skimming a cylinder head

[Rex Wyer]

Hi Chaps some information please.

My American TR6 engine is approx. 104 BHP and I would like to increase its power. At present it has a 219019 head which

is correct for a 75 car.

The engine already has a 125 cam in it and so how much do I have to skim off the head to increase the compression ratio

the head height is 3.560 inches at present and a PI head is 3.40. The car will run on carbs not PI so how much do I get the engineers to skim off. The head will also be unleaded.

 

All answers kindly received

 

Rex

.

[mjdearing]

Hi Rex get it over to Pete Burgess (Alfreton) and let him advise and work his magic.

[john.r.davies]

OR,

Assume nothing, measure the combustion chamber volumes, and calculate how much to skim. No doubt any expert will do the same, but why not extend your skill mix?

 

I've described how to do so, in detail here: http://sideways-technologies.co.uk/forums/index.php?/topic/7551-how-to-raise-the-compression-ratio-safely-and-effectively/

 

John

[PhilipB]

Hi Rex get it over to Pete Burgess (Alfreton) and let him advise and work his magic.

+1

He wrote the book - literally

 

Excellent job on OGB's head

 

Phil

[TriumphV8]

Often on Sideways I did not find Johns post up to now.

Very interesting! Will read that later!

 

https://www.goodparts.com/shop/index.php?ukey=auxpage_tr6-compression-ratio

 

This is a coarse way to combine head thickness with CR

It is only correct if chamber is not modified and bore is close to stock.

 

It is a good idea to reduce wall shroud and sharp edges

and also the valve seat area needs attention if the rise of CR

had also the idea to rise power.

[Motorsport Mickey]

Rex,

 

If you wish to use the car without having to use additives added to the petrol at every filling up be careful to what level you increase the compression. Normally on 4 cylinder cars I limit it to about 10.3:1 ratio, which still allows filling from pumps without extra jallop with noxious side affects and increased costs for every tankful to be added.

 

Be advised when you tell an engineering firm that 60 thou (1.5mm or thereabouts) needs removing from the head face that is what will be removed, right or wrong. By all means measure your existing chambers with a burette for volume and by allowing for the gasket you are going to fit and the piston size and cylinder block bore define what compression ratio you wish to come out at. But if work is to be carried out on the head, cutting valve seats, altering combustion chamber shape, etc it's likely that the compression ratio will change.

If you wish to take it further and alter the heads to improve performance, you can of course decide to have a go yourself using some of the tuning publications published, and judging by eye and intuition as to where material should be removed, grind material from head inlet and exhaust tracts changing the valve profiles or replacing with better shaped valves normally of a larger size.

 

You have to be careful, unthinking removal of material or even smoothing the edge in various areas can have unintended consequences and lose power or even worse puncture a head waterjacket. The only way you can measure improvements (or not)is by measuring the airflow on a flowbench where air is sucked through the cylinder head valves to quantify it.

Alternatively you give your head to a specialist and tell them what you are wanting the engine to do, full out competition, normal road use or the most difficult, fast road application. As long as you give all the information the specialist should be able to turn out the cylinder head and deliver the required performance from the engine, maybe even recommending a particular cam which he's seen give good results on the rolling road or dynamometer.

 

I build my own engines and use Peter Burgess for developing the cylinder head, no particular reason. (other than the unfair advantage)

 

1987 TR Register 4 cylinder Tuned winner and Overall Champion

1988 TR Register 4 cylinder Tuned winner and Overall Champion

1989 TR Register 4 cylinder Tuned winner and Overall Champion (engine builder only, driven by Chris Conoly)

1990 TR Register V8 Modified class winner

1991 TR Register V8 Modified class winner and Overall Champion

1992 TR Register V8 Modified class winner and Overall Champion

1993 TR Register V8 Modified class winner and Overall Champion

 

Mick Richards

Edited February 5, 2018 by Motorsport Mickey

[DRD]

OR,

Assume nothing, measure the combustion chamber volumes, and calculate how much to skim. No doubt any expert will do the same, but why not extend your skill mix?

 

I've described how to do so, in detail here: http://sideways-technologies.co.uk/forums/index.php?/topic/7551-how-to-raise-the-compression-ratio-safely-and-effectively/

 

John

 

John,

 

Assuming you have an accurate gauge is there a way of deducing/calculating the CR from a compresion test reading?

 

Darren

[john.r.davies]

I regret not, Darren, but life's not like that, else it would be too easy!

 

Thing is, compression gauges are not precision instruments and are notbcalibrated. Even a Vernier gauge can be said to be those. It'll give you an accurate, useful comparison between bores across the block or even between two engines, if done with the same gauge on the same day. But not between different gauges.

 

Mickey has taken the trouble to spell out the reasons for buretting and calculating your CR. My article shows you how to calculate the skim as well.

Enjoy!

John

[Tonyloz]

But if you calibrate your compression gauge isnt it the gas compression that gives you the correct combustion efficiency?

[oldtuckunder]

But if you calibrate your compression gauge isnt it the gas compression that gives you the correct combustion efficiency?

 

NO.

 

It would be dangerous even if you could, as you have no idea of what other wear factors are affecting any compression readings, a subsequent change of rings etc could alter the effective CR.

 

Its one of those jobs where there is just no substitute to accurately measuring and calculating what you have, and what you have to remove to get what you want. The Simplistic tables of skim this from this head to achieve this CR, might have worked when everything was close to standard, but these engines are all 40+ years old you haven't the faintest idea what has been done to an engine previously unless you measure and find out.

 

If you don't think accurate measurement is important, go ask one of the Mars Landers

 

Alan

[Motorsport Mickey]

But if you calibrate your compression gauge isnt it the gas compression that gives you the correct combustion efficiency?

Takes no account of independent factors for each cylinder, piston ring leakage, valve leakage etc all of which will vary and alter the compression achieved whilst telling you nothing about the chamber size in the head per cylinder which is what determines the compression ratio for the engine. Whilst appreciating the thought process this mousetrap has been built in the best way.

 

Mick Richards

Edited February 5, 2018 by Motorsport Mickey

[john.r.davies]

BUT, calculation of CR from buretting is theoretical.

It will be the maximum that can be acheived, but in practice less than that, diminished by the volumetric efficiency of the engine, how unobstructive its in- and out-let passages are, and by the leaks that Mick mentions.

JOhn

[Tonyloz]

But if you know your piston size and stroke then there must be correlation to the compression figure achieved AND it would take into account piston and valve leakage to give an exact figure for your particular engine not just theoretical

[Motorsport Mickey]

All you can do is start by working the compression out by theory, all the various leakages will compromise it but if the engineering is completed competently and as best as can be done, at least the various leakages will approximate the same compromise per cylinder and the compressions will be the same...about.

 

The individual measuring of chambers removes the imponderables of the various leakages we've mentioned, and at least gives you the same datum for development on an individual cylinder basis, and has been developed and refined by engineering departments from the major manufacturers of autos to the specialist engine gurus such as Cosworth et al.

 

There ain't no better way, if they could measure compression ratio by measuring compression of an engine these Titans of mechanical expertise would be using it, there isn't. Taking compression readings from engines is an imperfect science, because of just some of the variables we've talked about it's perfectly possible to have a 5 or 10lb difference across cylinders and there is NO mechanic I know that would suggest that an engine showing a variable of that order is anything but "ok". Whereas when you burette individual chambers the specialist head firms will break their balls to give as close to exact cc to within a decimal point per chamber as they can machine, and this is a fundamental component of calculating the compression ratio.

 

It's important to invest the appropriate amount of expertise and components into engines and then the real world takes over and you have to live with what you get.

 

Tony, I don't know what argument you hope to prove with this, the world's engineering genius's disagree with you, unless you can point me to the appropriate engineering dissertation or article that proves otherwise, source ?

 

 

Mick Richards

Edited February 6, 2018 by Motorsport Mickey

[DRD]

But if you know your piston size and stroke then there must be correlation to the compression figure achieved AND it would take into account piston and valve leakage to give an exact figure for your particular engine not just theoretical

 

That's what I would have thought. If the gauge had been calibrated correctly then you will measure the actual compression of the engine surely, leaks and all?

 

I agree you wouldn't want to rely on this if you were skimming a head, the only way would be by using a volume of liquid to avoid all the other issues that could have an effect. However once its done then its perfectly possible to measure the end result. NB: My recently refurbed head gave identical readings for all the cylinders, so pretty sure there were no leaks.

[oldtuckunder]

But if you know your piston size and stroke then there must be correlation to the compression figure achieved AND it would take into account piston and valve leakage to give an exact figure for your particular engine not just theoretical

 

What Mick says, but as a real world example an engine with a high engineered CR but with a "hot" cam with a lot of overlap is likely to give way lower real compression readings than a lower CR engine with a standard cam. Well at least until you get to the RPM range where the volumetric efficiency created by the hot cam starts getting more induction charge into the cylinder.

 

Alan

[oldtuckunder]

NB: My recently refurbed head gave identical readings for all the cylinders, so pretty sure there were no leaks.

 

No what you had was nearly identical leakage across all cylinders, which is good. However it could be way more or way less leakage than another engine that gave identical compression readings, you just wouldn't know.

Alan

[DRD]

 

No what you had was nearly identical leakage across all cylinders, which is good. However it could be way more or way less leakage than another engine that gave identical compression readings, you just wouldn't know.

Alan

No I guess so but it would still represent the actual compression which is down to the effective CR (and all the other factors etc).

[DRD]

No I guess so but it would still represent the actual compression which is down to the effective CR (and all the other factors etc).

 

This is useful, sort of answers my original question.

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=14&cad=rja&uact=8&ved=0ahUKEwjy8PKxn5HZAhWKAMAKHQh1BQ4QFgh1MA0&url=http%3A%2F%2Fwww.healey6.com%2FTechnical%2FNominal%2520compression%2520pressure.pdf&usg=AOvVaw1hvcsT1EIvMR7vsqBzgbFa

Edited February 6, 2018 by DRD

[ianc]

In Section P2 of the Technicalities CD can be found an article which I wrote for Newsletter #13 (Spring 1973), and this may assist.

Back then, I wasn't aware of 88mm and 89mm liners and pistons, so some computation will be required for these, but I have stated the formula, so not too difficult a task.

As Mick and others have said, there's no substitute for measuring the volume inside the head for each of the combustion chambers, and then computing the CR.

Compression measurements are a useful guide to the state of a particular engine, but dodgy if compared with any other engine.

Ian Cornish

[Motorsport Mickey]

I've just returned from a cylinder head specialist (prior appointment) and he's had the question posed before about engine compression ratio and does it relate to compression ratio ? and as stated previously there is a correlation.

However he doesn't know of any specialist who will rely upon the compression figures to state the compression ratio as being a definitive figure and correct, and more especially at compression ratios where there is a likelihood of pinking or detonation, calculation of the figures is the definitive method of stating the compression ratio, as used by himself.

 

Mick Richards.

Edited February 7, 2018 by Motorsport Mickey

[DRD]

I've just returned from a cylinder head specialist (prior appointment) and he's had the question posed before about engine compression ratio and does it relate to compression ratio ? and as stated previously there is a correlation.

However he doesn't know of any specialist who will rely upon the compression figures to state the compression ratio as being a definitive figure and correct and more especially at compression ratios where there is a likelihood of pinking or detonation calculation of the figures is the definitive method of stating the compression ratio as used by himself.

 

Mick Richards.

 

Done a bit more investigation myself. Yes there's a theoretical CR based on the geometry/size and an effective CR which is lower than the geometric one. The difference being the volumetric efficiency. This is were the uncertainty arises. I did some calcs based on my head which I know the parameters for, ie a CR of 9.5 and the comp pressure of 195psi. This resulted in a volumetric efficiency of 81% so I guess you could then use this and a comp pressure to estimate the new CR if it was increased assuming nothing else changed.

 

But I agree if you were going to cut metal then it's best to directly measure the head volume.

[YankeeTR5]

Or....if you're taking a factory NA thickhead, skimming it down to the 3.4 that the rest of the world got as standard, factory deck height is a simple method of increasing compression. Getting to the factory PI compression ratio will do a lot towards giving the car the power its missing right now.

 

Now, if you're taking a 3.4' thick head and skimming from there, I'd go the extra mile and measure twice, skim once since a mis measurement might get the car into a compression ratio that's street unfriendly.

[spyder dryver]

Done a bit more investigation myself. Yes there's a theoretical CR based on the geometry/size and an effective CR which is lower than the geometric one. The difference being the volumetric efficiency.

 

Its less to do with VE and more to do with inlet valve closing angle. IVC. No compression can occur before the valve is fully closed. This happens at some angle after BDC and can be well after BDC. This angle on my 16V engine is 65 deg ABDC. This means that the piston is part way up the bore and that distance is further affected by conrod length. I have measured this effective stroke on my part built engine and it is 61.5mm. The stroke is 86mm.

By then using 61.5mm in the normal calculation it gives a "dynamic compression ratio" of 8.2

Dynamic comp ratio, DCR, is a "big thing" in American V8 circles. Less so over here. Google it.

I was at Peter Burgess' with Mick today. Pete has never calculated DCR.

As regards calculating comp ratio from compression readings this all means that you are actually trying to calculate the DCR.

In the previous post DDR has seemingly found out that the DCR of his engine is 81% of 9.5.

If you stand this on its head it seems that the piston might be roughly 19% on its way up when the valve closes. It might be possible then, if you know the closing angle and conrod length to derive the static comp ratio.

Or thereabouts.

Well nearly.

The main spanner in the works will be the inevitable heating that occurs during compression. This imponderable alone is enough to scupper what may have been a meaningful exercise.

There are various DCR and static comp ratio calculators online. I have just burretted my chambers as well as measured the ccs of piston dome.

Its the only way to be sure.