May Need Rebuild

[PokerJoker]

Hey guys,

Well after finally getting the Check Engine Light off and the computer working properly, it looks like I might need a rebuild. Started driving a little more than I have been and noticed a pretty significant knock. I thought it might be timing related but my timing is right on. It's not as noticable when cold, but as it warms up it gets worse. It's real loud when its in gear, when I put it in park it quiets down a bit...also, oil pressure starts off around 40 cold, then as she warms up drops to around 20-25ish, but then if sitting at red light or coasting into turn it drops to 0, then as soon as I give her the gas, it jumps back up to 20ish.

So, changed the oil and noticed a lot of metal flakes in the filter. Took her to a shop (different shop than my previous post) and they listened to it, looked at the metal in the filter and concluded it's probably time for a rebuild.

Question is, if I do this would you rebuild the original engine and keep it stock? Or, drop in a GM performance engine (nothing too radical) to get more horsepower...and if I do that, can I keep the original intake, distributor, carb and computer? I feel like I'd like to keep those parts, they seem to be working fine.

Any advice?

 

[Wajulia]

wow -- like loosing an old friend.

If you need to replace it -- the 1981 corvette will never be a top dollar car (Lets hope I eat those words someday!) So do whatever makes it more fun -- a reasonable price crate motor will sure add a lot more power.

 

[Tommy Almond]

Last summer my fuel pump cam lobe went "toast" and replaced engine. Brought a GM crate motor 350/195HP and dropped in. Engine was $1600 with 3 year, 100,000 miles warranty. Not sure if GM still offers the 195HP but they have the 290HP for about $400 more but extra horses do not kick in until 4300 rpm I was told.

 

[PokerJoker]

Tommy,

Were you happy with your decision to replace rather than rebuild? Did you keep the original carb, dist, computer?

 

[Tommy Almond]

For me no brainer to buy new crate motor since the vette had a Jasper motor in it when I brought it so was not original anyway. But yes we swapped over carb, dist, intake and even valve covers etc and put her as close to factory as we could and have all emission and CCC/ECM parts on and working.

Only wierd thing is still new crate motor cames with flat tap not rollers so now using AmsOil to protect it better.

 

[Peer81]

First think what you want, cruiser, stoplight shark etc etc.
Second look into how much you want to spend.
Then look into what kind of 350ci engine you want.
What I would do is take a 350ci with 10:1 static compression, much torque no need of full top hp I think. It's always possible to reinstall the carb and other things. Only thing you need to think about is when you want a very modded engine, the carb needs to be altered to

Greetings Peter

 

[PokerJoker]

Almost there...

Okay, I've pretty much decided to go with a GM Performance Crate engine. There are three I'm looking at...the basic 195hp, the 260hp and the 290hp. I'm leaning toward the 290hp engine. Question: Summit says, everything will work fine with it...carb, dist., intake and ECM. Jegs says, may have trouble with computer because of the cam in this engine...he says it can be done, but he suggests going down to the 260hp engine with a milder cam.

Are any of you running a modified cam with the original ECM...any problems? Can the carb be tuned to fix any idle issues? Maybe a different prom chip in the computer? I finally got the ECM working correctly with good codes and no check engine light, I don't want to screw that up now!

Here's the cam specs on the 350/290hp engine: Lift: .450"intake, .460"exhaust - Duration: 222* intake, 222* exhaust @ .050" tappet lift - 114* ATDC intake, 114* BTDC exhaust

Here's all I know about the cam specs on the 350/260hp engine: .383"/.401" and 112* lobe separation

Thanks for your help with this decision.

Jeff

 

[jj_long81]

hello. i would rebuild mine like origional. once you change it it will be much more difficult to change it back. if the origional parts are gone, they are gone. that makes a 1981 origional l81 more rare every time somebody dumps the oem parts. but on a second note if i had to i would keep everything and go big block with a five or six speed trans. but why stop there...

 

[Peer81]

One thing you need to keep in mind, the carb is controlled largely on two sensors. The first is the o2 sensor for the lean/rich control. Second is the barometer (intake vacuum meter) when the vacuum drops the ECM knows your on the gaspedal so will alter the carb settings. The ECM has somekind of base setting when idling (minimum vacuum). When you put in a very modded engine with a big cam the idle vacuum will be lower because of the intake overlap. In return the ECM will think your already on the gaspedal so will put in some extra fuel to correct that. So with a big cam the engine will run rich. I think Cris (sensei) has build something to bypass this problem (input a fake vacuumsignal) but you better contact him on that one. I know some people here are running XE268H cams without a problem (I think?). I think I would draw the line somewhere around or above the XE262H.
When you do a forum search on L81 engine vacuum etc etc I think you'll find the minimum idle vacuum somewhere so you can look into the cam charts to see how big there idle vacuumsignal is.

Greetings Peter

 

[LannyL81]

Probably to late to add to this thread, but:
As stated above when using a higher lift cam / less vacuum the ECM will change the mixture using the MAP sensor input. It is a simple circuit to add that modifies the MAP sensor output so that the ECM will keep the idle mixture leaner.

The circuit can be built for about $5 using Radio Shack parts and I found an easy spot to locate it.

Original poster, if you are interested, reply to thread and I can give more details.

LannyL81

 

[PokerJoker]

Yes! I am very interested. Please post any more information about building and locating. I've ordered my new engine, GM Performance 350/290 hp, planning on using my original intake, carb and dist. Computer seems to be working fine right now and I would rather keep it as long as its still communicating with me via codes. So if this modification will help with that, I'm all for it.

Thanks for your help.

Jeff

 

[PokerJoker]

Update

Well, picked up the car from the shop on Monday. It runs great!

Engine swap went well, with no major issues. Had to use my original oil pan on new motor, the pan on the crate engine wouldn't fit into place. But other than that things went pretty well.

No, check engine light, so I'm assuming the new cam wasn't too radical to throw any codes. Temp is running perfect, just under 200*. And going from 190hp to 290hp is pretty cool. She responds really good when I give her the gas...havn't felt that in a long time...um, never actually.

Now, a paint job and a few odds and ends and I'm good to go.

Thanks for everyone's help and input.

Jeff

 

[Wajulia]

Great! Glad it is running again. It is great you left the ccc in it with that much hp. Shows the CCC haters that the CCC setup can handle a lot of performance upgrades.

If you are ever in md, I would love to feel what 290 hp feels like. Good luck in the future