Will I need to reject my carb?
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=363
Printed Date: May-16-2024 at 9:00am
Topic: Will I need to reject my carb?
Posted By: AWhite70
Subject: Will I need to reject my carb?
Date Posted: May-10-2004 at 3:11pm
I'm rebuilding the engine in my '79 SN and am just about finished. I'm curious as to whether I will need to rejet my carb? Here's what I've done to the engine:
Stock heads, ported w/ 1.94/1/6 valves
Edelbrock Performer Intake Manifold
Hotter Cam (Comp Cams 218/224 .494"/.513")
Slightly increased Compression Ratio (8.7:1)
GM HEI style distributor
Thanks in advace for your advice
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Replies:
Posted By: danman
Date Posted: May-10-2004 at 11:38pm
I assume your running a 4160 - 600cfm carb. I have the same basic set-up after my rebuild as you do except the cam, and mine does just fine. I don't see running any more than a 600 on that motor. by the way awhite, I have my distributor dialed in at 15 degrees btdc for base timeing and seems to run really strong there....
I'm running 9.17:1 compression ratio after rebuilding my motor...
what kind of pistons did you buy, and are you running the stock bore?
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Posted By: AWhite70
Date Posted: May-13-2004 at 2:24am
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My pistons are Speed Pro Hypereutectic's (H336P 30). They are a flat top with 4 valve reliefs. The block is bored .030" over. I never cc'd the heads but they should be the stock 69cc assuming the valve work didn't effect bowl volume much.
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Posted By: danman
Date Posted: May-15-2004 at 2:54pm
awhite - that is the same pistons I used -
did you have them pressed on backwards (dot on top of piston facing the rear of the engine instead of the front?) and also what did your machine shop set the piston to bore clearance at? It needs to be .0025"-.003"
If by chance he set the clearance to .0015" for a automotive application... You might have a problem.
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Posted By: AWhite70
Date Posted: May-16-2004 at 12:26pm
The pistons were not installed backwards. I read in several places that suggested not to do that, and since the marks on the old piston were facing forward the new ones are as well.
I'm not sure what the piston to bore clearance is set at, I'm guessing it's the .0015" that Speed Pro suggests. I'm almost positive the higher clearance specified in the PCM manual is due to the higher expansion rate of the factory cast pistons. Hypereutectics have a lower expansion rate so they can run tighter clearances. I don't think it has much to do with a difference between auto and marine.
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Posted By: danman
Date Posted: May-17-2004 at 10:33pm
awhite- CHECK with your machine shop!!!! I have a document from federal mogul on our h336 speed pro hyper pistons - for marine the piston to bore clearance needs to be .003"-.0045" I am trying to save you the headache I am going through right now. My machine shop set my piston to bore clearance to .0015" and I had a problem with the motor last weekend. It started knocking lightly. I pulled the motor last monday, and tore it down to find that cylinder #1 piston inside skirt started rubbing against the cylinder wall causing a big mess! I delivered my block to a different machine shop today to have it TORQUE PLATE HONED! I may have to go .040" over size if he can't get the scratches out. but we have .0015"-.0020" of material to work with so I think well be fine.
As for the piston theory....I put mine in with the dots facing forward also. I took a survey and there were only a couple places that installed the pistons backwards
awhite, there is a differance between auto and marine. The difference is the cylinder temps are alot higher in a marine application than an automotive.
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Posted By: AWhite70
Date Posted: May-18-2004 at 3:11am
Danman-now you've got my attention. If you could please email me the document from Federal Mogul stating what the piston to bore clearance should be for marine engines.
I'll be contacting my machine shop and Federal Mogul tomorrow.
Thanks again!
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Posted By: AWhite70
Date Posted: May-18-2004 at 10:53am
Danman,
I got to thinking about piston clearances more an more as I worried myself to sleep last night and I thought of a couple things I thought I'd share with you.
From your other posts I've gathered that your engine has higher compression ratio, your base timing is set at 15deg with an advance curve of 24deg @ 3000 (39deg total), and you haven't changed your jetting.
All of the above items can increase cylinder pressure, which causes increased piston loading and heat. They also can cause detonation, which isn't always audible. I'm not saying that bore clearance wasn't part of the problem, but maybe this helped it along. I don't have the original distributor specs in front of me but 39deg is quite a bit more advanced than what I remember reading, and your compression ratio is up close to 12%. Something to keep in mind when you put your engine back together.
I'm an engineer in the heavy duty diesel engine business so I know a little about high load factor engines (although I'm not a power cylinder expert). There definitely is a difference between the load cycle of an automotive engine and a marine engine. An automotive engine may reach max power during a hard acceleration, but once desired speed is reached the load on the engine drops way off. A marine engine on the other hand, due to the drag of the water, operates under a heavy load all of the time. I don't think the cylinder temperatures of the marine engine would be any hotter than auto for the same load, but a marine engine would operate under those highly loaded conditions much longer.
Hopefully my long windedness provides someone with some useful information. Sorry for the long post.
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Posted By: AWhite70
Date Posted: May-18-2004 at 6:16pm
Just talked to my machine shop and my piston to bore clearance was set to .0025"-.0030"
Also in doing some searching and reading on the net it seems that the increased bore clearance is not needed necessarily because the piston runs hotter, but that the cylinders can run cooler. Since you're sucking in lake water for cooling you could be running a hot piston and a cold block. The block doesn't expand as much as the piston and thus the piston scuffs the cylinder.
I would think if your thermostat was functioning properly you shouldn't get a slug of cold coolant in a hot engine.
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