Thanks. I was thinking around 12:1 or so if I make the switch.

Dann, how much over 25psi? 26, 30, more?

 

Added some timing and smoothed it out a little more. Only briefly ran this map being cautious looking for any signs of knock but I didn't hear any and the logs dont show any more noise than the first map. Definitely pulled much harder and actually got rid of my little stumble/misfire at higher rpm and high boost. I have been trying to figure out what was causing that and couldn't come up with anything. I couldn't imagine 14 degrees or so of timing up top would be to retarded for it to pull cleanly but maybe so. 18psi does this table look a little better for a 10.5:1 high boost e85 engine? Yes its built with supporting mods and at least e70 in the tank. EGT also appeared to drop 100-150 degrees or so in cruise but need to drive around more to be 100% sure

 

Much better.

 

Yeah 26-27

I didnt get to the tuning laptop today also guys

 

Bummer. I'm curious to see your map. You said you run 20 degrees at 25psi? So your map still must be considerably higher than my second one

 

Maybe, Im just picking numbers by memory.

Ill show you the exact map.

Dann

 

Yep better, and make sure your AFR's aren't dipping below 11.8 or you might get misfires from that too. You've still room for more advance too, this is on the conservative side

 

Really? Damn e85 must really be some magical unicorn gizz. It only takes 11.8 to be rich enough to misfire? I figured it was closer to 11. Because it's definitely dropping below 11.8 at some points. It's already pulling much cleaner and harder so I'll keep playing with it. I'd still like to see what other people are running

 

nah it's probably fine at 11.8, but once you start dippin lower by too much, you'll start losing spark. you're pumping ~30% more of the stuff into your cylinders, it overwhelms the spark

 

Even while running boss d585 coils and a reasonable spark plug gap?

 

with those you're likely fine, but I'd still run 12:1 on the dot cause it should be a wee bit faster and waste less fuel.
On Ryan's built 1.6 we could only go up to 21-22psi due to weak spark, but we still ran 20* midrange tapering up to 30* up top. that car put down 360whp on a 1.6 and a log manifold and a chinacharger. we've done BP4W's similarly and it was over 400whp.

the stuff is pretty nuts

 

Pat I think you're capable enough that I can remind you to disregard static, and focus on dynamic compression ratio. If you **** up and end up with too much static, so you have to pull so much spark it makes less power fix it is just a cam timing adjustment away.

 

I was just wondering what everyone thought would be doable. Not a lot of high comp, high boost builds in miataland. I haven't looked, but I'm sure there's some folks in other 4cylinder platforms doing it, probably evos and STIs if I had to guess.

I run 17* @ 38 PSI with no knock. Not going higher till I get it on a dyno.

 

Year long bump. I searched the forum to see if anyone is running high compression + boost, and found my own thread discussing this...

Getting bored, thinking of putting together a new shortblock to experiment with some things. One being high compression. Current thought is to try AT LEAST 12:1 with E85/Boost. Actually thinking about going to 13:1 or even 14:1 and see what happens. Would have to get custom pistons made to do this, looking into that now.

 

YES!! do it for science.

Although a local buddy of mine is running 12:1 already on a miata that traps 132mph currently.

So you gotta go higher to blaze new trails

 

12:1 BP? What fuel? How many passes has it made and not blown up?

I am tempted to go higher.

 

Yes, Yes, cornjuice of course. It's been at least a year and I wann say a couple dozen passes? I'll ask him to chime in or at least fwd me the details. but he beats on it like a redheaded stepchild very very often. this is by no means a garage queen

 

Thanks, yeah would be great to hear from him.

 

I run 11:1 with 9psi with a Rotrex on E85. The cams have shorter seat timing than stock further bumping static compression, Not extreme but it is a track car that gets flogged hard!

I suggest you start from the top and work your way backwards. Determine whatever you plan to run (15-20-25psi) to meet your HP goals and then fill in the compression that keeps your dynamic numbers sane. Somewhere around 18:1 is what I shoot for. I have another marque I build using the same formula. Been running 10+ years strong on the street with some track days sprinkled in.

This is a pretty good tool to help you play with some numbers. Wallace Racing: Dynamic Compression Ratio Calculator

The extra compression buys you more HP per PSI of boost. The motor tends to be much more lively in transient phases and just rips WOT. Good Luck Pat!

 

Using your calculator, my current setup with 84mm bore, 85mm stroke, 5.233" rods, 9:1 compression, 50* ABDC inlet valve timing, and 40 PSI boost, I get
7.96 dynamic compression ratio
29.62 effective boost compression ratio.

That is already significantly over your 18:1 sane numbers.


If I ran 12:1 and 35 PSI boost:
10.57:1 dynamic compression ratio
35.74:1 effective boost compression ratio.