500 or the later 4.0/4.6 based 400/450's. TVR predated LR in use of composite head gaskets and skimming the heads to suit the gasket thickness.

[carsy]

Dont forget the good old copper head gaskets on the 430`s. These are actually the same thickness as composite.

Why did they use copper on these and not composite?

[spend]

carsy said:

Dont forget the good old copper head gaskets on the 430`s. These are actually the same thickness as composite.

Why did they use copper on these and not composite?

They were composite gaskets? Only the fire ring was copper to accommodate the wire ring in the head in that particular gasket ~ it's just that those particular gaskets are no longer available so folks tend to either use solid copper or skim the ring off the head. I think it was Rob that mentioned std comp gaskets could be use with the ringed heads, I did try clamping some & wasn't convinced by the 'bite' pattern so generated my own outline pattern for cutting copper.