High Compression Ratio Problem

[GeofP]

I had a problem with my 2.0l Pinto cylinder head and bought another as a less-cost option to reconditioning mine. The head is unleaded with new valves and guides and has been skimmed “for maximum compression ratio”. I thought it a good opportunity to fit a cam that is better than the standard Cortina one. My Robin Hood S7 is only used for road and I’m considering the Kent FR31. However, I am concerned that there will be sufficient clearance between valves and top of piston.

 

The engine is a 2.0l Pinto from a 1980 Ford Cortina. The block was skimmed when I built the car some 22 years ago. The engine uses a standard Weber downdraught twin venturi 2V carburettor, throttle barrel diameter 32/36 mm and an after-market 4 branch exhaust.

 

Comparing the head with my old one, it looks like it has been skimmed by about 2.5mm. I note that the spark plug electrode will foul the piston unless it is inserted with the electrode at the top. Is it possible to tell from the following measurements if the valves will also foul the pistons? I asked Kent Cams but they weren’t really interested in helping.

 

Cylinder head thickness, rocker cover to face: 91.95mm

 

With valve closed and measuring from the edge of the valve closest to the face of the cylinder head:

distance from exhaust valve to face: 6.68 mm

distance from inlet valve to face: 5.84 mm

 

With piston at TDC, and measuring from top of piston to face of block: 0.2mm

 

Inlet valve diameter: 40.6 mm

Exhaust valve diameter: 35.9mm

 

I think the problem, if any, will be when the piston is rising and the exhaust valve closing but without knowing the cam profile I can’t predict this (not sure I could do the maths anyway). I could do a trial build and check for contact using Plastercine on the piston top, but that means buying the cam first which is a costly gamble.

 

I would also appreciate opinions as to whether the Kent FR31 would be suitable or if there is a better option for my set-up.

 

[Guest lotusPaul]

You can trial build using the standard cam that you have and plasticine on the piston crown. The crush marks will give you the clearances on inlet and exhaust valves.

 

This will tell you how much lift you can get away with.

 

BUT BEWARE

 

If your chosen cam has different seat-to-seat values it will not be as simple.

Also you will be doing this on a 'cold' engine so you must allow for exhaust valve growth more than inlet, typicaly use same dimension tho for ease.

Also no oil pressure floating the crank, so it will be sitting lower.

If were getting to the n'th degree then understand growth factors of con rods at max opperating rpms.

 

However looking at your dimensions given you have a 0.2mm deck height,

If this is accurate id be looking to use a thick head gasket and not the ultra thin items that seem to crop up. Note this is thick AFTER installation.

 

As for a cam, the standard replacement 2.0 injection is actually a great torquey cam and with your setup its right on the money.

 

Id look to get one of those as theyre much cheaper and do a trial build with it, if the values look ok to you then stick with it. I doubt you'd notice much difference between the two.

 

Standard valve sizes should be 42/36 so check your inlet again.

 

Whats your heads combustion chamber size?

38cc is 1.6 head, 48cc is a good 2.0,

If you have a heavily skimed head it may increase your compression ratio.pintos like this, shame standard pistons dont.10-1 is fine up to 10.7-1. But you need a solid bottom end and accurate spark timing.

 

Lastly you can fit different plugs. On Ford 302/351 cid i fit super 4 plugs dry, then cut off the electrodes that will foul. Maybe worthwhile checking, unless yours are even closer.

 

Sounds like the making of a good package.

Enjoy

 

Hope it helps

 

Paul

[GeofP]

Hi Paul. Thanks for your comments.

 

I re-checked the inlet valve and it is 1.65" i.e. 41.9mm, so is standard.

 

I have just measured the combustion chamber size with a 10ml syringe and it is about 38ml, i.e. 38cc. The head is definitively from a 2.0l but skimmed a lot. Does that make it suitable for my engine or would the CR be too high?

 

Is the standard replacement 2.0 injection cam still a good choice and if so are you able to advise where I could source one and if you have a part number.?

 

Regards... Geof

[theduck]

38cc on a standard block would be around 11:1, on a skimmed block it will be way too high for stock pistons.

[Guest lotusPaul]

Geof,

 

I think head is skimed for a 1.6ltr. On a 2.0 its gonna be way to high, as said above. Easy maths is 2000/4=500cc per cyl, 500/38=13.1cr, never happening on standard pistons or forge in a road pinto. You'll need a thick gasket to bring it down to a manageable level.

 

Id look for a standard head and cam off a 2.0i and use that instead. Just clean it up and get a new spray bar.

 

Ive said before in other posts that you may have missed that a 2.0i cam is a good upgrade over a carb cam, with a good scavenging exhaust and a high power ignition system and even using the standard 32/36 carb you can get good numbers especialy torque.

Your most of the way there.

 

Paul

[GeofP]

Thanks to you both. I wish now I had got more info about CR before I bought the head. Still, at least I know a bit more about the subject and will start my search again for another head and try and sell this one.

 

Geof

[Guest lotusPaul]

Geof,

 

Your head at 38cc is fine for a 1600 and also a 1800,(depending on deck height and gasket volume when compressed)

 

Theres a few heads on fleebay worth a look at.

 

Best of luck

[GeofP]

Paul

 

With regard to a high energy ignition system, would the Powerspark High Energy Distributor plus Lucas DLB198 coil as sold by www.simonbbc.com be suitable?

 

Geof

[Guest lotusPaul]

Geof,

 

It entirely up to the end user and mostly its down to cost.

 

Im a big fan of Aldon ignitor 2 with thier flame thrower coil and leads. I have used this on my lotus twincam and the difference was night and day.

Ive also used H and H systems which use a reliable late model mitsubishi electronic package.

 

Bosche and lucas coils are good as upgrades to points, but i generally just use the full 12volts and leave the resistor in the tool bench drawer.

 

Just using the Aldon electronic module is also a good upgrade on a standard system.

 

Not used them for a long while but lumination were top items, no idea on them now.

 

You can always think out the box and go DIY, ive had great results by using the late Ford OEM electronic ignitions as fitted to the last cvh engines and pintos, also the crossflow based endura fitted in the KA had a setup that can be donated but needs a little more work.

I have no experience with powerspark but i would see how thier prices compare with the more established packages and make a choice.

 

With reference to your cam choice, in theory lift should be no more than 1/4 of valve diam. So give or take its 10mm valve lift,altho its not 100%.

Keep overlap degrees down as it increases chamber pressures which would also keep duration down.

So were looking at short durations and high lift and as fast a ramp angle as possible that doesnt destroy valve train in 10k miles or less.

2.0i cam is good and cheap

FK34k is an upgrade to this i cam but works well on carbs too.

Piper are good in the 268 to 275 range.

I prefere to keep a torquey engine, more duration tends to destroy it at low revs and moves powerband higher. For this you generally stress other components.

[GeofP]

Paul

 

 

I also prefer an engine with good torque and less high revs. Doing a search for a Pinto 2.0i cam led me to one sold by Burton which they describe as "Pinto 2.0/2.0i cam". Will this be an upgrade to my 1980 2.0 cam?

 

Geof

[Longboarder]

GeofP. Can I recheck with you the method of measuring combustion chamber volume. This is critical as to wether this head is not practical. 38cc is too small.; However 45cc is OK with a standard head gasket.

Chamber volume is possible to measyre accurately if you lightly smear each valve seat with vaseline and then use a greased flat plate to almost close off the head surface leaving a small hole to allow you to squirt in the liquid. You should do all chambers in turn. Doesn't have to be with perspex as shown nor with a central hole drilled although it is nicer doing it that way. Usually done with paraffin as it flows nicely.

Don't forget the combustion chamber volume includes the bit in the cylinders with the piston at TDC and a bit for the head gasket.

 

Nigel

[Snapperpaul]

A lot of great information in this thread.

I have 2 heads that have around 42cc chambers on a 2.1 block so struggled to get a usable road compression ratio of around 10.5 to 1 however the standard Ford gasket is 1.65mm thick.

You need a compression ratio calculator so you can accurately calculate your compression ratio on your mix of components.

 

There are always or course variables and pinking/detonation is not only a compression/timing/fuel octane only problem.

For instance a longer duration cam will effectively lower the compression ratio that the engine sees in operation compared to a static calculated one.

The induction system also has a bearing on an engines susceptibility to detonation and counter intuitively a single twin choke carb will not det as much as a set of twin webers on the same engine.

You can dial out det with ignition timing but that is really cancelling a lot of the tuning work you have spent good money on.

 

A good point about valve size and cam lift but this is a measurement at a specific valve/cam lift and what is often missed is that a higher lift cam will present a longer period of maximum port flow as it passes through the maximum flow point and keeps the valve open for longer at this point because it takes more time to then return to the max flow point in its closing phase.

 

Love these technical threads

[Guest lotusPaul]

Geof,

 

Just putting in a cam wont help you. If c/r is high a short duration cam will not help. You would need a longer duration with low lift. This is because of how it would change dynamic compression values.

 

I think you would be better getting a 2.0i head in good condition and going for an Fk34k (cam kit) or the burton 134, how you drive will be the determining factor.

 

But this is when we need to look at the package, this is not just engine but drive train too. In this case what box, diff ratio, wheel and tyre size. Sticking with the engine, induction and exhaust systems will also need to be considered.

 

You can probably tell by now that properly building an engine, and developing it, is not just as simple as throwing in a cam and see how it goes.

 

Again, id look for a different head and 2.0i are better due to port shaping. Generally may even come complete with a usable cam to start.

[GeofP]

Nigel

 

I used the method described by you but with water and just measured one chamber.

 

I have taken measurements of the compressed head gasket and space above the piston in an attempt to get a better idea of the CR. Can someone advise please if what I calculate below is correct, because if it is then perhaps the head is suitable for my standard pistons.

 

My old compressed head gasket is 1.3mm thick and 9.3cm diameter

Volume of gasket = pi x 9.3 / 2 x 9.3 / 2 x 0.13 = 8.8cc

 

Distance of piston at TDC (measured with feeler gauge) is 0.53mm (correction from 0.2mm stated at start of thread)

Bore diameter is 9.12cm

Volume above piston = pi x 9.12 / 2 x 9.12 / 2 x 0.053 = 3.5cc

 

Volume of combustion chamber measured above = 38cc

 

Total volume = 8.8 + 3.5 + 38 = 50.3cc

 

Piston stroke length is 7.68cm

Bore diameter is 9.12cm

Swept volume of piston = pi x 91.2 / 2 x 9.12 / 2 x 7.68 = 501.6cc

 

So, is CR = 501.6cc / 50.3cc = 9.97 and therefore OK or is it not as simple as that?

[Guest lotusPaul]

Geof, very close but not correct.

 

Essentially and bottom to top, then divide by top.

 

500+50 ÷ 50=11c/r

 

Using your figures (thank god your pistons are down further) your shy of 11-1.

Still too much for my liking by around .3 but its doable.

 

If i were you id invest in a dremel and try to relive the chambers but ONLY TO DESHROUD THE VALVES. nothing else. This will add cc to top which will help a little. Thicker gasket too and your there.

 

In which case, and im happy to say, cam selection as above would be fine providing your ignition is set correctly.i would stress tho that you need to keep on top of maint and keep it running sweet.

 

Added this https://www.google.co.uk/search?ie=UTF-8&client=ms-android-samsung&source=android-browser&q=how+to+work.out+compression+ratio&gfe_rd=cr&ei=F7B3WaGQBMTHXq3ZhfAE

Edited July 25, 2017 by lotusPaul