Compression Ratios

[Ian Vincent]

I am rebuilding my TR3a engine and next on the list is the head.

 

I am going to convert it to unleaded and in addition to a very slightly hotter cam am going to mildly gas flow it by opening it up around the inlet valves and modifying the squish area in line with the later TR4a heads. All of which should give me the opportunity to adjust the CR to whatever I want. I have a burette and can do the maths (incidentally the answer to the recent question elsewhere on this forum is that for a 4 cylinder engine the unswept length of cylinder would have to be between 10 and 11 mm before the CR reduces as the diameter of bore increases) so what is the general view on the ideal CR for the fuels that are currently available? And how much can you sensibly skim off a TR head?

 

Any advice gratefully received.

 

Rgds Ian

[Alec Pringle]

Hi Ian,

 

personally I'd need a good reason to go any higher than 9.5:1, skim the necessary minimum, and I'd be looking for a good torque spread rather than maximising top end power.

 

Cheers,

 

Alec

[TR 2100]

I am rebuilding my TR3a engine and next on the list is the head.

 

I am going to mildly gas flow it by opening it up around the inlet valves and

modifying the squish area in line with the later TR4a heads.

All of which should give me the opportunity to adjust the CR to whatever I want.

I have a burette and can do the maths so what is the general view on the ideal

CR for the fuels that are currently available? And how much can you sensibly

skim off a TR head?

 

 

I think I skimmed over 100 thou off mine - too much for normal use,

although this does save a lot of work removing the shroud over the

inlet valve.

I also used a steel gasket AND removed the inner ring - with hindsight,

this is too extreme and I don't recommend it.

 

I reckon 70-80 thou should be OK, but take account of the thickness of

the head gasket you are going to use.

Use whatever head gasket you are going to use as a template, and have

fun with the burette etc. I think I found it easier to use a 50cc pipette

first followed by the final measurement with a more accurate but smaller

burette. Quicker.

 

Great fun also sealing the plastic sheet over the head and messing

about with the paraffin.

 

A highly satisfying experience, though.

 

Are you fitting larger inlet valves?

 

AlanR

[ianc]

I think I skimmed over 100 thou off mine - too much for normal use,

although this does save a lot of work removing the shroud over the

inlet valve.

I also used a steel gasket AND removed the inner ring - with hindsight,

this is too extreme and I don't recommend it.

 

I reckon 70-80 thou should be OK, but take account of the thickness of

the head gasket you are going to use.

Use whatever head gasket you are going to use as a template, and have

fun with the burette etc. I think I found it easier to use a 50cc pipette

first followed by the final measurement with a more accurate but smaller

burette. Quicker.

 

Great fun also sealing the plastic sheet over the head and messing

about with the paraffin.

 

A highly satisfying experience, though.

 

Are you fitting larger inlet valves?

 

AlanR

 

 

I have a CR of a smidgen over 10, but use 4-star leaded petrol - so the car doesn't pink, even if I open the throttle wide at low rpm.

If using that horrible unleaded stuff, it's probably better to aim just below 10 for a road car.

Ian Cornish

[wriggley monkey]

I think I skimmed over 100 thou off mine - too much for normal use,

although this does save a lot of work removing the shroud over the

inlet valve.

I also used a steel gasket AND removed the inner ring - with hindsight,

this is too extreme and I don't recommend it.

 

I reckon 70-80 thou should be OK, but take account of the thickness of

the head gasket you are going to use.

Use whatever head gasket you are going to use as a template, and have

fun with the burette etc. I think I found it easier to use a 50cc pipette

first followed by the final measurement with a more accurate but smaller

burette. Quicker.

 

Great fun also sealing the plastic sheet over the head and messing

about with the paraffin.

 

A highly satisfying experience, though.

 

Are you fitting larger inlet valves?

 

AlanR

 

 

 

 

Alan R

 

What compression ratio did that get you and did it work?

 

What cam are you running and what fuel do you use.?

 

 

An Interesting topic.

[TR 2100]

Alan R

What compression ratio did that get you and did it work?

What cam are you running and what fuel do you use.?

An Interesting topic.

 

 

wriggley,

 

Yes, an interesting topic.

I was aiming for a CR of 10.5, I think. Based on calculations,

of swept volume and head + steel head gasket volume, that is

what I should have got.

 

It's difficult to say whether 'this' aspect worked or not - I

did blueprint the whole engine, checked piston heights against

the block, lightened con rods (2/3rds of what Kastner book says)

everything balanced, including pistons and con rods assemblies to

the nearest gram, head volumes to 0.1cc.

And 42DCOEs, of course. What else?

 

The engine was powerful and smooth. It really took off after I had

run it in and dared to really rev it. Looking back, I don't recall

even getting the normal resonance in the 2800-3000rpm range.

 

What I did with the steel head gasket, removing the inner sealing

ring to allow more extreme head modification (mainly around the

larger inlet valves) was a bit extreme for fast road use but it

worked well enough - a very smooth engine.

That is, until I shipped the car out to Hong Kong, the shipping

company drove the car instead of the covered flatbed I had specified,

and cooked the engine. The head warped ever so slightly, not showing

with a straight edge but with an unforgiving steel gasket, enough to

keep blowing the gasket.

 

I was running a fast road cam (RCT 777), 87 (8?8) + 40thou pistons

(they were the largest you could get at the time.

This was 20 years ago now, the days of leaded petrol.

 

I have done very similar modifications to TR 2100, but 89mm pistons

and the head will be Stage 2/3, I think, done by TR Enterprises.

I don't have the time or facilities any more.

 

AlanR

[TR3BGeorge]

A slightly related anecdote, of no value other than amusement.

 

In 1958, a high-school acquaintance had a 1950 Ford flathead 6 with the head milled down to a little over 12:1 ratio. When it blew up, chunks of the head put big holes right thru his bonnet. Big crowd out in front of the school. He "whined it out" right in front of the crowd. Great theater.