The V8 Owners Forum -

I have found a set of the Ford Modular Pistons that are -24cc, can anyone work out the compression ratio with these with the buick 300 heads...again as earlier in the post would be using 4.0 rods in my 4.6 engine.

I dont have a clue lol My numbers keep changing...

Regards

Neal

Hi
I think the 300 heads are about 58cc chamber volume, with 24cc dish and about 7cc for the gasket if the pistons are decked flush with the top of the bores then you are looking at about 7.5:1 I think you want the flat top ones, that should give you aboyt 9.75 :1, maybe skim them a little to get you to 10:1
The formula by the way is CR= 1+ swept volume of 1 cylinder/ total chamber volume
Best regards
Mike
PS you need to select the rods based on piston pin height and deck height, post up some numbers when you get them.

PPS just re read your first post, you probably want the ones with about 7 cc of dish that should give you about 9:1 CR 16 cc dish gets you 8:1 CR.
your other alternative is to go with the 24cc pistons and build up a squish area in the heads with weld and skim them down to and skim them a bit to get the head volume down to about 40 to 45cc.

Hi,

Im back thanks for all your help so far. My pistons turned up the other day they are the Ford Modular Manley 3.700" Forged Flat Tops

It says the compression height is 1.220 on the box

Is there a way of working out what rods I should use on my 4.6 engine? Either the 4.0 or 4.6 ones?

I will be using the Buick 300 heads. Looking for 8.5.1 compression I think but open to suggestions / advice...

Many Thanks

Neal


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Hi
At this point it would be a good idea to measure accuratly the chamber volume of your heads. Also has the block been machined yet? if it has then get an accurate measurment of the deck height, if not machined yet then it gives us another variable to play with. . .
I presume you are not reboring as you have bought standard bore heads so are you having the decks trued up?
In an ideal world the distance between the flat face of the head and the top of the piston (squish distance) would be between 35 and 45 thou of an inch, this is piston distance down the bore plus gasket thichness, you have 2 gaskets to choose from, tin and composite, tins are 18thou and composites are 48 thou.
If I assume you are going for composites then you want to have the pistons ending up at + 3 thou out the block minimum.
standard deck height un machined is 8.96", your piston height is 1.22" and your stroke is 3.2".
So rod length= 8.96 -(half stroke, 1.6 + piston height, 1.22) = 6.14" plus 3 thou = 6.143"
Now to look at the compression ratio CR = 1+(swept volume of 1 piston/volume above piston) = 1 + 575/(58cc (aprox chamber volume) + =9.72
So you are looking for some rods of about 6.14 to 6.15 length (aftermarket chevy ones?) and a CR in the region of 9.7 to 9.9 to 1 or less swept volume and longer rods but a lower CR
As I said in the last post on this I would have looked for the ones with a dish in them if you wanted the 8.5:1 cr and 4.6 litres, or go for a 4 litre crank and I'll recalculate the rod length.
Best regards
Mike

You will definitely need to get eyebrows cut into those pistons if you want to avoid a load of bent valves!

Even the standard pistons which are around 40 thou down the bore cause problems as soon as you up grade the cam or have the heads machined to up the CR. With your pistons possibly being +3 thou out of the block they must have eyebrows cut into them!

Machining the block (decking) does have another benefit which is that it will get rid of the horrible chamfer which is way too big at the top of the bores. This chamfer can cause the head gaskets firing ring to fail. (Of course you do need a small chamfer in order to get the pistons and rings down the bores)

If you do get the squish band working properly on this engine I suspect it will be one of only a few Rover engines where the squish actually does work!

Hi Sidecar
I see what you are saying about valve clearance, but at this stage I think he needs to sort out a few other priorities first. Looking back, if he is going to make use of the 4.0 rods then he needs to deck the block by 30 to 35 thou, and get them bushed otherwise it is a case of looking for Chevy ones, can you get ARP bolts for 4.0 rods?
I think this will be a case of trial assembly of cylindrers 1,2,7 and 8 once the machining is done to work out how much clearance there is for valves (We don't even know what cam is being planned here!) and we still don't know what CC the heads are (some places say 54cc some 58cc, these could have been skimmed previously!). Lets get the basics sorted first, how much boost, what CR is required.
Best regards
Mike
Ps the pressure side of those turbos look very small by the way

kiwicar wrote:Hi Sidecar
I see what you are saying about valve clearance, but at this stage I think he needs to sort out a few other priorities first. Looking back, if he is going to make use of the 4.0 rods then he needs to deck the block by 30 to 35 thou, and get them bushed otherwise it is a case of looking for Chevy ones, can you get ARP bolts for 4.0 rods?
I think this will be a case of trial assembly of cylindrers 1,2,7 and 8 once the machining is done to work out how much clearance there is for valves (We don't even know what cam is being planned here!) and we still don't know what CC the heads are (some places say 54cc some 58cc, these could have been skimmed previously!). Lets get the basics sorted first, how much boost, what CR is required.
Best regards
Mike
Ps the pressure side of those turbos look very small by the way

Yep all very true! I just would hate to see a nice engine get built only to then bend all the valves when its cranked over!

The last engine I had a hand in building ended up getting dry built about 10 times in order to check out various bits and bobs.

A little reflection about Ford pistons : they are designed for 3.700" bore but Rover V8 has a bore of 3.702".

Most forged pistons are designer to run with 0.004" bore clearance in NA application (if you measure piston skirt diameter you should have 3.696").
For Turbo/supercharger application, they advice generally to run between 0.005" and 0.0055" bore clearance.

Do you have some special recommendations in fitting instruction with this pistons ?

sidecar wrote:

kiwicar wrote:Hi Sidecar
I see what you are saying about valve clearance, but at this stage I think he needs to sort out a few other priorities first. Looking back, if he is going to make use of the 4.0 rods then he needs to deck the block by 30 to 35 thou, and get them bushed otherwise it is a case of looking for Chevy ones, can you get ARP bolts for 4.0 rods?
I think this will be a case of trial assembly of cylindrers 1,2,7 and 8 once the machining is done to work out how much clearance there is for valves (We don't even know what cam is being planned here!) and we still don't know what CC the heads are (some places say 54cc some 58cc, these could have been skimmed previously!). Lets get the basics sorted first, how much boost, what CR is required.
Best regards
Mike
Ps the pressure side of those turbos look very small by the way

Yep all very true! I just would hate to see a nice engine get built only to then bend all the valves when its cranked over!

The last engine I had a hand in building ended up getting dry built about 10 times in order to check out various bits and bobs.

Hi Mate
only 10 times! at this level of custom building that sounds about right, my chevy had at least ten dry builds before I did a final assembly it took about 6 to get the crank and rods to clear, 4 or 5 to get the thrust bearings bedded right, 2 or three to check the valve clearances. . . the list goes on, just part of the joy
I think this can be sorted and have a good squish clearance with cam, maybe valve cut outs, he may be able to dish the pistons a bit or maybe just drop to a 4 litre or a 4.2 crank, it is not a big problem, just needs a bit of a planned build and sorting out a full spec as I said earlier we don't even know how much boost is planned for here, it may be fine at 9.7:1 CR. . .
Best regards
Mike

kiwicar wrote:Hi Sidecar
I see what you are saying about valve clearance, but at this stage I think he needs to sort out a few other priorities first. Looking back, if he is going to make use of the 4.0 rods then he needs to deck the block by 30 to 35 thou, and get them bushed otherwise it is a case of looking for Chevy ones, can you get ARP bolts for 4.0 rods?
I think this will be a case of trial assembly of cylindrers 1,2,7 and 8 once the machining is done to work out how much clearance there is for valves (We don't even know what cam is being planned here!) and we still don't know what CC the heads are (some places say 54cc some 58cc, these could have been skimmed previously!). Lets get the basics sorted first, how much boost, what CR is required.
Best regards
Mike
Ps the pressure side of those turbos look very small by the way

Thanks for all the replies so far..

I wanted to run around 20psi, I have a very mild cam in there at the moment from real steel think it was called a cyclone but dont quote me I have the paperwork at home. But was thinking of changing to there supercharger/turbo cam offering.

I would like to stick with 4.6l..I have a top hat linered block with the standard pistons Im guessing its standard deck height but thinking the only way to do this properly is to pull the block and see. I have the buick 300 heads that I will be getting re-furbished soon with the bigger valves, then I can cc them and let you know.

I have a friend that can machine the pistons for me if needed. Im prepared to take the build anyway I can with rods from chevy, 4.0 etc there was talk of cometic thicker head gaskets to drop the compression a touch...Its open idears at the moment but for the price of the pistons I had to have them even if I have to sell them on.

If you deck the block will the inlets etc need modifying...

Let the tinkering begin

Hi
if you are planning to run 20 PSI on petrol then you will need the squish band at the tighter end of tollerance so 35 thou or so gasket plus piston done the cylinder. Trying to lower the compression on a turbo engine with thick gaskets has not been done for about 20 years (by serious tuners) as it was realised opening up the squish band lost more in detonation resistance than any gain in reduced CR made. Even Allard commented on this in his original book on turbo engines (first edition) and that was writen about 35 years ago.
You will need good head gaskets!
If you are going to get pistons machined for dishes then get a "reverse crown" done like JE do on their pistons so you can maintain the squish band. I recon you will need between 9 and 13 cc of dish plus valve cut outs depending on what size the chambers are, so that is about 6mm deep out of the centre of the piston crown, I hope there is enough meat there.
I think I would try more cam, more CR and less boost and time the cam to close the inlet valve very late, say 16 pSI, 9:1 maybe 9.25:1 cr and a cam with lots of inlet duration, say 235 at .05 with about 225 of exhaust on a 114 degree LSA timed 4 to 6 degrees late, then you won't have to machine those pistons so far.
Best regards
Mike
PS I hope you have a nice strong gearbox

Thanks for the reply mike you are baffling me lol

I am currently around 8.0 on my engine thats with original TVR heads but I am using de-compression plates. Some people like them some dont it was a mixed review when I got them. But they have stood up fine at 10psi now for 3000+miles. They are 1.5mm thick and mated to the head with non setting high temp sealant. I am using 1 x composite gasket. And it has been to 15psi once or twice.

I dont think 6mm out the pistsons is going to work they would seriously weaken them alot I think.

So many options....

Hi
I'm sorry not trying to baffel you in any way. What I was trying to say is rather than take a low base tune engine and boost it like mad to get the power you want, go for a higher base tune level, you are already half way there with the better heads, and then close the inlet valve later to reduce the dynamic compression ratio.
There are many ways of getting 500 bhp out of a turbo engine, start with a base engine of say 230 bhp and boost it with a pressure ratio of about 2.5:1 (22 psi ish boost) and use a big intercooler, or go for a higher initial state of tune, your buick heads and a bit more cam and get 310 base engine add about 14 psi of boost and a not quite so big intercooler. Both will give you the same output, I would suspect if you got the buick heads set up and working correctly with the better cylinder filling from the heads and a longer duration cam you would end up with a much "crisper" engine with alot less lag and alot more fun day to day.
Best regards
Mike

Hi
Just rechecked my maths, for a 13cc dish in the pistons it would take about 2.5mm of depth with 1/4" squish band around the outside and D shaped over 5/6 of the remainder of the piston crown. I think as most valve cutouts come in at about 5cc you may need only 1.5 to 2 mm of depth to achieve what you may need to. It all really depends on the chamber volume and how much boost!
Best regards
Mike

Thanks Mike.

I have the pistons in front of me now I reckon the crowns are 6mm thick according to my steel ruler.

Sounds like I am best the develop the heads first in case they have been skimmed then I can come back with some true readings.

What do you think about de-comp plates?? I liking the ideas regarding cams and compression.

**on another note could you work out my current compression ratio from the following info** (from my pistonheads post)

4.6L
Bore 94mm

CC of heads 28cc (They have been refaced twice but can still get 28cc in each one)10 bolt late heads
CC Pistons in block at TDC 30cc

I will be running a composite gasket and according to the web the compressed thickness is:
3.9/4.0/4.2/4.6 Composite gaskets - 48thou (1.2mm)

Decompression plates 1.5mm thick

Many Thanks

Neal