18 replies to this topic

[MB1380]

Im currently rebuilding my 1380 and am wondering what is the typical Compression Ratio used in them? Im looking to run standard pump fuel and for road use....

[Paul Wiginton]

10.5:1 is a good starting point for road use. For a track day car 11:1 is good. I run 12:1 in mine with limited road use and mainly for Sprinting 1/4 mile

Paul

[MB1380]

10.5:1 is a good starting point for road use. For a track day car 11:1 is good. I run 12:1 in mine with limited road use and mainly for Sprinting 1/4 mile

Paul

Running at 12:1 are you using hi-octane petrol ya?
The reason im rebuilding it is that i was running 12:1 and ive managed to damage my pistons but i suspect the octane additive i was using was useless.

[Paul Wiginton]

No, just BP super unleaded. Been fine for 9 years now, only just replacing the pistons because there are lighter ones available. I do make sure it is well set up though

Paul

[bigmotherwhale]

you can run up to 15 to 1 compression with water and or methanol injection, whats the psi you get when you run a compression test paul?

[Paul Wiginton]

Cant remember off the top of my head, the motors stripped at the moment

Paul

[mini_mentalist_chris]

Mine is running an 11.5:1 compression ratio.

[Prawn]

my 1380 also runs around 11:1 compression, although I'm going to raise it to 12 when I get a new cam.

Surely the compression ratio you choose to run should be based on the cam and the rest of the engine spec in general?

If you're running a standard fuel pump I guess you won't be revving it very high, or going for a massive power output?

[Paul Wiginton]

Yes compression very much depends on cam, Mine has a 310SP, when I used to run a 296SP, it was 11.5:1.

A standard fuel pump is going to restrict the engine to a maximum of 100hp maybe less.

Paul

[Prawn]

I've never trusted the standard fuel pump at high revs either! I once bought a car where the pump had been fitted without a spacer in the past, the arm on the pump soon snapped off, and caused a LOT of trouble in the box.

[MB1380]

Yes compression very much depends on cam, Mine has a 310SP, when I used to run a 296SP, it was 11.5:1.

A standard fuel pump is going to restrict the engine to a maximum of 100hp maybe less.

Paul

Im using an MD286 with a 12G940 Stage 3 Head and 1.5 Roller Tip Rockers?
But after a couple of hundred miles i found this. Im putting it down to running 12:1 on shitty fuel but i could be all wrong? I also dident really stick to the running in procedure, giving it full throttle a bit which probably dident help

[MB1380]

my 1380 also runs around 11:1 compression, although I'm going to raise it to 12 when I get a new cam.

Surely the compression ratio you choose to run should be based on the cam and the rest of the engine spec in general?

If you're running a standard fuel pump I guess you won't be revving it very high, or going for a massive power output?

Sorry i dident mean standard fuel pump, It was standard petrol i was on about "pump fuel" as opposed to hi-octane

[Paul Wiginton]

Odd! What make are they?

Ive only ever run in my engine for 20 mins at 3,000 revs then nail it.
If it was compression I would have thought there would be signs of fatigue or melting on the crown, particularly on the edges.
This looks more like an oil issue to me, crap pistons or maybe the ring was damaged upon assembly

Paul

[MB1380]

Odd! What make are they?

Ive only ever run in my engine for 20 mins at 3,000 revs then nail it.
If it was compression I would have thought there would be signs of fatigue or melting on the crown, particularly on the edges.
This looks more like an oil issue to me, crap pistons or maybe the ring was damaged upon assembly

Paul

Powermax pistons, i have a new set of them arriving this week. theres no sign of melting at all, pistons are immaculate otherwise and onlt 2 were affected, does that rule out a compression problem so?
Now that you mention it about ring damage, if i remember there was a problem with the ring compressor when it was done, this could be my whole problem?

[Paul Wiginton]

I would think so. Get a new one

Paul