Booty_Con

Hi all, hope everyone's doing well. I've always gotten lots of good advice and feedback here at thirdgen.org.

I have a 1991 Camaro RS with 5.0 TBI and 700R4, and I am finally getting ready to start a new engine build. I know a few things but I am not a mechanic by trade and don't have any hands-on experience working on engines. (I can change oil, battery, radio, other minor stuff, etc.--you get the picture.)

Over the past 5 years, I've had the following mods done to my car:

- exhaust headers; dual cats; 3-inch Flowmaster exhaust
- Ford 9-inch rear with 1:2.75 rear gears; TruTrac worm gear differential
(I'd prefer to keep my 2.75 rear gears, I like getting decent MPG)
- open element TBI mod
- front & rear suspension rebuilds (including wonderbar)
- LS1 disc brakes x 4
- aftermarket aluminum radiator


After long and careful consideration of budget, what parts are likely to work well with each other, ease of installation and ECU tuning, etc., as well as what my own needs are (mainly weekend cruising, no drag racing) here is what I am considering for a new engine build:

- L31 longblock with addition of roller rocker arms to heads
- TBI with 50mm bores, vortec TBI intake manifold 12496821 (also bored
50mm), and 80 lb/hr flow-matched injectors, new fuel pump, AFPR, etc.
- RamJet camshaft (14097395) vs. Comp 260HR (want a fairly smooth idle,
more ephasis on low-mid torque than high RPM horsepower)--or I could
just stick with the L31 camshaft
- Rbob's EBL computer


I already have the 50mm throttle body (from xtremefi), and the injectors (from dctrumpet).

I also already have a 50mm factory TBI intake from xtremefi, but it is of course not the proper bolt pattern for the iron vortec heads that come with the L31. Alternatively I could get an L31 shortblock and aftermarket cylinder heads that would be compatible with the intake manifold I already have, but it's hard to beat the price of the iron vortecs.

I've also considered the possibility of getting a junkyard 350 sbc, but I like the fact that the L31 (and it's iron vortec heads) would be brand spanking new and come with a strong warranty.

Does anyone have any thoughts/advice? Can anyone spot any potential problems with this build? Will the camshafts I've mentioned work okay with TBI?

Thanks in advance.

Atilla the Fun

I suggest you do the ram jet 350 cam, these valves: http://www.competitionproducts.com/S...ductinfo/8430/ with 30-degree back cuts added by your local machine shop, polosh the exhaust ports, do NOT do ANY grinding in the Vortec intake ports, and spend the $ for the GM 12496821 intake manifold. It is an awful steep price, but you want to cruise at 1600 rpm without surging, right?

Booty_Con

Yeah, it will be for mainly just cruising down the highway and a few back country roads, taking the wife out on dates, etc. What do you mean by surging--would that be the engine speed intermittently going faster and then slower rather than remaining steady? How does the manifold affect that? Thanks.

Atilla the Fun

too low of a cruising rpm combined with too big of an intake port ( not using Vortec heads), in an EFI application, surges. This feels like you're going from 1/4 throttle to 3/4 throttle and back, kind of rapidly. Your rpm doesn't surge, it's just a feeling. It's not far from lugging. The computer is having irreconcileable differences between the VSS signal, the throttle position signal, and the rpm signal. So it adjusts the timing, and then tries trimming the fuel, then tries the timing again, it's just not good. If you save money by not getting the TbI Vortec manifold and the vortec heads, then to get the power gain, you end up with 180cc heads. doesn't seem like much of a difference from the 170cc Vortecs, but it's not just the extra 10 cc, it's also the lower flow velocity through the 2.02" valves.
If you go with Dart's S/S heads, which are like cheap Vortecs that don't need Vortec intakes, then you don't gain the power you're looking for. If you use a non-Vortec intake with Vortec heads, again, you don't get the power you were looking for.
The only 2 resolutions are to spend more for the Vortec TBI intake, or change axle ratio, which you ruled out.
Think of the $370 intake as a one-time, up front, lump sum investment, which is dumber than changing axle ratios and financing the extra cost of slightly less mpg.
So, If you're willing to change axle gearing, you lose 1-2 mpg, but gain better acceleration, and can keep the intake you have, but use 180cc heads and find more power.
Sorry if you have to read it several times to grasp it. I'm not known for being good at explaining, (because I suck at it)I'm known for getting the results.

Booty_Con

I should mention another reason why I want to keep my rear gears as they are is that I've had that rear end worked on about four times in all before they finally got it right, i.e. quiet with no grinding, etc. So I am hesitant to have it opened up and worked on again only to have it making noise and suffering excessive wear and tear because it was set up incorrectly again.

Booty_Con

Also, I downloaded Comp's CamQuest 6 software and plugged in some conservative numbers:

1.94 intake valve, 1.50 exhaust valve, 500cfm flow rate, GM TBI intake, small-tube headers, mufflers with cat, 9:1 compression, and cylinder head flow rate for L31 iron vortec heads which I got from this page:

http://www.purplesagetradingpost.com...20Vortec%20855

The software is telling me that with the Comp 262HR-12 my peak torque would be 467 ftlbs at 2000 rpm--that's a boatload of torque. Is this software pretty accurate? (Also 318 hp at 4500 rpm.)

Another limit with the L31 iron vortec heads is the max valve life of 0.475, which I guess could be fixed with beefier valve springs...another upgrade I would have to invest in if I go with a comp cam. The ramjet cam is indeed looking like the way to go, with its lift numbers of 0.431 intake / 0.451 exhaust. It seems to make a nice broad, fairly flat torque curve with the Ramjet motor; wish there was a way to plug the ramjet cam's numbers into the CamQuest software.

Atilla the Fun

COMP's XR252HR10 with 1.5:1 is awfully close to what you get with the gMPP Ram Jet 350 cam with 1.6:1 rockers. Even so, I'd expect a real dyno to show about 410 ft-lbs at 3000 rpm, and about 340 HP at about 5000 rpm. If you polish the exhaust ports, and use good valves with back cuts, 360 hp and 415 ft-lbs could happen. Assuming 1.625" long-tube headers. and 9.5:1 static.

Rufus_911

....

Rufus_911

Hey Atilla, what do you think of these heads?

http://www.summitracing.com/parts/TFS-30310006/

The intake port is only 5cc bigger than the vortec's, the valves are 1.94/1.50, and their bolt pattern would fit the factory TBI (non-vortec) manifold. Their combustion chambers are 56cc versus the iron vortec's 64cc.

RazorN8

Couple quick questions. Why would you pay for an L31 longblock just to strip the heads and put aftermarket heads on? Seems to me the bottom end would be the same as any other gen I 350cid motor. Why not invest in a shortblock and then put the heads you want on? I swapped the L31 heads into my 350 this winter and used a comp xe262h cam (i have a carb so this may not be computer compatible) and it idles pretty smooth. Just a thought.

305sbc

iTrader: (3)

I like the 2.75:1 gears.
I think your idea of using an L31 longblock is a good one and I'd probably leave the L31 cam in there as well. You just need an intake manifold to make your TBI work on it.

I don't think this has been suggested yet, but you might find a Vortec style carbureted dual-plane intake for a bit less money, used perhaps, and use an adapter plate to mount your TBI on it.

The EBL will have all the tuning power you need to avoid surging or any other driveability problems that may be present after the swap.

Rufus_911

Earlier in the thread the possibility was mentioned of getting an L31 shortblock and placing aftermarket heads on it. I was simply suggesting the Trick Flow heads.

Their combustion chamber is 56cc. Using a dish volume of 13cc (0.793ci) for an L31 piston, cylinder swept volume of 43.73ci, chamber volume of 3.417ci, piston deck of 0.314ci, and gasket volume of 0.478ci, I get a compression ratio of roughly 9.7:1 -- not too high, not too low. Those heads might work nicely. And one could use the factory TBI manifold without having to spend dough on a vortec intake.