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Old 12-07-2013, 11:54 AM
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Help me critique this 450HP 350ci SBC build

Thanks for checking in on this thread, the engine build in the following article is identical to the engine I am currently constructing almost to the part. I already have the GMPP "604" heads, four bolt roller Gen 1 block zero decked with small dish hypereutectic pistons.

Chevy Small Block Performance Parts - Super Chevy Magazine

The block is done except for cam selection and this is where I'm thinking there could be a change to a more modern profile, the XR282HR has been around for a long time and I wonder if there isn't something better out there now.

The converter is 2800 rpm stall and the car is around 2700pds with a 4.33:1 ratio.

I'm limited to 0.530 lift with 1.5:1 rockers as this is the max the AL fast burn heads can take without springs and machining.

What do you say, can we improve on this 2003 build with the latest components and make more torque above 3-6K rpm. Peak HP isn't as important to me as long as it can run to 6500rpm.

Everything from the cam up isn't determined yet except for the basic heads with 3/8" studs and the self aligning rockers requirement. I'm not against losing vacuum with a tight LSA if it gains me torque in lower rpm range.

The car will be autocrossed so flat torque curves are preferred to peaky punch at high rpm. The 450HP mark is right where I wanted to be at 6000 rpm but if I can gain 30 ft/pds at 3500 rpm and lose only a little up top I would do that.

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Old 12-07-2013, 12:32 PM
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I think you can use a lot more camshaft. I would also consider a 4 hole ported spacer with a bullet nose.
I used a different carb in the past that has a very crisp throttle response;start with a 4777,replace throttle plates with the base from an 850,modify top as per usual(remove choke horn etc,
not sure if you use float bowl extensions or other internal mods???Im sure you have a favourite small engine carb,,,,
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Old 12-07-2013, 12:35 PM
How fast is fast enough?
 
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Smaller cam duration by about 4-5 degrees on the intake and exhaust.

That cam is too big for the rpm range and goals. Those are good heads which will require less duration than poorer ones for the same rpm.

If you want to use the cam spec'd then raise the rev limiter and narrow the LSA a hair.

The fact that they advanced it for best power tells you right off the bat the cam is too large.
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Old 12-07-2013, 02:14 PM
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quote:The fact that they advanced it for best power tells you right off the bat the cam is too large.

look at the dyno graph,the engine made peak torque at 4800 and peak hp at 6200 the engine is not getting enough air,bigger heads and bigger cam.or other parts to optimize the flow figures of the heads,,,

If those heads flow 250 plus CFM,we want closer to 500 hp.
I would use a bigger cam and maybe,,,a slightly tighter LDA, plus 10º on the exhaust side.


with limited lift this is what I would do,or make room for more lift,shorten the duration and spread LDA
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Old 12-07-2013, 04:38 PM
How fast is fast enough?
 
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***?

It makes peak power too high in the rpm band, 6200 as you point out, so you give it more cam?

That's like using gasoline to put out a fire.

Last edited by 68NovaSS; 12-07-2013 at 05:28 PM. Reason: Profanity
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Old 12-07-2013, 05:03 PM
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no sir,I think the "road race" engine needs to make peak HP at 6500 RPM not 6200.The peak torque figure and peak hp are too close together for a "roadrace engine"
This why a lot of RR cams have a wider lda. we are using a limited lift cam so I added duration to get the peak hp a little higher and shortened icl as it seems the article being followed was the guideline being used for this engine.
with short rear gears and only a 3 speed transmission the engine will likely need to make power to 7kor 7,200?
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Old 12-07-2013, 05:57 PM
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Did you read the original post or are you just saying what you'd do if you had a set of fast burn heads and no target?
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Old 12-07-2013, 06:01 PM
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ap72,re more torque 3-6: spacer/4777 modified
an engine that makes peak hp at 6500 (hopefully more hp and bringing the same or a bit more torque with it)
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Old 12-07-2013, 06:33 PM
How fast is fast enough?
 
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Run up to 6500 means red line not peak. Your peak should be around 5800-6000 for that redline, and a smaller cam would increase the torque in the 3-6000 rpm range.

So... How does a bigger cam come closer to any of his goals?
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Old 12-07-2013, 06:56 PM
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I dont think a smaller cam will increase the torque,,,,and with a 6500 peak power the car could go 120 mph(guessing at tire size) some tracks have straights that can go a lot faster than that. The Calgary track is a good example,
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Old 12-07-2013, 07:19 PM
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It's unlikely I will ever be out of first or second gear on most of these tracks, it's slalom racing between cones usually in a parking lot of a mall or park. There are very few straights, if there is one its usually a long reducing radius ending in a hairpin.

Power is important and carrying one gear even more as a shift slows you down and upsets the chassis. Big bumps in the torque curve aren't desirable, broad and flat is ideal.

This cam come the closest for an off the shelf roller but I'm no expert, I know it's an old grind...heck I had a 292H grind in my 69 Mustang back in HS although back then they had less lift.

It should be known that this head will valve float at 6800 with the stock springs, 6500 will be rev limited in the ign box. I've considered a custom grind but there are so many grinds out there right in this alley I don't think it's necessary.

As far as LSA goes, if narrower is better for torque down low it would seem a Thumpr style grind might be preferable...maybe the 283 THR7.

08-600-8 - Thumprâ„¢ Hydraulic Roller Camshafts for 1987-1998 305-350 Originally Equipped With Hydraulic Roller Camshaft & Most Crate Engines, Except LT1 and LT4

I know these cams are kind of thought of as "poser" cams but the few dyno sheets I've seen seem to support the area under curve concept even if you give up some vacuum and driveability.
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Old 12-07-2013, 07:26 PM
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parking lot pylon racing I think your first gear is too steep? If you hit the rev limiter before 50 mph,,,,you might be slow on the straights.everywhere else with that converter,you might need racing tires to hook.
how tall are the rear tires?
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Old 12-07-2013, 07:31 PM
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Sorry,missed some important components.

Carb: Holley 750DP

Intake: Weiand 7504 single plane with slot for divider plate.

Headers: 1 5/8" shorty, 2.5" collector.
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Old 12-07-2013, 07:32 PM
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Knock a bunch of exhaust duration off that thumper cam and you're close to a good one. The excessive exhaust duration is just sacrificing mid range power for peak- not ideal if you drive in the mid range rpm.

For your app something around 225-230 on a 110 or so will give you great results.
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Old 12-07-2013, 07:44 PM
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Not sure what SCR you have, but the mag build was 9.4:1 I believe.
The 08-600-8 cam closes the intake at 35.5* ABDC @0.050" tappet lift and will produce a DCR of 8.26:1 on the Keith Black calculator if using an SCR of 9.4:1. Very driveable on pump gas.
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