Ok so as many know, I an building my dream engine. With lots of Type 4 Store goodies including Nickies, 78 mm crank, and 102mm P/Cs, LE-200 heads and a decent cam, I am looking at where I want my compression to be.

With a normal engine and quality components it seems that 8.5 to 9.5 is about the limit before heat and detonation become issues.

Don't have any measurements yet but playing around with some calculations and assuming a low deck height because I have a slight dish and or valve pockets in the pistons I could easily get to over 11 to 1.

But I am think that might not be a long lasting engine. Looking at going for 10 to 10.5.

Thoughts?

So, after spending all that money at the type 4 store, they wont give you any advice on your combo?

Or the other way around, I assume you initially discussed your combo with someone over there before placing your order...

I have the same engine in my '67 bug, anything higher than 9.5 needs twinplug or race gas.

My bug is my summer DD a set of rear tires last me 3 seasons, just noticed one of my tires is now a slick.

180hp in a 1700lbs car, IIRC I set mine up at 9.2.

I did not ask them. But I am fairly sure they would not make a recommendation. It comes down to how you want to use the tools they give you. If they said set it a 11:1 if you want the highest HP and you want a nice daily driver then they share some responsibility for you using their tool in the wrong application.

Same LE-200 head?

At that compression just run E85

Its somewhat apples and oranges since my engine is a 2056, but in this most recent iteration I backed the compression from 9.2:1 down to 8.8:1. The engine has responded very positively to that. Not as punchy, yes, but head heat was an issue on the highway unless I kept it in 4th until about 70mph. With the slightly lower compression, I'm generating a lot less heat. (measuring heat on a Dakota Digital CHT tied to Cyl 3 spark plug.)

I think going 10:1 with the thin cylinder walls is going to be a challenge, unless you have a system in place to REALLY move that heat out of the cylinders and heads.

Zach

I would still ask them their thoughts for the highest compression they advise with the gas and ignition you are using. I don't think the high quality parts you are using really make that much difference when considering compression ratios.

My heads are OE with 44 x 38 valves, 12mm plug, dual springs, etc. that I built.

I know they are close to the LE, because I built a 2.7 with my heads, the customer over revved them, damaged a guide boss and I replaced them with LE200. Power was about exactly the same, the customer and I noticed no difference.

My heads before I installed the seats and did the chamber work.

Zach You are completely correct. The 10 to 1 was a bit tongue in cheek with the exploding icon. Certainly I see now that was not clear.

However higher compression can lead to more efficient combustion and up to a point does not equate to higher head temperatures. Problem is, for each engine and build that point is different.

Reading back through a huge amount of information with some of it written by Jake, the reality seems that 9 to 9.5 is the range. but that is just a guess on my part.

Everything is a compromise, to get a drivable DD you will have to leave some HP on the table.

I use the 163/86B web cam, my favorite for an engine this size. Lots of torque, bit lopey idle, but usable for street.

Pretty much going with the Type 4 version of the 86b. Slightly more duration on the intake at 284 and more on the exhaust at 300.

Based on a suggestion from Len I am having the ports ceramic coated which will help with the heat.

Type 4 confirmed the 8.5-9.5:1 range.

This engine is going to scream. I'm excited to see it come together.

Even with the engine I have now (which I am loving) I wish I had more knowledge and funding when I specked it out 5 or so years ago.

The next time I build it, its getting bigger. lol.
Zach

My engine is 10.3:1 2256 CC and it runs fine. But I dont see 100F ambient ever, more like 80 on a warm summers day and even those days are rare.

Remember that the cam makes a difference to how much compression the engine can tolerate. More aggressive cam grinds generally allow you to (and require you to!) run more compression, in part because of the overlap where both intake and exhaust valves are open at the same time.

Things like bore size also play a significant role. (I think that has to do with the propagation speed of the combustion flame front, but I'm on very shaky ground with that memory!!!)

Lots of variables, is what I'm trying to say.

--DD

Dave is spot on. With a more aggressive cam you can run higher static compression ratios because the overlap of the cam lower the dynamic compression ratio at low RPMs where you have a higher risk for detonation.

Larger diameter cylinders increase the risk for detonation because it takes longer for the flame front to travel across the whole cylinder. During the longer burn period the piston is still moving up to top dead center and increases the cylinder pressure to a point that it could detonate.

Changes in compression ratio and/or cylinder bore will change the ideal ignition curve as well.

Should be nice.
Make it thump, I'd aim for no less than 9:1, set your timing for no more than 28* total, run premium and detonation won't be an issue.

You could run regular gas at 8.5, but why the fuch would you want to do that for?


Nickies you can run tighter ring gaps and a tight deck. With big bore head chambers and unshrouded valves you'll find you have to run a tight deck to get the CR you want.

Buy the LN ARP head studs, I also have the ARP case though bolts, but IMHO they're not needed.

No regular for me!

Len is waiting till I get deck height measured before doing heads in case I need to tighten up the deck height.

All ready purchased the head studs. But I did pass on the through bolts.