articfriends

iTrader: (1)

What kind of egt gauge? I had gaffrigs and they read 1550+ at wot all the time, I poured fuel to it and could only get them to 1500 and boat was pig rich, plugs were almost BLACK, not even brown, put it back where it was and ignored them, Smitty

MILD THUNDER

iTrader: (3)

Eddie, something I noticed when I recently tore my engines down. The front cylinders appeared to be running quite a bit leaner than the rears. I set my dominators up with squared jetting, and even powervalves. Made me scratch my head about that, and do some research. Apparently, from what I read, that was a common problem on the b&m/holley 250 blower, when running the single carb. I found out that they came out with a new style single carb adapter, that put the carb a bit further towards the front of the blower. You can still get these adapters, sold under the weiand name, but they are for the old B&M 250's.

Something else I noticed, and didn't like. When mounting the dominator on the carb plate, you can clearly see how the plate was designed for a 4150 style carb. While someone did some grinding on my adapter plate, its still shoddy looking. When you open the carb to wot, the outer sides of the barrels pretty much hit a wall, and its not a smother transition at all. What I thought about there, is using a 4150 to 4500 carb adapter. More of a funnel shape spacer. Whether or not its worth anything, I can't see that hurting? I do like the simplicity of a single dominator. Engines idled, cruised, and went balls out without a sputter or hiccup. Also would idle all day long in gear at 700rpm.

James

I'm using Autometer # 6545 Digital pyrometer, I have a gauge on each engine and swap from bank to bank with the
o2 sensor to be sure all cyl are firing. When I lean out the engine to 14.1 the EGT climbs to 1550 F . Does anyone else have experience with EGT in a SC configuration ?

HaxbySpeed

Why are you leaning it out to 14-1!?

I see temps similar to Smitty's. Unless you can crank some more timing into it along with the fuel, the fuel alone won't make a big difference and will even start to go the wrong way if too rich and not enough advance. Running that lean at any engine load with a blown marine engine is gonna make some heat for sure.. I find even the idle stabilizes better at around 13.5-13.8

James

I only ran for few minutes one engine @ 14.1 A/F, I was testing the PV & jetting to understand how many jet sizes made a difference. With 2 750 carbs & 8 jets & 2 PV I was thinking one jet size would make a big differnece, it did not. One jet size was giving me about .3-.5 AF change on the FAST digital o2 sensor. I have been careful to test my octane with an electonic test instrument. I have been running 91.5 - 94.5 Octane through the sample period. I understand that Octane has nothing to do with EGT or A/F but the details remain important. When the pistons melt off the rods I will not have to wonder why

Anybody know what the A/F map is for merc SC 700 ?

HaxbySpeed

Next time you've got some good gas in there try 34 degrees and 11.8-12.0 afr and see what it does. What's your static comp?

supermx96

Returning to blower question...

I want to know what you think guys about the 250 blower size or a 420 blower size on a 454 ??? Is the 420 are too big?

Griff

A 420 is not too big for a 454. It just depends on how much power you are looking to make.
A 250 is plenty adequate to make 650hp or so and a decent improvement over the 177.
It won't make nearly as much heat running the same amount of boost and has more potential to increase flow.

articfriends

iTrader: (1)

My egt's were always sky high BUT my afr's are in the high 11's at low boost and mid 11's at high boost, never melted it down yet. I never based my motor around cranking compression BUT at 8.4-1 with the cam I am running I see between 150-155 on a gauge. If I cranked it up and saw 190 I would say the base compression is too high or cam is too small unless you were running race gas or VERY low boost.
Eddie, your post is very confusing!

articfriends

iTrader: (1)

FWIW too, I hear it's not unusual for a blower motor to have high egts because so much raw fuel is blown out the exhaust that it creates a secondary burn or something, I would be more concerned about seeing safe afr's on both banks, Smitty