[NWayne]

Whats the most compression I can run with 64cc Vortec heads and still use pump gas?
The combo I'm looking at is,

383 With Probe Forged Flat top 4 Valve Relief Pistons FPS3831

EAGLE SIR 5140 FORGED STEEL RODS SIR5700BPLW

EAGLE CAST STEEL/ NODULAR IRON 3.75" CRANK 2 PC REAR MAIN SEAL 103503750

Lunati Bracket master II #00010 230/230 with 480/480 lift.

Compression ratio 10.8:1
Is this too much compression for pump gas?

I can get this same setup with dish pistons but it drops the compression down around 9:1. I'm afraid that would be too low for the cam I want to run.

Thanks

[NWayne]

I just skimmed through another post and came to the conclusion that this is probably way too much compression for pump gas.
So would 9:1 work out on this combo, or with the cam I'm going to be using?
I already have the cam and heads so that's not going to change.
It's either this or go back to my orginal plan and build the basicaly stock 350 with 9.5:1 compression.

Thanks

[curtis73]

Careful tuning and you'll be OK. I've seen pump gas 10.5:1 with vortecs. 10.8 is asking a lot, but you have the advantage of the fast burning heads and you only need about 30* total advance.

Focus on ignition curve and fuel curves. Don't let it get lean anywhere. I also suggest (for once) running manifold vacuum. Mileage will suffer a bit, but idle tuning and detonation will be helped. Suggestions that may add up to help you..........
Run a 180 stat instead of 195. Don't even think about electric fans... a good mechanical fan and shroud. Use a higher volume water pump with a smaller pulley, and consider upgrading your radiator from whatever it is to something better. Taking the time and money to make it thermally stable means that once you set it, you can expect it to perform that way in summer, winter, idling, and on the highway. Polishing the chambers will do two things; add a couple cc to them, and remove the peaks and valleys of the casting. Removing peaks removes hot spots and prevents detonation. Cold air and cold fuel will help. Run one cooler range spark plug if you find they're too hot.

Many vortec engines run 9.5:1 on 87 from the factory. They of course have the benefit of EFI and knock sensors. 10.8 is on the ragged edge and you'll have to sacrifice some timing (meaning you'll have to crutch the combo and make it not optimal tune) but it should be do-able. The first thing I'd consider is polishing the chambers, grooving the quench (controversial, I know), and carefully tuning the fuel and spark curves. If you still find detonation after that, then consider the rest.

[NWayne]

Originally Posted by curtis73

Careful tuning and you'll be OK. I've seen pump gas 10.5:1 with vortecs. 10.8 is asking a lot, but you have the advantage of the fast burning heads and you only need about 30* total advance.

Focus on ignition curve and fuel curves. Don't let it get lean anywhere. I also suggest (for once) running manifold vacuum. Mileage will suffer a bit, but idle tuning and detonation will be helped. Suggestions that may add up to help you..........
Run a 180 stat instead of 195. Don't even think about electric fans... a good mechanical fan and shroud. Use a higher volume water pump with a smaller pulley, and consider upgrading your radiator from whatever it is to something better. Taking the time and money to make it thermally stable means that once you set it, you can expect it to perform that way in summer, winter, idling, and on the highway. Polishing the chambers will do two things; add a couple cc to them, and remove the peaks and valleys of the casting. Removing peaks removes hot spots and prevents detonation. Cold air and cold fuel will help. Run one cooler range spark plug if you find they're too hot.

Many vortec engines run 9.5:1 on 87 from the factory. They of course have the benefit of EFI and knock sensors. 10.8 is on the ragged edge and you'll have to sacrifice some timing (meaning you'll have to crutch the combo and make it not optimal tune) but it should be do-able. The first thing I'd consider is polishing the chambers, grooving the quench (controversial, I know), and carefully tuning the fuel and spark curves. If you still find detonation after that, then consider the rest.

Thanks for the suggestions.
If I can keep the same cam and do it with around 9:1 compression that's probably the way I'll go. Otherwise I think I'll just go back to the 350 with 9.5:1 and keep it safe.

[Richard H.]

Originally Posted by curtis73

Careful tuning and you'll be OK. I've seen pump gas 10.5:1 with vortecs. 10.8 is asking a lot, but you have the advantage of the fast burning heads and you only need about 30* total advance.

Focus on ignition curve and fuel curves. Don't let it get lean anywhere. I also suggest (for once) running manifold vacuum. Mileage will suffer a bit, but idle tuning and detonation will be helped. Suggestions that may add up to help you..........
Run a 180 stat instead of 195. Don't even think about electric fans... a good mechanical fan and shroud. Use a higher volume water pump with a smaller pulley, and consider upgrading your radiator from whatever it is to something better. Taking the time and money to make it thermally stable means that once you set it, you can expect it to perform that way in summer, winter, idling, and on the highway. Polishing the chambers will do two things; add a couple cc to them, and remove the peaks and valleys of the casting. Removing peaks removes hot spots and prevents detonation. Cold air and cold fuel will help. Run one cooler range spark plug if you find they're too hot.

Many vortec engines run 9.5:1 on 87 from the factory. They of course have the benefit of EFI and knock sensors. 10.8 is on the ragged edge and you'll have to sacrifice some timing (meaning you'll have to crutch the combo and make it not optimal tune) but it should be do-able. The first thing I'd consider is polishing the chambers, grooving the quench (controversial, I know), and carefully tuning the fuel and spark curves. If you still find detonation after that, then consider the rest.

That is a dead on answer Curtis, good on you! I fired up the same set up only at 10.5 cr last week. I now have a re-curve kit coming and with a box fan and the engine fan, new radiator ,180* stat I was still pushing 200*.

[Hippie]

Read this..........

Pat Kelley's DCR Page

and then this........

DCR Combos that worked.

I know most of these guys from Chevy Talk, this is the straight scoop, no BS. And by that I do NOT mean curtis73 response was BS! Good info there too, I mean these guys aren't spouting theory or quoting magazines, these are actual builds.

[UR50SLO]

The 406 I built with Edelbrock heads (RPM) and flat top's was 10.7-1 with a Comp Nitrous HP cam on a 113LCA (mid 5's lift)
Ran 92 and sprayed 150hp... No problems with correct timing and plugs.
Daily drove it for several years.

Just my experience... Can't go wrong with low compression... You won't gain that much power from it anyway.

Had a local guy running a 72 Nova go from 12-1 to 13.5-1 (355 race setup)
I told him he wouldn't gain a tenth... sure enough.. no more power.

I'm sure there's variables but I'm not a big believer in High Compression for power.
~
Scott

[baddbob]

You may be able to retard the cam some and get the cranking compression around 180-200 which should work on premium-93 octane, run a cool thermostat 160 degrees and pull some timing out with a total around 34 give it a whirl and I bet it'll work.

[SlowGTA]

A little more info about the car and what you wan't to do with it would help.
If this is a heavy daily driver with an auto and tall gears (3.08ish) than 10.8 is going to give you trouble. If this is a fairly light weekend racer/occasional street car with a manual or loose converter it won't be as bad as you think...

First thing I would do is CC the heads seeing as you already have them. The vortecs are a great head for the money and can make good power but they are a production part with GM production tolerances. I know they're advertised at 64cc's but I've seen them as small as 60 and as big as 70cc's out of the box. Buy a cheap cc'ing kit and see what yours actually are. This might change the whole picture for you.

[red65mustang]

here is a link to 100+(355 and 383) proven combos with build spec and CR and dyno results

http://ryanscarpage.50megs.com/combos5.html

note that none are higher than 10.0......most are 9.0....
why? a more aggresive spark timing curve yields way more HP and TQ over the whole rpm range

increasing CR only increases HP and TQ at the "peak" rpms

strip or race only car= high CR....you are always at the hp peak

question: you are at what altitude in NV, above 4,000' "everything" changes

[firestone]

Comp has a cam that pretty much exactly the same as the lunati cam you are looking at (280H 230 230@.050 .480 lift 110 lsa) and the cam one step bigger needs 9:1. That would tell me that 9:1 with yours would be fine.

Here is the comp catalog
http://www.compcams.com/technical/Ca...67_226-227.pdf

Adam

[NWayne]

Originally Posted by SlowGTA

A little more info about the car and what you wan't to do with it would help.
If this is a heavy daily driver with an auto and tall gears (3.08ish) than 10.8 is going to give you trouble. If this is a fairly light weekend racer/occasional street car with a manual or loose converter it won't be as bad as you think...

First thing I would do is CC the heads seeing as you already have them. The vortecs are a great head for the money and can make good power but they are a production part with GM production tolerances. I know they're advertised at 64cc's but I've seen them as small as 60 and as big as 70cc's out of the box. Buy a cheap cc'ing kit and see what yours actually are. This might change the whole picture for you.

The car is a 64 Nova, weight approximately 2900lbs. It will be mainly street driven, but with an occasional blast down the strip.
350 tranny
2500+ stall
3.08 gears for the time being. Will be switched at some point to 3.73's.

The heads have been cc'd and are right at 64cc. They have also had some minor port work on the exhaust side.

Thanks

[killerformula]

From the builds above and links, you can see that people are running high compression and still filling up at a normal pump. You should only build a motor that you're comfortable with, however. Some guys can run a bit more compression on pump gas than others because they have a lot of experience tuning carbs, working with heads and timing. If you're going to be on the street and we're talking about a small-block, I'm assuming you won't be Turing over 6000 R's. That means we're talking about a cam with maybe a 2200-6000 RPM range which probably needs about 9.5:1 to work properly. If it were my motor, I would let sleeping dogs lie for the extra few horses you're going to get off that extra point if you're not experienced running high compression on street gas.

K

[red65mustang]

NWayne,
you didn't answer my altitude question so I will just say:

CR ratio calculations are based on sea level air mass density.....

you need to adjust the calculations for each additional 1,000' higher altitude

ex: a motor that tests 180psi at sea level will test approx 150psi at 5,000'

[NWayne]

Originally Posted by red65mustang

NWayne,
you didn't answer my altitude question so I will just say:

CR ratio calculations are based on sea level air mass density.....

you need to adjust the calculations for each additional 1,000' higher altitude

ex: a motor that tests 180psi at sea level will test approx 150psi at 5,000'

That might be because I'm not sure. I've never paid attention. I would have to look it up.