Best way of raising compression on a small block Chevy on the cheap.....

Hi we'll I've dug myself into a bit of a corner with my build and spent ages porting/flowing my sbc heads only to realise they are low compression heads....doh!!!! Now to get the rumpty rump on idle from a camshaft I haven't chosen yet I need to up the compression some, are there any views on doing this, I have found some dome topped pistons on summit.....

http://www.summitracing.com/int/parts/slp-h617cp/overview/

Have anyone used these or any experience with hypertueretic pistons, I read that they can cause problems with the burn being so domed, I don't want a full race car just a nice lumpy idle but will lift a leg if I need a quick burnout lol. I didn't really want to go this far with changing pistons with the build but I also don't want a dog of an engine I'm running a turbo350 possibly getting a high stall converter and unsure of the ratio that my 10 bolt rear end is.

Cheers again hambers for any help, oh and happy new year one and all let's make it the year of getting those projects on the road!!!!!


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Head casting number will tell what they originally were from. No expert but lumpy pistons would raise compression ratio and compensate for heads. What is it now with the heads and your current pistons. Any idea what chamber 'cc' currently is? What is the engine 350, smaller engine bores can cause problems with larger valves.
Forged pistons are better as they have the ability to maintain size from cold to hot temperature without the common 'piston rattle' commonly ***ociated with cast or hypereutetic pistons.There are more knowledgeable people out there who will seek additional information.
No use IMHO having a high stall convertor without matching engine and transmissions to cam, rear end ratio etc etc.

 

I'm away from my workshop for a week or so at the moment so unable to get numbers, yes it's a good old 1989 hencho en mexico crate 350 motor, I presume they are the low compression 76cc heads, so I believe it to be 8.5:1, I will get a high stall converter but when I know what I'm running and match what I have to motor and cam. Thanks Jason.


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Cheap way :

Shave the heads
Use .015"-.019" steel shim head gaskets

You need to know what heads/cc you have to begin with, decode your heads casting number, then cc your chambers to confirm if your heads have already been milled or not.

 

Best way I know of is to find a pair of 305 H.O. heads. Small C.C. chamber, 1.88 intake valve, nice runners and most of all hardened valve seats. I know its kind of apples and oranges. But I put a pair of these on a .040 283 I did for a Chris Craft boat that belongs to a buddy. Really woke it up.

 

Angle mill the heads

 

The smog 350s like that usually had a dished piston, around 8 or so compression, with the big chamber heads. Putting in a flat top piston will get you around 9 something, and if you mill the heads a little and use the steel shim gasket, you'll get to where you might have to put good gas in it, ***uming the cam isn't too big.

Don't panic yet.

 

it's the cam that effects the sound and what stall torque convertor are used you should start by measuring the deck height and heads actual cc's to determine the present compression ratio and what you want are you planning to run pump gas or racing gas you go to start at the beginning.

 

Blower !!!

So this goes with your other thread ?

If those heads are the mega huge 76 cc chambers they aren't good for compression concerns. New Domed pistons will require a rod refitting, balancing, ring fitting and hone. So basically starting from scratch. Or bolt a blower on it.

The 305 ho heads will give you a hell of a compression bump cheap! great low end but the valve size will limit your upper rpm power. That rumpity music your looking for will come from the cam that comes alive and makes power right where the 305 heads start giving up on a 350. And you'll be shelving those heads you have countless hours on unless you go with a blower. I'd go with this option if its going to stay under 3500 Rpms

The vortec heads are a good choice but you'll need to work them a little to get more than mid 0.4** lift. As noted in your other thread I have $450 into mine and they will take 0.550 lift easy- maybe 565 but I need to check everything after I install the cam- haha buy a cam and see if I can use it ? Not.

My absolute favorite HP to dollar engine is a 307 with 305ho heads.

 

Yes do the math first, figure out the head cc's, deck height, compressed gasket thinckness before you order yourself a set of pistons. I like the hyperutectic pistons and have built a good number of engines with them.
I feel that a c/r of 9.5 up to 10.5 is just fine for todays pump fuel provided the effective quench area is about .040" or greater.

 

I think you've got that backwards. In general, cast pistons are more stable over a range of temperatures. This kept them quieter when cold and was one reason they were so widely used in OEM production on most street driven engines. They were also lighter and cheaper to manufacture. A lot of cast pistons were spec'd to run at less than .001" to maybe .0015" cold clearance.

Forged pistons are heavier and stronger, as well as more expensive to make. They also tend to expand and contract more with temperature changes. Cold clearances tend to be .003" to .004" or even more depending on the design and application. The rule of thumb years ago was approximately .001" of cold clearance for every inch of bore diameter. As a result they tend to be a bit noisier after a cold start but quiet down after they've warmed up a bit.

It's actually kind of a pleasant mechanical "chatter" they make when cold if you know what you're listening to. I think it was the older 375 hp 327 with forged pistons that made a good example of this sound when first started, provided that there was some kind of an exhaust system on the car. Then after a few minutes and a little warm up it was gone.

 

not uber cheap, but, 400 crank turned to 350 main size. Boost displacement about 8%, and torque similarly, then boosts CR, which adds MORE torque, etc.
Not sure which damper and flywheel are needed to maintain semblance of good balance

H860CP pistons

Small Block Chevy 383 with 5.700 Rod
Flat Piston with Two Valve Reliefs
Compression Ratio: 10.4 with 64cc Cylinder Head, 9.21 with 76cc Cylinder Head

About 55 bux cheaper per set than the domed ones

 

x2 on this one. You've spent time and money on your heads so this is the logical cost-effective way.

 

If you don't mind me saying this is a recipe for disaster. Instead of trying to build a motor that "sounds" good, You should just use the 268 or similar cam suggested in the other thread and have a motor that RUNS good. The whole concept of the longer duration and overlap that makes the loping sound you are wanting is the byproduct of a motor that needs more compression, more gear, more stall, and better flowing heads and exhaust because it makes its power at much higher RPM's . You already have the rest of what is needed to run the 268 or similar cam. There is nothing worse than an over cammed motor that wont get out of its own way on the bottom end and don't have the rest of what's needed to make power at high RPM's. In the end I think you will be happier with a motor that will run strong than one that's sounds good but don't go anywhere.

 

What cam are you planning to run? The duration and compression ratio have to be complimentary.

Someone else has said it but with the budget contraints and parts you are working with you should choose a very mild camshaft. You can try to get the sound you want from the exhaust system. Running a larger diameter tubing than necessary with headers can give you the lumpy sound you want.


With 76cc head on a 350 with flat top pistons you going to see 8.5:1 compression. Not ideal unless you running a very mild cam shaft. A shim gaskit will net you about 9:1 compression also not ideal. You'll want to be atleast 9.5:1 on a hot street engine with a performance cam. If you cam is over 212@.050 you'll want to be at 10:1 and if you're getting into the 220's@.050 you need to be a bit higher.

I'd skip the dome pistons they fragment flame travel and distort the combustion chamber design. Shaving the heads isnt going to matter that much. Dont bother, if it did you'd see 76cc heads actually be worth something.

Your options as I see it.
-Shim gaskit and flattop pistons should get you 9:1...good for mild cam. You will need a scale to make your new pistons weigh the same as your old pistons to avoid rebalancing costs. $300

-Domed pistons + new gaskits...$300

-New heads...750$ ...no reblancing, no engine short block diss***embly, better flow. should yeild 9:1 with your pistons and ok for mild cam.

- Stroker kit $$$ ???

Hyperutecnic pistons are cast with silica so they are dimensionally stable over a broad temp range but the silica makes them brittle. In specialized applications like nitrous, turbos and stokers where the ring lands are thin they are not ideal. If you're running a hot street engine they can be ideal becasue of low weight. Tight tollerances can be run to keep the pistons and rings stable in the bore for ideal sealing.

 

Regular vortec head moves you down to 64cc. $350 each

L98 "corvette" heads moves you down to 58cc. $650 each, but worth it in my opinion because you will lose a lot of weight being that they are aluminum.

However, i'm with Russco on this whole matter; Forget about the sound and concentrate on getting the engine to run well first and foremost.

If you really want the rump, then suggest fitting a manual choke to the carb. When you want that rumpy sound then richen up the mixture.

 

This is the cheap way to go. We've done this sevaral times. Really wakes up and engine. Back when we where into mud buggies we would do this and run the hell out of the engines till they blew....

 

Add a plus one on the 305 heads.
First of all they are easy and inexpensive to find.
I am running a pair on a .60 over 283 with a four speed and it has a lot of good ole fashion *****.
I am running a Melling cam (don't remember the exact number but is basically the old L79 350hp Corvette cam from a 327) with a 550 Edelbrock carb.
I don't get the thumpity thump you are looking for, but it is a really great running little engine.
Just my 2 cents.

 

It seems that so many on the forum don't read the original post fully. OP has a set of heads he has spent time and money on that he wants to use but they have large chambers. The only way I know of to compensate for the large chamber is to fill that chamber with something. The easiest way is to use a piston with a dome to take up some room. Shaving the heads will help but won't raise it much with the large chamber heads.

You will need to know the chamber size, figure out what compression ratio you want and maybe talk to one or two of the piston manufacturers tech lines so you fill enough space but not too much. Check the bore to make sure you know if you need oversize.

 

Oh I read it alright and just fine.
What's cheaper ? Results in a better engine?

 

Then play around machining an intake to fit? $$$$

 

Re-read the O.P. Did not read the whole thing at first. Understand wanting to use you heads. Can only give you another example of something I know works. My buddy has a '69 Camaro, he and his brothers run in NHRA SS/JA. .070 over 350, flat top zero deck pistons, steel shim head gaskets and #441 (76 cc) heads. I know this is not your normal 350. But math is math as far as the C.R. goes. Also with 1.8 rockers it has over .700 lift at the valves.

 

Hey thanks for that yes the topic seems to have drifted from where I am a bit, I need to use these heads really as I've spent so much time on them etc. plus being over here in the uk parts are very hard to come by and very expensive to import. I've actually found a second hand set of the domed pistons on eBay that are cheap at the moment and would take compression up to about 10.13:1 with my heads. But if I keep it low I've decided to get a comp cams 268h cam and not have the lumpy idle I've been after but have a quite streetable motor. Thanks for the help I'm going back down to the shop where my car is Friday so can get a definite size of chambers etc the get bits ordered. Thanks again Jason.


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Ditto. I wouldn't care WHAT my car sounded like, as long as it pulled like a rabid bull.

The engine I have now, is a 350 SBC with worked over big valve 305 heads and a good street cam, so I run a stock stall speed with 3:73 gears. It doesn't have that "rumpity rumpity" sound, but I've skinned cars that made that sound. Build for performance and get a turbo muffler, if you're looking for "sound".

 

If you are looking for a lumpy sounding cam, I may have what you are looking for. I have a Big Mutha Thumper cam by Comp Cams. I have the lifters and the correct springs and retainers. You will need a mild converter if you are using an automatic. I had this cam in a Mexico crate motor for a short while and sounded great and was fairly snappy with some good heads and a tunnel ram. Send me a PM and we'll talk.

 

Probably a little late to this party but your rear end ratio is more important than your compression ratio. I know this from personal experience as my rear end is a 10 bolt with 2.56 gears and smart cars are faster than me on the highway. Sure I’ve got rpm for days but my wheels can’t mechanically compete with a 3.08 or say 3.55.
Car accélérations rapides Speed is = Engine RPM x Transmission Gear x Rear gear

 

Fel-Pro makes a steel shim head gasket with a very thin rubber coating for a 4.00-inch bore 350 that is only 0.015-inch thick. When added to your 0.020-inch deck height, this produces a 0.035-inch piston-to-head clearance. This is a little tight but should be fine for a mild street engine that does not see engine speeds beyond 6,500 rpm.

The good news is that this gasket will bump the static compression ratio up to 8.97:1 or essentially 9:1, which is worth roughly a half a point in compression

 

If you have good pistons a exhaust driven supercharger will make the engine seem twice as powerful. Remember those supercharged V6 Buicks? However a supercharger can melt pistons.

 

Those were either a 8.2 of 8.6:1 set of pistons. You can plug and chug using the little calculator that is on the Summit link you provided. Myself, my 350 was the 8.6 version and I went with Speed Pro flat-top pistons, with a 64cc set of heads I'm close to 10:1 cr. Like Squirrel says, you would be limited with the cam at under 9.5:1 cr.

 

The #601, 305 heads had 53 CC chambers and either 1.72 OR 1.84 intake valves; both had 1.50 exhaust valves. next in line were the #416 305 heads with 58 CC chambers and 1.72 intakes/1.50 exhausts. A lot of dirt track, roundy rounder racers used these heads on 350 engine for a cheap bump in CR, but they were also limited to "stock, production heads"; most guys would give these heads away. I believe DART has some 49 cc heads with decent ports and valve sizes, but you're no longer in the "cheap" range going that route. Might be easier to just go with a higher stall torque converter, and lower gears in the rear end. I am Butch/56sedandelivery.