350 sbc reduce compression ratio or retard cam?

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68maliblue
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350 sbc reduce compression ratio or retard cam?

Post by 68maliblue »

I am building a low budget 350 for a 74 Camaro. I have not purchased pistions yet but will be sticking with the standard rebuilder hypereutectic pistons to keep costs down. I would like to use a set of Silvolite 3437h-030 pistons which have a 1.560" compression height and 6cc volume to keep the pistons close to .025" below the deck - theoretically. I will be using 63cc Vortec heads for this build and a .026" thick head gasket which gives about 10:1 compression and .051" quench. Its not perfect but the best I can do without putting $$$ into decking the block. I already have a cam that I would like to use - Comp 260AH-8 212 int/218 exh @.050" on a 108 lsa and it is ground on a 108 ICL. Plan is to keep the stock driveline and put a 2200 rpm stall converter in the TH350 and keep the 2.73 gears in it to reduce cost. The cam is a little on the small side for 10:1 compression but with the 2.73 rear gears in it I don't want to go much bigger. Not sure if this combination will live with 91 octane on the street without pinging. I'm in the San Joaquin Valley of CA and temps here are around 100 deg most of the summer. I'll definitely be paying attention to the cooling system but am concerned with the 91 octane gas around here. Which method would be better to reduce cranking pressure for 91 octane - retard the cam 4 deg or step down to a dished piston and 9.4:1 compression to stay safe? I'd rather not use a dished piston but I don't want to have to tear this apart again to get it right.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by rfoll »

I have had nuisance pinging issues over 9.25:1 with small cams. The Vortec heads help because they don't require as much timing. I got real tired of being price gouged by the difference between regular and premium. That being said, I have been told by a serious engine builder that retarding ignition timing works out better than lowering compression.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by travis »

I’ve used that cam on several low compression builds...it’s one of my favorites. I think with 10-1 compression you are too close, if not a little over, the edge. Hypereutectic pistons do not handle detonation well...they tend to shatter...and you can’t always hear it. I also don’t like retarded timing as not only does it make the engine a bit lazy, but it also tends to make them run hotter which is not what you want. The vortec’s help in that they don’t need as much timing as older, non fastburn style chambers which does help a lot.

Me personally, if it was mine, I’d probably be trying to get low 9’s on the compression. Those generations of F-body’s aren’t exactly light, and you are running a taller gear. The slightly looser than stock converter will help a bit though.

I think even with a dished piston it will be a great running driver.

I’ll also add that my daily driver for several years was a ‘78 Chevy 1/2 ton with a 9.4-1 355 with vortec heads, a comp XE268 cam, 2000 stall converter, and 3.08 gears. 180 psi cranking compression, and even with terrible quench it ran fine on 89 octane fuel. I initially ran a performer rpm vortec intake with both 600 and 750 vacuum secondary carbs, but eventually switched to a performer vortec (2116 iirc), with a 9895 Holley 650 vac sec q-jet replacement carb. I loved that setup...it would blow the tires off at anything under 30 mph, and would get 16 mpg on the highway.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by mag2555 »

There's a simple fix for this that can be easily done now since your motor is not machined yet, and this mod will add power to some degree even while getting you down below 10 to 1 compression.

Bolt a head to the block and check when the tops of the bores can be ground to put a valve notch into each side for both the Intake and Exh valves.

In making these notches on SBC 355 motors I find that they add 3 to 4 CCs to what would be the chamber volume which is enough to make a 1/4 point drop in compression and yet does not reduce the needed quench action, but can add to it.

With running a 91 octane fuel even a hot 175 psi of cranking compression should not make for knock or ping if the tune is right!

Also in terms of fighting off knock and ping I would save up the added small amount for a real forged piston since they dissipate head heat faster then the hyper ones.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by bobmc »

wikipedia says the elevation of Stockton CA is 13 feet? use the dish piston and install the cam at 110, should be very similar to stock L31 for the so called dynamic CR
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Re: 350 sbc reduce compression ratio or retard cam?

Post by BillK »

Why not just use the dished piston that all of the Vortec engines used and be done with it ? Sealed Power H815P or Silvolite 3470HC if you prefer that brand. My 99 Tahoe runs just fine on 87 octane and has a ton of power.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by CGT »

BillK wrote: Fri Jan 15, 2021 8:06 am Why not just use the dished piston that all of the Vortec engines used and be done with it ? Sealed Power H815P or Silvolite 3470HC if you prefer that brand. My 99 Tahoe runs just fine on 87 octane and has a ton of power.
I agree. Plus that piston is 1.5 1.5 3mm. Its been awhile since I looked, but it used to be considerably more expensive than the generic flat tops ..worth it to me though.

My experience with the vortecs is that they seem to be very detonation resistant. That cam with the 10:1 compression wouldn't bother me, especially if it was my own car....
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Re: 350 sbc reduce compression ratio or retard cam?

Post by 68maliblue »

I am avoiding most of the dished pistons due to their reduced compression height at 1.540 which combined with a 12-14cc dish puts compression at or slightly below 9:1. I did find these pistons which look interesting

https://uempistons.com/i-19960-single-s ... er-v8.html

1.558 compression height and 9.5 cc dish should be around 9.6:1 Compression with a .026” gasket. Not sure if quench is even a thing to worry about on a build like this but those pistons should be around .053”.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by CamKing »

Here's a crazy idea. Have a cam made to fit the engine, not the other way around. #-o
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Re: 350 sbc reduce compression ratio or retard cam?

Post by CGT »

68maliblue wrote: Fri Jan 15, 2021 9:41 am I am avoiding most of the dished pistons due to their reduced compression height at 1.540
Yeah that sucks that they do that. You could use the felpro 1094 which is .015 thick to get some of it back. That style piston really doesn't have any quench area anyway. I wouldn't get to worked up about quench using that style of dish piston.

I just personally have a hard time putting 5/64 rings in anything anymore :lol: , but I understand budgets are budgets.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by panic »

Worried about knocking and not worried about quench are opposites.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by 68maliblue »

CamKing wrote: Fri Jan 15, 2021 10:21 am Here's a crazy idea. Have a cam made to fit the engine, not the other way around. #-o
I know that would be the right thing to do but I’m trying to use what I already have to keep the costs down. I have a good set of vortec heads and gm performance manifold I took off a ht383 when I upgraded the heads. The crank on this block is fine with a polish and new bearings and I already have a cam so my major costs are pistons and bore/hone. Probably only keep this car a couple years as a driver so I’m not looking to invest much but still want to do as good a job as I can with it.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by rfoll »

It just occurred to me that I have a very similar engine on my test stand. I stumbled into a set of Speed Pro 2256 forged flat tops for $50. With the .028" Vortec head gasket, and Vortec heads, compression came out to 9.63:1 and it ran fine on regular if i was careful with the timing curve. I want to suggest fat top pistons are not likely to make 10:1. Advertised piston volumes and theoretical piston deck clearance can easily be high by 1/2 compression point.
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Re: 350 sbc reduce compression ratio or retard cam?

Post by BillK »

68maliblue wrote: Fri Jan 15, 2021 9:41 am I am avoiding most of the dished pistons due to their reduced compression height at 1.540
The H815's are 1.550 By the way the stock 1999 vortec engine is listed as 9.4 to 1
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Re: 350 sbc reduce compression ratio or retard cam?

Post by CamKing »

Sell the Comp cam, or keep it for another project, with lower compression.
Buy a GM cam # 24502476, for around $150 new. 212/222 @.050", .435"/.460", 112 LSA
https://www.gmperformancemotor.com/parts/24502476.html
Installed on a 110 ICL, it will handle the 10:1

When you get done using the engine, you can sell the used cam for as much as $100, because it's the cam used in the 602 Crate engines, and circle track racers are always looking for them to have reground into cheater cams
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