Building a New Engine, What CR?

Working on a new engine for my T-Bucket and I have no idea what compression ratio I'll end up with...

I have just got my 1979 350 5.7 block back from the shop, it's been bored .30 over, I have new flat top pistons and I'm gonna run a 327 350hp cam. I'm hoping this configuration will be at or under 9.5 compression ratio so I can use pump fuel and drive it anywhere.

The intake is a Edelbrock Performer with a Edelbrock 600 carb. The heads are 1.94 valves & 76 chamber

Sure hope I haven't goofed on the configuration.

Can someone give me a ballpark on what the heck I'm putting together here? Thanks

 

You're good to go.

That combo will vary between 8.7 and 9.3ish depending on the exact piston, if the block or heads have been surfaced, and the gasket type. So if anything you're estimating on the high side, which is the way to go for pump gas. Don't be suprised if it's happy on 89 octane mid grade.

Good luck

 

Thank-you for the quick response!

 

If I were you I'd check the deck height of the pistons and choose a head gasket that will tighten up the quench. I believe most folks shoot for .060". This will promote turbulence in the chamber and give you more octane tolerance, then you should be able to use regular grade unleaded to full advantage with no penalty (like retarding the timing to avoid knock).

P.S. Look for any sharp edges in the chambers and round over a bit, this will avoid hot spots.

 

Actually if you are using the typical flat top with 4 valve reliefs, a composite style gasket, and the deck height is the typical .020 to .040 with a 76cc head you're probably in the 8.3 to 8.4 range. Easy to get a burrette and check it to be accurate.

Don't know if you care about the hp or not but by our experience on our engine dyno a combo like this only makes about 250hp. We actually just did a motor with that cam and 8.3 and that's exactly what it made. 8.5 is a dead zone on a naturally aspirated engine. It'll never run really strong.

Spend the extra money to zero deck it and do what you have to do to get it up to about 9.5 (thinner gasket, mill the heads etc). You'll likely get 300hp out of it and it'll still run great on 89 octane if you tune it right. You may actually have less detonation trouble with the extra compression and get better mileage if you get some quench with this set up.

Hope this helps and I'm not trying to criticize your build. You may be entirely happy with what you've got. Just offering advice to help you get some extra bang for your buck. PM me if you have any questions. Have fun with your project.

 

What HP range will I end up with? Is that cam OK or outdated? I purchased it about 4 months ago. Thanks

 

That cam is OK but the grind is pretty old school. It sounds decent with a very slight lope to it. It will make a super flat torque curve which is good but the power curve will be all over at 5000rpm. Keep in mind with a T-bucket torque aint that important cause the car is so light. A cam that really thumps will still pull it around fine even on the low end. Getting the compression up is the main thing. At 8.5 you can put just about any cam you want in it and you're still gonna make 250hp. Build something 9.5, put a cam in it around 230-235 at .050 with an LSA of 106-108 (that'll make it sound awesome), MSD ignition, nice dual plane intake, headers, and you can make 350hp easy with no driveability problems and a sound that scares kids and old people. If you want to spend some money on set of aftermarket heads that will flow some air it'll be more like 400hp and you'll scare yourself to death in that T-bucket. Later, Greg