Increasing compression ratio

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Britboy
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Increasing compression ratio

Post by Britboy »

The cr on my stock '75 Jensen Int 440, is I believe, 8.2. I'm thinking I can gain more oomph (excuse the technical language) by switching to pistons that would provide closer to 9.
I'm sure some of you have made that switch and I wondered if anyone could suggest a suitable replacement piston and a source.
Thanks for any advice on the subject.
Cheers
Forever hooked on old British iron.
1954 AH BN1, 1960AH BN7, 1955 Jaguar xk140 fhc, 1967 Jaguar e type fhc, 1967 Sunbeam Alpine, 1960 Jaguar Mk2, 2003 Jaguar xk8 fhc (had).
1975 Jensen Interceptor MKIII, 2010 Jaguar xf (have).
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Grant
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Re: Increasing compression ratio

Post by Grant »

Britboy wrote:The cr on my stock '75 Jensen Int 440, is I believe, 8.2. I'm thinking I can gain more oomph (excuse the technical language) by switching to pistons that would provide closer to 9.
I'm sure some of you have made that switch and I wondered if anyone could suggest a suitable replacement piston and a source.
Thanks for any advice on the subject.
Cheers
Your are about correct with the later cars having around the compression ratio, that is what was aimed at and the later cars with the smog cams also like yours has are fairly slow from my experience.. we have a 75 convertible and that was the same, we pulled the heads and measured down the bores to carry out an exact comp ratio and sure enough it was 8.2:1, although it seems they did vary as the tolerances of the deck height were a little off when they were manufactured in the Chrysler factory, so if you want to up it I would aim for around 9.7:1 to get good performance without too much detonation, although then you can eradicate that by adding a mapped ignition system and then mapping out any detonation :wink:
To work out which pistons you need you would need to pull the heads and measure the piston top > deck height, then using that measurement and the volume of the head pocket you would need to add that into the formula in ie Summit racing piston selector to aim for the Comp ratio you require Britboy .. if it were me doing this I would then change the cam for something similar to a 69 roadrunner.. >> You'll be flying then My Ole Fruit Bat :P
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Steve Payne
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Re: Increasing compression ratio

Post by Steve Payne »

Keith Black KB237 are the ones you want if you are aiming for 9.7 to 1. Bullet proof and are well made.

You will also need to rejet your carb and fiddle with the ignition timing, even with this modest increase in compression it means all the factory settings are way off.

Don't go to wild on the cam, these are not Ferraris so don't try and make them go like one. A good cam with plenty of low down torque and a good idle vacuum or the brakes will be terrible.

Steve
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Dino Fritz
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Re: Increasing compression ratio

Post by Dino Fritz »

Hi everyone

Grant raises an excellent point - before you go out any buy anything, just be sure that your heads or block haven’t had any material taken off them which may have increased the compression ratio over the years.

Dino
Jensens.........when does it ever end!
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Britboy
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Re: Increasing compression ratio

Post by Britboy »

Steve Payne wrote:Keith Black KB237 are the ones you want if you are aiming for 9.7 to 1. Bullet proof and are well made.

You will also need to rejet your carb and fiddle with the ignition timing, even with this modest increase in compression it means all the factory settings are way off.

Don't go to wild on the cam, these are not Ferraris so don't try and make them go like one. A good cam with plenty of low down torque and a good idle vacuum or the brakes will be terrible.

Steve
Thanks for the advice Steve. I don't ever want (or expect) this baby to behave like a Ferrari. More like a 1970 Newport or Chrysler 300 that used this same engine (CR= 9.7) and 727 auto gearbox. 4400lb land yachts that moved with grace and poise, quietly powered by 350hp and 480ft.lb torque. I'd like my MkIII to feel, more or less, the same way. A smooth, reasonably quiet GT cruiser with some "get up and go" when called upon.
Forever hooked on old British iron.
1954 AH BN1, 1960AH BN7, 1955 Jaguar xk140 fhc, 1967 Jaguar e type fhc, 1967 Sunbeam Alpine, 1960 Jaguar Mk2, 2003 Jaguar xk8 fhc (had).
1975 Jensen Interceptor MKIII, 2010 Jaguar xf (have).
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Britboy
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Re: Increasing compression ratio

Post by Britboy »

Given modern fuels, is 9.7 a bit much to ask for? I am concerned about detonation/pinging with, at best, only 91 octane gasoline available.
Forever hooked on old British iron.
1954 AH BN1, 1960AH BN7, 1955 Jaguar xk140 fhc, 1967 Jaguar e type fhc, 1967 Sunbeam Alpine, 1960 Jaguar Mk2, 2003 Jaguar xk8 fhc (had).
1975 Jensen Interceptor MKIII, 2010 Jaguar xf (have).
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Grant
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Re: Increasing compression ratio

Post by Grant »

Britboy wrote:Given modern fuels, is 9.7 a bit much to ask for? I am concerned about detonation/pinging with, at best, only 91 octane gasoline available.
:shock: 91 Octane :shock: .. Maybe you should leave the compression as it is, is it the States that vary then out there in the US, or is that the only fuel available in all States?
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Re: Increasing compression ratio

Post by Steve Payne »

US 91 Octane is the same as UK 95 so it is not to bad.

Yes as I said earlier you might have to do some mods to get it to run nicely but there are some great devices now available so you can have mapped ignition.

9.7 is not that high, these engines back in the 60's were running far higher compression ratio's than that.

Cam choice will be the factor that gives you power or problems.

Just a thought, when was your convertor last refurnished? A tired convertor can make the most powerful engine feel slow as it will just absorb all the power the engine is producing.

Steve
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Philip Lochner
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Re: Increasing compression ratio

Post by Philip Lochner »

Britboy wrote:The cr on my stock '75 Jensen Int 440, is I believe, 8.2. I'm thinking I can gain more oomph (excuse the technical language) by switching to pistons that would provide closer to 9.
I'm sure some of you have made that switch and I wondered if anyone could suggest a suitable replacement piston and a source.
I have now done exactly that. KB237 pistons and a Comp Cam 1500-5500. VERY happy with the result. See: https://www.joc.org.uk/forum/viewtopic.php?f=4&t=26892
Best regards
Philip
Mk3 7.2 #128/8120 Jan 73 http://tinyurl.com/hsjucm5
Mk3 7.2 #2240.9677 May '74 modified:
Fuel injected: http://tinyurl.com/qyrx93f
GM 4L60e 4-sp transmission: http://tinyurl.com/qxlwk95
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