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mattyboy
07-28-2016, 09:25 AM
I just need to check my logic here
On a raw water cross over cooling system with no cold water dump into exhaust water on the risers the exhaust water at the end dump is warmer than the water in the block correct???

I have been troubleshooting an issue that the motor runs up to 200 or so when on plane then back down to 180 or so when not on plane I thought it might be air in the system coming from a bad seal in the drive but now I am thinking it might be a gauge issue

If I measure the water temp at the back of the boat it should be warmer than the block after having cooled the block and then the exhaust no?

So if I see temps at 200 at the back can I say the block is no higher than that?
The water is hot but not scalding hair to the touch

My lake is also about 80 degrees now

Ghost
07-28-2016, 10:59 AM
You're not talking about dry exhaust, right? Rather, you're talking about the water from the block going to the manifolds and risers, and then getting dumped into the exhaust. If so...

Short answer is that I still don't know. What you're describing makes sense about how it should only go up because it's effectively additive. But one other wild card is that evaporation absorbs heat. Once water, even hot water, is introduced into the exhaust gas, I'd expect there was some rapid evaporation, which would provide a temp driver in the other direction.

Not sure how they'd stack up against each other, though I'm thinking the water leaving the boat would be warmer than leaving the block, just mitigated by some evaporative cooling.

Pat McPherson
07-28-2016, 01:54 PM
Matt, you don't run a thermostat? If no, do you have some sort of a flow restrictor?
All the cooling water goes thru the block first and then to the exhaust?
I would think that if all the water goes thru the block and then to the exhaust, the exhaust would continue to heat up the water... so yes.

I ran a cross over setup on my 75 18C with 350 chevy. It did not have a thermostat but had a restrictor were the thermostat would be. The cooling water went into the block at the front and out thru the thermostat housing then into the exhaust manifolds. She ran up to about 200deg when I pushed her hard and would cool down to like 120 at 1000rpm...

mattyboy
07-28-2016, 05:43 PM
This is the way the old Holman Moody's were plumbed I don't run a t stat the old HM's did run a bit warmer than a Chevy with two pumps

It is a crank driven jabsco pump from there to the HM. Front mount where it has two water inlets into the block then thru the block and up out of the intake into the T style HM Tstat housing and into each manifold and the out the back of the boat
My 351 in the 16 would run 190 or so in warm water and stay there on plane then would spike to 200-210 for minute when I came off plane then drop back to 190

This 302 runs at 180 but if I stay on plane for a minute it starts to climb to 200-210 and continues to climb if I come off plane and fast idle for a minute it goes back down to 190 and stays there as long as I am at non planing speeds
I want to double check the temp gauge I just installed all new Farias and have a digital multi meter that will act as a thermometer
I think I am sucking air but not from the drive it may be coming from the flush valve

I just want to make sure what I see on the gauge is correct or in the ballpark

210 to me would seem to hot enough not let your hand stay in the exhaust flow which I can do. It does produce some steam after a run on plane

yeller
08-01-2016, 02:18 PM
My 16 was plumbed to the exhaust first, then the block. This was the recommended plumbing as per Glenwood. I didn't run a thermostat and the exhaust was cooled first in order to pre-heat the water for the block. Temp would never register on the gauge, even with the exhaust heating the water. I never boated in 80deg water though. Max was probably 65deg.

turbo2256
08-01-2016, 10:08 PM
Have you checked the impellers in the water pump? I run racing water pickups and crossovers My issue is low speed can run hot especially off plane and idle. On plane no issues. Have also had issues with sea weed restricting flow and have had to jump overboard to unclog the pickups.

chip w
08-02-2016, 09:17 AM
This is the way the old Holman Moody's were plumbed I don't run a t stat the old HM's did run a bit warmer than a Chevy with two pumps

It is a crank driven jabsco pump from there to the HM. Front mount where it has two water inlets into the block then thru the block and up out of the intake into the T style HM Tstat housing and into each manifold and the out the back of the boat
My 351 in the 16 would run 190 or so in warm water and stay there on plane then would spike to 200-210 for minute when I came off plane then drop back to 190

This 302 runs at 180 but if I stay on plane for a minute it starts to climb to 200-210 and continues to climb if I come off plane and fast idle for a minute it goes back down to 190 and stays there as long as I am at non planing speeds


When I had my '69 '16 with the HM 302 it ran exactly as Matty describes. I used a T-stat with the guts punched out to restrict the flow a little. I'm in Tampa so water temps ranged from around 60 to 90 depending on the time of year. Never had an issue once we set it up this way. I also ran the same Jabsco pump and put a new impeller in every year even if the old one still looked good. It was cheap insurance.