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SBC 350 feeling slugish

35K views 276 replies 13 participants last post by  04Mullis 
#1 ·
Hey everyone. I have a Vortec headed 355 in my 86 olds cutlass. It have a howards roller 180325-08 camshaft and 10.25:1 compression, scorpion full roller rochers, edelbrock performer rpm intake and edelbrock thunder series 650cfm carb. Has an 2800-3200 stall converter rebuilt th350 by a very reputible racing trans builder in the area (for 450hp) and 3.73 gears with 25.5" tall tires.
Exhaust is hooker 1 1/2" primaries into 3" collectors which get squeezed down to 2" and then adapt into 2 1/4" flowmaster 40 series mufflers. (on the to do list for bigger exhaust)

I really expected this combination to pull harder but it feels like it pulls hard from 3500-4500 then just pulls flat and gives up around 5500-5800. I only have maybe at most 1" of vacuum at WOT which most people recommend upgrading to a bigger carb and other say a couple of inches is ok.

I have a wideband in the car and at WOT I am right around 12:1 in the top of 1 into 2 and into 3 gear. Last I checked I thought the timing was at 36* which I believe when we dynoed the engine it was at 34* (might have been moved during install) also running 93 octane since that is what it was dynoed on. Could retarding the degrees 2* back to 34* help pickup some top end? I had the plugs out yesterday and there were no signs of any issues like speckles from detonation besides a rich idle that I have fixed but the porcelin was a nice light tan.

Innovative recommends 12.5-13.5 at WOT which I will shoot for Wednesday when I take the car to the track. Can anyone suggest what may be wrong? I am assuming the exhaust and carb could go bigger.

Thanks for your input,
Shane
 
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#2 ·
Any chance you could temporarily open up the headers and wire the pipes up out of the way for testing?? Squeezing down into 2" is pretty small.

1-1/2" header is rather small too, and are these a "shorty" header and not a full length header??


Have you made sure the advance is fully in by around 3000 rpm, and doesn't keep advancing at 4000 or 4500 rpm?, because if it does it will kill upper rpm power.

What distributor are you using??
 
#3 · (Edited)
When I am tuning and looking for power, if it is slow , I give it gas and timing.

Chances are if it is not yet blowing blue clouds of raw fuel out back, and the problem still exists, jet up.

If your not yet at 38 degrees total, no det , then advance to win.

An edelbrock 650cfm size carb should be fine for a hydraulic 350 cam rev-erage. Ending around 5500-5800 rpm is normal.

That package should do all the things muscle cars are known for , burnouts and strong V8 launches.
 
#4 · (Edited)
. The 650 CFM carb. sounds big enough... 225/233 cam should be pulling strong into 6000+ RPMs... the exhaust restriction down to 2" and 2 1/4" mufflers (stock-type?) going to hurt above 5000 RPMs... as is running a smidge rich... will run leaner as is with better flowing exhaust...

. Does the aircleaner lid clear the top of the carb. by an inch or more? Choke opening fully when warmed up? Did you use the same distributor on the dyno? Is car one good to 6500 RPMs?

. 1/2" vacuum at WOT prolly a bit better, but 1" shouldn't be making the engine noticeably fall on it's face too soon... what kinda HP did you see on the dyno? Did it pull to 6500 on the dyno?

. Vortec heads usually like about 32-34 total timing, but 36 shouldn't be making a seat of the pants difference...
 
#6 ·
Vortec heads will usually perform best @ 32 to 34*.Goin any higher is most times a waste.I would do as Eric said & make sure the timing is coming in as needed by 3000 rpm or so.I also agree that your headers & exhaust are holding you back.1 5/8" headers & freeing up the exhaust would make a good improvement.
 
#7 ·
. Yes, at his stated 10 1/4 compression ratio, usually don't need so much timing, but if putting Vortec heads on lower compression engine/dished pistons may need more timing advance... should prolly verify that 1 1/2" primaries, may actually be 1 5/8" already... unless headers came off an earlier 305" engine...
 
#8 ·
Obviously finding the tune is a big element here but what sticks out like a sore thumb is the stock port Vortecs which are too small without porting, as well as the carb and exhaust being too small.

What you have here should on an engine dyno at sea level on a standard day ought to make a solid 400 horses maybe a bit more with a bigger carb and exhaust.

The L31 Vortec is iffy getting into the 400 horse zone without porting or better yet most anybody's 190 cc intake port aluminum head. Otherwise the L31 is only good to about 380, it can't really use a high lift cam as the ports just aren't there to support it with flow.

You can squeeze 400 horses out of a 350 with a 650 CFM carb but you gotta be damn lucky or be a really good carb tuner, this is a lot more than getting F/A ratios you end up fighting fuel velocities in the passages and getting the emulsions just right. A 750 CFM makes getting the potential of what you've built a lot simpler and more stable.

The exhaust is too small from the headers on back. The headers should be at least 1-5/8ths but preferably 1-3/4qtrs. The 3 inch collectors are OK but 3.5 would be better. Head pipes to the mufflers if running duals should be 2-1/2 to 2-3/4qtrs inches. After the mufflers if not too many cuves 2-1/4 if a lot of bends then 2-1/2 inches. When the cam starts getting big the whole engine get really sensitive, small misses in tune and external components like the carb and exhaust costs big time power. Exhaust can be very quirky, peak power especially with a cam that carries a lot of overlap is very sensitive to exhaust wave tuning. This is a variable of trying to get the waves to hit the chambers at just the right time to pull fresh mixture in and dispell residual exhaust gases. The total elements and length come into play, if they are bringing the waves to the cylinder at the wrong time they are just as effective at negating power as they are in building power in the right circumstances. The other issue is capacity which is a result of the ability of the exhaust stream to exit with a minimum of pressure buildup in the pipes.

You've said noting about pistons or the block as to how the Vortecs are getting over 10 to 1 compression. All these bits and pieces need to play together so it's useful to know what's in it and what's been done to it.

Bogie
 
#9 ·
Wow thanks for all the responses everyone. I will try to answer all the questions.

*Headers are full length.
*timing is all in by 3000rpm.
*Dist is a MSD pro billet with a MSD6 box with a 6500 rpm chip.
*engine has 6cc KB pistons with 1.561" CH which are .025" in the hole (decks were squared) and I believe the gasket was 4.100x0.015" which with a 64cc head should right in the neighborhood of 10.25:1.
*I changed the header gaskets yesterday and verified that the 1 1/2" gaskets fit the headers perfect.
*I can try to do something which will allow me to remove the 2" pipes and dump the exhaust down so it is not hitting the floor.

*There was some conflicting information on the dyno results which I could never get a straight answer for from the dyno operator. The uncorrected numbers seemed right to me I believe it was around 385hp at 5500hp. The corrected numbers were like 480 at 6400?
http://www.hotrodders.com/forum/dyno-results-234587.html
If you read further on the lb/hr and O2 rating were off. I believe they were not hooked up since I brought it to the dyno to have it broken in under controlled circumstances.

Let me know if you have more questions. I do not remember the jet and rod sizes off hand. (They are on notepad on passenger seat.) I can get that information on Wednesday
 
#12 ·
I'm thinking don't worry about the collectors and turndowns, just a quick open header test on a country road near your house, ...if it is possible for you to do this in your location....just to see if restrictive exhaust is an issue that is making the power go flat above 4000 rpm.
 
#11 · (Edited)
. Bogie, I think from his 3rd paragraph he's saying the engine was tuned on a "dyno", but is wimping out now installed in the vehicle, so engine internals not a question... external boltons a concern...

. OK, I see more was posted while I was posting/writing... if this was dyno'd in New York at near sea level, I wouldn't expect the corrected numbers to be much different from the raw dyno info... big difference between 385 (seems lowish) and 482 (seems unlikely high) HP... and the RPMs should be about the same, not 5500 vs 6300... or did you buy the engine from mile-high Denver, Colorado?

. This buildup using stock, unported, 1.94" x 1.5" valves Vortec heads (except valve springs and rocker arms) indicates their power potential:

GM HT383 Crate Small-Block - Engine Build - Hot Rod Magazine All Pages
 
#19 ·
From a 355 with this set up I'd expect about 380 hp around 5600 revs. The lack of porting the 650 cfm carb and the apparently what are 1-5/8ths headers are holding this back. The correction to 480 at 6300 seems like somebody is having a bad math day.

Certainly the problem is in the installation. This not only requires the use of adaquate parts in the exhaust system as well as proper tuning for the realities of the installation which are quite different from the dyno room. The problem is getting back to something as colse to the dyno room as possible.

As to be expected with high strung engine the power is very senstive to the support systems like the exhaust as well as the air intake. Small problems with flow either into or out of the engine can have very large consequences with power output that just aren't issues with an engine.

I also think that more effort needs to go into the timing, all in by 3000 with this cam seems slow to me. I think he needs to push this up amount (sooner in RPMs not necessarily amount) till the engine hits detonation then back off theRPM point where it peaks that is push full advance up the rev range. This has to be done in the vehicle as it is going to be sensitive to load on the crankshaft which changes to the gear selections of the tranny. Usually top gear proving to be the limiter.

Bogie
 
#14 ·
What lifters? Sloppy lifters can pump up and/or bleed down causing valves to bounce off the seat if the lifters is collapsed or hang open if they're pumping up. Poor valve control can seriously limit RPM potential. You're on the right track with the WB02, but I'd lean it up a bit and see how it responds.
 
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#21 ·
. 3" collector is plenty big enough for this engine...

. Yes, narrow 108 LSA cam will cause the engine to punch hardest in 4000-6000 RPMs range and then fall off faster above that range than a wider LSA cam would... but still think small pipes/mufflers are blocking the revving even more above that range...

. Enjoy the engine the way it is... and when get around to bigger pipes/mufflers, so be it...
 
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#23 ·
I specified all that information in the first post. When I foot brake the car I can get a 2600-2800 stall out of it. Flash stall seems to be low 3000 but I can confirm closer numbers tomorrow. Those are numbers from just playing around in the drive way seeing what it can do. It is a ******* racing torque converter.
 
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#31 ·
Nitrous is nice.. short term. But you never own the power.
Its like renting. And soon costs more than a blower.
Be fair now, blowers have hidden costs. Don't forget to add in spinning that blower any time the engine runs plus the extra weight of carrying it around everywhere you go.
With a blower you are paying more every time you fire it up.
With n2o, you only pay more when you use it.
 
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#30 ·
I think that would have to be dependent on the amount of lb/min of NO2 that you use. Which you could easily figure out if you flow x lb/min and your run is ten seconds you have flowed convert to lb/sec and multiply by seconds that you ran. I image a 12 second car (with nitrous activated) Could easily get 8-10 passes from a 10lb bottle only using 150hp shot. I don't know let me look up nitrous requirements for different hp level.
 
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#33 ·
Spinning a blower is not free. Just spinning the drive with nothing attached costs fuel/power. It's a hidden cost that adds up over time. The more street miles you put on your hotrod, the more the cost/benefit ratios shift to favor nitrous.

No need to go out of your way to pay high retail nitrous prices. For the price of a blower drive alone, one can purchase a couple big mother bottles and everything it takes to top off your own 10lb bottles at home, any time you want. The bottles for my car are always full ;)
 
#34 ·
Just the initial upfront cost for a blower puts it out of reach for the average guy with a home & family to support,but'still trying to keep a hot rod of some sort.it more than doubles the cost of most average builds upfront.It"s just not a viable option for the average street car guy.
 
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