Hi everyone. 71 LS5 +0.030, aluminum oval port heads, 10.5:1 CR, 0.567/0.578 lift, 224.6/230.6 duration @ .050 on 112LSA, original steel crank, headers, Torker 2 intake, Holley sniper, Factory BB hood. TKO 600, 3.70 gears. tanks inc. and pump (possibly a walbro 255 - not sure)

Issues: I finished this car sometimes in 2018. Did everything in my power to tune that sniper to best of my knowledge. Still surges, 3.5" display randomly navigates on it's own, heat soak issue after a hot start and many other random annoying issues. Also feels like it doesn't breath after 3K rpm due to 2" filter element. Not very impressive under 3K either. Made only 400 RWHP back in 2018. Engine makes about 18 inches of vacuum warm idle. Timing is not controlled by sniper.

I want better street manners and reliability. I believe motor should make more power than this. I understand my intake sucks, but besides this and pancake LS6 nothing else will clear the hood (and I want to keep BB hood). Thinking to get rid of sniper and put together a MPFI setup, a decent wide range (idle -5500) intake with a TB, cold air ducting and IAT ( like the average LS stuff - next to air filter). I'm also thinking to put an elbow with TB backward and get fresh air from cowl area. Anyone knows if this will work or not, or has any other suggestion on a tried and true MPFI intake setup that I can fit under stock BB hood and it flows well in lower usable RPM range. All offerings I see for MKIV BB are either a single plane 4150 style or a tall tunnel ram. I also considering converting a single/dual plane intake to MPFI, as far I can close the hood. Someone made some custom intake some 20 years ago based on Ramjet 502 lower plenum, but never mentioned how it performed.
Any idea greatly appreciated. While this is a matching number A/C BB, If I can't really make this engine to act better, I'm afraid I get tired and pull it out and drop a 427 or 454 LSX in it. feel bad to do such. Thanks.

 

your duration is short for performance, thats why you have great vacuum. What diameter are your headers and your exhaust system. if the headers arent 1.75 or larger and our exhaust is less than 2.5 inches, ou are chiking your engine and the exhaust is choking the motor. THe only differrence between MKIV motors and new LS motors is the heads on the LS breathe better from the factory than stock ones did on the MKIV. All you need is a longer duration cam to get the air fuel in and the exhaust out.....
This is the cam I run with 1.8 rockers so its a bit bigger than advertised https://www.summitracing.com/parts/c...make/chevrolet
Also what is your timing set to....going to 14-16 btdc and setting it to be all in before 3000 rpm with help alot

 

FWIW:
My local Corvette Shop will no longer put a Sniper on any Corvette. Way too many install, tuning, and running issues. SB or BB.
They go straight to Holley Terminator X style with a 4150 style throttle body.
A 4150 fits like a glove, just like it was designed for. Just EFI now.
ECU fits in a cooler spot, like side vent area, etc.
Many advantages.

You need at least a 3" tall filter for that 454 to breathe.
And a 2.0" drop AC Delco base is really the only one that fits with a BB, and a stock BB hood.
But that won't fit on a Sniper TB.
Also recommend the LS6 459 intake, that is the only one that fits a 4150, correctly, under the stock hood.
Use stock LS6 parts and it all fits under that hood, just like it did in '71. With maybe 1/2" of clearance.
Many have used the R-Port LS6 intake on a oval port head, and apparently the mismatch does not cause any major troubles.

Not a bad cam, milder, certainly not too big.
You should be making 450-480 crank HP with that combo, if running right. Depending on which heads you have.

One of the best advantages to EFI is computer controlled timing.
If you have EFI, and are not doing it, you should switch.

One guy here runs a 496 with a custom port injected tri power intake.
DDawson runs a 496 with carb and that intake.

 

Thanks for the response. Idle vacuum I believe it was 15 inches not 18, my mistake. Hooker headers (runners look pretty thick - definitely not smaller than 1 3/4") dual 3" exhaust . More duration will be translated to more overlap, therefore even worse drivability, unless LSA also widened. Currently, I'm more focused on EFI/induction improvement rather than camshaft itself. Never been a "lope" lover much. Prefer better drivability.
Motor likes it about 18 degrees initial and 36 D total. comes in 2750 RPM or so. no vacuum advanced.

 

Hi, Thank you for the response. Yes, that sniper has been a nightmare ever since I put it on. I just don't like the Idea of open air filter element under the hood and Integrated IAT into TBI because of heat soak issue I have. I want a functional MPFI with TB and ducting either to the front of radiator area or cowl area. Also want to get rid of distributor and run COP setup. So far, any possible scenario boils down to put an elbow forward or backward on some sort of off the shelf MPFI intake or converted dual/single plane intake - which I have no idea how it'll work - and need input on this area. Made 400 to rear wheels last time. I even consider a wider LSA if that helps. Hydraulic roller setup currently.

 

Agree with everything posted above. If you want to keep stock hood, you will be limited. A possible solution may be a multiport setup with a low profile (~1” tall) 4150 throttle body and then a drop base air cleaner. Though, the EFI rails may interfere with the drop base. Also, based on my issues with heat soak, I recommend a return less EFI setup. Works great for my 496.

BTW, I am guy running the custom Tripower EFI. Fits under the stock ‘75 hood but I am limited to a custom 2.5” tall filter (stock Tripower is 1-3/4” tall).

 

Very cool. So, you've converted a 3X2 intake to MPI then what TBs used? Didn't know they make 2 BBl dry throttle body.
As I mentioned in first post, I have problem with sucking air with an open air filter element under the hood, especially a thin one. I'm suffering from a 2" filter already. It looks cool/classic (and I have such set up as I'm writing this). The other issue is the IAT sensor integrated into TBI. On a hot start in summer, IAT will be close to 200F and will take a long time to come down to around 120F. I have this issue for the past 7 years. I have return less fuel delivery.

This is what I'm interested to know more about. Obviously with a lower profile elbow. I'd like to know how such setup will perform. With this I can make a custom fresh are box somewhere in front (or in the cowl area) , put IAT sensor there and bring fresh air to TB with ducting. A holley dual sync distributor will give crank/cam signal to be used with any EFI ECM. current setup:

 

There are lower profile elbows out there but I am not sure what the flow is. I would also look at the low profile 4150 throttle body with as much of a drop base as you can fit.

My intake is not a Tripower. It started life as an Edelbrock dual quad, welded up and remachined for three two barrel throttle bodies on a progressive linkage. It was done by Bruce at FB Throttle Bodies who started doing Mopar Six Pack EFI setups. It is a true custom setup using FAST XFI to control it.

 

Appreciated. Sent them an email. Will see what they can do. Personally, I'm not opposed to this setup, as far It has low end torque (idle-5500) and doesn't give me headache. Doesn't need to be 3X2 like yours.

 

I'm I missing something here? 400 HP at the wheels is like 480HP at the flywheel when you figure in 20% drive line loss. Engine installed in the car would be net HP. Rule of thumb is to add 20% for net to gross HP like the way engines are tested in a dyno cell. That would bring that same 400rwhp engine to 576HP gross HP. Doesn't sound to shabby for a 454 with a 224.6/230.6 duration @ .050 on 112LSA camshaft. I'm I wrong?

 

These were the first 3 pulls. later I added 2 more degrees to make it 36 total. Hot august day on 89 octane pump gas, air cleaner on the engine. Only hood was open. A/Fs started going all over the place but was able to hit 400 WHP. Wanted the number to be as realistic as possible. Damn. 576 HP gross motor will only put this much on the ground? Then this originally LS5 365 gross HP was making how much to tires back then. Regardless, I'm fine with whatever it makes, just want to be able to "use" it in a broader range of RPM. There was fueling issue at the top as AF numbers suggest.

 

Normal musclecar BB parts do not fit on a C3. This car has the lowest hood line that GM ever put a BB under.

My LS6 clone. Air filter almost touches valve covers.


Sideways visual of how flat a LS6 intake is. This is the ONE 4150 intake that fits well.


BBC intake heights.

The LS6 intake is the only 4150 version that fits under the hood. That is exactly why GM made it. For the very low 70 Vette and Camaro hoods. Even the LS6 Chevelle got it, even tho it didn't need it. (That car used a 3-4" spacer to reach the cowl hood.) The 569 intake is frequently reported as 1/2" taller, but it is not. I measured an original. The Torquer II has smaller runners, restricts flow, and is purported to fit. But it only does with a compromised air filter. It is too tall. The top 3 are all QJet intakes. Nothing below the top 4 fit.

The 569 leaves 1" to 1-1/2" over an factory open element air filter. That drops 2.0" low, and hits the throttle linkage on non factory carbs, or the air intake pipe hits on double pumpers. The lid on the correct air filter only sits 3/4" taller than the choke tower on a 4150, you need the correct factory style domed lid. There is a repro flat one does not clear the choke horn and restricts flow. You may have room for a 1/2" spacer under that carb. But that's pushing it. That is all the room there is, it isn't much.

I severely doubt that throttle body you showed will fit, or could be made to fit. But you could always have another elbow machined, that should not be too hard.


Here are two cars that DDawson and I measured.

I measured a 70 LS5 with a open element air cleaner and it had 1-1/2" clearance. The common twin snorkel is taller, it has less clearance. 1/2" less IIRC. So your total space from the china wall on the block, up to the top center of the hood above the air cleaner, varies from 9.4 to 9.9". As we know all these cars seem to vary a bit. In this case if the front clip is sat on the firewall just a hair different, or the frame, it affects your hood clearance.


DDawson 1969 Vert 502 with 569 intake


This is the huge 2.0" drop air cleaner base that is required to make everything fit. And a sniper will not fit under it.


This is the 4150 style Holley Terminator X that does fit under that air cleaner, and works on a SB or BB C3. Based on my local shop's experience, I would lean this way, and still use both a thin phenolic spacer and a aluminum reflective shield for heat control.

If you chose to go another way, I guess you have 6-1/4"-6-3/4" from the top of the 569 to the bottom of the hood, to fit both an elbow and / or a traditional MAF in there. I will never say anything can't be done, but it will be very tight.

Good luck!

 

This elbow says it is only 5.0" tall. So it may work with a 569. Even after you move it up an inch to go over the therm outlet. You'll be at 6" then.

 

Wow. great info you've provided. many thanks. Helped a lot to make a decision. So, you basically suggesting:
1 - 569 intake + 4150 style Holley Terminator X + special air cleaner base + 3" air filter (preferred)
2- 569 intake + elbow/TB
- I have (big) oval port heads. 569 I believe is rectangular port. Is that OK? how about what they say EFI doesn't like dual plane intake? Do you know any real example of this setup (569 intake + holley terminator) that works good on a BB?
- Elbow thing is just my idea as seen on mostly LSX stuff. the only reason I'm thinking about it is fresh air from outside and IAT/MAP separation from TB. Otherwise, I yet to see one example of (successful) such setup on a dual plane BB or how it flows in RPM ranges. Also, a dual sync holley distributor can be used as a cam/crank sensor to clean up spark plug wires and go COP. Holley HP (or even an early LS ECM) can manage it. Not sure how this scenario will work. Not willing to throw yet $$$ into it and not be happy.
- I see they sell repro 569s...will these do the trick?
- looks like brodix also have a low profile dual plane intake. Also rectangular port. Any experience with that?
- What's the PN on that dropped base air cleaner you mentioned. Thanks.

 

All the Corvette Vendors carry them. I like ZIp Corvette:
https://www.zip-corvette.com/68-71-a...placement.html
AC-211
Just see who has it in stock.
Corvette Central,
Top Flight / Corvette America, etc.