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Old 02-08-2011, 01:35 PM   #1
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Default TBI with thumpr cam = no good

My buddies and i recently swapped the motor in his 88 blazer, we put another tbi motor in it, but thought we would put a cam in it, just so it wasnt still stock. ( changed valve springs as well). And im not sure if it is because the vaccum is low now with the cam, but it is getting way to much fuel. I guess im asking for suggestions if there is anything we can do to correct it, or just put a carberated intake on and carberate it. The cam we put in is one of comp cams thumpr cams, it has a 107* lsa. Thanks.

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Old 02-08-2011, 01:50 PM   #2
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What are the full specs on the cam?

You need to have a new chip burned for the PCM to lean it out at part throttle.
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Old 02-08-2011, 03:52 PM   #3
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Yeah, needs a chip, or convert to carb. If you convert to carb, make sure you get a fuel pressure regulator as the TBI runs at something like 12psi which is too high for a carb.
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Old 02-08-2011, 05:59 PM   #4
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yea, i knew it would need a regulator although i have seen carb swaps done without a regulator, but i would run one. i dont know the full specs off the top of my head, the cam card is at my buddys shop. i didnt know if there was anything we could do to fool the computer, because quite frankly we dont think it is worth getting a chip made for it, because afterall it is still a tbi. and those tbi throttle bodies only flow about 450 cfm. i think we are putting a edelbrock performer intake and 600 edelbrock carb on it. that should be plenty flow for those crappy tbi heads. it would save us 200 bucks if we could keep it tbi, but thank you for the input

edit: cam specs are
duration 287 305
lift 489 476
lsa 107

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Old 02-08-2011, 09:28 PM   #5
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You could go either way. The chip would be easier, but the carb would probably be better in the long run.
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Old 02-09-2011, 01:20 PM   #6
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TBI is a "speed density" system, meaning no mass air flow sensor, and meaning programing assumptions are made about the operating conditions such that if you change any of them (like exhaust flow, engine displacement, cam duration, etc.), you need to reprogram the assumptions in the computer.

I'll have to respectfully disagree with you, 95Z. A properly programmed TBI is superior to carb in all ways. I would not say carb is better in the long run - it may be argued that it is better in the short run (no particular skill required to get it to run with carb).

I have to say, though, that you made a poor choice for a cam. The Thumper series is all about the sound - that "old school muscle car sound", whatever that is. It doesn't have anything to do with performance. Even a skilled programmer might not be able to get TBI to run properly with that cam - but whatever compromise you'd have to make with TBI would only be worse if you put a carb on it.

There are so many good computer-friendly grinds out there, it doesn't make any sense to try to work around that cam. Ditch it would be my advice.
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Old 02-09-2011, 06:43 PM   #7
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all we were going for was sound, its just a stock motor in a rotted out 88 blazer. and its not that we dont have any skill, we dont wanna waist time and money getting a chip made for a stock motor since that is what we have to do, and there is no guarantee that it wont just dump the fuel at it like it is doing now, since the pressure regulator still wont have as much vacuum to keep the pressure down.

and i respectfully disagree about tbi being better. it only flows around 450 cfm, and even though tbi heads were the worst flowing heads gm ever produced, a 650 edelbrock will do fine. yes at times carbs can be a pain in the butt, but so can tbi. its personal preference i guess. i have had bad experiences with tbi units, and carbs as well.
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Old 02-09-2011, 10:57 PM   #8
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I see where you are coming from five7, the TBI will start better in any situation and be more fuel efficient, but I'm really not a fan of SD unless its tuned with a wideband and couldn't imagine having to tune one for a cam that large and on a 107 LSA. I would think the carb would be easier to get things running as good as it can with that cam and it wouldn't need a re-tune with every future mod (if they do so).

But I do agree with you about the cam, its just not going to run great in that platform. I pulled up the full specs-

http://www.compcams.com/Company/CC/c...?csid=108&sb=2

Gackle, what torque converter are you running? Too tight of a torque converter can put a lot of load on the engine and kill vac at idle also, if its a manual just ignore this.

Also what year is the new engine from? That cam is for a non roller block.

The engine needs a lot more timing, idle speed, and less fuel so no doubt even a cheap mail order tune would help a lot.
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Old 02-10-2011, 02:22 PM   #9
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The first step in TBI performance is to put a BBC TB on it.

Even 450 CFM will make more power than people give it credit.

But, regardless, the cam will make tuning a computer system very difficult. I'd change the cam rather than switch to carb.

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Old 02-10-2011, 02:40 PM   #10
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Quote:
Originally Posted by five7kid View Post
The first step in TBI performance is to put a BBC TB on it.

Even 450 CFM will make more power than people give it credit.

But, regardless, the cam will make tuning a computer system very difficult. I'd change the cam rather than switch to carb.

I agree with you five 7. I have a mild (330HP ish estimated) V8 S10 that originally came with TBI so i kept it. I have a TBI throttle body bored out to twin 52MM, Holley pro jection 2 barrell intake manifold, BBC chevy injectors, custom AFPR, Walbro 255 Pump, and a custom chip burned from Brian at TBI chips.com. It took time and some money to make it work, but I would never go back to a carb at this point. It was almost 10 below and that thing started right up without a hitch. Performance wise I might have a little left on the table on the high end, but drivability couldn't be better. Believe it or not I get 22 on the highway with it, AC on high. My cam is much more friendly though. I don't have the specs on hand but nothing like that thumper.

To the OP in your case if you have a carb and the right intake, and an HEI distributor then go ahead and put the carb on. IT will be your easiest way out. I don't know of any body burning chips that would wanna take a stab at that radical of a cam without a wide band and some datalogging software.
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Old 02-10-2011, 03:31 PM   #11
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Yes it is a very big cam for what we are doing, my buddy ordered it after having a few drinks but since we have it now we just put it in anyway.

The new motor is out of a 92 with 65k miles on it. We gave it 8* of base timing when we tried running with the tb. It did idle for a while at 450 rpm (on the timing light)then it all went down hill from there. adjusting idle screw, unplugging iac, trying to relearn iac all came with no luck of course. We put a new converter in that was made for a grand national. stall is supposed to be around 2500 but it seemed to be a little higher than that. And we did throw a set of long tube headers on it.

Tuning this thing is out the window, we have, hei dist. carb and intake for it.
And yes, cold starting is a lot nicer with throttle body, but with what we have and what we wanna spend in time and money it seems to be alot more effective to put a carb on it. Fuel efficiency isnt really a concern since it has a plow hanging off the front of it anyway.
Thanks for the info, i appreciate the info
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Old 02-10-2011, 03:38 PM   #12
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I agree 100% now that I know a little more of your situation. The carb will work fine and its a full size blazer so who cares about gas mileage. I had a 79 K5 with a 383 stroker and I was lucky to see 10 MPG ha ha. The good ole days is what i like to call it.
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Old 02-10-2011, 06:39 PM   #13
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we are just using an older style intake tho, all you have to do to make them work is take a die grinder to the center bolt holes so you can get the bolts in the heads. a lot cheaper than buying a brand new intake for it. should have it running on saturday

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Old 04-03-2011, 05:05 PM   #14
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Just an update on this, with the carb on it and 17* of timing with vacuum advance hooked up it runs pretty good. But it is still a stock 350, it has quite a lope at idle which is what we were looking for. No huge power gain though.
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Old 10-31-2011, 09:39 PM   #15
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It will all work due to the 2500 stall convertor, the saving grace. Hopefully the gear ratio isn;t to tall and if it is a 4 spd auto the lower first will help a bit too.
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