I understand that, but how much compression is too much for a street car? Not all of us are after max effort track setups. I was under the impression that around 11:1 static compression is about ideal for a pump gas motor. I am not sure I would want to push past 11.5:1 on pump with todays inconsistent fuels. With E85 different story of course.

 

I'm running 11.5:1 compression on 93 and there are no issues whatsoever

 

Yeah, 11.5:1 is a good goal, but quite a few are running 12:1 and greater on 93. All in the tune I guess.

 

wow did not realize that folks were running 12:1 on pump.

 

Yeah, it can be done. Like I said, tjne plays a major factor. I would shoot for 11.5:1. That's basically where my setup is at. Haven't had any issues yet, but I buy my 93 at the same gas station all the time.

 

You can run 12:1 if your cam is big enough with a late enough IVC @ .006" to determine your cranking compression/DCR. I used the Polluter as an example. It's a 239/244 112+3. It's IVC is 73.5 degrees at advertised. With a 58cc Chamber, .040" Cometics and stock flat tops, you're around 11.95:1 CR and 8.8:1 DCR with that cam. Really that's an ideal setup with that camshaft. Had I know about the TFS 205s when I bought my heads, I would have gone that route and did something similar to the Polluter which is slightly bigger than my cam and has a little more overlap - but the later IVC needs more compression.

You need a pretty big cam to get by with 12:1 on 93. Or you're going to run into issues with timing and KR. And I'd rather run 27-28 degrees without KR and a safety margin with slightly less compression than 12:1 at 20 degrees with little to no safety margin and the possibility of KR with a tank of bad gas.

 

Ok I see...so for more of a max effort setup going with the 205's and a 240 duration cam would be a great combo, but the 64cc TFS-220's will work better with a 229/231 cam or something in that area?

 

I'd like to see Martin comment on this, but even with big big cams there's a limit on static/dynamic compression.


Say you're at 12:1, even if you have a can that drops you down to around 8.3 DCR, you'll be fine at lower rpm's, but you might run into problems in the higher RPMS when the engine gets more efficient, or in tune, with those cam specs. It will start sucking in more total air/fuel that the larger duration and later IVC will allow. Actual compression and cylinder pressure will increase because of scavenging, pressure waves, and what not.

I'm not the best at putting thoughts into words, but basically the DCR isn't truly a fixed number, it will increase nearing peak torque, which is where you'd run into problems with a big cam and that much compression.

Martin might comment on this

 

Also if you ever go on a road trip you'll discover that 93 octane is getting hard to find. I feel lucky that I can still get ethanol free 91 octane.

 

I think you did a great job explaining it!

I personally would keep the SCR around 11.5-11.7:1, possibly 11.8:1 tops.

The DCR is not a set number that will stay put as the engine runs through the operating range is the simplest way I can put it.

I will say I have had a number of customers run 11.8-12:1 SCR with 93 pump gas and run fine, but there are many variables that can come into play in allowing it to work.

 

I'm assuming quench would be one of the important variables, along with combustion chamber design.

On that note, how would you say the as-cast TFS 220 heads do in that regard? And the 205s? Figured I'd ask since its the topic of the thread.

 

That is correct. Without going back and looking through a lot of old emails, I believe the highest CR I've had someone run with the As Cast 220's is 11.7-11.8:1.

I don't have any experience with the 205's. I had a customer purchase a camshaft for a set a while back, but I believe he changed combinations.

 

That pretty much sums up my setup.

~11.7:1 SCR on a 408 with as-cast 220's

I run a 239/247 comp cam currently on 93 with no issues.

 

I've run the TFS220's with PM valve guides, Ferrera valves, stock rockers (with trunnion bearings), 63cc chambers and a VRX5 .609/.605 cam for 6k miles no issues whatsoever on 93 octane. I opted out of porting as the extra 10hp wasn't worth the price. To each their own, though.

 

Where you been brotha? Ain't seen you around here in a long while.

 

do you have any power numbers or track times? that is very similar to the setup I am considering. thx

 

We had a baby boy arrive it is taking up all my time lol.

440/403 no track numbers. I've added a few other tweaks it will probably be around 450whp now. This is with small 1-3/4" headers and an LS6 intake also. It's a good setup.

 

Congrats Dirtbag. Kids are pretty awesome, but do take up the majority of your time.

 

Congrats and Nice numbers! I have 1 7/8 kooks with their true duals now. Debating on keeping the LS6 or ponying up for a Fast.

I have a small issue to sort before going forward. Currently tracking down a tick that runs down the passenger side header. Top side of the engine is dead quiet, no signs of leaks anywhere, but I am noticing a persistent ticking sound when fully warm at idle.

 

I had a broken inner valve spring with your symptoms before. Just fyi