engine jerks when i put the plugs in and turn over

[mini64]

I'm worried i built a 20million:1 compression engine. I have built a "stoker" motor using a 1600 crank and the pistons to match such a build. my calculations showed 12.36:1 compression, higher then i wanted but still within reason for a racer.

my problem is this - when i turn it over plugs out all is good. when i so much as put 1 plug in the engine damn near jerks out of its mounts on the compression stroke, making a bit of a knocking sound which i cant tell if its all the movement or internal to the engine. If i turn it over slowly by hand with plugs in everything is good, although difficult at compression. pistons are not hitting the plugs - this happens even with them screwed in a few turns.

any ideas? did i mess up? or am i just not used to a high compression motor? when i measure compression the needles gets to about 210psi on the third stroke. numbers below:


piston 25 thou proud of block at TDC. calculated to 3.51cc on an 84mm bore

piston moved down .395 inch to calculate piston volume, measured 56.0 cc on a 84mm bore. should have been 55.6cc so a .402 cc piston shape volume.

head volume 37.5cc
gasket volume ~ 3.5cc
stroke 3.059"; vol 430.56cc

Vb=430.56+37.5+3.5-3.51+.402=468.45cc
Vs=37.5+3.5-3.51+.402=37.89cc

12.36:1

[rgh0]

It appears you have built a high compression motor without an intruder dome on the top of the piston and have the piston coming up above the block the same thickness as the head gasket. The calculations look OK to me without having measured up the motor myself.

BUT -You must have close to zero clearance on the piston to head face squish area and may be getting contact here. Not sure why you only see and hear a problem when cranking with plugs in but I would be worried about this lack of clearance.

cheers
Rohan

[mini64]

are you saying normally the pistons would not be proud of the deck at TDC? as in my block deck height was too little?

[mini64]

slight update:

the pistons being proud not withstanding, I've found the knocking/jerking is in time with the starter trying to kick in and out. its an engaged type, tilton super starter. i can watch the gear bounce around to the engine jerk and there is a lot of metal filings coming out of the bell housing. dont really see and gear mesh problems - all is brand new and there are slight burnishing marks on the starter gear but it seems to spin fine.

but again, this only happens when the plugs are in, causing significant load on the starter of course with the compression. is this starter business the cause of the racket, or a result of engine problems?....

[rgh0]

are you saying normally the pistons would not be proud of the deck at TDC? as in my block deck height was too little?

Yes I would normally aim in a high compression race engine to have the piston top deck no higher than flush with the top of the block. In operation the piston will come up higher as the rod stretches under inertia loads at TDC and the bearing clearances reduce. You want around a minimum of 1.5mm clearance at all locations between the piston and valves and the head which you do not have with the piston proud of the block. Assumng you have the right pistons and rods it sound like you block has been decked or line bored to reduce the block height from the crank centreline.

The standard engine piston top deck was about 0.6mm below the top of the block

You shold not be getting metal filings out of the flywheel / starter pinion engagement so you may also have an alignment problem when its under load. Some of the pre-engage starters have faily poorly designed adapater plates and connecting bolts to the adapter that can cause unwanted flex and also do not fully engage the pinion with the ring gear. There are lots of posts around that topic.

cheers
Rohan

[7skypilot]

I'd agree with Rohan about checking the starter motor and ring gear mesh. I had a similar result (swarf in the bell housing) when the pinion and ring-gear made only about 2mm contact!

[Galwaylotus]

I had issues with my starter too and found the ring gear had slipped on the flywheel. The new ring gear is now tack welded to the flywheel!!

[mini64]

thanks again for everyone's help. here is a photo of the piston to block deck. opinions? i'm measuring 35thou+ sticking proud of the block. wonder if i can pull the pistons and machine them down around the outer diameter to give some squish volume. I'm going to call the piston manufacturer (JE) tomorrow and ask.

[holywood3645]

Have you considered a thicker head gasket? I think Kosmic or Cosmic make them in various thickness. I had intruder pistons machined flat with (zero deck) the top of the block when I built my motor. It was a 711M block with about 1/4inch taken off it. The CR is 10.6/1

James

[rgh0]

Cometic is the company that can make you a MLS ( Multi Layer Steel ) head gasket in varying thicknesses. Unfortunatley they dont seem to seal to well on a twin cam so i would not recommend them in this aplication. I just think the head bolt loads or head flex you get in a twink is insufficient to compress them correctly

You need to check the intruder diameter and that it fits up into the combustion chamber with adequate clearance on the sides as well as machine some of the top of the piston in the squish area to bring that down to flush with the top of the block at the highest. Depending on the compression ratio you want you also may want to machine some of the top of the intruder. JE should be able to tell you whats safe to take of in the squish area, machining the intruder section should be no problems.

Depending on the cams you are running you also should check the valve clearances in the pockets at TDC with the piston up higher than it should be these may to close also.

cheers
Rohan

[Quart Meg Miles]

Are we sure we haven't got detonation here? There won't be much rod stretch at starter speeds, I would have thought. Can you fit a Colortune and have a look inside the cylinder while pressing the starter solenoid?

[CBUEB1771]

wonder if i can pull the pistons and machine them down around the outer diameter to give some squish volume.

Sure, that is an option. I did that with the pistons in my +2 to arrive at a sensible compression ratio with a cylinder head that had been faced too much.