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Maximum Dynamic Compression Ratio for 93 octane pump gas
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jeep_406
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1972 Chevrolet Nova

PostPosted: Fri Dec 26, 2014 12:52 am    Post subject: Maximum Dynamic Compression Ratio for 93 octane pump gas Reply with quote

Hope Santa was good to you. Boy it's been too long since I checked in. I'm finally rebuilding the 408 SBC for my 72 Nova. Switching from Sportsman II cast heads to AFR 195 Competition angle plug heads. Figuring on going with flat top pistons. Can't find a defining limit for DRC using 93 octane pump gas. Think it used to be around 8.5-8.7 with aluminum heads. I've heard the number 9.0 bandied about with the 93 octane with 10% ethanol. Using SMOKEmUP auto math with the cam I'm looking at the DRC would be around 9.0 What do you think?


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osmiumfoot
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PostPosted: Sat Dec 27, 2014 1:05 am    Post subject: Reply with quote

I run 91 octane, 34 total degrees of timing and 10.3 CR with aluminum heads and to my knowledge have never rattled. A lot of overlap helps.
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jeep_406
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1972 Chevrolet Nova

PostPosted: Sat Dec 27, 2014 2:01 am    Post subject: Reply with quote

The cam I was looking at (12-450-8 hydraulic roller) has the intake closing at 69 ABDC.
That would give me 11.32-1 compression and a DCR of 8.728 with 65cc chambers.

I think David Vizard suggested 8.5 to 8.7 on pump gas. That was before the switch to 10% ethenol. I've heard guys talking of running 9.0 DCR on 93 octane but I haven't found that in print anywhere.

I drive the car on the street more than I do at the track now. I pretty much only run Nostalgia Events now with some Wednesday afternoon Test & Tunes thrown in. With the old setup I've gone 11.07 @ 123+mph. I want to keep the torque downstairs as the car is pretty heavy and my 60 foots are usually 1.50-1.52 with ET Streets. My best 60 foot was 1.48 with a 3718 pound car with my not so skinny body on board.

With the Sportsman II's I was running 10.14-1 compression with the heads cut to 56cc's. I has a dished piston. Cam was the 12-433-8 Hydraulic Roller. It made 530 HP at 5900 RPM. The heads flowed 249 at .550 on the intake and 214 at .550 on the exhaust. I use 1.6 rockers on both intake & exhaust.


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SMOKEmUP
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1979 Chevrolet Camaro

PostPosted: Sun Dec 28, 2014 12:15 am    Post subject: Reply with quote

Hey Jeep...long time. Merry XMas to you as well.

I'm running 8.5:1 DCR with my LS motor on 93 and haven't had any issues with knock.

I've seen auto-ignition temperature ratings for 93 octane to be around 860 degrees F. If the intake air temp entering the motor on a warm day is 100 degrees and no heat from the motor is transferred to the intake air charge, only raised by the compression. Then the limit would be around 8.6:1.

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jeep_406
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Joined: 12 Sep 2002
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1972 Chevrolet Nova

PostPosted: Sun Dec 28, 2014 12:56 am    Post subject: Reply with quote

Great to hear from you. I've decided to freshen up the 408. Leak down was around 12%. Going to go with a set of AFR 195 Competition Heads which will not only save about 60 pounds, they'll flow considerably better than the Cast Iron Sportsman II's.

I was running the stock block with 2 bolt mains. I'm having Golen Engine install splayed 4 bolt caps on it. The main caps were moving around a little.

I'm also thinking of running a Weiand Stealth Air Strike Dual Plane intake. Engine Master Magazine did an article where they picked up quite a bit of low end without losing much at 6000 rpm.

I run an air box that Isolates the carb intake from the engine compartment heat. I don't plan on running in extreme heat. The first Nostalgia race is in May. See if we can make it by then. Who knows. . . I'm getting old!

Chad bought a new Superflow dyno. Maybe we'll try a few things. I'll keep you updated.
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af2
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1933 Willys Coupe

PostPosted: Sun Dec 28, 2014 2:17 am    Post subject: Reply with quote

Gene, you have a PM as I don't want to quote on here. But will say you need a bigger/ overlap cam.
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af2
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1933 Willys Coupe

PostPosted: Sun Dec 28, 2014 2:37 am    Post subject: Reply with quote

SMOKEmUP wrote:
Hey Jeep...long time. Merry XMas to you as well.

I'm running 8.5:1 DCR with my LS motor on 93 and haven't had any issues with knock.

I've seen auto-ignition temperature ratings for 93 octane to be around 860 degrees F. If the intake air temp entering the motor on a warm day is 100 degrees and no heat from the motor is transferred to the intake air charge, only raised by the compression. Then the limit would be around 8.6:1.


I blew right threw Smokes post..... He answered the question.

With the heads you are going to run the chamber should be close to optimal to what Smoke is using.
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jeep_406
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1972 Chevrolet Nova

PostPosted: Thu Jan 01, 2015 11:03 am    Post subject: Reply with quote

Smoke,
I isolate the carb inlet from the engine compartment temperatures. Couldn't see myself running at New England Dragway with ambient temps anywhere near 100 degrees. With the package I was running the engine ran cold. I could go round & round and the temp would get no warmer than 180 degrees. I run it with the cooling fans on. Never made any difference adding heat or with the dual cooling fans off.

A 67cc chamber would work out to 11.11 Static with a DCR of 8.55
A 66cc chamber would work out to 11.21 Static with a DCR of 8.64

Probably best to stay safe with the 67cc head. It's only a couple of HP difference. Either package should get me into the 10's even on a bad air day.
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af2
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1933 Willys Coupe

PostPosted: Sun Jan 04, 2015 10:12 pm    Post subject: Reply with quote

jeep_406 wrote:


A 67cc chamber would work out to 11.11 Static with a DCR of 8.55
A 66cc chamber would work out to 11.21 Static with a DCR of 8.64

Probably best to stay safe with the 67cc head. It's only a couple of HP difference. Either package should get me into the 10's even on a bad air day.
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I would go with a 64 cc chamber and a little more cam to keep the static high. But that is me and my thought of the blow down theory.
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Paul P
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1971 Chevrolet Chevelle

PostPosted: Mon Jan 05, 2015 3:10 pm    Post subject: Reply with quote

Hi Gene,
How much RPM do you plan for this combo? Just curious.
Paul

_________________
2001 Focus 2.0 Zetec
stock cams, bolt-ons and tune
15.63@87 MPH 1/4mi

1971 - Chevelle 408 SBC N/A
6.86@102.5 MPH 1/8mi
10.78@122 MPH 1/4mi
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jeep_406
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1972 Chevrolet Nova

PostPosted: Tue Jan 06, 2015 12:18 am    Post subject: Reply with quote

Paul I was going through the eyes around 6400. I've been looking at 3 cams from Comp Cams. 12-468-8 has quick cam lobes. According to Comp it should make a bunch of HP @ 6500 RPM with 565 Torque at 5500 RPM with 11.86-1 compression. The Dynamic Compression Ratio would be 8.645 I'm worried that it might give up too much on the lower end.
I made 530 HP at 5900 RPM on Chad's dyno with the old setup.

Another new cam 12-467-8 also has quick lobes but makes power much lower. It'll make just under 600 HP with over 500 foot pounds of torque from 3500 through 6 grand. The Static Compression would be 11.2-1 with a DCR of 8.63. I think this puppy should be more streetable. Being streetable is important. I only run at the Nostalgia Events now and a some Test Sessions, maybe a total of a dozen or so days.

One more cam I was looking at 12-450-8 is a little bit of both. The first two have a Lobe Centerline of 113 with an Intake Centerline of 109. The last one has a 110/106 setup.

I'm a little dizzy from looking at cam profiles.

I'm going up to see Paul Rinaldi at Chad's and see what he thinks.

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Paul P
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1971 Chevrolet Chevelle

PostPosted: Tue Jan 06, 2015 5:07 pm    Post subject: Reply with quote

That torque number is really big! Say Hi for me. I have Patrick at Pro systems helping me today. Might pull the trigger on a carb....
_________________
2001 Focus 2.0 Zetec
stock cams, bolt-ons and tune
15.63@87 MPH 1/4mi

1971 - Chevelle 408 SBC N/A
6.86@102.5 MPH 1/8mi
10.78@122 MPH 1/4mi
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jeep_406
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Joined: 12 Sep 2002
Posts: 1661
Location: Tewksbury, Mass 01876
53123.48 points


1972 Chevrolet Nova

PostPosted: Fri Mar 27, 2015 10:44 pm    Post subject: Reply with quote

We decided on going with a Comp Cams Hydraulic Roller using cam lobes 3017/3038 with the 4/7 firing order change. The machine shop I use, Golen Engine Service in Hudson, NH, has a new Superflow Dyno. Hope to check out my 408 in the next month or so. Looking to improve the whole power range. I think it's going to make my big boat dance.
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Paul P
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1971 Chevrolet Chevelle

PostPosted: Fri Mar 27, 2015 11:12 pm    Post subject: Reply with quote

Hi Gene,
What rocker ratio are you using with that cam? 1.7 would get you over .600" lift. 236/242 duration intake/exhaust @.050 should be very driveable with that displacement. My Crower cam that I first had in the 406 was in that range and it didn't even sound like it had a cam in it. Let me know when you will be on the dyno. I might be able to come by for a look.
Paul

_________________
2001 Focus 2.0 Zetec
stock cams, bolt-ons and tune
15.63@87 MPH 1/4mi

1971 - Chevelle 408 SBC N/A
6.86@102.5 MPH 1/8mi
10.78@122 MPH 1/4mi
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jeep_406
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Joined: 12 Sep 2002
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Location: Tewksbury, Mass 01876
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1972 Chevrolet Nova

PostPosted: Fri Mar 27, 2015 11:47 pm    Post subject: Reply with quote

Paul, I'm going to try 1.6's because that's what I have.
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