355 blower engine, two 850's or 750's

Hey guys, this is deno. I have a 1968 Camaro, and I just found out that I have two 850 Demon carb's on it. The motor was built by Big Al's toy box. This is a stage two motor with a M .L. 671 blower on a 355 ci. chevy. I called Big Al's with questions, and I get different answers every time I call. I think the carb's ruined the oil rings. The blower puts out 10 lbs of boost as I was told at 4,500 rpm. I don't know about the history of the engine, but when I bought the car from Rons classic cars the engine smelled like gas. I changed the oil,adjusted the carbs,timing,changed the plugs, and adjusted the rockers. I will have to take the pistons out to see what damage it did. Do you think that the engine is over carbed ?

What exactly is the problem? Fuel in the oil? smokes blue oil smoke?

It's not overcarbed. It's pretty much impossible to overcarb a blower.

There are optimum carb sizes and you are probably a bit over optimum.

If you washed down the rings, you are probably overtuned or have a problem keeping the fuel in the bowls.

If you are wondering if you need to buy new carbs because of size the answer is no. You can tune around less than optimum carb size on a blower.

A little more info would help. You didn't mention head or cam specs. Two 850's on a 355 is a little much, but that depends on what the engine was built for. If it is a high RPM engine that may be fine. I run two 750's on an 8-71, the engine is 388cubic inches (12lbs of boost). I think two 750's would be a better fit for what you are doing. I would try to tune the Demons before I went out and bought two new carbs though. As Infomaniac stated you can tune around the over carb'd situation. As far as washing down the rings, are you sure they're bad or are you just getting gas in your oil? With a blower and two big carbs it's easy to contaminate your oil.

Royce

As far as sizing carbs go. I have learned a lot over the years. And I will have to admit. Buying several thousands of dollars of top of the line carbs from Pro Systems with outstanding tech support is one way to do it.

If the carbs are too big. They actually meter too little fuel with stock jetting. Carbs meter fuel according to the vacuum signal given to them. If the carbs are too big the signal is insufficient for the carb design and they do not meter the proper amount.

Soo.... carbs too big for the engine or power being produced do not cause a rich condition as would probably be assumed. Too big a carb has to be jetted UP and to little carb has to be jetted DOWN.

I doubt you carbs have been jetted way up but could happen I guess.

Hi guys, this is deno. When the engine warms up I get blue smoke when I rev it up. I did a leak-down test and I could hear some air coming out of the dip stick. A loss of 8 to 10 lbs. of air. I have the build sheet, but no specs on the cam, it said customer supplied. The heads are 202's, Speed pro forged L2441F-30. The rings are Sealed power E 251K-30 ring end gap top .018 2nd .018 oil is .022 The plugs from front look OK, but as I go towards the rear the plugs are black and have some oil on them. I think that I will pull the pistons out and send the carb's out to get them checked out also. Thanks Guys. deno---

Info, I was saying any dual carb'd blown set-up can easily contaminate the oil. Not that becasue his 850's are to big that was causing a problem. With a blower over carb'ing is different that when naturally aspirated.

In either case I assume pulling the pistons and re-ringinr it wouldn't hurt. Sending the carb out to be checked isn't a bad idea either if you are not familiar with them.

I highly recommend All State Carbs in NY.

My post on carb size was just thrown out there as a general statement camaroman. Not directed at anyone or I would have quoted ya.

8 -10 lbs of leakdown is not a disaster. You were using 100 lbs of air?

If you are tearing it down, I would suggest taking the heads off before pulling the engine. The guides may not have any seals on them causing the oil smoke. Or guides worn out. And sounds like a bowl draining out as fuel is running to the back of the engine.

If the guides were bad wouldn't I be getting smoke out of the breathers. I have no smoke coming from the breathers at all. Thanks deno---

Not necessairily man. Smoke out the breathers could be caused by a number of things and no smoke from them does not indicate something is not wrong.

I'm just trying to save you some work. if you intend to tear it down anyway why not try? You could even take the valve springs off of one cyl and take a peek. You have a leakdown tester so you have the means of keeping the valves in place.

A lot of builders will leave valve stem seals off on a blown engine to help keep valves lubed and cool. On a daily driver I will put a seal on the intake but take the spring or whatever clamping device off of the exhaust seal. Big HP or race deals I leave the exhaust seal off.

I did have a guy that was in a big *** hurry and the wrong size seals were sent. He could not wait. We put it together without any and it ran fine. Would smoke a little blue at idle.

Thanks Infomaniac, I'll try that next week. The engine doesn't smoke at idle, but when I run up the rpm's it smokes. What grade oil should I use, 20/50 wt. Thanks deno---

Oil grade would depend on your clearances. Since the engine was already built you have no idea what the clearances are. The best way to determing the correct oil would be to watch your oil pressure gauge. I typically run 10-30, I found that 20-50 would have my oil pressure way to high for my tastes. Of course your climate and driving conditions will come into play as well.

Hey Camaroman7d, Thanks. I have some info. on the engine. Main bearing #5 is .0025. #4 is .0027 #3 is .0026. #2 is .0023 # 1 is .106 ? I don't know if there is one more number after the 6. The rod bearing clearance is .0022 Crank end play is .004. The ring end gap top is .018 2nd is .018 and the oil ring's are .022 Piston to wall clearance is .0045 Deck height is .0045 . Spring pressure 135 LBS@ 1.900.350 Lbs open. Does this help you to let me know what oil to use. Thank you deno----