A question about swapping heads

[BA80]

I recently purchased another SG and one of the problems I had to fix was 2 spark plugs broken off in the head. I removed the head and got the plugs out but then I started thinking.............(scary ain't it)........I have a good head off of an SF with smaller combustion chambers and bigger cams. Since the later pistons have a higher dome if I put that head on this engine it would raise the compression a bit and I would have the bigger cams. Possibly a + power combo.

I've heard of combos with the later head with bigger valves and trying to use the earlier thumpy cams but it didn't work out well, but not of switching the head and cams as a package.

Ivan has been tossing this around for a while but hasn't done it yet and I thought this would be a good opportinity to give it a shot.

Any input?

[crazy steve]

IIRC, Dan Hodges has done this, with a slight HP gain. I know I've seen a few posts on this and that it can work. I think to get the best results with the early cams you need the '78E timing curve, either by swapping in the '78 mechanical advance assembly, or altering the stock later unit to give the same curve. Basically, that's just reducing the amount of advance by 5 degrees on the later units, then timing the motor for 5 more initial degrees of timing.

[BA80]

Thanks Steve. I don't have an E engine but I have several Fs. That should be the same. Right?

[natemoen]

No, even the f had a different curve than the E.

[crazy steve]

Thanks Steve. I don't have an E engine but I have several Fs. That should be the same. Right?

Yeah, Nate's right; it's got to be a '78. The funny thing is the parts fiche show the same advance unit for all '78-79 motors, but the manual shows two different curves and different initial timing.

I'll try to pull the advance unit out of my spare '78 motor in the next few days and see just what the difference is....

[BA80]

Which manual are you looking at? The only one I have is Clymers and it only shows proceedure and setting for all 80 & 81 models.

[crazy steve]

Factory manual; I don't think the Clymer shows the specific year curves....

[81xsproject]

Greg, I believe I have a 78 advance unit on the shelf if you're interested. I'd love to hear the results of the swap. I know the early cams/late head is a definite gain for late motors.

[81xsproject]

I should add to this after being contacted by Todd, that the late motor/early cam configuration does not increase power throughout the RPM range. It will detract from low end torque up to around 3.5 grand. It's not much different than stock from 3.5-4.5k. However, when you hit 5k, it is a very noticeable improvement over stock. So, for you final-drivers, this is not for you.

[Incubus]

Those are exactly the symptoms I saw when I had F cams in an XJ head. Scary fast torque over 5000 rpm. Revving at stoplights like a damn Harley to keep it from dying cuz it isn't firing consistently on all four cylinders. Getting passed by mopeds until you hit 3500 rpm.

If you live to race, make the switch. If you plan to ride for enjoyment or transportation, don't mix and match.

Patrick

[BA80]

What I'm planning to do is swap the entire head a cam assembly from an SF onto the engine in an SG. Not mixing cams and heads.

Except for the timing curve as was mentioned it is for all intensive purposes the same engine. the earlier engines with this head and cam configuration were a little quicker so it seems that with slightly increasing the compression it would do the same for HP.

Steve, was there a difference in the timing curve from the SF to the SG?

[crazy steve]

Steve, was there a difference in the timing curve from the SF to the SG?

The E had one curve, the F/SF had another, and the G/SG had yet another. The G/SG curve is the most aggressive of the bunch and may be too fast for the early head/late piston combo. Tomorrow I'll try to compare the three advance units and post some pics....

[BA80]

The E had one curve, the F/SF had another, and the G/SG had yet another. The G/SG curve is the most aggressive of the bunch and may be too fast for the early head/late piston combo. Tomorrow I'll try to compare the three advance units and post some pics....

Thanks Steve.

I have the stuff for an F/SF and the G/SG. Might have to play with it a bit.

[crazy steve]

Mechanical advance differences

Greg, I posted this as separate thread so it doesn't get lost in the pile, so here it is... http://www.xs11.com/forum/showthread.php?t=34277

After looking at these, you might want to try the 80 advance unit; if it doesn't cause detonation, it should be worth a bit of power. Personally, I'd try to shorten the reluctor slots to the 78 specs and run 10 degrees initial timing as that will really help with low-speed response. The initial timing and the curve is the only difference between the 78 and 79 bikes and that was worth almost .4 of a second in quartermile times, so there's definitely some power there. If you shorten the slots on the 80 unit, that should give you a curve that starts at 1800 rpm and will be all in at about 3500. It may be too much, but would be worth a try.

The other thing I noticed (again, according the FSM) is there's a pretty big difference in the vacuum advance units. The 78/79 deliver full advance at 150 mmhg, while the 80 has it all in at only 90 mmhg (the 'average' of the range shown in the manual), so if you get part-throttle pinging with the late vacuum advance, swap to the early unit. If you don't get pinging, the late unit should offer a bit better mileage.

[BA80]

Many thanks for the info Steve. I really appreciate the help.

I'll give it a shot with what's in there to start. Don't want to make too many changes all at once.