TR4/4A TR4 Milled Cyl. Head Suggestions

[jimitro]

Hi,
Thanks to everyone for all the past help you have given me on my posts. Now my next question. I removed my head due to it leaking and now am going to get it converted to unlead.
My engine is stock with header, dual exhaust, elect.ingition. I'm thinking of having the head shaved .20 for more compression. Any other suggestions???
Thanks

 

[Simon TR4a]

I like the idea of increasing the compression ratio as a means of improving performance, but I doubt you will make much difference by milling off only 20 thou.

What you really need to di is to take the head off and take it to the machine shop to be measured- either the thickness compared with stock or the volume of the combustion chambers . This will tell you if the head has already been skimmed in the past, and if so, how much.

Then you want toi decide what compression ratio you want to acheive, I would suggest about 9.5 to 1 , which will give you a little margin for safety as carbon will gradually build up in the combustion chamber, the engine may need to be rebored at some future date, or may run hotter than normal for some reason such as silt in the radiator or coolant passages in the block.

The combustion chamber of the TR4 is roughly D shaped, with a more or less flat roof and vertical sides, so the effect on the chamber volume is close to proportional to the thickness of metal removed, unlike a hemi head, where the efeect is progressive. In spite of this, be cautious, as the remedy for removing too much metal is use of a steel head gasket in addition to the regular one, or removing material from the squish area and the "eyebrow" shrouding the inlet valve, to compensate.

Unshrouding the inlet valve is also very worthwhile, by the way. When you get the head off you will see what I mean.
Simon.

 

[TRMark]

If you have not already, make sure you check head thickness before you do any millwork. Stock is 3.330 The converting to unleaded question should lead to plenty of debate, it usually does.

 

[Bob Claffie]

You might look for the old Kastner book on preparing TR's for racing. Amazingly I was able to take .125 off of the head of a Spitfire I raced in F Prod with the SCCA back in the 70's Never did figure out the compression ratio but it ran great on the track. Bob

 

[trfourtune]

if you do any combustion chamber modifications, do them first as much as you can. make sure the chambers are exactly the same cc's for all four. measure volumes uncluding head gasket volume and check each piston at tdc to top of cylinder for volume of each one. then mill your head based on the numbers you have to get the desired compression ratio. then re-check all volumes to see what you really ended up with. remember that the valve job affects the volume of the combustion chamber. mike each valve from head face to valve to ensure they are at the same depth. you want to end up with exactly the same voulume on each cylinder. this is the base line. the nascar guys actually vary cc's to compensate for other things but that is beyond us mere mortals.
rob
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[Alan_Myers]

Hi,

I agree with the other responses, you need to figure out what you've got before making any changes.

.020" from stock is very little change. .060" is usually the minimum done on the 4 cyl. TR motor to increase CR slightly, and .090" is pretty common, too. More toward the racing end of the spectrum, .120" is starting to get radical, .150" is a whole lot radical, but possible with the later (post-TR3) heads.

With radical milling, there can be concerns about making the wall or ceiling of the combustion chamber too thin at some of the coolant passages, which might lead to a collapse. And with too much milling the earlier heads (pre-TR4) can see an interference at the front, between the water pump housing and thermostat neck.

MRichlen is correct, the head measured from the face to the flange where the valve cover gasket seals should be about 3.30" tall, assuming it's never been milled at all. Any less than that dimension means some shaving has already been done. Do check this first.

Rob is correct, there is more to it than just milling the head. The process of improving the combustion chamber shape removes some material and each cyl. needs to be cc'ed to make sure they are all the same. Once the size of the "squish area" is known and added to the volume of the cylinder, CR (compression ratio) can be calculated.

Other major considerations include the deck height of the pistons you are using, not to mention the bore. Larger bore = larger volume = lower CR. This is one place where milling the head comes into play, adjusting CR back to where you want it.

The head gasket itself can be used to adjust CR a little, too. A thicker gasket (mostly available in solid copper) can be used to raise the head and reduce CR, especially useful for cyl. heads that have been over-milled in the past. On the other hand, the "factory" racing modification was to use the special, thin shim steel gasket that was available, either in conjunction with milling the head or by itself. This will increase CR (it this gasket was actually originally offered to decrease CR, i.e. for use in combination with the stock, sandwich type head gasket, intended for cars in countries where fuel quality was low and a slightly lower CR was required).

I agree with Simon that many folks try to set CR to around 9.5:1 for street cars in the U.S., to run well on widely available fuels. For comparison, Greg Solow at the The Engine Room builds TR/Morgan engines for race use that have 12.5 to even over 13:1 CR and produce up around 200 HP. But those wouldn't be much fun on the street and you'd better have access to *very* high octane fuel! To achieve these really high figures, in addition to milling the head, Greg fill welds in some of the combustion chamber effectively moving the ceiling down, a process that requires a lot of hand work and special stem-length valves.

By the way, the calculation of CR is usually done "statically", meaning simply by calculating and comparing the minimum and maximum volumes of the cylinder (Google for calculators on the Internet to help you do this, but you need to know all the volume factors, first).

However, there is also "effective" CR, perhaps a more realistic figure that is what the engine actually produces while running. "Effective" CR is effected by valve head diameter, camshaft duration and lift, and various other factors effecting gas flow in and out of the cylinder.

"Static" and "effective" are my own words here, trying to explain this. I don't know if these are truly correct terms. My point is that when using a high lift/long duration cam and bigger cyl. bores it might be possible to set "static" CR a little higher than would be otherwise practical.

So - back to your question - taking into consideration any and all modifications being made to your engine that change the volume of the combustion chamber, you need to determine what CR you are ending up with, then mill the head as needed to achieve the CR you want.

Hope this helps!

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[TRTEL]

Unless something else gets changed like going back to a stock head from a heavily milled head your compression ratio will go up with an increase in bore diameter. Since the CR is equal to the piston displacement volume plus the combustion space volume all divided by the combustion space volume any increase in piston displacement will increase the CR units. Going from an 83 mm bore to the 87mm will raise the CR from about 8.5 to 9.3 or something in that neighborhood.
Tom Lains

 

[martx-5]

[ QUOTE ]



Other major considerations include the deck height of the pistons you are using, not to mention the bore. Larger bore = larger volume = lower CR. This is one place where milling the head comes into play, adjusting CR back to where you want it.


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[/ QUOTE ]

Alan,

I was under the impression that INCREASING the bore would INCREASE the CR all other things being equal.

This online CR calculator such as you mentioned seems to bear this out.

 

[jimitro]

Thanks for all the info.
It took the suggestions to measure the head first and was surprised that it had been worked on previously. The car only has 84,000 miles on it and I believe that it is the original motor although you really can't tell since Triumph numbers don't match. By the time a flat plane was achieved,
the measurement ended at .318. This should work well for a stock TR4 engine.