cranking compression and gear reduction starters

[AC sports]

Just did 300 miles on a newly build engine. Forged bottom end with thin rings. 42/82 inlet, 40/80 exh cam timing on 110* lobe centres. 10.4:1 static comp ratio.
Pistons tops look dry, plugs same.
Hot cranking compression is 175 psi across all 4 cylinders. It seems a little low to me.
The only thing I can think of is this engine runs a gear reduction starter.
Could this be the cause? I'm thinking the lower flywheel speed due to the gear reduction is offset by more torque as the piston approaches TDC on the compression stroke.
Thoughts?

[Stan Weiss]

What is your bore, stroke, and rod length?

Stan

[AC sports]

Bore is 84.5mm
Stroke 79.2mm
Rod length 136mm

[Stan Weiss]

Larry Meaux or David Redszus maybe able to talk to the difference due to starter RPM. But for that bore, stroke, rod length with 10.4:1 CR and IVC 82 ABDC I get 167.4 psi.

Stan

[AC sports]

The calculator I used gave a bit less at 158psi but similar engines I've built crank 200+ psi on stock starter, which is why I suspect the gear reduction starter plays a role.

[Biteme]

I have an engine with slightly more compression and bigger cam.
Cranks 175 with reduction starter.

[David Redszus]

In addition to the physical parameters of a given engine (bore, stroke, ConRod, IVC, CC) the cranking
pressure must consider the pressure difference between the cylinder and ambient pressure inlet air.

For the engine parameters listed, the PSIA readings are calculated as follows:

@ 14.7 psi = 181.3 psi
@ 13.7 psi = 169 psi
@ 12.7 psi = 157 psi

Inlet restrictions such as a throttle blade not fully open will affect cranking pressure.
Similarly, at running speeds, resonant tuning will have an effect on pressure ratios.

[leahymtsps]

The calculator I used gave a bit less at 158psi but similar engines I've built crank 200+ psi on stock starter, which is why I suspect the gear reduction starter plays a role.

Cranking speed can have a big effect on cranking compression. People have been circumventing cranking compression
rules by slowing cranking speed for decades.

Tom

[mag2555]

You are doing the test with the throttle wide open right?
If you want a tad more cranking speed then yank the other 3 plugs out if you have not done so already.

[AC sports]

All plugs out. Wide open throttle. Ignition disconnected. The only way to do it.

[makin chips]

Cranking speed depends a lot on reduction ratio of the starter itself. What's the ratio on yours? Lower numbers numerically will spin the engine faster where higher numbers provide more torque to turn over high compression engines. What's the ratio on your starter? If you've already got something like a 4.4:1 ratio, then it's quite possible it's not spinning fast enough.

[dwilliams]

Almost all aftermarket gear reduction starters use imported motors. US electronic motor manufacturers lie like rugs about their motor ratings; there's not even "industry practice" to control imports. That "3kW" starter you just bought might actually be only 1kW, if that.

FWIW, all the OEM Ford gear reduction starters I've used cranked noticeably slower than the starters they replaced, even though the old ones were pretty tired.

[dwilliams]

I'll go ahead and repost my Starter Rant:

The starter depends on a complete circuit, positive and negative. Krylon and rust at the joint where the starter and engine meet isn't a good electrical connection. There should be bare metal there, wiped with Vaseline to inhibit corrosion.

Seeking a ground across the U-joints, throttle cable, and sensor wires isn't good either. For that matter, cast iron isn't a great conductor either. Man up and run a nice fat ground cable from the battery to one of the starter mounting lugs.

Grounding the battery and starter to the body isn't good either. Now you're seeking a path through sheet metal, spot welds, and whatever corrosion is hidden by the seam sealer.

Bigger is better for starter cables. Forget ampacity ratings on the wire; the starter looks like a dead short to the battery. And those cables get HOT after a few minutes of cranking, and the copper's conductivity goes down as temperature goes up.

I have my local welding shop make up cables from 4-0 welding wire. (not #4) They have the proper equipment to crimp the lugs. Some NAPA stores have lug crimpers. Also, some hydraulic shops can do it.

You don't care about a pound or two of extra cable. You're not going anywhere if the thing won't start. And further, don't even *think* about aluminum cable; the connections corrode with a hard look, it's not very flexible, and when it does flex, it's liable to break.

That little quarter inch wide "ground strap" screwed to the firewall or frame? That's for radio noise, not starter current.

Welding cable isn't rated for underhood use. I've used it for decades and never had a problem. You can order underhood-rated cable, but it's a lot more expensive and you have to wait. At least, nobody in my area keeps useful sizes in stock.

For the battery ends, Belden sells collet-type terminals. They're made of cast copper. You fill them with vaselite or dielectric grease, push the wire in, and turn the collet nut with a wrench. They're not only reuseable, they allow you to disassemble the connection and inspect the wires for corrosion.

Vaseline works better as a corrosion inhibitor than those colored felt washers. It should be applied to every connection.

For cars with fender-mounted starter relays, the relay depends on adequate ground too. With a poor ground connection, the voltage drop when the starter is engaged can be enough to cause the relay to disengage - that's the chattering sound you get when the battery is almost gone. There might be enough zap left to start the car, but you can't get it if the relay won't work.

The starter cable should be anchored so it can't swing around. I've watched them swing six inches when someone kit the key when I was looking under the hood. Twice. With a big POP! and flash, and a knot on the back of my head from the hood immediately after. You shoot enough juice through them and they try to unwind like a piece of string, and they head right for the nearest exhaust tube or sharp edge.