Since my heads are off and I am SLOWLY converting my 2056 to megasquirt, I am rethinking some of my stuff.

What is the ideal compression ratio for my motor.
I am willing to run 93 octane fuel - I run it now.
I have a 9530 Raby cam and valve gear.
2056cc
spark is going to be electronically controlled (maybe EDIS, maybe something else)

I was thinking of dropping compression to 9.0 or maybe even 8.7.

I want more compression then stock, but I want cooler head temps then I currently get (the engine has always run warm at the heads, and I can never use 5th on the freeway to keep temps down). My normal operating temps tend to be 360-380 and I'd like to back them down ~10 to 15 degrees. Head temps were measured on Cyl 3 with a compensated Dakota Digital CHT gauge.

When I built the engine, I just set the deck height to .040 and figured out the CR later. I know better now, and am at a great point to correct it.

Zach

I have 2056, dJet, 9550 cam and 9.0 compression and my heads stay plenty cool. I have a CHT gauge. It does seem happier with 93 octane.

I ran 8.3 with the dished. Never got hot in the 100*+ days here in central valley, Ca.
Are you still having register issues?

Stock was only 7.6 USA & 8.0 Euro/RoW - so 8.7 isn't much more, and "modern" engines are running up to 10 & a bit more on "Premium recommended" (not required), so 91 or 93 AKI Premium would be plenty at 8.7-9.0 - if not even 87 or 89 AKI.

The 7.6 US & 8.0 Euros only needed 85-87 AKI (same as 91 RON on the gas tank/wheel house sticker).

There's a good plain language gas octane ratings article in the April 2012 edition of Hemmings' "Classic Car" on pg. 74 on this. Unfortunately it won't be available online for a month or two, & I was planning on posting a link to it online on here when it does. So in the meantime get to B&N or borrow one.

For your 2056, it's more a matter of how hot do you want it to run & how long to last, cuz the higher CRs is how they're getting 200+ HP out of today's 2.0Ls & Jake's & FAT's built-up T-4s.

That will also be dictated by how you want to drive it, since the octane rating will only prevent knocking & keep the resultant heat down a bit, but you'll still run hotter the more CR & HP/TQ you produce, which may require you to take cool down breaks a Denny's along the way of long trips!

What's the spec of that cam? The dynamic CR depends on valves overlap.

I will not be inspired too much from what modern cars have as CR since you don't know what cam they have and they all have knock sensors today. Can you add a knock sensor to your new EFI?

I ran that cam at 9:1 and it was about right. I wouldn't run more myself. With the 9130 I'm runnin 9.4:1. Just where the numbers fell and it's good for up to 9.5:1. I have lot's of extra cooling though.

My 2056 in the 912E ran a 9530 with 9.7:1 and did so for 160K miles. My head temps were 25* less than yours and I never had an oil temp more than 225F without an external cooler.

You still haven't found the sweet spot for tuning yet would be my guess... Play with it and give the engine what IT WANTS, it won't like you telling it what to do.

Exhaust selection plays a role in this as well as any small deficiencies in the cooling system, hell a dirty fan can cost you 10 degrees!

Jake, what were you running for timing? Just as a data point...

With programmable engine management I had the timing all over the place based on MP and RPM. Even when that engine ran carbs and a mallory for its first 59K miles I ran similar temps with timing optimized at 28* full advance.

BUT that was way back before ethanol enriched fuels which have changed everything and elevated CHT on every engine we have experienced.

Exhaust this time around are going to be 2.0 SSIs. I have a bursch but may use somehting else for the muffler. I need to be able to clear my windshield on humid evenings. Too many times I drove home not being able to see from all the condensation on the windshield on a cool humid night.

Selling the Eurorace at Hershey.

I know I never had the tune right. But I was at the limits of my tuning knowledge. I had just welded a bung into the Eurorace for an O2 sniffer when I decided to tear things down again and go EFI.

You can't relate what works on a modern engine to the T4. Not even a little as they have nothing in common.

Modern engines have 4 valves/cylinder and a centrally located sparkplug. The pent-roof chamber shape and uninhibited flame travel lend itself to much better combustion characteristics which drastically increase the pressure threshold before detonation occurs. This allows for much higher compression ratios with a given octane rating than the T4's 2valve wedge chambers.

The presence of modern ecu's mated to knock sensors provides real- time tuning adjustments when loads (cylinder pressure) exceed the fuels limits of controlled reaction. This allows for even higher CR's.

To illustrate just how limited the T4 chamber is by it's shape and plug location consider that when I prepped twin plug T4 heads for Jake's R&D he found that compression ratios could be elevated to 11.0:1 on combos that were limited to no more than 10.0:1 with a single plug.

As an aside to this issue keep in mind that increasing deck ht leads to decreased combustion efficiency which results in fuel reacting in the exhaust port and elevated ex gas temps. Elevated EGT's are a real killer with the T4 due to its weak ex port area. Engines with excessive deck (anything over .060) require more timing to combat the issue.