View Full Version : 671-dd Johnson Towers Cyl Specs


TomTDM
11-15-2010, 11:49 AM
Can Anyone guide me in selecting the proper cyl ratio 17:1, 18:1, 19:1 for Detroit 671-JT engines. I have been getting different answers. I have heard that if I use 19:1 ratio vs 17:1 that the engines will not smoke as much!

Bglad
11-15-2010, 07:07 PM
Can Anyone guide me in selecting the proper cyl ratio 17:1, 18:1, 19:1 for Detroit 671-JT engines. I have been getting different answers. I have heard that if I use 19:1 ratio vs 17:1 that the engines will not smoke as much!

The specs available to me show the 6-71TI as 17:1. I have heard that you can increase the compression ratio to make them smoke less but that requires rebuilding the engine. If you install block heaters they will still smoke some but not nearly as bad. Even engines in good shape will smoke on start-up. If it clears up after they are warmed up then they probably don't need an overhaul.

TomTDM
11-16-2010, 11:52 AM
Thanks for the info. I'm now rebuilding the engine and trying to get the best results. In your opinion, would installing the 19:1 ratio cyl create any negatives?

Bglad
11-16-2010, 12:13 PM
The local DD guy (30+ years experience) I use on surveys and recommend for overhauls says the higher compression pistons will shorten the engine life and you will have to mess around with the injector timing. His comment was to install the 17:1 kits and block heaters.

FAST FRED
11-17-2010, 06:58 AM
THe CR is matched to the turbo boost you will be cruising at.

The 6-71 with no turbo is rated at about 235 hp at 2100, reduce the power to 180 at 1800 , and the engines life is beyond belief.

Double the power with a big turbo boost and you are lucky to even see 1000 hours to rebuild., with a 450 -550 hp turboed version.

This works fine for the day charter fish killers , but unless someone else is paying for the fuel and engines , not common on most cruisers boats.

What are you attempting?

FF

TomTDM
11-17-2010, 07:31 AM
Hi Fred. Thanks for the reply. This is a 1983 42 Ocean 671-JT Turbo rated at 450hp I usually cruise at 2000 rpm. I have owned the boat for over 10 years and 8 years ago I rebuild one of the engines with a mechanic that is no where to be found today. At that time he told me that he was the only one in town that could rebuild the 671-JT with a no smoke results. HE WAS RIGHT! I ran that engine for over 6 years. He mentioned at that time that the trick was to use the right cyl ratio for the JT engines. The mechanic I'm using now says that the 18:1 CR is what I should use. Others are saying to use the 17:1 I'm confused!

FAST FRED
11-18-2010, 06:55 AM
You are on almost full throttle most of the time , but since you are happy with the results of the last cylinder set replacements , take them out and see what they are!

All the DD parts will be had in a few days of waiting , so the cylinder/piston kit is not required before the tear down .No thinking , just COPY.

CR with turbo boost is less a smoke problem than a longevity problem .

FF

View Full Version : 671-dd Johnson Towers Cyl Specs