P6B Pistons

NickDunning

NickDunning

Active Member
Folks:

Question from John Windwood which I can't answer:

"I have here a set of BL boxed pistons for the Rover V8 engine. The part numbers on the boxes indicate that they are 10.5:1 CR, BUT I have discovered that they are not actually new, but barely worn second hand, which implies to me that they might not be the right part numbers. The only guide I have to identity is a Hepolite number cast into them-614500-and they have the letter B" stamped into the crown.
I have been unable to confirm what they are-can you help?"

Who's going to be the first with the info :)

Thanks
Nick
 
The September 1977 3500/3500S Parts Catalogue I have doesn't have 614500 listed.
 
Can't be certain, but as they have a grade letter on the top, then they're standard bore, and there are 3 types, early, and late 10.5 & 9.25. I'll try and find a pic of the difference between early and late. The CR difference is the difference in the dish in the piston crown.

Early type

SpeedMechanics-B.jpg


late type

pic836.jpg


Sorry about the small second pic, it was the only one Icould find.

They could be someting other thanP6, but the difference would be the same. Pics would help.
 
Found this pic of Hepolite 614500 pistons.

614500-PISTONS-AE-HEPOLITE-DIA88-84-1.jpg


Which would make them the later type, so it would be a case of measuring the dish in the crown to calculate the CR.
 
Thanks guys for the help. So how would I measure the piston crown in order to determine the ratios?
 
John W said:
Thanks guys for the help. So how would I measure the piston crown in order to determine the ratios?
Click to expand...

Put a straight edge across the top and then measure the depression using feeler guages.
 
As near as I can get it with the limited tools have here, it's 2mm. How would I work out CR ?
Thanks for your advice.
 
John W said:
As near as I can get it with the limited tools have here, it's 2mm. How would I work out CR ?
Thanks for your advice.
Click to expand...


I've got some 10.5 pistons here I'll measure those, and depending on what I get we should be able to make some sort of progress. I may have difficulty finding metric feeler guages though, it's all Imperial around here.
 
harveyp6 said:
John W said:
As near as I can get it with the limited tools have here, it's 2mm. How would I work out CR ?
Thanks for your advice.
Click to expand...


I've got some 10.5 pistons here I'll measure those, and depending on what I get we should be able to make some sort of progress. I may have difficulty finding metric feeler guages though, it's all Imperial around here.
Click to expand...

2mm equates to 0.0787401575 inches (78.74 thou), if that's any help! :)
 
John W wrote,...
As near as I can get it with the limited tools have here, it's 2mm. How would I work out CR ?
Thanks for your advice.
Click to expand...

Hello John,

You'll need to calculate the volume of the depression within the crown. Measuring the depth with a feeler gauge won't help unfortunately going from the pic that Harvey posted. To measure the volume you can fill the depression with grease, place a piece of glass across the top to ensure that it is perfectly filled with no excess, and then carefully remove the grease and place into a graduated burette which will allow you to read off the volume.

Ron.
 
or you could just fill with water far less messy than grease but might need to allow for the Meniscus.

Graeme
 
Hello John,

To calculate the compression ratio, the formula is (Cv + Cctv) / Cctv where Cv is the cylinder volume and Cctv is the combustion chamber volume which is composed of the volume of the combustion chamber within the cylinder head, which is a nominal 36 cubic cm for the P6B head, plus the volume within the head gasket, plus the volume above the piston to the block deck when at TDC, plus the volume within the dish of the piston.

Ron.
 
Hmm...complicated....
I think I'll just chuck them in the bin-they are worth nothing anyway.
Thanks gents.
 
SydneyRoverP6B said:
You'll need to calculate the volume of the depression within the crown. Measuring the depth with a feeler gauge won't help unfortunately going from the pic that Harvey posted.
Click to expand...

It will because all the depressions are roughly the same diameter, it's just the depth that varies, and from that, as you only have a limited number of choices there's no need to take a volume purely for identification purposes. I can tell what CR they are just by looking at them when they are at hand, but I wouldn't be quite so keen to do that just from a pic.
Things like that always have a value, you just have to find the person that wants them.
 
I've measured the 10.5:1 pistons I have and the dish is about 0.45-0.5mm deep, so yours are definitely not that high, as you have approx 4 times the depth of dish in yours, so that leaves you with 9.25 if they're P6B, plus all the SD1, export, & Range Rover variants to choose from. Chances are they're a low compression set changed for something better performance wise.
 
Thanks. I'm pretty sure they are P6 as they have all been put back carefully into the original boxes, all printed with the correct Rover part number for 10:5-1 P6B pistons. So most likely 9.25-1 as you say.
Anyone want a set, with rings and pins, clearly VERY low mileage and virtually unworn?
 
I would love a set to upgrade the 8.25:1 pistons in my 1972 pre sd1 range rover engine if they are going in the bin otherwise :D

Are you going to go looking for 10.5:1 pistons elsewhere?
 
fett wrote,...
I would love a set to upgrade the 8.25:1 pistons in my 1972 pre sd1 range rover engine if they are going in the bin otherwise
Click to expand...

Hi Fett,

You may well find that even with 10.5 : 1 CR pistons fitted, your engine doesn't deliver the sort of power increase that you might be expecting. This is especially so if you have to run the initial timing at TDC so as to limit pinging as a result of the high compession. It is much better to have a lower compression engine and be able to use more ignition timing than have a higher compression engine and have to use less.
My original engine was a 10.5 : 1 version, and it always pinged although if I used an octane booster (at $20 per tank extra a bit on the expensive side!!) it did make a noticeable improvement. Still, I had to run it at TDC, very inefficient, much better in my humble opinion to limit your compression to 9.25: 1 if you really want an increase.

Low compression engines may deliver less maximum power and even slightly less maximum torque, but the real difference lies in how that power is delivered across the rev range rather than just the peak figures. Certainly it is the case with the 4.6 litre engines, the low compression editions put their power down more rapidly than the high comp versions. It could well be the same story with the 3.5 litre engines too. Given that your engine came from a Range Rover, in such a vehicle you would want power delivered rapidly in the low to mid range, such would make for a much more useable and driveable vehicle.

What initial ignition timing are you running with your engine? Have you ever had the distributor regraphed to suit your P6B?

Ron.
 
You must log in or register to reply here.
Back
Top