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SBC lacking power

2488 Views 37 Replies 18 Participants Last post by  Moosecountry

I am hoping someone can help shed some light on my current set-up. I know this has been raised a lot on here before but I couldn’t find anything that really gave me an answer.

I have only just got my 66 C10 with a 350 on the road and it feels like a bit of a slug/like it is still a stock motor (before all the upgrades happended). I have measured the 0-100 and its around 10 seconds (shifting manually on the TH350) which is slower than my 1.2 litre Volkswagen Polo and from a rough calculation would equal around 150 hp at best. The engine is as follows:
  • Stock bottom end – standard bore and compression test is all within 4% of each other
  • New alloy heads with 195cc runners, 64cc chambers 2.02 intake and 1.60 exhaust which are a little big but looking to go a 383 in the future. Flow numbers are:
  • Lift Intake Exhaust
    200 145 106
    300 192 135
    400 232 160
    450 242 168
    500 242 178
  • XE262h cam – Was a comp cams kit with timing chain, lifters and springs
  • Edelbrockperformer RPM intake
  • Brawler 650 carb (brand new) vac secondaries
  • Flowtechheaders – 1 ½ primary, 3 in collector but runs a duel 2 ¼ in exhaust to Flowmasterstraight through mufflers
  • MSD StreetfireHEI distributer – new MSD leads and spark plugs
  • Initial timing is set at 14 degrees and total 34 degrees (all in by 2800 rpm)
  • Stock TH350
  • Stock Converter
  • Rear gear ratio 3.73
  • Rear tire height is 28 inches

The car starts, idles and cruises perfectly and sounds strong but just doesn’t seem to have much go at all. The plugs are not fouling or over rich or lean. My question is whether I am just expecting tomuch out of the engine (although looking at the dyno sheets from comp cams for the cam I am not) or whether it sounds like something is a misssomewhere? While I feel like it would benefit from a looser converter I don’t feel like this would change the accelationthat much.

I have ordered a cam degree kit to check whether it has been installed retarded and is the correct cam. No loose lifter and pushrods spin when cranking so don’t think I have wiped a lobe.

Hopefully I have provided enough info for someone who knows more then me.


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Cam is too mild the XE262 is basically an RV cam, you need at least the XE268 which doesn’t seem like much but is a day to night difference. The 650 carb ain’t nothin to write home about either, nor is your 2.25 exhaust system.

Aluminum heads typically use very thick head gaskets, if you follow the manufacturer’s selection of .049/.053 thick gasket along with the standard SBC piston being .025 in the bore at TDC you end up with with inadequate compression and no effective squish quench this exacerbated by a stock bottom end that might have worn out rings and bore walls plus that deep round dish piston better suited to making Chicago deep dish pizza than contributing to compression and squish/quench. To get power out of aluminum heads you have to make them work harder which Vis-a-Vis cast iron which is easy to do without getting into detonation. otherwise they run the cylinder cooler than iron and that costs buckets of power. So just bolting them on a stock bottom end doesn’t do much more than empty your wallet, you gotta build into their strengths and likes most of which comes in getting the compression way up along with the squish/quench function. Guys pay far too much attention to port flow, not that it isn’t important but it’s only one player and it alone cannot overcome other technical deficiencies. To a big extent compression and squish/quench can be improved with a .023 thick thick Cometic C5245-023 or C5269-023 MLS head gasket, these are the high price spread. A bit easier on the budget is the GMPP composition gaskets at .028 inch thick part 10105117 or 14096405.

In the real world your truck is heavy and final rear gearing is subject to tire size, big tires take away from the 3.73 axle ratio so if this is like a working farm truck with tall tires the 3.73 is not near as effective as if it was in a Z28 Camaro.

My experience with pickups is my old 34 got a 256 out of a 53 Merc. The 62 with a 292 got a 430 out of a 60 Lincoln. The 69 with a 352 got a 428, my 78 Chevy with a 350 got a 454. My 89 S15 with an Iron Duke 4 got a 350. You can see the current running through this.

Then comes the tranny where old automatics strapped to frisky young engines don’t do a good job of transferring that power as they prepare to die.

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If you’re going to a 383 in the future I say your just stuck till that happens.

When you say it’s a stock and apparently will used bottom end I’d hazard to guess that the botton end is tired. Obviously a 66 C10 predates a 350 being the original engine, so it’s something from somewhere else. 350’s from the factory cam in variations from 170ish horsepower to 375, there is a lot of difference in the Potomac ends of these engines. The thing most affecting power of these engines is the combination of cam and compression.

So without knowing exactly what version of 350 is installed in this truck nor its condition relative to ring sealing, blowby, oil consumption and how the details of your top end rebuild in terms of exact head used more specifically chamber volume, the thickness of the head gasket used and you must have seen the pistons so can deliver some insite as to what is the crown shape of the pistons. Basically the average 350 will be low compression, not every 350 is a Corvette as the popular press would lead you to believe, so there will be a round dished piston with 4 valve reliefs. A flat top with 2 or 4 valve reliefs perhaps even a small pop up dome would be a high compression piston.

In normal factory builds even with the more typical 64cc chamber iron head these would use a fairly thin shim gasket. If you installed aluminum heads with the recommended thick composite gasket you lost compression ratio. Added to that aluminum pull heat out of the burn faster than iron. Heat and pressure walk together through the Gas Laws. So here you get a double whammy of lower compression generating less heat therefore pressure thus less force on the piston.

Changing to aluminum heads and adding cam is not a guaranty of adding power. The Devil is in the damn details of how good the original bottom end is at holding compression and combustion pressure, ie are the rings and cylinder walls any good. What compression did you start with therefore we need to know the original head, they have a casting number between valve spring sets. The original head gasket and what the piston crown looks like from this we can determine pretty closely the original compression ratio. This then extends into the heads you put on and their chamber size and what thickness head gasket you used so we can calculate the new compression ratio.

This is not to say the carb or the ignition is not yet tuned adaquatly nor even if the cam is timed correctly. I’m trying to understand the physical constraints of the build to get an idea as to whether proper tune could ebven make a difference if there are oddities about the basic build which are not all that uncommon an event.

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