Question on loop charged systems. (Johnson / Evinrude)

[60Looper]

I recall back when I had my Johnson 1992 175 V60 looper that due to the nature of the loop charged induction the lower two cylinders would read a little bit lower compression than the middle or upper two. This came about because my numbers were 98's on the lower two, 101 or so on the mids and 105 on the upper two.

Again don't recall who or where I heard that info from but had someone looking to buy a 1996 150 ocean pro with the similar variances ask about it.

Also was not sure how to answer the question is compression numbers matching on the same height cylinder is more a more important factor than the number itself. So for instance the lower two cylinders being 98psi and the upper two being 105 is a more important / better reading factor than if the lower left was 98, lower right 105 and the upper left being 98 and the upper right being 105.

[60Looper]

For anyone reading here is what I found out from some pretty knowledgeable regulars at the Evinrude/Johnson facebook page.

The domed combustion chambers in the heads in the loop charged motors (understood for both the 60 and 90 degree setups) were at different depths from the lower, mid and high cylinders of each bank, therefore giving different volumes, therefore causing lower PSI readings in the cylinders with more volume. The purpose of this was for improved idling efficiency (maybe a fuel saving measure back when they came out). Exactly how this improved idling I still cant explain. I also had a few other people confirm that their 200/225 powerheads were similar with higher numbers at the top cylinders and lower numbers at the lowers.

Hope this makes for a good reference if anyone needs and please correct any of the info above if it looks wrong.

[H2OPERF]

It wasn't done for idling, Cylinders 5 and 6 typically run hotter so they made the head chambers slightly larger to lower the temps on those cylinders at high power levels.

[FMP]

Slightly lower compression helps smooth out idle. The likely chose those cyl because at speed dynamic pressure isn't equal in all cyl by port to port blowdown distances and cooling system characteristics. Doing this probably helped on all issues. Yamaha and FORCE also did it on certain models.

[H2OPERF]

The holes or slots above exhaust ports help with idle and starting.

[60Looper]

It wasn't done for idling, Cylinders 5 and 6 typically run hotter so they made the head chambers slightly larger to lower the temps on those cylinders at high power levels.

Thanks for the great info! Was the enlargement of the combustion chamber in the overall depth or diameter? I don't have an extra set of heads (nor a looper motor any longer, currently have a 88spl crossflow) to get a micrometer to to check overall diameter and depth myself.

And just keeping the technical talk for upping my knowledge on it... loopers, J/E, Merc, Yahmaha etc, does the enlargement of any combustion area's volume always equal a cooler burn? It seems that the lower burn not being as powerful just means less power. Also because the bottom two cylinders aren't pushing as hard doesn't that cause a harmonic imbalance? It sounds minimal but if your top cylinders are firing at 100psi and the lowers are 90psi that's a 10% variation of power from side to side on the crankshaft.

[FMP]

You're referencing cranking psi not running dynamic psi which is not the same. The goal is near equal dynamic at speed.

[60Looper]

Slightly lower compression helps smooth out idle. The likely chose those cyl because at speed dynamic pressure isn't equal in all cyl by port to port blowdown distances and cooling system characteristics. Doing this probably helped on all issues. Yamaha and FORCE also did it on certain models.

does that mean that the DP is not equal because of the distance the exhaust has to travel from the uppers cyls is greater than the lowers? I always understood that dynamic pressure was relative and equal when all points share a single chamber (one main point of exhaust vs say an exhaust pipe per cylinder like the old racing mercs).

I can also understand the cooling system not working as well for the lower cylinders because of the thermal transfer and cooling capability loss by the time the water reaches the bottom but would think the better solution to hot cylinders would be to address an additional cooling path rather than in a way down-tune but lowering PSI's.

I definitely don't understand it all have a ton to learn but just like being able to explain it when it comes up.

[60Looper]

You're referencing cranking psi not running dynamic psi which is not the same. The goal is near equal dynamic at speed.

Ok thank you; I'll admit I've never thought much about the difference between the two conditions.