Hot Rods SBC won't rev - are rebuilder pistons the problem?

Last year I hurt the motor in my truck while having too much fun. I replaced it with a 4 bolt main 350 that came with another project car. It was supposedly a fresh rebuild with flat tops. I stuck a comp cams 268H in it and reused everything else from my previous motor (Edelbrock RPM intake, carb, HEI).

....it has never really ran right. It just won't pull he way the old motor did. I lays over after 4,000 rpm. I've never ran a 268H, but it should do better than this looking at the specs.

I tore into it this weekend with plans to replace the heads and cam....and found out that it did not have flat top pistons in it. It has shitty 4 relief dished pistons with a large bevel around the top of the piston. In addition to being dished and beveled, they sit way down below the deck.

Is the screwed up quench why this motor runs so bad?

 

what heads does it have?

how big was the cam in the other engine?

 

Cam was the second smallest Lunati voodoo .454/.462 lift 213/219 duration. The 268h is .454/.454 lift 219/219 duration. I know modern grinds are more aggressive...but this much difference?

Heads are shitty 1.94/1.5 76cc 70's heads with "Z28 springs" on both combos.

The old combo had TRW 4 relief flat top pistons.

The old combo would pull to almost 6,000 rpm.

 

It sounds like the c/r could be dismally low.
Supposed chamber c.c.?
How much deck height?
Gasket thickness?
If you can i.d. the pistons, dish volume?
Doubtful it would " pull" too good, with your current setup.

 

Possibly truck or stationary engine pistons?

 

76cc chambers. Deck height unknown. Gaskets were Felpro .040"? Pistons are dished with 4 valve reliefs and have a god awful 1/4 taper around the outside of the piston top.

Yeah, it runs bad.

 

How far is "way down below the deck?"

 

Sounds like you hav about a 6-1 compression ratio. A buddy did that by mistake with 76cc heads and it hardly moved his van. The good thing is if the battery goes dead you can hand crank it over.
The cam probably dropped the cr to 5-1...

 

The good news is... no detonation. The bad news is... no detonation.

 

I believe you have another problem than lack of compression. I've used that cam before and the engines had no problem revving up to at least 5500 rpm. Even a lower comp. engine still makes power, just usually looses bottom end the most. There are c.c. charts on the internet that gave give you a close idea of what your ratio is.

 

Have one of the 70 era 350s with the dished pistons. Added 461 heads. Runs great.

 

Sounds like 8.5:1 pistons with the 76cc heads.... No wonder it's a dog...

 

Did you by chance center line the cam to see if the timing is correct?

 

Some rebuilder pistons have a lower deck height figured in assuming one has had the deck surfaced. You could put a supercharger on it now..

 

My guess is 0.060" just eyeballing it.

 

Add that to the .040 gaskets and that's why.

Sent from my SM-G900P using The H.A.M.B. mobile app

 

Its ready for 15 lbs of boost.

 

8.4 with a flat piston..Could be 7 to 1 !

Agree the 4000 business is a different problem

 

It's all ready for a big supercharger or turbo.

 

Did you put a vacuum gauge on it? You can tell if cam timing is correct by doing that.

 

We had just bought our new house, moving out of the leased one with 2 car garage. (roadster all apart, engine out of my F100) Mad hurry!
Got a low mileage '89 Chev 350 with swirl port heads, (free from a customer) did cast iron rings, rod & main brgs, valve job, and a new Comp Cams 268H cam/lifters. Installed it, w/Edelbrock intake & 650 Holley.
Never ran like it should have, we got moved in and I switched cams to Comp 290H, well...
Big difference...6500 RPM and change, got there fast, too.
Just my experience... But! Make sure ports are aligned with the right intake gaskets. Match them closely.
Obviously, you have to address the compression issue first...(or 'during'...)
'Good hunting.'

 

Just about all rebuilder pistons are a further .020" down the bore. So add that to the stock GM clearances and you end up with .040 to .050 down the bore.
On my 283 , I milled .050 off the deck to get zero deck clearance.

A bolt-on remedy is Corvette L98 heads [58cc with 1.94 intakes]

Or find a set of old power pack heads, or 305 heads and install larger intake valves [and a good 3 angle valve job]
Get a shorter duration cam , it will pull hard from down low and still rev out.

 

My thoughts too. I think there is some other problem.

I had a low compression LT1 (order mistake) and put a big cam in it. I ran it in the sprint car and it didn't have any bottom end, but it would make power up high and rev fine. Went to a milder cam and got the bottom end back.

 

By your description it sounds like the cam timing is off, probably 1 tooth advanced.

 

Well, it is going to get real flat tops (Sealed Power H631CP w/ 2 valve reliefs and 1.560" compression height) and an old set of GM pink rods. I hope this makes for a fun combo.

 

Take ONE complete piston/rod/rings and install in each corner, using the same piston/rod combo. Measure deck height at TDC on each corner, and determine how much to have your deck cut; and also if it's parallel to crank centerline.

 

If you're keeping those same heads you're still going to have low compression, why not get a domed piston and at least shoot for 9.5 compression, this is a short block I have and the head cc to tame the compression down, I'm probably going with a 72cc head to get around 10-1 compression.

 

This motor is getting a set of World Sportsman II heads with 64cc chambers and a Summit 1105 cam.

 

That's good to hear, this was a 11-1 motor with 64cc heads, but I don't need that in the truck it's going into.

 

Dad measured and said it was .120^ down in the hole!!!