My basic understanding is this: Compression helps a F/I setup either turbo or supercharger by increasing off boost drivability, helping a turbo spool faster, overall % increase in power for the same level of boost. As boost levels climb, higher compression and increased charge temp results in detonation. E85 will help a lot with this but where is the limit?

Whats the maximum workable compression for various levels of boost when running on E85?
Where is the trade off point where retarding timing = less power because of compression levels?
Pushing the compression adds more risk to running safely, eg on a hot day with higher engine temps. At what point does the tune safety margin get too small?

Emilio ran Deviate on a 11:1 compression engine (I think) with around 9-10PSI via a Rotrex on E85 for approx 330whp. Where would a 12:1 compression engine compare? How about a 10:1 compression engine? Could that same engine work safely at 12PSI, 15PSI?

 

lower compression = more boost safely

idk what the sweet spot is for our engines. But I do know the answer is not hard to find. Shoot an email to FM or TSE/Sav/Andrew. They should help you out.

 

Depends on the engine really. I can refer to my vg30dett builds. We have run as high as 24 psi at 10:5 compression on ethnol. We were running 26 deg of timing up top. No knock.

In the end there is no correct answer to your question. It depends on the following parameters.
timing
head design
afr
compression ratio
etc etc

just run a really good knock sensor and you will find out.

 

yeah I suppose there is really no way of knowing. My existing engine has 10.5:1 Wiseco pistons with a decked head raising the compression to between 11:1 and 12:1. Is that too high for boost or is it worth giving it a shot to see if it will work? I would probably be only aiming for 10-12 PSI.
I know thicker gaskets are really a no-no when it comes raising compression, but it would be one way of dropping the compression a little to make it safer. Alternatively I could just switch out the pistons and be done with it.

 

Depends on what timing you run. I suspect it will run fine. You should be able to run within a couple degrees of MBT which IMO is fantastic for turbo. Also why only 10 or 12 psi ? Personally I would leave cr alone and run more boost. I don't think you will will gain much in spool up going from 10.5 to 12:1. More is not always better. If I was in your position I would simply run more boost if you want more power.

btw thicker gaskets makes the squish area progressively worthless. I would prefer to use the stock thickness head gasket.

 

I'm looking at options for my existing engine, which is 200whp N/A. It's an older BP-05 head but it has been extensively worked and it's built for high revs, so one option is an all out supercharger build with a large Rotrex. The other option is an EFR turbo. Target power would be around the 350-400whp mark.

I'm trying to figure out if I can actually make 12:1 compression work with the existing engine running E85 and large Rotrex? I don't think a turbo would suit it this engine at all so if I go down that route I'd probably drop on a BP-4W head and keep the 10.5:1 bottom end.

 

You are extremely, extremely optimistic to think that 10.5:1 with a slightly decked head nets you anything above 11:1, you are absolutely nowhere near 12:1 so I would consider not spreading that around as fact.

In my opinion, for boost and E85, 10.5:1 is a happy medium.

 

The head is decked near 2mm. I have to take it off to measure it properly which I'll do next rebuild. I did do some estimations at some point and remember it raised the compression a decent amount but I can't remember the final total I got. It wasn't accurate however as it depends on how much volume the piston takes up and how much bowl work was done to the head. It's definately over 11:1 though. Anyone roughly know how much extra compression you get per thou?

 

Values for combustion chamber volume I've seen are anywhere between 50.2-52 cc (You will have to measure this yourself). Using the fattest value:
K590M84 is labeled as a 10.03 CR piston, I can't seem to find a 10.5 listing according to wiseco's own spec sheet.
This would have a -7cc dome
Stock gasket is 0.79mm (.031") - I don't know if this is crushed value
85mm stock stroke
deck clearance is -0.5mm (wiseco spec)
cylinder displacement = (84mm/2)^2*3.14*85mm = 470811.6
Engine displacement = cylinder displacement * 4 = 1883246.4 = 1883.25 cc

Plugging this info into the Summit calculator, it looks like the compression ratio should be around 11.17:1

I don't know if you mean you shaved 2mm off the head, but that sounds like a lot. A LOT. If you're using 84mm pistons, shaving the head 2mm decreased cylinder volume by 11.08354 cubic centimeters. Taking that difference would throw your compression above 14:1 unless my plug and chug math is super faulty. Find your combustion chamber volume and calculate compression ratio = (displacement + combustion chamber volume)/ combustion chamber volume.

 

Thanks for the calcs. I need to pull the head off to measure it correctly.

This head is off a fully developed race engine I purchased. I don't know the full specs of what has been done to it ... the head is decked enough so that the timing belt is a little slack at full tensioner position. The stock idler was replaced with a larger idler pulley to get more tension... even still it has skipped teeth during backfires when cranking. Measuring it on the engine with a flat edge and some verniers gets me around 2mm but it's hard to get an accurate reading.

My EWP replacement water pump plate moves the idler in 2mm to get a bit more tension which should stop the risk of backfires skipping teeth in the future. The valves are a few degrees from touching the piston so any teeth skip is bad.

I doubt the compression is more than 12:1. I used to run this engine on 98 octane which is 95ron??? I'll have to have a look at my timing maps.