81 vette vent

I know you should run a blower with 8:1 compression. The ZZ4 motors boast 10:1 compression. Do they really have this much compression? I have a blower setup with 7 psi of boost for the 350 motor and would like it to have 9:1s. I know GM always boast numbers that aren't exactly true.

If they are 10:1 pistons, I will need to go get lower compression pistons.

bluedawg

I would figure gmpp would be close. Two things to consider, ring gap and combustion chamber cc. If the combustion chambers are now 64cc you could lower the cr by going to 76cc, it would be a good excuse to put a set of afr 195's on.

3JsVette

I believe the compression rating is very close also. The ZZ series engines seem to live up to their specs when dynoed.
Why not buy a ZZ4 short block and pick out a set of heads that best suits what you want to build the motor into?

81 vette vent

What if I used the old 194 heads from my stock 81 vette? Would that get me down to a 9.5 ~ 9.0:1?
I'm not wanting to race the car. I have read where people have ran the ZZ4 with 7 psi boost and claim they've had no problems.
I basically have the original 1981 corvette motor, a ZZ4 motor, and a 6-71 blower with 7 boost. I guess I can change the pulley to make 4~5 psi to be safe.

bluedawg

If your super charging your looking for power my friend....Hello my name is Blue Im an addict.....if your changing heads, spend the coin, buy heads that make power, other wise buy a low compression mule and save the zz4 for if you grenade the mule.

lionelhutz

My guess would be 7-8lbs intercooled is safe so 5lbs without an intercooler is likely in the ball park. Good tuning will make all the difference between it running fine or cracking a piston. Expect maybe around 450-475hp give or take.

If you did upgrade for the blower then I'd say that forged pistons would be a better investment before changing heads. Forged rods would be a good idea too.

CanadaGrant

I think if you check you will find a ZZ4 crate motor from GM uses 58CC aluminimum heads to get that 10-1 compression. Mine is stock except for the addition of a GM "hot cam" and 1:6 to 1 roller rockers and it dynoed at 402 hp with those extras so with a blower you should be at 500 easy. It's a really nice motor and has been in my wife's 78 for 7 years without a problem ever. My only costs have been oil and filters.

bluedawg

I didnt look the zz4 up, but lionel has a good point, what pistons are in the zz4? And you could get the cr dowm by changing heads.

billla

Specs are here. Wouldn't be my first choice as the foundation for a blower motor...

http://www.chevrolet.com/performance...-350.html#dyno

lionelhutz

Pistons are hypereutectic. Not the best choice for boosted abuse.

bluedawg

Yeah deffinately, forged piston uprade and you could knock the compression down to 8 to 1 gap the rings for 8 to 9 psi.

billla

Keep in mind that it's pressed pins - and that the rods are PM and even GM doesn't recommend more than about 450 HP. Plus the heads are the L98 with a 168cc intake runner and "eh" flow numbers...which is going to hugely limit a blower.

Not sure if the OP has a ZZ4 and is asking how it would be, or if they're looking at buying one - but as noted it's just a lousy platform for a blower motor for a bunch of reasons.

CanadaGrant

The 4 bolt short block with forged crank and other goodies are worth it. Change the heads for the blower but I agree with you, this isn't a blower motor but can be with minimal cost. GM got the 10:1 compression by using 58CC heads. Use something else....

billla

I don't understand your math. I can get a better block and a better forged (383!) crank for less money without buying a crate engine and then selling most of it (rods, pistons, heads) for pennies on the dollar used. Minimal cost?

CanadaGrant

My math is that the engine is very strong. What is a forged crank worth? 4 bolt mains? New GM block? A 383 might be the way to go but it's still the same block. Would that stroker crank be forged or cast?
All I'm saying is at the time I paid about $3200 for a new roller motor with forged crank, four bolt, nice pistons and rods, aluminum 58cc heads and if I wanted boost, I could reduce the compression by dropping the good heads exchange at a profit. It isn't the pistons. It's the 58cc heads that give a ZZ4 10-1.
The 383 is nice too, really nice but the OP mentioned ZZ4....?

billla

Nope, still bad math as the only thing the OP would be keeping is the block and crank, which as noted can be purchased for less.

As for selling the rest "at a profit"...well, folks that have been doing this for a while know better. Especially the heads - someone might think they're interesting, but with 163cc intake ports and flow worse than a set of stock Vortecs any knowledgeable buyer is going to pass.

The pistons and rods aren't "nice" - the PM rods are rated for no more than 450 HP and no engine builder worth their salt would use them in anything other than a mild daily driver...certainly not in a forced induction engine that will make more than that power level. Forced induction on hypereutectic pistons is a Bad Idea as well. And good luck selling the rods/pistons "at a profit" as they're standard bore, so the market is kinda limited. The cam is also not the right choice for a blower motor.

The OP asked an open question on a public forum, so I gave a good answer - bad choice on the suggested engine, and if you're spending money on forced induction get more CID. I'd add that generally when building a forced induction engine, it's a good idea to start from scratch so everything in the shortblock is right - blowers tend to find anything that's not just about perfect and go boom.

cv67

ring gap is a biggie with blown motors
Guys have stuck the little B and M type blowers on and gotten away with 4-5 lbs with a cast piston bottom end not saying its a great idea though.
When a hyper piston goes (esp with too tight a ring gap) it will take out everything else in the motor, EVERYthing. Its like a bomb went off inside.
Takes your chances.
At least get a head with a larger CC and regap those rings. Where does it stop though

ajrothm

If you're talking about throwing a little mini blower on it, pulley it for 7-8 psi, it will be fine.. 10-1 compression is fine for boost as long as the fuel system supports it and the timing is set correctly. Read plugs, run good quality 93 octane and you will be ok..

8-1 compression will make that engine a dog.. This ain't the 70s any more...

True hyperutectic pistons don't like to be rattled and they will break ring lands if you detonate them or put so much heat in it that you butt the ring ends together but... It's not likely its gonna happen on 6-7 psi of boost if the timing is right. Tune up is everything here.

If you really wanted to get trick with it, convert it to E85, then run more boost and timing safely.. You could also go with a meth injection setup and stick with 93 oct but... That's really overkill for 6-7 psi...and 450 crank hp..


All this being said, you would likely make more power and more safely doing a head/cam/intake swap.. The problem with that is finding a good aftermarket head that you can keep the compression up with since you have 58cc chambers now. Maybe the AFR 195 eliminators with 64cc, then cut them down to 60cc.. Run a thin shim head gasket.. Probably still only end up at 9.5-9.7 or so.. You could easily make 450hp with that 350zz4 bottom end with the right stuff on top.

brando1118

http://www.whipplesuperchargers.com/...sp?ProdID=1171

CanadaGrant

8-1 compression will make that engine a dog.. This ain't the 70s any more...

Why would anyone reduce compression just to install a blower to increase things? Kind of a waste of time.