Compression horespower and engine numbers

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lowact
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#1 Compression horespower and engine numbers

Post by lowact » Fri Feb 09, 2018 3:30 pm

Hello, does anyone have this knowledge? My car is 1972 US spec with all the emission control paraphernalia and 9:1 compression ratio (engine number suffix is “SA”). As I understand (from www.automobile-catalog.com), European cars with the same compression ratio produce ~9% more power and ~7% more torque while burning ~1% less fuel. Do we know why this is, is it mainly due to the air pump?

My spare engine is from a European 1986 XJ S3 Sovereign (non catalyst). According to www.automobile-catalog.com these have 12.5:1 compression ratio. The engine number suffix is “HB”, which indicates the compression ratio, except for this era engine I don’t know the code, does anyone? Does anyone know what compression I would/should measure with a 12.5:1 engine?

Thanks for any info,
Regards,
ColinL
'72 OTS manual V12

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politeperson
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#2 Re: Compression horespower and engine numbers

Post by politeperson » Fri Feb 09, 2018 9:19 pm

I have found all the data on these engines pretty unreliable.

A difference of 7% or 9% is pretty small compared to the claimed and measured power of the XK engine in my amateurish experiments. The SAE and DIN claims being the main source of misunderstandings.

My 8:1 compression ration pistons produce a compression reading of 190 psi on each cylinder. That is high. The head has not been skimmed much and neither has the deck. How does that work? I haven't a clue. I was expecting 160 psi?

All I know is my car goes like stink. Thats the important bit.

I am trying to get more power though, fool I am.

I cant see an road bound S3 XJ6 4.2 running a CR of 12.5 :1 ! I think Rob Beere would have something to say about that one.
James

L.J.K. Setright was right.
"You just cant beat a good E-type"

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#3 Re: Compression horespower and engine numbers

Post by AussieEtype » Sat Feb 10, 2018 1:45 am

lowact wrote:
Fri Feb 09, 2018 3:30 pm
My spare engine is from a European 1986 XJ S3 Sovereign (non catalyst). According to www.automobile-catalog.com these have 12.5:1 compression ratio. The engine number suffix is “HB”, which indicates the compression ratio, except for this era engine I don’t know the code, does anyone? Does anyone know what compression I would/should measure with a 12.5:1 engine?
Being 1986 - I would assume that is a HE engine with different pistons and injection system to the earlier injected engines which is different again to the e-type v12s.

Take all the pollution crap off your US spec engine and convert it to UK/Aust spec - basically no pollution stuff and different tuning.

Garry
1971 Series 3 E-type OTS

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#4 Re: Compression horespower and engine numbers

Post by jagwit » Sat Feb 10, 2018 1:46 am

Colin, so often replies do not answer the issue raised and are opinion based rather than supported with verifiable data - as this reply will be :shock:

I used to have a '71 S3 OTS (low compression). When I got the car, the first thing I did to it was to fit it with fuel injection, using the EFI hardware from an HE engine but the ECU was a Megasquirt-2 and ignition was with dual EDIS-6 distributorless systems. This is the very car: https://creativerides.co.za/listings/19 ... -roadster/S3 OTS EFI

My point is this setup worked very well and I thought the car was quite powerful- until it seized a big-end forcing a rebuild. I had all the hardware, so I rebuilt it as an HE engine using 11.5:1 pistons I got from the US and HE heads from a scrap engine. The end result was astounding!!

This little OTS could keep up with my friend's 4.2L Supercharged XKR!! But as is well known, the HE engine did not really want to rev beyond 5000rpm (did not need to either will all the torque lower down) whereas the pre-HE was quite happy all the way to 6 grand.

I also have a '80 XJS with the pre-HE engine with the 10:1 CR engine - considered the most powerful of all standard Jag V12 engines. One can feel that this car is powerful, but the heavy body does tame the power.

My point is:
Upping the compression ratio on the pre-HE will bring significant power gains. If I have time tomorrow, I'll go do a compression test on my 12.5:1 DD6 - if I can find an easily accessible spark plug.
Best Regards
Philip
Jag: 72 S3 XKE, 73 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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#5 Re: Compression horespower and engine numbers

Post by AussieEtype » Sat Feb 10, 2018 1:51 am

politeperson wrote:
Fri Feb 09, 2018 9:19 pm

I cant see an road bound S3 XJ6 4.2 running a CR of 12.5 :1 ! I think Rob Beere would have something to say about that one.
I think we are talking V12s not 6 cylinders. - 86 is the HE era of V12s - May Heads 10.5 to 12.5 compression ratio.
1971 Series 3 E-type OTS

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#6 Re: Compression horespower and engine numbers

Post by lowact » Sat Feb 10, 2018 4:04 am

Wow James, 190 psi seems a lot from a 7.8:1 V12, if that is what you have? What is yr engine no. suffix, LA or LB? Why were you expecting 160 psi? I found this discussion: http://www.jcna.com/engine-compression- ... e-v12-1973 which includes “The general rule for cylinder cranking prussure is 14.7 multiplied by the compression ratio of the engine.” So, I was assuming that the 140–160 psi that I get from my 9:1 engine (original, 70k miles) wasn’t too bad? Mind you, imo it does NOT “go like stink” (yet), the Sovereign (yes is a V12, not 6, purchased complete and stripped) would have eaten it.
The only info I can find about the engine number suffix’s that are compression ratio codes is from the old jag-lovers: http://jag-lovers.org/xk-lovers/library ... umbers.htm which says: ”Note that for V12s 1971 through 1979, LA and LB suffix was 7.8:1 CR and SA and SB suffix was 9:1 CR. For 1980, suffix HA was 10:1 CR. From 1981 ?? suffix was 11.5:1 CR and ?? suffix was 12.5:1 CR.” My “??” is “HB” So I wonder if it is 11.5 or 12.5? Pretty sure it will be 12.5 as it was a Brit car and the 11.5’s were US?
Phil, I know all about yr old car; it’s my inspiration. As well as the compression, what is the suffix on yr DD6? Does anyone have or know of a 5.3L HE that has 11.5:1 C/R? What is the suffix on that? Maybe we can solve the Jag-lovers “??”?
Yes Garry, ultimately the E will be getting an upgrade, is the reason for the Sovereign engine. Hopefully with a Drivenman 5speed and my 3.54 diff … Is that you I sometimes see around town in the white OTS?
Regards,
ColinL
'72 OTS manual V12

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#7 Re: Compression horespower and engine numbers

Post by AussieEtype » Sat Feb 10, 2018 5:08 am

lowact wrote:
Sat Feb 10, 2018 4:04 am
Yes Garry, ultimately the E will be getting an upgrade, is the reason for the Sovereign engine. Hopefully with a Drivenman 5speed and my 3.54 diff … Is that you I sometimes see around town in the white OTS?
From discussion in other threads you might want to consider going to a 3.07 or even a 2.88 diff ratio.

Do Driveman do a E-type 5 speed - they seem to list XJ12 and XJS where I believe the gear lever is in a different position - and is the bell housing a different length?

Not me driving around - are you in Canberra?

Garry
1971 Series 3 E-type OTS

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#8 Re: Compression horespower and engine numbers

Post by lowact » Sat Feb 10, 2018 6:35 am

Yep - I'm in Canberra. Not u means there's a 3rd - breeding like flies...
I should not have mentioned 5speed, can we move this to a separate post?
Regards,
ColinL
'72 OTS manual V12

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#9 Re: Compression horespower and engine numbers

Post by politeperson » Sat Feb 10, 2018 11:04 am

Yes boys, wrong section. I am talking about the 6 cylinder, sorry!
James

L.J.K. Setright was right.
"You just cant beat a good E-type"

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#10 Re: Compression horespower and engine numbers

Post by jagwit » Sat Feb 10, 2018 5:14 pm

Measured two cyl's on my S3 today:

2A: 700kpa / 101psi
2B: 750kpa / 109 psi

I had run the engine a bit before measuring, throttle wide open. Its interesting though that such low figures produces the performance she has - which I recon is as good as they come. I decided that I should measure the 10:1 XJS pre-HE as well to complete the picture.

I can not measure the DD6, because its parked in by my XJS which is locked in by a heap of creosote poles. So it will take a while to come back with those figures. Sorry :roll:

But for interest, the rebuilt Chrysler 440 I installed into my Jensen Interceptor calculated to be 10.9:1, measured as below.

(kpa / psi):
#1: 1200 / 174
#2: 1050 / 152
#3: 1150 / 168
#4: 1200 / 174
#5: 1250 / 181
#6: 1100 / 159
#7: 1050 / 152
#8: 1100 / 159

Again, note that I stay at 1400m AMSL where barometric pressure is 85kpa. This must surely have an impact on the figures.
Best Regards
Philip
Jag: 72 S3 XKE, 73 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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#11 Re: Compression horespower and engine numbers

Post by lowact » Sun Feb 11, 2018 12:00 am

Thx Phil. Low barometric pressure will have a direct effect. Also you said you had a low compression engine? If your engine number ends in LA or LB it has 7.8:1 CR, then assuming that 14.7 "rule of thumb" and factoring for 85 instead of 101 kPa barometric pressure gives 7.8 x 14.7 x 85/101 = 96.5 psi. So yr measurements are good? Still keen to know what are the letters on the end of yr DD6 engine number. If u know for sure that the DD6 engine is 12.5:1, and if the letters are "HB" (same as mine), it would confirm for me that my engine is 12.5 also ...
Also if anyone has access to an 11.5:1 CR engine and can report the two letters on the end of the engine number?
Regards,
ColinL
'72 OTS manual V12

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#12 Re: Compression horespower and engine numbers

Post by helge.kassel » Wed Feb 21, 2018 10:26 am

Hi Philip, thank You for the info.
When You build the high compresion engine, did You use the HE-heads or the flat heads of the pre-HE engines?
And why did You change from Edis with MS2 to the original Ignition system.
BTW: do You still sell the DESC-board, that You developed for the Edis system? :yellow:
Best regards from Seville
Helge
If You do it, do it right
1971 V12 Aut 2+2 with MX5 Seats :bigrin:

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#13 Re: Compression horespower and engine numbers

Post by jagwit » Wed Feb 21, 2018 12:18 pm

helge.kassel wrote:
Wed Feb 21, 2018 10:26 am
When You build the high compresion engine, did You use the HE-heads or the flat heads of the pre-HE engines?
I presume you are referring to the 11:1 engine I built for my (now sold) OTS E-type? Yes, the engine was standard a 8.2:1 pre-HE, but I used the original block with the new 11:1 HE pistons and HE heads to build it as an HE engine with the original block (thus having an HE, but still "matching numbers" engine FWIW).
helge.kassel wrote:
Wed Feb 21, 2018 10:26 am
And why did You change from Edis with MS2 to the original Ignition system.
Not sure which car you are referring to now :bigrin: Let me put it like this: ALL "my" project EFI V12's that are using MS2 are using the dual EDIS-6 ignition systems. Even if I have to do another one, I would again use the dual EDIS-6. The reason is simply that, although MS-2 CAN be built with 6 ignition outputs (for V12 wasted spark ), I believe that using a continuous barometric sensor is imperative IF the car will experience more than 300m in altitude variation. MS-2 can not have BOTH 6 ignition outputs AND continuous barometric correction due to it not having enough inputs and outputs. For the dual EDIS-6, only one ignition output is needed.

I did rebuild a S3 Coupe recently for my friend and on that car I used the "original" ignition system from a HE Daimler Double Six, complete with dual coils. The reason for this was:
1) Eliminates OPUS rubbish;
2) Very cheap, proven, reliable and std jag parts;
3) Distributor now has vacuum advance to optimise the engine under partial throttle (which is most of the time!!).
helge.kassel wrote:
Wed Feb 21, 2018 10:26 am
BTW: do You still sell the DESC-board, that You developed for the Edis system? :yellow:
Yes, I do, but its worth noting that:
1) MS3 can offer 6 or 12 ignition outputs making the need for dual EDIS (and DESC) redundant;
2) MS2 can now be built with 6 ignition outputs (for V12 wasted spark) also making dual EDIS (and DESC) redundant BUT then you can not have continuous barometric fuelling correction (which is very important if the car will experience more than 300m in altitude variation.)
Best Regards
Philip
Jag: 72 S3 XKE, 73 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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#14 Re: Compression horespower and engine numbers

Post by helge.kassel » Fri Mar 16, 2018 3:46 pm

Thank You for the extensive explication.
I was following Your route with MS2, but the development of the MS3 will help me a lot. :bigrin:
Best Regards from Seville
Helge Kassel
If You do it, do it right
1971 V12 Aut 2+2 with MX5 Seats :bigrin:

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#15 Re: Compression horespower and engine numbers

Post by Mado » Thu Nov 25, 2021 11:18 pm

I’m in the process of rebuilding my Series 3 1974 pre HE 5.3 litre V12 flat head (LB suffix). Was running JE M92 pistons and 6 twin barrel Weber’s but now need to replace pistons and liners. Can I simply order new pistons to achieve 9 : 1 CR. Can the flat heads be machined. I’m also considering fuel injection. Any thoughts appreciated.

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MarekH
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#16 Re: Compression horespower and engine numbers

Post by MarekH » Fri Nov 26, 2021 8:21 am

Pistons and liners were matched on the original engines - the "A" or "B" suffixes indicates diameter.

A flat head can be machined - it makes no difference to the compression ratio as that is determined by the dished top of the piston.

The "H" or "S" in the engine number is a reference to the compression ratio.

Mostly missing from this discussion was any mention of ignition advance. Broadly speaking, Jaguar will have dropped the ignition timing when they increased the compression ratio or dropped the fuel quality and comparisons between the various v12s exist on the Roger Bywater website.

Conversion to a modern fuel injection system also opens the door to fitting a fully mapped ignition which can have the benefit of making the car more driveable at all revs and engine loads, whereas a simple distributor is compromised as it has to be set up to not over-advance the engine at the rpm where maximum torque occurs.

kind regards
Marek

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Craig Balzer
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#17 Re: Compression horespower and engine numbers

Post by Craig Balzer » Sat Nov 27, 2021 6:04 pm

Phil said: "Again, note that I stay at 1400m AMSL where barometric pressure is 85kpa. This must surely have an impact on the figures."

There are some variables that affect the readings obtained from compression testing. They are :

cranking speed,
altitude
temperature,
worn camshaft lobes and
high-performance, long-duration profile camshafts.

The cranking speed needs to be maintained the same for each cylinder. This may mean jumping your battery to maintain the speed. There are factors to compensate for the different altitudes and the corresponding temperature differences. These are as follows:

1,000 feet = .9711
2,000 feet = .9428
3,000 feet = .9151
4,000 feet = .8881
5,000 feet = .8617
6,000 feet = .8359
7,000 feet = .8106
8,000 feet = .7860.

The equivalent compression reading for a cylinder that should be 135 psi by the data at 5,000 feet would be 135 x .8617 = 116.33.
Craig Balzer
Colorado Springs, CO, USA
1972 Series III OTS, 4-Speed (soon-to-be a Guy Broad 5-Speed), A/C, CWW

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#18 Re: Compression horespower and engine numbers

Post by lowact » Sun Nov 28, 2021 4:45 am

You want to increase compression if you can. Otto cycle thermal efficiency potential = 1 – 1/CR^0.4, where CR is compression ratio. So for 7.8 compression ratio, thermal efficiency potential is 56%, for 12.5 CR it is 64%, i.e. a theoretical 14% increase in power and/or economy.
This is why, for HE engines, Jaguar increased compression ratio up to 12.5.
There are practical limits, pre-detonation (pinging) occurs at higher compression ratios, necessitating high octane fuel. If that is not enough, less ignition advance, i.e. detuning the engine to not achieve its maximum efficiency potential.
This is why, for US HE engines compression ratio was less, 11.5, 98 RON equivalent fuel was then not universally available. Is also why, for 6 litre engines, CR was universally reduced to 11.
There are other consequences, higher compression engines have increased piston ring blow-by, this is why, for HE engines, the crankcase ventilation system was supersized.
Flat head engines, all the combustion chamber is in the cylinder, cannot be reduced (CR increased) by skimming the heads, only by changing pistons and/or connecting rods. Weirdly, high comp alteratives seem not to be readily available. SNGB sell 7.8 CR versions, hopefully everybody knows not to buy these. What SNGB should sell for flat-head engines are the 10:1 CR versions, the final and reportedly best iteration before the HE. Imo everyone should want to buy these.
I’m following Phil’s lead, HE heads and pistons (& efi) on my e-type block. Also changing from petrol to dedicated liquid propane (>105 RON) and doubling up on the crankcase ventilation:
Image
Regards,
ColinL
'72 OTS manual V12

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#19 Re: Compression horespower and engine numbers

Post by 74jagguy » Sat Jan 08, 2022 6:24 pm

I am new to the group and have a 74 Series 3 V12 OTS LHD that is being restored. I am in the early stages for the engine rebuild and am researching the change to pistons. I currently have the 7.8:1 CR with all the emission controls stuff that will be going away. My initial thought was to simply purchase new 9.0:1 CR pistons and new liners. Apparently there are other options to consider so thanks for all the information in all the previous posts.
I am interested in lowact's comment regarding the pistons sold by SNG. I am not familiar with any reason not to buy the product SNG offers. Can someone enlighten me please?

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#20 Re: Compression horespower and engine numbers

Post by lowact » Sun Jan 09, 2022 12:50 pm

That "pollution control" equipment you are planning to remove, consider that, in practical terms, it is also 1970’s era "stench control" equipment.

Very recently SNGB have started offering 9:1 comp pistons, i.e. from them there is now a choice, 7.8:1 or 9:1. With 9:1 the combustion chamber is not just the dished piston, there is also a space, ~3 mm, above the piston, so with slightly longer connecting rods you could easily increase the compression further. The result would be better performance and worse emissions. The most physical manifestation of emissions is the stench. With e-type aerodynamics you travel in a cloud of your own exhaust, not that you notice until, at the end of the trip your hair, clothes and luggage all stink of hydrocarbons.

Higher comp. engines could not be made to burn cleanly at low load. So during 70s and early 80’s, compression ratios generally fell. E.g. Ford replaced their higher compression/ performance Cleveland V8’s with lower comp/performance Winsors. And my car, ’72 US spec 9:1 CR was replaced by your car, ’74 US spec 7.8:1 CR.
Etc.
These days modern (2020 era) engine management has enabled compression ratios to be increased to the max that the fuel can accommodate, without any of the stench that was previously a consequence.
Regards,
ColinL
'72 OTS manual V12

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