The V8 Owners Forum -

Hello, im in the middle of building another engine, its a 3.5 rover, 9.75 pistons with ported 4.6 style heads and a crowler cam. Which do i use, tin or comp. Does anyone know the cc they take up when fitted?
Thanks.

The tin gaskets are about 3cc, the comp ones are 6cc.

If your heads are 28cc rather than the normal 36cc ones that the 3.5 heads are then the comp gakets will work well without your heads being skimmed. The tin ones may push your compression over the top. (Some of this depends on what fuel you are prepared to use).

Your 9.75 pistons are ment to work with 36cc heads so 28's will be very high on standard tins.

The proper way to work this out is to burette each piston dish at TDC and each combustion chamber then sit down with a calculator and work it all out. I found that standard pistons and heads can vary by about 1cc. That makes 2cc in total which could push a cylinder over the top if you only used one cylinder to determine the CR and it happened to be a "low one".

Alot of people don't like the tin gaskets even though others have said that they are OK if you don't use the outer head bolts. (The row below the spark plugs on a 14 bolt head). I don't like them and I would not use the outer head bolts with tins or composites. As your heads are 4.6 jobbies it may not have the outer row.

If you do not have a burette or don't want to go to the hassle of doing all that measuring then stick with the comps and you should be OK.


HTH,

Pete

if they are 4.6 heads use the comp gaskets unless you want to up the compression, you will get around 10.3:1. Be warned though if you do that you might run into sealing problems with the inlet as the V will be smaller. What do you mean by 4.6 style heads by the way?

Hi
I just made it 10.72 ish with comp gaskets and 28cc chambers assuming the 9.75 pistons are intended to work with tin gaskets. This to me would be a little high working with a standard cam and supermarket unleaded, though not sure what you are planning on this front.
Mike

kiwicar wrote:Hi
I just made it 10.72 ish with comp gaskets and 28cc chambers assuming the 9.75 pistons are intended to work with tin gaskets. This to me would be a little high working with a standard cam and supermarket unleaded, though not sure what you are planning on this front.
Mike

Dont get how you came about your figure? 4.6 heads with comp gaskets have the same volume as older heads with tins. Therefore 9.75:1 will still stand. Its only if tins and 4.6 heads were used the the CR would go up by about .7

katanaman wrote:

kiwicar wrote:Hi
I just made it 10.72 ish with comp gaskets and 28cc chambers assuming the 9.75 pistons are intended to work with tin gaskets. This to me would be a little high working with a standard cam and supermarket unleaded, though not sure what you are planning on this front.
Mike

Dont get how you came about your figure? 4.6 heads with comp gaskets have the same volume as older heads with tins. Therefore 9.75:1 will still stand. Its only if tins and 4.6 heads were used the the CR would go up by about .7

Surely the CR will go up more than .7 if you use tin's and 28cc heads as its an 8cc reduction in combustion chamber size, that is quite a large percentage difference when the total head volume is normally 36cc.

Ignoring the piston dish (as I don't know the volume and it does not matter for what I'm about to say) the standard 3.5 lump on tins would have had a 39cc chamber. (36+3). (+dish vol)

28cc chambers plus comps would be 34cc (28+6) (+dish vol), this will be a fair increase in compression. (Maybe the 10.7:1...I can't be bothered to work it out!)

28cc chambers with tins (28+3) (+dish vol) would be 31cc, thats miles away from the standard 39cc.

The thing about the 4.6 lumps is that even though the head volume was lower than the 3.5 lumps the piston dish was much larger. (My 3.5 on 9:35 pistons has a piston dish vol around 15cc. The 4.6 on my bench with 9.35 pistons has a piston vol around 30cc, but then again it has a bigger swept volume to squash into this space).


If anyone knows the piston dish vol for 9.75 pistons or can be bothered to work the maths out backwards then the real actual CR could be worked out....I think that it will be pretty high!

(You could try running it on diesel!....call it the "iceberge project" )

Pete

Guys

Assuming a deck height of 20thou, piston Vol. 7.5cc then with 28cc. heads and tin gaskets, compression approx 11.376/1 !!

Comp gaskets = 10.7/1

So Shell V Power or BP Ultimate only.

Kevin

Nowt on the tele so I've worked a few things out.

9.75:1 pistons must have a dish volume of 11.5cc to give a standard 3.5 lump that compression on tin gaskets and standard 36cc heads.

(11.5cc is the total vol above the pistons including the vol due to the deck height)


Those pistons with 28cc heads and comp gaskets are going to give 10.67:1 CR.

The same pistons with tin gaskets are going to give 11.35:1

Personally I think that even the lower CR is pushing your luck, Russell Ram reckons that you can run 10.25:1 put I've chickened out of pushing my 4.6 to that. I'm going to go for 10:1. (Obviously the cam has to be considered).

I also think that too much "weight" is given to getting the CR really high when tuning an engine. There is a formula kicking about on the web that calulates the BHP when the CR is increased. (I know that you need to use it with a "pinch of salt"). The BHP does not go up a massive amount for each increase in CR. The risk is that if you do go too high and end up having to retard the ignition to kill detonation the engine will feel crap compared to one with the right CR and a decent amount of spark advance.

You could speak to V8 developments and get the chambers opened up to 31cc, this will give you a CR of 10:1 or 32cc to give you 9.88:1
(Assuming that you piston dish really is 11.5cc).

Pete

Pete.

I think you had it right the first time, it's unlikely the pistons are level with the decks.

Kevin.

There must be an awfull lot of engines on this forum running very high compression then as there is loads of 3.5 and 3.9 engines running 4.6 heads and comp gaskets.

I've just had the pistons measured, they are 8.5 cc and the heads are 29cc. does that help. What is the formula to work this out?

Paul V8 wrote:I've just had the pistons measured, they are 8.5 cc and the heads are 29cc. does that help. What is the formula to work this out?

I presume that the 8.5cc is just the dish in the piston. You need to know how far the piston is down the bore at TDC or if the engine is part built you can just measure the total volume above the piston.
1.You need to seal the piston to the bore with a smear of vasaline or grease.
2. Get the piston on TDC, don't guess as this is critical, use a dial gauge.
3. make a perspex cover to go over the bore with a small hole right at the top edge of the bore.
4. bolt the perspex plate to the block and seal it in the same way as the piston.
5. carefully burette a liquid like white spirit into the small hole. The amount that goes in is the total vol above the piston.

If you know the deck height (say 20 thou or 0.5mm) and you know the volume of the piston dish you can work out the total vol above the piston.

Vol=height x area.

Bore = 88.9 mm (radius "R" = 44.45= 4.445cm)
stroke = 71.1mm (7.11cm)
Area= RxRx3.14= 62.04cm2

Cylinder vol = 62.04 x 7.11 = 441.1 cm3
Tin gasket Vol = 62.04 x 0.05 = 3.1 cm3 (20 thou is 0.05 cm)
Comp gasket vol = 62.04 x 0.1 = 6.2 cm3 (40 thou is 0.1 cm)

Total vol above piston and chamber=
TvPC= total piston vol+Gasket+Chamber

CR = (TvPC + cylinder vol)/TvPC

So say your 8.5 pistons are 20 thou down the bore at TDC. That gives a volume that you could burette of 8.5 + 3.1 = 11.6 cm3

The TvPC for tins would be 11.6 + 29 + 3.1 = 43.7 cm3 (29 being your head vol and 3.1 being the gasket vol)

so CR = (43.7 + 441.1)/43.7 = 11.09:1

CR of 11.09:1 on tins

TvPC for comps would be 11.6 + 29 +6.2 = 46.8 cm3

so CR = (46.8 + 441.1)/46.8 = 10.42:1

CR of 10.42:1 on comps

(These figures are slightly more accurate than the ones in my earlier post as I used a stroke of 71mm not 71.1mm and your head vol of 29, not 28 )


Pete
[/b]

Show off.
Thanks for that, i think comps is the way to go.

katanaman wrote:There must be an awfull lot of engines on this forum running very high compression then as there is loads of 3.5 and 3.9 engines running 4.6 heads and comp gaskets.

If there running 9:35 pistons then they will be fine!

If they're running 9:75 pistons then I'm not saying that they will detonate to pieces. Its up to the owner of the lump to decide what CR they want to run! In my opinion (which ain't worth much) going much over 10:1 is a bit dodgy!

Pete

Paul V8 wrote:Show off.
Thanks for that, i think comps is the way to go.

"Show off"....You asked "how do you work it out!"

Anyway I think that even on comps it's a bit too high but it's your motor, only you can decide!

Pete