Maximum compression on 87 octane?

[Tim_K]

Title says it all. How high can the compression ratio be pushed and still run 87 octane in a V-6?

Let's assume stock cams, ported intake, 52mm throttle body, and a 2-1/2" exhaust with gasket matched manifolds and no cat, if that makes any difference. I may eventually put in Diamante cams if that helps any. I think the longer duration cams would allow a bit more static compression ratio.

And if you mill the heads down to raise compression, how far can they be cut? Then you have to cut the manifold side of the heads some also, what sort of ratio would it be?

Like for every 0.010" off the deck surface you have to cut "X" off the intake manifold surface.

I used to know this stuff for Mopar 90 degree V-8s.

[Ondonti]

well, in the cold weather my 3.0 will take 20 deg base timing on 85 octane, but Ive proven myself to make about 28whp less here then I do at sea level.

Im not really sure what causes the knock. If its the plugs, then you could try running recessed plugs.

I guess the onlyway to answer this question would be if someone knew what is actually causing the hotspot in the combuston chamber to make it knock in the first place.

Anyways, if you could find the main hotspots and fix them, Im sure you could run decent compression and timing on low octane.

[87turbodance]

What about a fine water injection only under boost to cool any hot spots?

[Boogieman142]

yup, as he said above, except that he never said anything about boost...

[Ondonti]

i would first look into fixing hot spots before bandaiding with water injection.

[Tim_K]

I'm planning to port & smooth out a set of V-6 heads for my engine, and in the process I will take my Dremel grinder with some sanding drums and smooth down the entire combustion chamber and the edges of the piston valve reliefs. If there are no sharp edges, that should help a lot in controlling detonation.

During the years I spent working on Mopar V-8's, when the aluminum heads started to come out on the market, it was widely written that iron head engines with 92-93 octane fuel could tolerate only a certain amount of static compression ratio, like 9:1, but with aluminum heads, you could safely go 10:1 static compression on the same fuel. And of course, longer duration cams affected the cylinder pressure, so "X" engine with "X" octane could handle more static compression ratio with a longer duration intake lobe on the cam.

But since this car is the work car / beater, I want to only run 87 octane in it. It MIGHT get the Diamante cams if I can get them for the $55 price Odonti said he got his for.

This engine will not get boost or nitrous at all, and if it did, I would run 93 octane in it for that.

[crazyace]

Well, the 92-93 Diamante runs 10:1 compression and they do not have a premium fuel requirement. The mitsu engines do have a knock sensor though. Our computers (being Mopar) tend to be on the easy side of timing. So I see no reason to run any more octane than you normally do if that compression ratio is used with stock ignition timing. Do not get carried away with milling the heads though, you have to remember that unlike the older V8's our cam timing retards as the amount of milling increases due to being SOHC.

[Boogieman142]

so, how does timing retard with milling, u mean by planing the head?

[crazyace]

Nope, when you mill the head it gets closer to the block therefore the tension applied to the belt rotates the cams to the point they become retarded.

[Ondonti]

so you could still degree the cams a few degrees advanced................

I think Im gonna degree my turbo cams 4 deg adv.

[crazyace]

Only if you can make some adjustable cam sprockets. You need to remember that .020 off usually relates to 1-2 degrees.

If you can manage to get everything matched up for good high flow (on an N/A engine) a few degrees retarded can actually make your engine think the cams are a bit bigger. How many degrees it will like always depends ln how the engine was built though.

Ondontii, you may want to reconsider advancing that much unless you are planning to run it that way with no boost at first. Remember that is the same as getting both cams off one whole tooth.

[Ondonti]

Dont you just need a degree wheel and a dial indicator or something like that?

I dont follow what you mean by running no boost at first.

[crazyace]

Our cam gears are not adjustable, so you will need to find a way to make them adjustable. You can use a degree wheel to find out just how advanced one tooth makes the cams. The "run no boost at first" statement was for incase you were going to try that with the new boosted engine you are building. If you are considering the attempt on your stocker then go for it, the only bad thing you should notice is a drop in horsepower.