Dynamic and static compression ratios

[promotor]

I've been doing a few sums and am struggling with what I think are/aren't acceptable figures relating to dynamic compression ratio.

A standard Lotus T/C with 9.5:1 static and 22/62 cams based on my maths gives approx 6.5:1 dynamic.

A 10.5:1 static 83.5mmx72.75mm T/C with QED420 42.5/62.5 cams based on my maths gives approx 7.2:1 dynamic.
Do these dynamic figures look correct, or are they too low?

For example my Yamaha WR426 (95mm bore x 60.1mm stroke = 426cc and 11,000RPM!!!) motorbike had 12.5:1 static and 25/55 cams and my maths gives me a 9.0:1 dynamic compression ratio. I used to run that on 95 octane fuel on the road and 98 when I used to race and never ever had an issue with it pinking, detonating or melting anything. I put about 30,000 miles on that bike (which is a ridiculous amount for an enduro bike but I used to use it every day for work, and play on the weekends) and it never even needed a new piston or major overhaul.

With the discussions I have seen on here regarding Lotus T/C race motors running on race gas and needing 12.5:1 static compression (which I have worked out to give approx 8.4:1 dynamic ratio on a 65 ABDC inlet closing event) my question (if my maths is correct) is there scope for more compression in a Lotus Twincam?
Has anyone any experience of higher ratios with/without catastrophic results?

[rgh0]

The dynamic compression ratio calcs look about right to me. However dynamic compression ratio is just one indicator of pinking / knock potential so not really possible to extrapolate to far from your bike engine to a twin cam.

Other factors affecting it that varies between engines include:
cylinder head and piston shape and squish gap between piston deck and cylinder head
cylinder head cooling and hot spots
inlet port design and pulse effects on the inlet reversion
rpm at which actual maximum dynamic compression occurs
ignition advance curve

Having said that I know you can run a race twin cam up to 13:1 OK on Avgas with a relatively short duration race cam say 290 to 300 degrees and perhaps a little higher with a 320 degree race cam. I design my engines based on 12.5:1 to allow for build tolerances and carbon build up. I have heard third hand of people using 14:1 but never gone that high myself.

cheers
Rohan

[SJ Lambert]

One of the guys running an open wheeler in Sydney makes the following comments in reference to my current build where I'm aiming at 12.5:1 compression.

Paul's motors have been pretty reliable for a number of years.


[quote name="Paul Hamilton" post="7014423" timestamp="1421796674"]That looks about right, James.
?
My no.1 engine is running a 12.8:1 comp ratio with a smaller intruder on the piston and a chamber volume of 38.6ml. ?My valve sizes are 42.14 inlet and 39.56 exhaust and I am using Wade 160 B and C cams in that engine which are claimed to be Hart equivalents. ?The performance both on the dyno and in the car is pretty much on par with my old Hart engine which is now the spare after a very long and active life.?
?
The secret to top twin cam performance is all in the cylinder head!! ?Valve timing, valve sizes, comp ratio, combustion chamber ?and porting. ?Its a tricky combination to get right as there are clearance problems to address when you stretch it at the margins and some compromises to accept.[/quote][quote name="Paul Hamilton" post="7014423" timestamp="1421796674"]That looks about right, James.
?
My no.1 engine is running a 12.8:1 comp ratio with a smaller intruder on the piston and a chamber volume of 38.6ml. ?My valve sizes are 42.14 inlet and 39.56 exhaust and I am using Wade 160 B and C cams in that engine which are claimed to be Hart equivalents. ?The performance both on the dyno and in the car is pretty much on par with my old Hart engine which is now the spare after a very long and active life.?
?
The secret to top twin cam performance is all in the cylinder head!! ?Valve timing, valve sizes, comp ratio, combustion chamber ?and porting. ?Its a tricky combination to get right as there are clearance problems to address when you stretch it at the margins and some compromises to accept.[/quote]

[rgh0]

I was just talking today with a friend who is currently building a new race twin cam and looking for a little more power. He is trying a 9:1 dynamic ratio which will be about 13.5:1 static with the cams he is using. This is higher that he has used before so its a bit of an experiment but he is looking for that mythical over 200 hp in a 1600 cc twin cam.

He is also getting a special set of gears cut for his box so he can keep the engine in what will be a high and narrow power band!

cheers
Rohan

[promotor]

Having looked at 290 degree cams I'm guessing with 13.5:1 static the engine would still come out with approx 8.4:1 dynamic so it would seem there's still a bit to be gained!

I appreciate the comments on my motorbike being a totally different "kettle of fish" due to the intake runner being short - maybe 4 inches from the carb mouth to the back of the valves, and it ran 5 valves (3 inlet 2 ex) so filling the chamber would be excellent, and it also ran 2D ignition to help it along.

As my idea revolves around an engine to be used on the road with 95-98 octane fuel I think I will be looking at fuel injection if I build a high compression engine to help get around the spark timing issues. I'm going to need next to no spark advance on cranking to even get it to start for one thing!

[Chancer]

I'va had that problem when I have built engines with dynamic compression corresponding to the stock motor that had over 13:1 static compression. It was before the days of mapped ignition so I used a latching relay circuit for the ignition.

I would turn on the ignition (except it remained off) crank the engine, shout "Contact!" and press the momentary N/O?button to latch the ignition on, starting was never a problem after that, I learnt the trick with my 6 volt Veedub that went like a rocket when the ignition was Advanced but would not crank against it.

2 other benefits of the simple circuit, if you tuck the activation button away out of site, Under/beside the steering column it becomes a security device, if you fit a second momentary N/C external switch it will act as a battery cut out and pass scrutineering without you suffering the contact problems of those that have to pass the starting current.

I'm surprised that you didnt know your problem was fuel related, the way an engine cuts out through fuel starvation is very different to ignition even an intermittent fault, the very second I feel an engine miss a beat I look at the tacho which on older vehicles will tell me if its ignition or otherwise.