I was wondering how much compression can you get away with on 93 octane? I'm going to build my motor soon and I'm getting my heads milled for around 11.1. Will that be alright? I'm building a somewhat mild motor for cruising and weekend dragstrip. Naturally aspirated. No turbo or N20.

 

Your good...11.5 even on a good tune.

 

ditto that. There are guys running close to that on the 91 octane available in some states

 

Thanks for the feedback. Happy New year also.
I am going to run the 317 heads on a stock lower 02 ls1. Do you know how much I have to mill to get 11.5?

 

I run 11.5 on 93 with no issues

 

Good to know. Hope to see ya this summer.
Think 11.5 is what I'm shooting for. Like I said I got 317 heads and they have 70cc chamber's I believe. If I have them milled .020-.025 should that put me around 11.5? Also my cam is .627 int. 228 duration and .613 ex. 234 duration. Will I be safe without flycutting the pistons after the mill?

 

I'm 11.5:1 on 91 with no issues.

 

Like everyone else said, you should be fine. But understand, if you use a cam with an Intake Valve Close (IVC) near to Bottom Dead Center, this will raise your dynamic compression ratio. For example, I'm running 11.5:1 on 91 with no issues, but my static compression ratio isn't what determines whether I'll be safe on 91. My cam closes my intake valve a bit of the way up the cylinder above bottom dead center giving me a dynamic compression ratio of about 9.3:1 so this is significantly lower due to the cam profile.

J-Rod made an awesome spreadsheet that will spit out your DCR as well as your ideal intake runner and exhaust setup by plugging in different cams and motor specs. here it is:

https://ls1tech.com/forums/advanced-...readsheet.html

 

Good point about DCR. My SCR is 11.5 and DCR is (still being figured out; used bad input data) and running good. All about the total build specs though to be sure. Combustion chamber design, piston profile, plug heat range, and a million other things come into play. If designed properly though 11.5:1 on 93 should be no problem.

 

How are you calculating DCR? Should be using the 006 IVC not the 050. Typical cammed DCR is in the lower to mid 8's. 8.7 is about as high as you want to go on pump gas.

 

CRAP! Good catch.

I've got to go back and look at what i got from degreeing the cam. Now where did I put my notes...

 

I am not that up on what chamber volume it takes to get what compression, but I think you are off track here, think you will need a lot more milling than you think to get that compression and are going to have PTV issues once you do.

 

My calculations put you around a 66cc chamber with 317's milled .030. on a 3.898" bore and 3.62" stroke, piston ~.007 out of the hole, flat top with no valve reliefs, and a .040" headgasket I get 10.7:1-10.8:1 SCR. Most likely you will need to flycut though since your PTV clearance will be very tight with a decent cam. Figure 3cc's taken out of the piston for clearance and you get to 10.4:1 SCR.

Frankly, you would be better off finding a smaller chamber to start with and mill that.

I am making some assumptions here though that may not be the same with your exact setup, so results may vary.

 

I am trying to figure this out. Help me see the light.

If I have 11.5:1 compression, and I am running 91 octane. And I am "np brah I do it all day because my cam timing...."

My question is this. Yeah sure, at idle and low rpm you bleed compression and everything is peachy.

But at some point, a well designed naturally aspirated engine will top 100%VE. You hope this is true. And in that situation, your compression will be 11.5:1 or higher, dynamically.

So the argument "my cam timing bleeds CR" seems irrelevant to me, since at some point you will have more DC than SC regardless of cam timing, if your engine is made after 1990, or so, anyways.

I must be wrong, but why.

 

You find the truth when you tune the car on the dyno or track and have to reduce timing to eliminate knock.

 

That doesn't answer the question. That is a bandaid for people who build an engine with too much compression and then say "ooops" and the result is reduced power output, compared to if they simply went with less CR to begin with.

 

I think I see what you're saying, and here is where it comes into play - the momentum of the air filling the cylinder. While the piston is near BDC and on its way up, air continues to fill the cylinder, resulting in VE greater than 100 under the right conditions - typically max torque. Only other way you get over 100% VE is with FI. For this purpose, the OP is talking NA.

At idle, your VE is terrible on a cammed motor, so if you're comparing DCR to SCR, the cylinder pressures the motor sees at idle is around 30% what it is at peak torque / peak VE. At 100% VE, the motor sees the calculated DCR pressures. You're calculating your compression ratio based on the position of the piston when the valve closes.

Also at higher RPM, the overlap event is so quick, you don't get the short circuiting intake to exhaust that you do at idle, which is why the VE changes so much with RPM.

The only way the NA motor will see pressures equal to greater than SCR calculations is if your intake valve close event occurs exactly at BDC AND you have >100% VE with that stupidly early IVC. Which will never happen. So, your DCR never exceeds your SCR.

Now, to throw a wrench into things. Sometimes, someone will replace a cam calculating a DCR of say 8.8 with a larger duration cam to reduce DCR to 8.5, but ends up increasing their detonation instead of decreasing it. This is because they have increased their VE, resulting in higher cylinder pressures.

I've had several prominent cam designers tell me to quit worrying about DCR, because it's actually cylinder pressure that matters. And more likely it's heat, not pressure that's the real issue. But I keep looking at DCR. I just don't use it as anything more than a rule of thumb.

Sorry for that long dissertation. There was not an easy way to answer the question.

 

Ok. I tried downloading this spreadsheet but its not readable. 1/2 of it is in English and the other half is in jiburish. Maybe its my computer. IDK.

I see what your saying about milling the head and then having clearance issues. I have 317 heads here and was told they are the same as a 243 with a bigger chamber. I thought I could get away with milling the head to make up the difference but I guess I was wrong.

Guess I'm looking for 243 heads so I can start with a smaller chamber. There is a lot of technicality's on compression I did not know about. Thanks for the insight.

 

Should I try for 11.1 so I'm safe in both direction's as far as not to much compression. I have a cam card but the way you guys are talking I cant make sense of all the factors that go into compression.

 

I was under the impression even OEM camshafts have IVC points after BDC because there is very little piston motion and the ramps of the camshaft take time to set down the valve on the seat gently.

Like you say a bigger cam on right combo gives us higher VE and less DCR around idle. Thats my point- you guys are all calculating low speed (idle~) DCR Like it somehow has anything to do with WOT cylinder pressures which is where detonation is really troublesome.


Heres my opinion, after tuning for 15+ years, I never had anybody regret going a little lower on compression ratio for safety, and I have had plenty of folks wish they had a bit less compression for various reasons- sometimes it was ambient temp, gearing changes, future FI plans, etc... they all reflect back to your compression because peak cylinder pressure (as was mentioned) is truly the final say, as you can move timing around for a bandaid (like the OEM comptuer does) whereas in a true high performance application you do not want any variable (such as compression) forcing you to back off timing, raising EGT and wasting fuel, because the builder put an extra point of compression into your combination to get 4% or whatever more power.