[tom s]

You can play with all the things BUT IMO 215 pumping compression will be a issue.Putting a "bigger" cam can bleed off pumping compression but we put bigger cams in to make better cyc filling so what happens at 2500-3500?Been through this too many times.My 91 gas line in the sand is 190 MAX,180 better.Carry on.Tom

[steve25]

Well this new piston info changes things since now we know you are not running the factory 1975 chamfered piston that would add 6 CC's or more ( I forget the exact number) of of volume to the compression equation.

Here's another question , what do the plugs look like, can you post up a photo?

Also just for ****'s and giggles, if you pull all the plugs out and hook a torque wrench to the Crank bolt how much does it take to get the motor spinning?

[Stan Weiss]

Originally Posted by Stan Weiss

My guess would be about 10.75:1 CR

Stan

That was just a guess on my part, but why I said 10.75

Head Gasket Thickness = 0.028"

Head Gasket Bore = 4.160"

Combustion Chamber Volume = 72 cc's

Dome Volume = -7 cc's

Zero Deck

CR = 10.752

Stan

[steve25]

Stan those 2262 Pistons have 6.7 CCs of valve notch volume and a ring land back space volume of 1.5 CCs, so if we roll all of that into the chamber volume number we end up in round numbers with 80 CCs, and now if you really want to get down to it the E heads stated 72 CCs most times come in a near 74 CCs.

[Stan Weiss]

Steve,
I did not include the ring volume as I had no idea what to use for the depth of the top ring nor what the diameter of the piston was above the top ring.

4.15" bore and 3.75" stroke @ 10.75:1 Cr has a total volume of 85.25 cc'c

Stan

[steve25]

Here's my numbers.
Piston volume 50.7

Deck clearance volume .5199

Valve notch volume .4088

Head gasket volume .5736

Ring land back volume .09154

Chamber volume ( 72 CCs) 4.939

Adding all this up you get 57.23, then minus out the PV number and you have 6.532
Now divid with that and you have a compression ratio of 8.76 to 1.

If your running a gapless top ring you can half the ring back land volume.

[Formulajones]

Originally Posted by tom s

You can play with all the things BUT IMO 215 pumping compression will be a issue.Putting a "bigger" cam can bleed off pumping compression but we put bigger cams in to make better cyc filling so what happens at 2500-3500?Been through this too many times.My 91 gas line in the sand is 190 MAX,180 better.Carry on.Tom

That's been my experience as well.

Really doesn't matter what the static compression ratio of the engine is at this point. It's the dynamic compression that's going to be the concern just as Tom mentioned. Simple compression test would give you some ideas, as in this case.

Every engine I've ever had that would pump 210-220 psi was a real bear to run on even 94 octane pump gas. I had to be spot on with timing and fuel, but even on hot summer days that wasn't enough.

More camshaft was the solution, and even though larger cams are better at cylinder filling, it happens higher up in the rpm range where the engine isn't loaded as much so detonation becomes less of an issue. It's down low, off idle, slow speed, where most of the pinging problems lie, and couple that with a heavy car that has no rear gear, tight converter, it just exaggerates the issue.

Personally, that is a very very tiny camshaft for a 400, probably more suited to a 9:1 engine and/or 350 cubic inch.

[promptcritical]

So 8.76:1 CR and 215 psi cranking compression. I would think the 215 psi could still be an issue down the road after the not revving problem is worked out.

[Stan Weiss]

Originally Posted by promptcritical

So 8.76:1 CR and 215 psi cranking compression. I would think the 215 psi could still be an issue down the road after the not revving problem is worked out.

If the CR is 8.76:1 then the 215 psi cranking compression is the problem. To get 215 psi and 8.76:1 CR with that cam. The cam would need to have been installed with an ICL of somewhere around 70 ATDC.

Stan

["QUICK-SILVER"]

Originally Posted by steve25

Here's my numbers.
Piston volume 50.7

Deck clearance volume .5199

Valve notch volume .4088

Head gasket volume .5736

Ring land back volume .09154

Chamber volume ( 72 CCs) 4.939

Adding all this up you get 57.23, then minus out the PV number and you have 6.532
Now divid with that and you have a compression ratio of 8.76 to 1.

If your running a gapless top ring you can half the ring back land volume.

Steve,
You seem to like doing this kind of stuff..

Can you use the formula you posted and work backwards?

If you can, I'd go as far as using intake close for swept volume and then see what kind of combustion area is needed to get the 215 cranking psi.

Given the limited build info and known parts info the OP has access to, on this done built, as is project... I'll go out on a limb and guess the original owner wasn't looking for high rpm power. Then not knowing, had the cam advanced to get more low rpm, off idle power.

"This is only a test"

Clay

["QUICK-SILVER"]

Originally Posted by Stan Weiss

If the CR is 8.76:1 then the 215 psi cranking compression is the problem. To get 215 psi and 8.76:1 CR with that cam. The cam would need to have been installed with an ICL of somewhere around 70 ATDC.

Stan

Stan,

Cranking compression for 69' 400's factory rated at 10.75 CR, was 185-210.
Guessing low number was for 041 cammed engines and high number for 068 or 067?

OP's 278° cam with 105° ICL has got to be adding to effective stroke and running the cranking compression up.

Don't remember now but did the math in my head when you recommened the 068 cam for this. Big difference in intake close between it and whats being used.

Clay

[steve25]

The Cam in advanced is exactly what I have been alluding to from my knowing even without doing the math that he can not have over 9.5 comp with the parts he has at there stated stock spec!
I don't know that the formula I posted can be worked in reverse and used to come up with cranking psi since it can not account for pressure bleed due to any given Cams overlap duration.

The other wild card/ question here is are his heads at the out of the box CCs as even if the motor was zero decked your not going to see anything more then 1/4 of a point in compression gain.

The other thing to note here is as I have posted many times is that the highest compression factory motor 400 motor was the RA4 which was 10.5 at best when you run the numbers, not the 10.75 as you find on line and in repair manuals.

All other 400 motors where atleast .5 down in real life from there stated compression ratio.

[TAQuest]

Installed a new battery and hotwired the HEI. No change. The bog seems to be getting worse. I'm rigging up a camera to record the fuel pressure when it bogs.

[mchell]

Now it’s a bog?....

[Stan Weiss]

OK, Pull the valve cover so you can see the valves for number 1. Rotate the crank until the mark on the balance lines up with timing mark. Make sure that this is the overlap TDC where both valves are open. If the intake valve is open more than the exhaust valve, the cam is installed to far advanced.

Stan

[TAQuest]

The video of the fuel gage shows a solid 5 pounds through the whole test beat. Tomorrow I'm going to swap the carb from my truck onto it.

[TAQuest]

Originally Posted by mchell

Now it’s a bog?....

Bog. Loss of power. Will not go any faster. Like valve float. Maybe a bad carb float setting? Messed up carb? I'm on it tomorrow. Learning what it's not. It has the right power to the HEI. Fuel is being delivered correctly. The heads seem fine. It has great compression. Runs great right until it hits the rpm wall at about 3K now.

[TAQuest]

Originally Posted by Stan Weiss

OK, Pull the valve cover so you can see the valves for number 1. Rotate the crank until the mark on the balance lines up with timing mark. Make sure that this is the overlap TDC where both valves are open. If the intake valve is open more than the exhaust valve, the cam is installed to far advanced.

Stan

Stan,
It runs too good for that to be it.

[Navy Horn 16]

Have you tried it in all gears? Same thing at same RPM in each?

[TAQuest]

Originally Posted by Navy Horn 16

Have you tried it in all gears? Same thing at same RPM in each?

Yes. Same thing each gear.