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Old 02-17-2020, 02:48 AM
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Default SNIPER EFI Quadrajet on a 455 Pontiac ?

I'm considering installing the sniper spreadbore EFI on my iron manifold on a 469 pontiac. The engine has a 236/242 @50 roller cam and makes 500hp/577tq with its holley 800 spreadbore. On the engine dyno, the hat said the engine was only using about 650 cfm of air. The Sniper spreadbore is rated at 715 crm. While the EFI is rated for 500hp, do yall think 715 cfm flows enough to support similar hp figures the 800 holley spreadbore made?

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Old 02-17-2020, 12:35 PM
darbikrash darbikrash is offline
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When you say "hat" I assume this measures the airflow going into the carb. This is really not enough information.

Not sure if you can look back at the dyno sheet, but if so I'd suggest looking at manifold pressure (MAP) at WOT. You'd like to see less than 1" Hg MAP at WOT, this tells you if (and how much) the carb/throttle body is acting as a restriction in the air flow as this is measured below the throttle plates.

If the manifold vacuum at WOT throttle is less than 1"Hg, and the "hat" says 650 CFM, I'd have more confidence that this number means something.

I don't know the rest of the details on your engine, but I would expect an engine with that cam and that displacement to be upwards of 540+ HP. As the Qjet Sniper has four 100 lb/hr injectors, it would seem to have plenty of fuel capacity if you can confirm the air flow numbers from your dyno run.

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Old 02-17-2020, 01:07 PM
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I just recently built a 455 that made 507/571 HP and TQ with the stock Q-jet on the stock intake manifold, with stock exhaust manifolds in place, using a similar 239/243 @ .050 hydraulic roller cam.

I'd say the sniper is more than capable and if you want to go the fuel injection route then I certainly wouldn't hesitate trying the spread bore sniper on an engine like this. Long as you have the fuel system to support it.

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Old 02-17-2020, 02:59 PM
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What RPM was the 650 seen at? And was it at peak torque or peak horse power?

469 CID @ 100% VE and 5500 RPM's is pumping 746 CFM of air through it.

IMO How far the engine keeps making good power past peak HP is what I'd be looking at. Then CFM accordingly.

With 500hp/577tq it doen't appear to have been over carbureted.

Clay

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Old 02-17-2020, 04:15 PM
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i was told at peak hp it was only drawing 650cfm thru the hat. I'll pull the dyno records and see what is said. I had a discussion with the engine builder/dyno operator re carb size. His opinion was the ideal carb size was 750 for this engine, but the 800 was close enough. He said everyone knows you want 25% more CFM than you use for the pressure drop. (I never heard of that, but he knew 100x more than I running a dyno for years)

The EFI's aren't supposed to hurt hp if oversized since the injectors send the fuel for what the motor sucks in, not what the fi is rated for. My concern is would say a 900cfm EFI have a greater pressure drop thus better acceleration that a 715cfm due to its increased size. This is similar to the double pumper mechanical sec carbs being faster than a vacuum carb due to the pressure from created when the throttle is snapped open. I'm unsure, since the EFI sprays fuel into the manifold verses being drawn from the boosters, if this advantage still applies. I need a Engine Masters video: 715 CFM EFI verses a 1000 CFM on the same 450- to 500 hp engine. Which accelerates harder? The same?

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Old 02-17-2020, 04:27 PM
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Too big of a throttle body can create driveability issues at fast opening rates. If you have a lot of tunability with acceleration enrichment, you can overcome it. But being close to correctly sized has benefits. If the sniper has four 100lb injectors, looks like they'd have sized the throttle body for around 550ish horsepower at ~80% duty cycle. Maybe give them a call to discuss your particular situation.

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Old 02-17-2020, 11:15 PM
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At 5600 rpm the 469 was using only 610 cfm with an af ratio of 13.7 to 1. Using 203 lbs of fuel an hour. Peak tq was at 3900 using 461cfm. Hp peak 5500. Seems like 715 cfm would be enough to feed it? All this on the factory intake with a 800 Holley spreadbore.

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Old 02-17-2020, 11:50 PM
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The airflow can also be messaged a bit if you need more. Just like carb mods, you can knife edge the throttle blades, trim the butterfly mounting screws etc.

I can’t comment on the sniper spreadbore but on the FiTech throttle body simply trimming the mounting screws has repeatedly shown a 20cfm increase. Radius of the bottom of the throttle bores will give you a couple cfm as well.

Point is, if you’re wanting to go the throttle body injection route, but you’re in the edge with airflow, you can get a bit more out of it without a ton of work.

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Old 02-18-2020, 01:03 AM
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I would probably go a different route with it. Accoring to Holley, the Q-jet version of the sniper is good for 500hp which would be maxed out on your motor. However, the standard sniper 4150 is good for 650hp and the flange is dual drilled to work on spreadbore manifolds. I would probably use the 4150 then just port whatever spreadbore manifold I am using to fit the larger sniper 4150 primary throttle plates. The only only dpwnside I can see is with factory air cleaner clearance but that can be addressed as well. On the upside, you have plenty of capacity to deal with any changes you may make in the future.

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Old 02-18-2020, 08:42 PM
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Spoke to holley today, they didnt recommend the spreadbore sniper. Said if my engine made 500hp that sniper wasn't large enough. Possible hp overrated. Spoke to Butler, they say either a sniper 900 or the 1200 cfm unit. While I dont think they are wrong, it is puzzling an engine that uses 600cfm should run best with 900 cfm. Butler thought the torker 2 would be the manifold to use with the efi. The rpm too tall, the factory iron too hard to port to match the 4150 carb openings, and the performer too restrictive for the 469.

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Old 02-19-2020, 12:10 PM
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My read is judging a carb flow capacity vs. HP capacity is different for FI. I believe what Holley is saying is that the Sniper is more limited by it's fuel injector capactiy than by airflow. The Q jet version may be further limited by both so it is rated 100hp less than the regular sniper. Between the increased airflow capacity of the sniper and Torker II my bet is that you are probably going to see a HP increase which is why Butler is recommending the Sniper 4150.

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Old 02-19-2020, 01:53 PM
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I wonder how bad the Torker 2 will impact torque below 3000rpm?

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Old 02-23-2020, 11:14 PM
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Quote:
Originally Posted by Stripes View Post
I wonder how bad the Torker 2 will impact torque below 3000rpm?
I would hope not as much as it would with a carb as FI does not rely on the pressure differential to draw fuel out of the main jets.

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Old 02-24-2020, 09:11 AM
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Quote:
Originally Posted by Formula8 View Post
I would hope not as much as it would with a carb as FI does not rely on the pressure differential to draw fuel out of the main jets.
Bigger plenum and runners moves cylinder stuffing to a higher RPM all the same.

Clay

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Old 02-24-2020, 11:09 AM
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Quote:
Originally Posted by Formula8 View Post
My read is judging a carb flow capacity vs. HP capacity is different for FI. I believe what Holley is saying is that the Sniper is more limited by it's fuel injector capactiy than by airflow. .
That's pretty much the answers I got back from Holley when investigating the 4150 sized Snipers. They only rate those to 650HP which isn't quite enough for my application. They went on to tell me it wasn't the body of the unit that limited the HP, it's the small injectors they supply.

And just an FYI, they do offer injector upgrades, but they have to do the swap there for you and there is of course an additional fee for it. I just told them I'll wait until they start offering the Sniper with more injector to fill the HP void they currently have in their offerings.

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Old 02-24-2020, 02:10 PM
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Quote:
Originally Posted by Stripes View Post
I wonder how bad the Torker 2 will impact torque below 3000rpm?
I asked Jim Butler that exact question when considering a T2 EFI to replace my OE iron intake and Quadrajet (P400 with 230/236/112 HR), and his response was: "No difference at 3000rpm, maybe a small reduction in torque for the FI at 2500".

This surprised me but I went ahead and got the FI, should be running in a couple months.

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Old 02-24-2020, 08:34 PM
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Is the airflow for these TBI FI units measured the same as airflow for a carb?

Keep in mind that carb airflow is measured at a specific pressure drop. Industry standard is 1.5" vacuum. What that means is that air is being pulled through the carb hard enough to generate 1.5" vacuum below the throttle plates, so if you have an 800 cfm carb, that 800 cfm is what it should flow with 1.5" vacuum below it.

A high performance engine running on the dyno should never pull that much vacuum at WOT. Really, if you're pulling over 0.5" vacuum in the manifold at WOT your carb is under-sized.

Therefore, a running engine will never actually use as much CFM as the carb is rated for with a properly sized carb. That doesn't mean the carb is over-sized, it's just a difference between how the carb's airflow is rated and what a running engine actually does. Whatever that 800 cfm carb flows at only 0.5" pressure drop, it's considerably less than 800 cfm.

Hope I explained that clearly enough.

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Old 02-24-2020, 11:39 PM
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Thank you for that. Now I understand!

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