My lt4 (complete lt4 transplant) with automatic and hotcam shakes quite a bit. Is this normal with the hotcam? I had the computer tuned via mail order and it helped but, I was wondering if it should be smooth like the factory lt1 was. You know that old saying that you can put a cup of coffee on the engine and it wont spill...well, I doubt it will spill, but it will show some big ripples at idle. Also, there is a significanly greater raw gas smell out of the exhaust...I attribute that to overlap, but again, my friends say...you know.

Anything over say 850 rpms is smooth, but while sitting idling say with the door open you can see the door vibrate or even the engine shakes noticeabley more than an idling stock lt1. Friends tell me it is missing, but I tell them I think it's the cam and just the tradeoff one has to accept with a cammed car. I am running stock exhaust and you can hear the cam lope, but since the exhaust is so quiet, they see it as a miss.. What do you guys think?

Any ideas to tell if the spark plugs are all firing at low rpms?

thanks
sam

 

By this do you mean that you have an LT1 with the GM LT4 Hot Cam, 218 / 228, .525 / .525 112 LSA Lift #'s w/1.6 Rockers ???

 

No it is not and never was an lt1..I bought the complete lt4 engine last year on this site. the engine itself is out of a ce or gs lt4, and I put the gm hotcam in it before dropping it in my a4 that had a spun bearing. Yes, if I remember those are the specs of the cam with the 1.6 rockers.

Also, I had the AIR and EGR and rear o2 sensors programmed out from the lt1 computer.

thanks.
sam

 

The LT4 Hot Cam will have a noticable lope at idle. But, it sounds like yours may need a little fine tuning.

 

Yes, i definitely notice the lope, but I guess my question is more like this: When the engine rolls over..barabada dabada dabada should it vibrate ...like it has a miss?

The computer though mail order was done by a reputable person who has done hundreds of lt1 hotcammed conversions and/or hotcammed lt4s.

If I take it to a shop like expert tune or one of the chains, can they hook it up and tell me if it is misfiring by listening with a probe to the exhaust?

 

Before you take it to a shop, make sure all of your spark plug wires are seated properly.

 

I would say if it is smooth above 850rpm you are probably ok motor wise.

You probably should go to a higher stall converter and set your idle up to 850.

I would quess most of the hotcams have gone into six speeds not automatics.....maybe if someone has put one in an auto can chime in, but my quess is up your idle & higher stall and you will be fine...

BTW I like the idea of you stock exhaust......talk about stealth.....

 

thanks for you help stl94. They are seated properly, and I see the usual glow around the wires, but no obvious misfire sparks at night.
Also, since it smooths out completely at around 900 rpms, would that indicate a miss to you?

And lt4bud..yes, I like it too. In fact, I wanted people to think it was stock, but the most people say either it's missing or it's modified. The engine really is smooth and accelerates effortlessly past the stock 6000 tach and keeps pulling until it hits the rev limiter if I don't shift (manually). I forgot what the shift points were raised to.

I guess it's not nearly as bad as i'm making it sound, but it is noticeably choppier than my friends stock lt1 vette. It is on the verge of creeping forward now with the stock converter at the present idle. Any higher and I would really have to press on the brake harder at lights, and that to me makes driving unpleasant. I'd rather it shake a little and me not have to work so hard to be comfortable at intersections.

How much more stall do you think I would need if I wanted to raise the idle rpm another 100 or so, so that I wouldn't have the creep problem if I do decide to raise the idle? Also can I do this without a new reprogram?

 

I'm not sure just how "bad" the idle is; it should be noticable but not objectionable. It certainly won't idle like a stock LT1 though since you've moved up at least two steps in duration from the stock cam.

I use, as a rule of thumb, 6 degrees intake duration as one step up/down in duration. So if the stock cam is, say, 204 @.050, one step longer in duration would put you at 210; a second step would be 216, etc.

You can check a few things first.

Attach your timing light to each plug wire in succession and monitor the flashes. The light should flash a regular intervals.

Check for a vacuum leak. You can attach a vacuum gauge and monitor the needle to see if it remains steady at idle.

Check your lifter preload. Too tight on even one will screw up the idle.

I know it's a PIA, but check the plugs for color and gap settings.

Hope this helps.

Jake

 

You can also add a little "closed TPS timing", to clean up your combustion. But, this would have to be done through the tune.

 

If you were expecting it to idle close to a stock LT1 or LT4 you were gravely missinformed. It will have a noticable lope or chop to the idle and yes that is the trade off for the performance gains. Most tuners will raise the idle a bit to help smoothe it out. I used PCM for Less and it seems ok. You could put it on a dyno and get the data and send for a better tune for your car. As for your friends...how old are they? Most young people never heard a cammed car as they mostly deal with modding imports or turbo charged cars...they will think it has a miss..cammed cars and the sound it produces are very old school muscle car era kind stuff they usually know nothing about. I even debated installing the cam for fear it would be too far from stock at idle, as I did the cammed up thing when I was a kid and really didn't want a choppy sounding idle...I almost did a Pro Charger instead...But I don't regret the cam...

 

Trying to creep....Definitely need a loser converter. If it were me Id try a 2800 or so, it wont shake as bad at idle (in gear) and will leave much better.

 

Take your Corvette to a a good tuner shop and have it put on the dyno and tuned to the specifics of your motor. The Hot Cam has a lope to it at idle and that is the trade off for the performance gain but the correct lope at idle should not shake the Corvette to the level you seem to be experiencing. The mail-order tuner was a good start now have it correctly finished using a tune shop that knows Corvettes and has a dyno - I doubt you will find a need to change converters once the Corvette is correctly tuned using a dyno.

 

I did my base tune with LT1-Edit.

You need to have the idle up around 800 or it will bounce around alot.

Also, you'll need the 2,800 stall converter for it to perform properly.

Good luck with your new toy

 

"They are seated properly, and I see the usual glow around the wires, but no obvious misfire sparks at night."

TorchRed,

Your plugs actually glow? That could be really bad, I think?

 

Carpenter..no, there is an "ora" at night around the plug wires. I have noticed this before with the old motor, and read about it here on the forum. You can't see it in the day, only in the dark of night. Once I even switched brand new wires in and it did the same thing. I think I made it sound like there was heat...but there is none...it runs cool with a 160 stat and standard waterpump.

Toohottvette. I wasn't expecting it to idle so WELL actually. I have heard cammed cars before, but I wasn't sure how much shake was normal for this cam. I was expecting low vacuum, hard brakes, stalling with the air on and thinking I would have to raise the idle way up to compensate and put in a much higher stall converter in. It actually didn't stall even with the lt1 tune but it runs significantly bettter now. I intentionally did not put the converter in because I wanted to see how much I "needed" first and then go from there. I was surprised that I actually don't "need" a much higher stall converter for my purposes unless I really want to bring the idle up a couple more hundred rpms...but at least now i "know" that am won't be guessing as much. Plus, it was expensive :-)

Jake, I never thought about the vacuum. I am going to try that, and new plugs too. As for the preload, they are the stock lt4 rockers that you torque down and that's that, I don't believe they adjust at all.

I thank you all. I don't believe I really have a serious problme, but I don't think it is optiminzed yet either. Actually these guys are an older crowd like me mid 40's but a bit more conservative than me, and they believe that since the car is "computer controlled" that it should be able to smooth everything out so that it runs like a stocker. I have to fight them constantly.

It's not perfect, but I like it...correction, LOVE it....and there is NO way I would put it back to stock.. To be honest, I am not sure when it stops pulling...it seems like the only thing stopping it is the rev limiter and fuel cutoff points that he programmed in because it pulls hard right up the point that it hits the rev limiter...all of those have been pushed up slightly from the stock lt4 limits and lt1 shift points.

 

Thanks for the reply Torch,

I never read about the 'glow'. (I'll make a point of checking that out with mine once it's running). Thanks for the heads up.

Re: the 'torquing down' of the rockers....again, I'm alot limited with that issue as my '95 came with Crane RR's that needed adjusting (aka my '60's & '70's small block experiences) I would have assumed that the GM stock works the same way now-a-days???

Oh, btw....before I built my 383 motor I had a GM hotcam and it seemed to need about 850 RPM @ idle to run with no engine shake. Lots of uneven idle sure....but still no engine shake.

Actually......when I rebuilt it and first started it up, the engine shook quite a bit. Then I adjusted the rockers after warm-up and the shake went away....just the uneven rumble of the cam was heard.

 

have you checked the firing order? Its easy to criss-cross 2 of the wires on the opti.

 

Should there be any sputter out of the tailpipes if you're running true duals?

 

thank you all for your replys. I have some things to check. I really think it is a normal condition with the idle just a hair too low. But there may be a idle speed miss.

I will do all of your checks suggested and get back and let you know how it turns out. Well, except for the dyno tune..can't afford that one right now.


sam