MKI / MKII S type 240 340 & Daimler 1955 - 1967

3.8 s type compression ratio and performance

 
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Old 06-19-2019, 09:10 PM
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Default 3.8 s type compression ratio and performance

Hello all. Still trying to learn about these vehicles. I notice that the compression ratio on the 3.8 is quoted as 8:1 which seems quite modest to me.
Do many members up the comp ratio a little for increased power? I understand this would require slightly higher octane fuel and slightly more wear on the big end bearings.
What sort of increased out put can get and still maintain longevity and reliability?
What is a typical life of a 3.8 engine? I understand this is a difficult question to answer because it is so dependent on how the car is driven but all things being equal it should be possible to give some estimates. For example an old 1964 1200 beetle you would expect around 100,000 miles from. A 2014 Nissan Patrol Diesel you would expect closer to 200,000 miles.
In this link https://www.automobile-catalog.com/m...type/1965.html there is a pretty good collection of performance data. If we look at the 3.8 S Type M,O/D then we see that there is quite a difference in claimed performance by the factory and by this other mob "ProfessCars" eg
https://www.automobile-catalog.com/c...overdrive.html

Which figures do the members here feel ar the more accurate please?
Cheers
Tim
 

Last edited by wouldbeowner; 06-19-2019 at 11:56 PM.
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Old 06-20-2019, 06:14 AM
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Hi again, Tim. The S-type was made in 7:1, 8:1 and 9:1 compression ratios. 8:1 was standard for the UK and most countries with decent fuel in the 1960s, but you could specify a 9:1 CR engine as an option. The original CR of the engine is indicated by the last digit of the engine number, eg 7B 55555-8 would indicate 8:1. However, almost all of these cars will have had engine rebuilds by now and, if that involved skimming the head and/or the block, the CR would be higher than original. You can run a compression test to estimate what CR the engine actually has. My engine now has 9:1 CR (by measurement), but started life as 8:1. People who've driven plenty say that mine 'goes well', but, by modern standards, it is not a fast car!

It's said that these engines will do 100,000 miles between rebuilds, but that would be on the assumption of the vehicle being used continually and having regular maintenance and oil changes. The thing they hate most is lack of use, when, after a period of lay-up, you will get sticking/bent valves. My engine has done 82k miles and has had three rebuilds in that time, the first two because of bent valves and the most recent because of sticking piston rings.

The estimated performance on Automobile Catalog is an estimate based on the quoted power output of 220hp gross. This was a rather quaint English way of measuring horsepower with none of the peripherals attached to the engine - gearbox, water pump, dynamo, PAS pump, etc. The actual bhp is probably around 150bhp using a modern approach. Jaguar rated early XJ6 4.2s as 160bhp, so you get the picture. Because of all this, there is no way on earth that a standard 3.8S would hit 60mph in 8.4 secs! To get a picture of the figures obtained back in the day, I suggest having a look at the period road tests on the S-type Register website.
 

Last edited by Orlando St.R; 06-20-2019 at 06:16 AM.
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Old 06-20-2019, 06:37 AM
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The XK engine was considered a tough old beast for the day but life highly dependent on maintenance. I'm going to leave typical mileage claims to others here as I know some have fleet experience which would be much more indicative. I would refute that the average 1200 Beetle engine did 100K miles. It did not which is why VW had a rebuild programme & exchange units available in many countries. The rest of the car would do multiples of that.

Compression ratios of 7/8/9 to 1 reflected fuel octane availability of the day in different markets. Most cars today have shaved heads. I would not fiddle too much but you can read up on Coombs performance modifications for racing ~ CR, porting etc. The likes of Eagle today do a lot more but they even replace the crankshaft.

Performance figures of the day with 8 to 1 CR of 3.8s MOD by the likes of Bowler at Autosport were typically 0 to 60mph in approx 10.2 seconds & a top speed just in excess of 120 miles per hour. This backed up by many road tests in the day by many scribes in different countries.

Modern materials & lubricants allow many engines today to achieve 1 million Km's in taxi & similar service etc before rebuild.

Modern lubricants, filtration & sensible servicing will substantially increase XK engine life. Most of us here preserving these great old cars will never do enough mileage to wear out a properly rebuilt XK engine. For what it's worth the engine in my car had done 90,000 miles & was still serviceable. I rebuilt it anyway because I was restoring the car.
 
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Old 06-20-2019, 06:49 AM
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Sorry for some repeats Orlando. We posted over one another.

EDIT: I agree wholeheartedly with Orlando that these cars like regular use.

Tim, Buy this book.





 

Last edited by Glyn M Ruck; 06-20-2019 at 07:45 AM.
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Old 06-20-2019, 07:15 PM
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Thanks so much Glyn and Orlando. Would be a really interesting conversation around a nice open fire and a bottle of red I think.
Cheers
 
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Old 06-22-2019, 11:29 PM
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My 1966 S Type 3.8 is a measured 8.6 compression, and when I dyno tested it I achieved 171 HP @4300 rpm and 236 Ft-lbs at 2700 rpm. I have fuel injected it and converted to distributorless ignition, and by the seat of the pants meter the ignition made a big difference to the torque, the car felt quite a bit livelier.

For a 4.2, about the most you can get with available pump fuel is about 230 hp. A 3.8 would be slightly less HP. The hemi combustion chamber is actually not a great design, it requires more octane for a given compression ratio than other types of head designs, so available octane will always be the limiting factor for an XK engine.
 
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Old 06-23-2019, 06:25 AM
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Yes ~ the squish behavior of hemi designs does not provide high turbulence for the best air fuel mixing in the combustion chamber but does have other benefits.

Octane sensitivity/requirement of these engines also seems quite variable for reasons other than just CR. Correct timing & advance curves are crucial. I suspect that different installations suffer hot spots for varying reasons from deposits, to plugs, to cooling system maladies.

My engine is newly rebuilt & thus deposit free. CR between 8 & 9. Seems less susceptible to knock than some friends cars that always seem "on the edge" from a knock perspective. (At coast, 95RON gasoline/no alcohols)
 
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Old 06-26-2019, 02:44 AM
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Originally Posted by Jagboi64 View Post
My 1966 S Type 3.8 is a measured 8.6 compression, and when I dyno tested it I achieved 171 HP @4300 rpm and 236 Ft-lbs at 2700 rpm. I have fuel injected it and converted to distributorless ignition, and by the seat of the pants meter the ignition made a big difference to the torque, the car felt quite a bit livelier.
Would love to learn what injection system you employed, along with the ignition, ECU etc. Any chance of a tutorial?

Pete
 
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Old 06-26-2019, 11:35 AM
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Originally Posted by redtriangle View Post
Would love to learn what injection system you employed, along with the ignition, ECU etc. Any chance of a tutorial?

Pete
I used the Megasquirt ECU, and it controls both spark and fuel. For spark, I cut the trigger wheel off a 1.9 Ford Escort and then machined a shoulder on the back side of the main crank pulley. The system requires what is called a 36-1 wheel. It has 36 teeth ( 10 degrees apart), but missing one. The missing tooth tells the system where TDC is. It's a wasted spark, so no cam position sensor needed, the system fires on both compression and exhaust strokes. I built a bracket to mount the sensor where the timing pointer bolts to the oil pan. There is a Ford module needed, as well as a coilpack. The Ford EDIS modules are available in 4, 6, 8, and 10 cylinder versions, so have to get the right one. I mounted the coilpack on a bracket I made that attaches under a head stud and it sits at the back of the engine. It came from a 2002 Ford Taurus. The distributor is obsolete, so I blanked off the hole.

For fuel, I used carb adapters that mount a throttle body injector in place of the dome and piston of the SU carb. I built a bracket to adapt a throttle position sensor at the rear of the throttle shaft and blanked off the Otter switch, and instead mounted the coolant temp sender there. For the air intake, I used an air filter assembly from an X300 and mounted that in the left front wing, right in front of the tire, and ran piping up and and along the underside of the wing, and then drilled a hole to go through into the engine bay. I used the cast elbow from a Series III XJ6 intake to make the turn, and then went across the engine to a Ford mas air flow meter from a Crown Victoria.

On the fuel supply I used Walbro TBI pumps mounted in the same place as the originals. Most EFI pumps need to be gravity fed, but the Walbros can lift fuel up to 40", so they are suitable to mount in the original location. I used Series III changeover valves plumbed in a similar way to the SIII, and a relay to switch power to each pump with the position of the dash switch. That feeds a nylon fuel line and filter and pressure regulator. I used the original steel line as a return line, and the fuel returns to a fitting I threaded into the bottom drain bung on the tanks.

At the moment I have an idle air controller feeding the extra air into the plumbing on the underside of the intake manifold for the original starting carb, but it doesn't seem to work that well. When the engine bay gets hot after a long run I have an air leak somewhere and I can't control the idle speed, it comes up to 2000 rpm and can't be brought down. My next plan is to actually get rid of the SU carb bodies, and I am going to use the complete GM TBI unit off an early 90's Chevy truck. The injector rates for the 5.7 application are just about perfect for the 3.8 and It's a complete package of fuel pressure regulator, throttle position sensors and idle air control and injectors in one. I have to make a small manifold that adapts to the carb opening and then turns up, as the GM TBI is a downdraft application. Should basically just be a pair of elbows and a mounting flange. I already have the mounting flanges laser cut, just need to cut as weld the elbows and make a throttle cable. Always more projects!

 

Last edited by Jagboi64; 06-26-2019 at 12:25 PM.
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Old 06-27-2019, 04:31 AM
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Thanks so much for all that information. Looks like a whole new learning experience for me. Should be a sure-fire way of avoiding Alzeihmers!
 
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Old 06-27-2019, 05:03 AM
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Thanks for mentioning that book Glyn, I shall have to go on the hunt for one, I found this amusing though...


 
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Old 06-27-2019, 06:54 AM
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Yes ~ they botched up with the cover on one edition!
 
 
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