Late 70 9:1 engine #7R-9760 -9
So starting to acquire the long list of parts needed for my rebuild.
I need a set of 40 over 9:1 pistons.
Any more competitive recommendations than SNG and Martin Robey?
Thanks
John
Keep your eyes peeled, on eBay: sometimes they show up there.
You might also ask this gentleman if he has any in his stock.
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John I’d be sticking with brand name pistons; Mahle, AG etc.
You’re looking at $10k to rebuild your engine and cutting corners for fifty bucks Isn’t worth it.
I’d also be considering how much your head and block have been skimmed before buying 9:1 pistons. You don’t want to have too much compression or you’ll run into problems with pinging. We routinely fit 8:1 pistons on our rebuilds to avoid this problem.
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Certainly not cutting corners but then again not paying premiums unless necessary.
So interesting comment on the rebuild. So even though you rebuild an s engine you use 8:1 pistons?
Other than potential pinning how does this effect performance? What feed back have you had from those that have gone from 9:1 to 8:1 ?.
I have to check with the machine shop but I’m pretty sure the block got a generous shave.
I’m not looking to build a performance engine just the most reliable and drive-able one I can.
Thanks much !
PS: who is AG?
Or cc the head and tdc piston volume and calculate the correct Cometic head gasket to achieve the 9:1 ratio.
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I’m planning to possibly use the Payen/felpro composite head gaskets rather than the standard copper so not sure what effect that may have on the overall set up??
On the XK engine the standard is a steel gasket, not copper…?
That said, the steel one works perfectly well, if you have two very clean, very flat surfaces, and no corrosion in the water ports. If I were doing it myself, I would use either the Payen or the Cometic head gasket.
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Mahle is good, others probably too, little wrong with 8:1; I could have chosen 9:1 but I‘m actually happy.
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Given the limits of the upper chain adjustment, that does have its limits, but it is certainly something that needs to be taking into consideration.
The power drop from 9:1 to 8:1 would be, at most, about 3%. The average driver will not notice that, if at all.
P.m. Ray Livingston…
I figure that what was taken off the head and block deck will make up a considerable amount of compression loss from 9 to 8:1. Ya think??
So far I can find +40 9s in Mahle but 8:1 4O over only in Hepolite
Out of home will PM you later when back at base camp.
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Maybe best to see what needs to be done, because if the surfaces are within spec. or just a light skim is required you don’t even have to consider increased compression. Hepolite is fine, too, I‘m sure.
Yes I agree. I waiting for feed back from the machine shop but based on 90 miles and the engine was seized before the start of my resto I’m thinking there was some metal removed .
If there is enough taken off the block in the head, correct: you could use 8:1 pistons and end up with a 9:1 calculated mechanical ratio.
Modern fuels are now better than they were 20 years ago, but if you are at sea level, you definitely do not want to exceed 9:1.
Exactly. That’s why we used 8:1s. The boys had been doing this for years before I met them and we did probably 20 engines while I worked for them, plus 5 engines of my own.
Taking 1mm/40 thou off the deck/head removes 6cc from the chamber volume, so a couple of previous rebuilds can make a big difference when the average chamber volume is 90-95cc. From memory I think that there’s about 12 or 13cc difference between 9:1 and 8:1 piston crowns ( 6cc vs 19cc).
There isn’t any real difference in power because what you’re doing is bringing the compression back down to around 9:1 as opposed to using the 9:1 and ending up with it too high.
With my very first E Type I used 9:1 pistons ( I bought the parts without asking the boys’ advice) and ended up with compression around 190-200 psi. I spent months trying to fix the pinging and eventually replaced the head with an extra thick cometic gasket that brought compression down about 20 psi. Pinging gone.
Alternatively, as suggested cc the head.
Finally, if you’re out to 40 thou it’s worth considering resleeving the block back to standard and fitting top hat liners. This will add maybe $1k to your rebuild cost but does solve the problem of cracks between cylinders which are common. The additional benefit is that standard pistons are more easily available.
I put Hepolites in my MK2 engine recently. Seemed fine.
FWIW they’re made in the UK.
I installed Mahle pistons during my recent rebuild and was impressed with the quality. The rings come pre-installed and they where exact. I measured the stink out of them and they where exact. They fit perfectly and because they’re low expansion, I’m expecting very low oil usage and blow-by. It was money well spent.
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If I re-sleeved back to standard would I still go with 8:1 pistons to compensate for the head and block resurfacing ?