Hello Jaglovers, I am rebuilding an XK 150 engine 3.4Litre and I want to buy new pistons I it and the rest new, too.
Saw on the site of Sng.Barrett STD’s pistons, but also a higher compression piston set.
Is it wise to change the compression for this old/new engine?
Advise is nice.
Frank.
Unless you’re a purist for originality, I say why not?
Depends on the quality of available fuel, and your tolerance for spending more per gallon to satisfy the octane requirements of high compression. If you are a “cheapest per gallon” person you are not a high compression candidate.
8:1
Presuming you’ll deck the block and skim the head.
You’ll also have room to advance the timing.
I don’t know that 7:1 pistons are even available these days. I’d go with 8:1 which very many export cars were built with, anyway.
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with decking the block and skimming the head and on many cars it will have been done to one or both before you really already have with 8-1 pistons a compression ratio greater than 8-1 so forget 9-1 unless you are on a race track
terry
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Thank you Mike, now I am sure to buy just the original pistons, however there are two different piston sets, one with the extra oil ring and the other just with only one oil ring.
Two oil rings makes more friction I think, why did they make them?
Thank you all.
Frank.
On a street car, the increase in friction is nil: they used 2 oil control rings in an attempt to cut oil consumption.
Modern 3-piece oil control rings are far better at controlling oil consumption.
Hi Frank,
Take a look at the ongoing thread “3.4 lire engine upgrades?”, that has many good suggestions from many listers who have gone through the engine rebuilding drill recently.
I was faced with the same piston selection dilemma while building my XK-120 engine a few years ago. Another friend and fellow lister, Steve Turschmann, was also rebuilding two XK-120 engines at the same time and has a close relationship with a very experienced local engine rebuilder. Our three engines were simultaneously rebuilt by that shop, and we worked together to source parts and make decisions on what to have done.
As we looked at pistons we found several alternatives:
- AE pistons, similar to original, cast aluminum, with 4 rings, including one on the lower skirt
- County pistons, an aftermarket three ring cast aluminum reproduction piston
- Mahle pistons, a high quality cast aluminum piston of modern design
- Arias and Ross forged aluminum racing pistons of modern design
After looking at the design, cost and benefits of each piston I chose Mahle.
I was sold on the three piece oil control ring design and here is why:
Mahle has been a frontrunner in the development of rings, and the above table documents the improvement made in oil scraping capability that has decreased oil consumption to a quart (or less) in 10,000 miles. It’s to a point where checking oil on new cars is no longer a weekly task, as it was in the past. Imagine an XK engine where you don’t have to worry about burning oil. (Leaking is another story and can also be virtually eliminated with the addition of front and rear crankshaft seals and careful attention to recognizing and sealing leak points during engine assembly.)
I chose 9:1 compression ratio in order to maximize the power from the engine. I added a few other pretty much invisible performance improvements, including a “B” head, custom curved valve seats, and dual exhaust. I am well satisfied with the results. I have always used high test gas in my Jaguars, it just feels right and the engines run better. The additional cost for the few thousand miles I drive them in a year is worth it to me.
One last thing, if you go with Mahle, make sure you get the latest piston design with three piece oil control rings, there are older models still being offered that have a single piece cast iron oil control ring. Take advantage of what some would argue is one of the most significant improvements in piston engine technology in the last 50 years.
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Thanks a lot Brad, that is a good explanation about the issue. I am building my 3.8L engine ad the same time with the 3.4L and saw a three piece oil rings in a piston set I bought on ebay, but did not liked it, it was different than your picture and didn’t fit right, so I used the old three piece of the original piston.
For the 3.4L I saw the Mahle normal 1; 8 compression four rings and a Japanees piston 1 ; 9 with three piece oil rings, so I to decide now.
I think the oil consumption comes from the valve guide’s, they are not protected from the vacuum near the inlet valve’s.
I want to change the inlet guide’s to collect a rubber hose on them, just like in the XJ engine 3.4L I restore and sold, see picture.
I will renew every thing in the XK 150 engine.
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Thank you Brad, did bought Mahle, Hepolite, the best they had.
Install them the same day, they came with the post, beautiful.
Frank.
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I’d always reach this point, in an engine rebuild, and think to myself, “Those pistons are so pretty, it’s a shame to hide them with a cylinder head!”
Yep: QED.
Gorgeous things of art!
Nice!! They’ll do 6000rpm all day!
Hi Paul, yes you are right, it’s a pretty to look at them, when the head is not there.
It’s the most expensive piston set Sng.Barratt had, they from Mahle, Hepolite, but I looked fore the front sign, did not find any arrow, is that right?
Frank.
Can anyone tell me how high the dome on the 4.2 pistons are with a compression ratio of 9-1.
The pistons I just removed from a 1966 4.2 Mk 10 with 9-1 comp. have a dome height of .223 inch.+/-. but they are not the original pistons
I am assuming this engine is the same as a 4.2 E type.
morris
FWIW, the pictured piston has a smaller dome than the original 8 to 1 set that came outta my 3.4. Also, I seem to recall the 9 to 1 jobs had a taller and “wider at the base” dome than this one pictured.
I cannot answer your question directly, but for comparison, here is the original piston from a garden-variety North American S3XJ6 4.2L. The part number is EAC 2043
Thanks Mike and Lee, so it looks like my 4.2 has 8.1 or even less compression ratio. Time to order some 9.1 's.