High Compression Woes, good fix??

[41Draggin]

My Engine, a 4 bolt main 350 bored over .30 (so a 355). I’m running a summit 1705 cam with flat tappit hydraulic lifters, high flow oil pump, 2.02 camel hump heads, an Offenhauser dual quad “360” intake with two edelbrock “1404” 500 cfm carbs dressed to look like Carter AFB’s and an old Mallory dual point distributor.

Recently I did a compression check and all cylinders are running in the 200psi range and higher. Most are at 212psi.

Now here are my issues, because of the high compression I’m getting a lot of blow by both out of the valve cover breather and obviously through the PVC and into the intake manifold/combustion chambers. To fix this I'm installing an open circuit oil catch can, PVC to inlet and breather to out. I’m hoping this will stop the engine from puking oil at higher RPM’s as well as cut down on deposits in the combustion chambers that I believe are partly to be the cause of a run-on/deseiling issue I have at shut off. To help combat the run-on issue I also am considering running a cooler spark plug then I have been running (Autolite 86 rated at a (7) with one of these; Autolite 3924 (8), Autolite 85 (8) or Autolite 24 (9).

The last bit of this is to drastically lean out the carbs, I know the dual quad set up is more than I need and everyone is going to tell me to just run a single four but I am more concerned about the looks than the performance.

Has anyone been down this road before and have any advice on these processes, the catch can hook up, the seafoam, the different sparkplugs? Any help would be greatly appreciated. Thanks!

 

[xm20k]

What is your static compression ratio? Cam specs? That part number does not show up.

 

[fast 99]

Probably should perform a leak-down or alternately look at top of pistons with an inspection camera. Blow by is more often than not caused by rings leaking.

Run on can have many causes, low octane, high idle, vacuum leak, hot engine, to name a few. Leaning out would likely make that worse causing the cylinder temperature to increase. To some extent fuel is a coolant.

 

[Ricko1966]

Your blowback is not a result of compression. Your blowback as @fast 99 Said most likely ring seal. Dual 4bbls is fine for looks ir performance but leaning them out isn't a good idea. I learned that the hardware more than once. Flat tops and double humps that's factory LT1 or L79 they didn't have blow by problems.

 

[Chevy 88]

If you are looking to lower the compression - Ya might be able to get spark plugs that do not protrude as far into the cylinder, and ones with larger cavities around the center electrode. You can increase or decrease compression a little by using longer or shorter plugs, at least to some degree. Thicker oil might help seal the cylinders better. Might be able to install thicker head gaskets.

Compression is much different when the cylinder fires, VS a compression a test. So I think a leak down test is in order.

 

[Ricko1966]

And a big Amazon order with a bunch of right stuff silicone.

 

[Sad Sack]

My Engine, a 4 bolt main 350 bored over .30 (so a 355). I’m running a summit 1705 cam with flat tappit hydraulic lifters, high flow oil pump, 2.02 camel hump heads, an Offenhauser dual quad “360” intake with two edelbrock “1404” 500 cfm carbs dressed to look like Carter AFB’s and an old Mallory dual point distributor.

Recently I did a compression check and all cylinders are running in the 200psi range and higher. Most are at 212psi.

Now here are my issues, because of the high compression I’m getting a lot of blow by both out of the valve cover breather and obviously through the PVC and into the intake manifold/combustion chambers. To fix this I'm installing an open circuit oil catch can, PVC to inlet and breather to out. I’m hoping this will stop the engine from puking oil at higher RPM’s as well as cut down on deposits in the combustion chambers that I believe are partly to be the cause of a run-on/deseiling issue I have at shut off. To help combat the run-on issue I also am considering running a cooler spark plug then I have been running (Autolite 86 rated at a (7) with one of these; Autolite 3924 (8), Autolite 85 (8) or Autolite 24 (9).

The last bit of this is to drastically lean out the carbs, I know the dual quad set up is more than I need and everyone is going to tell me to just run a single four but I am more concerned about the looks than the performance.

Has anyone been down this road before and have any advice on these processes, the catch can hook up, the seafoam, the different sparkplugs? Any help would be greatly appreciated. Thanks!

Look into Autolite 3923 also. I ran those in my ATK HP94 383. Also read something about keeping the choke open when testing compression. Spin engine about 4 times and take readings. And with over 200 psi I'd be thinking about race gas.

 

[GTX63]

I've run motors, with those heads, at similar compression rates and higher with no blow by. GM put a lot of cars on the street running high compression motors without oil blowby.

This probably isn't the answer you are looking for, but the most likely causes aren't due to compression. Poor ring gap, misaligned rings, out of round cylinder walls, a poor crosshatch hone on the cylinder walls, fuel washing and glazing the cylinder walls. Did you run a leak down test to see if it is a ring or valve issue?

How was the motor broken in?

Lastly, today's gas at the pump is not my dad's gas, even the highest octane stuff. Our farm truck is a 1980 k15 with a smoggy 350, and there is a noticable difference in performance using regular unleaded over modern pump fuel.

 

[fast 99]

Cam design will effect compression ratio, 200 is to high on todays fuel. A friend just built a 406 Ford and had a heck of a time with high compression ratio. Took many phone calls to cam suppliers before he was able to find a grind that would relieve the ratio.

FYI, regular in the muscle car days was 94, premium 98, white pump 105 all leaded. Closest that can be reasonably purchased today is AV gas at 100.

 

[Sad Sack]

Cam design will effect compression ratio, 200 is to high on todays fuel. A friend just built a 406 Ford and had a heck of a time with high compression ratio. Took many phone calls to cam suppliers before he was able to find a grind that would relieve the ratio.

FYI, regular in the muscle car days was 94, premium 98, white pump 105 all leaded. Closest that can be reasonably purchased today is AV gas at 100.

A few Caseys around here also carry 93 not all do but some, otherwise a trip to the local airport with a couple 5 gallon gas tanks.

 

[xm20k]

I run 93 here from the BP. My 388 is a fully forged, aluminum headed, roller at 11.25:1 static but with my cam it's dynamic is 8:1 and makes 160 psi cranking pressure.

Cam can make dynamic compression too high for pump gas this could explain the dieseling/run on (Why I asked for specs) especially if it's making hot spots in a cast head, add a set of poorly setup dual carbs ie way too rich, can also cause the rings to wash out

Blow by could be a combo of ring seat/blowby this gets worse with higher compression as it forces more blowby elevating crank case pressure. Cam can cause so little vacuum to the point PCV can't keep up.

Possible parts combo and/or setup issue.

 

[fast 99]

A few Caseys around here also carry 93 not all do but some, otherwise a trip to the local airport with a couple 5 gallon gas tanks.

Another benefit of AV gas is it's much more stable. I rotate through my cars to give all of them equal use but that usually ends up being 3+ years of inactivity. Last fill up they get a good load of AV gas. Smells good to.

 

[MikeB]

I have installed dual 1404 setups on a Ford 302 and Chevy 350. I added electric choke kits to the rear carbs. Both engines run great. No issues at all except access to rear carb idle mixture screws and adjusting front carb throttle linkage!

Send me your e-mail address via a private conversation and I'll send you a Word document with an article by Jeff Smith on Edelbrock's jetting recommendations. There are also comments by another guy, whose name escapes me, and some comments from me too me based on the first such installation I did.

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[JBswth]

My Engine, a 4 bolt main 350 bored over .30 (so a 355). I’m running a summit 1705 cam with flat tappit hydraulic lifters, high flow oil pump, 2.02 camel hump heads, an Offenhauser dual quad “360” intake with two edelbrock “1404” 500 cfm carbs dressed to look like Carter AFB’s and an old Mallory dual point distributor.

Recently I did a compression check and all cylinders are running in the 200psi range and higher. Most are at 212psi.

Now here are my issues, because of the high compression I’m getting a lot of blow by both out of the valve cover breather and obviously through the PVC and into the intake manifold/combustion chambers. To fix this I'm installing an open circuit oil catch can, PVC to inlet and breather to out. I’m hoping this will stop the engine from puking oil at higher RPM’s as well as cut down on deposits in the combustion chambers that I believe are partly to be the cause of a run-on/deseiling issue I have at shut off. To help combat the run-on issue I also am considering running a cooler spark plug then I have been running (Autolite 86 rated at a (7) with one of these; Autolite 3924 (8), Autolite 85 (8) or Autolite 24 (9).

The last bit of this is to drastically lean out the carbs, I know the dual quad set up is more than I need and everyone is going to tell me to just run a single four but I am more concerned about the looks than the performance.

Has anyone been down this road before and have any advice on these processes, the catch can hook up, the seafoam, the different sparkplugs? Any help would be greatly appreciated. Thanks!

Leaning out the air-fuel mixture will make the pinging and running on worse, as well as destroy performance. With today's fuel, you can't run super-high compression. Modern engines get away with it because their ignition systems are computerized and instantly retard the timing at the first hint of pinging, which, of course, means that they do not put out their advertised horsepower and torque, unless you can get true high octane fuel.