Can you have too much comprssion?

[MikeB]

Folks

I have two cylinder heads for my 240 engine, both have been skimmed (I can't remember exactly what the head thicknesses are), but think one has 40 thou off and the other is 60 thou (or maybe even more). Both are the 73 spec 240 heads

I have a Piper Stage 2 cam fitted (266 duration inlet 23/60 exhaust 60/26)

With the higher compression head I have to run 97 Octane fuel to stop the engine running on when stopping. So am wondering is the extra compression of that head giving any benefit? Can you have too much compression for a camshaft?

 

[ben240z]

Have you measure the head cc and worked out actual compression ratio.
Dont forget that ignition timing/advance curve may need looking at as well due to higher comp.

higher compression gives bigger bang gives more power in simplistic terms. I know there are more factors to consider but that is the basic

Sorry if teaching to suck eggs.

 

[pmac]

Have you measure the head cc and worked out actual compression ratio.
Dont forget that ignition timing/advance curve may need looking at as well due to higher comp.

higher compression gives bigger bang gives more power in simplistic terms. I know there are more factors to consider but that is the basic

Sorry if teaching to suck eggs.

As Ben has already said you need to cc the head.

Compressed gasket thickness and whether they are standard pistons flush with deck or not also feature in the calculations.

 

[jonbills]

I'd also expect you're making extra power with the higher compression.
With it being on the border line of running on, is it pinking heavily under load?

Assuming it is a 0.060" skim, it's similar CR to my engine with the same head.

 

[pmac]

I'd also expect you're making extra power with the higher compression.
With it being on the border line of running on, is it pinking heavily under load?

Assuming it is a 0.060" skim, it's similar CR to my engine with the same head.

I thought you were running an L26 or L28 engine??

 

[jonbills]

I thought you were running an L26 or L28 engine??

yes - L28 (@2915)
I'm still running my old head which is an E88 from a 240. It's skimmed about 1mm, and if Mike's is skimmed about 1.5mm then we might be similar CR depending on gasket.
My gasket is 1.5, giving me a CR of about 10.8:1 and if Mike happened to have one of them Felpro jobs at 1.1mm compressed, then he'd be about 10.7:1

But all a lot of speculation without measuring the head volume, as you & Ben say!

 

[Rob Gaskin]

Both my old trackday car and current blue car 'run on' .

My old car was 200 psi, I haven't measured the blue one yet.

Never had any other issue and both perform well.

 

[MikeB]

Gents

Thank you for responding, I appreciate your time.

But you are missing the question.

That is, can you have too much compression?

There's a head on the engine, I can't measure the cc, so don't get bogged down in the specifics of what CR I'm running, in this instance it's not relevant to the question asked

If I asked what CR suits my engine then we can get into specifics, but do we need to do that to answer the question?

 

[jonbills]

You can have too much compression. The higher the compression, the hotter the burn and eventually it gets so hot you have pre-ignition and/or run-on.

https://en.wikipedia.org/wiki/Engine_knocking

I'd think if you're running on you may have too much compression or something else wrong.

 

[Rob Gaskin]

And I said my car was 200psi so good, ran on and was fine!

So run your engine, test the compression and report back.

If you want a basic answer then YES, the engine won't turn over.

 

[jonbills]

I don't think your old car running on Rob is evidence that running on is desirable!

 

[Alphabettispaghetti]

Both my old trackday car and current blue car 'run on' .

My old car was 200 psi, I haven't measured the blue one yet.

Never had any other issue and both perform well.

I'm 87.3% sure engine compression and compression ratio aren't the same thing.

 

[Rob Gaskin]

I don't think your old car running on Rob is evidence that running on is desirable!

Agreed Jon, but I wasn't going to cut my throat over it and after 12 years of it, it just got quicker!

I go from first-hand experiences, my blue car (DJ Engine) does it too.

Sign of a good old school engine in my books. Bit like a race horse, they just want to keep running!

 

[Rob Gaskin]

I'm 87.3% sure engine compression and compression ratio aren't the same thing.

Your point being?

 

[Alphabettispaghetti]

Your point being?

the post I quoted. you mentioned your engine compression, the op was about compression ratio.

 

[Rob Gaskin]

the post I quoted. you mentioned your engine compression, the op was about compression ratio.

Where does the OP mention CR apart from trying to avoid specifics?

 

[Alphabettispaghetti]

Where does the OP mention CR apart from trying to avoid specifics?

"Folks

I have two cylinder heads for my 240 engine, both have been skimmed (I can't remember exactly what the head thicknesses are), but think one has 40 thou off and the other is 60 thou (or maybe even more). Both are the 73 spec 240 heads

I have a Piper Stage 2 cam fitted (266 duration inlet 23/60 exhaust 60/26)

With the higher compression head I have to run 97 Octane fuel to stop the engine running on when stopping. So am wondering is the extra compression of that head giving any benefit? Can you have too much compression for a camshaft?"

Hi whole post I thought? About one head being skimmed more than the other, is it too much.

Maybe I misread it?

 

[MikeB]

Well the question was about compression.

It comes in two forms, but whatever way it is calculated, the basic question remains the same.

As we have worked out, yes if you get detonation then you have too much.

So I'll refine the query

The cam I have is relatively mild, so would a wilder cam require a higher compression, and if so would that higher amount be detrimental to the performance of the mild cam ?

Slide rules out gents............now where's that maths lecturer when you need him ? in

 

[ben240z]

Yes to both questions in varying degrees. They will both run with higher or lower compression.

 

[Farmer42]

...........With the higher compression head I have to run 97 Octane fuel to stop the engine running on when stopping. So am wondering is the extra compression of that head giving any benefit? Can you have too much compression for a camshaft?

Higher compression makes the engine run hotter and this in turn makes everything that is in and around the compression chamber hotter (Spark plugs, valves etc.)

I had a similar problem on an old Triumph dolomite that I had the head skimmed on a few years back and cured the problem with a bit of tweaking on the timing and running with cooler rated spark plugs ( a tip an old mechanic gave me). Hot spark plugs act as glow plugs do on diesels and if they get too hot and retain that heat when you switch off the ignition, they ignite any unburnt fuel. This turns the engine over and thus pumps more fuel into the piston chambers and this continues until the plugs cool down enough.

Most L Series run with BPR5ES sparkers (or equivalent). Try a lower rated sparker.