Can the compression ratio be too high for the cam that us used?

[Arnoud]

Hi!

What would the effects be if you have a compression ratio that is higher than the suppliers recommendation?As per Aircooled.net's info the Web 86a cam that i'm using should be run with approx.8.5-9,0:1 compression ratio.I'm running 9,6:1 on 95(RON)octane which is 91 octane in the US.
Can the higher CR have a negative effect on the engine's performance?

What are your thoughts on this?

Any input is appreciated!

Cheers,Arnoud

[Bruce.m]

Personal opinion warning …..

If you go outside the suggested cam duration-to-compression ratio, you are most likely to have an issue with the typical 009 distributor curve. With higher charge density you need less advance (ignoring all other factors). So if you are going with programmable ignition you can tune to suit.

[Arnoud]

Hi Bruce!

I'm running programmable EFI/Ignition.I'm worried that the high CR will cause unneccesary heating of the heads(the oil stays below 100DegC(210F)).
I ran the car at high speed(140-185Km/h) for quite a while on the Autobahn this weekend and after that the engine idled unhappy for a quite a while.
There is no CHT guage installed,just a sensor on the underside of the head to give signal for the afterstart-warmup.
Maximum ignition advance is 31Deg.

Cheers,Arnoud

[Piledriver]

If you are not knocking the CR is probably OK for the cam, perhaps mitigated by a tight deck etc, but that doesn't mean the timing is ideal, what Bruce mentioned is absolutely correct.

You can easily have more timing than ideal, this can make the heads run hotter but oddly will usually cause the EGT to go down a bit. If the engine is significantly altered from stock the stock ignition settings are simply wrong.

Modified T4s also typically like far more idle /initial timing, which greatly improves bottom end/off idle performance, and has reduced idle CHT for me as the factory settings are very retarded from optimal. (admittedly for emissions reasons)

The unhappy hot idle after a long run is typically a couple tight valve adjustment or two, you have aluminum pushrods?
With CrMo pushrods the gap gets larger as the engine heats up, which is a engine saving safety feature as burnt valves are bad. The "advantage" of aluminum pushrods is they are quiet. This is not a selling point if you know why.

Without a proper under-spark plug CHT gauge, knock sensor setup, EGT gauge or trip to the dyno (or some combination) you will never be sure. I log EGT, CHT (K thermocouples), CLT with a non contact GM sensor under the heads in the airflow around the exhaust port (attaches to pushrod tube) , and the MS3 setup has an internal programmable dual sensor knock system, also logged.

The MS2setup I'm currently installing also has EGT setup, also I switched to waste spark with 4 tower VW coils (non-logic type) so I could use a used J&S Safeguard Vampire 2 channel knock subsystem to monitor/control knock, this has a knock LED output that can be logged on the ECU. (The reason for this is it is getting an M62 Eaton blower, off topic as its going in an 86 Cabriolet)

[Bruce.m]

If you are running efi, the VE table gives you hints where the cam comes into tune & where the torque should peak. Higher VE typically needs less advance, ignoring all other factors. As the VE rolls off, you can add a couple more degrees since the charge is less dense.

However… once you get an old high aero draggy car up to 85mph and beyond, the engine is pushing and even a good tune doesn’t guarantee the cooling system is capable of removing the amount of heat being generated. If you are dropping back to 85kpa or less of MAP, it may need a bit more timing to minimise the losses to heat but you are probably above that MAP id guess. Another option is to add water misting nozzles to the cooling fan intake or directly into the head shrouds. Maybe just for high speed/ load situations to increase the cooling. It can be quite effective since it takes a fair amount of energy to vaporise water.

[Arnoud]

Ok..that's some information!

@Piledriver:I'll look for a decent CHT guage and plumb in a K-thermocouple(I'm not shure if the EMU supports multiple ones-I'll have to check).The Pushrods are stock aluminium ones and run 0.15/0.10mm lash.Your comment about initial timing supports something I saw/read recently.about airflow at idle.I'll bump the idle up to 10Deg and see if it idles more stable.
@Bruce:this is the VE-table in use.How can you see where the cam comes in?My knowledge of tuning in limited

2025-11-13_20h56_09.png

Cheers,Arnoud

[Bruce.m]

If you look at the full throttle line on the right side and see how the VE changes between cells. Above 5500rpm it flattens but doesn’t really fall away. Potentially you can add a couple of degrees above 5.5k but its unlikely to make much difference since the cam doesn’t drop away (VE reversing).

Looks like it picks up in the twos but VE increases more around 3K as the cam gets advantage from the intake inertia?

There is a peak at 3.5k which might be peak torque or some intake / exhaust tuning benefit.

There does appear to be a fat line at 80% throttle around 3.5K+ & VE falls slightly towards full throttle? If the map is well turned that can be a sign of the increased turbulence of a slightly angled butterfly in the intake which is better for burn than the extra airflow.

Its a bit harder to “read” a Throttle table than a MAP table because you have to infer the MAP changes (sort of diagonal).

[Piledriver]

note that water injection works even better on the inside, and you don't need boost to use it.