Compression ratio concerns

[miller]

So I think I have to settle with 9.01:1 CR.

Online there's CR calculators you can play with. It's all just a numbers game.
The pistons you use, and head gasket thickness are the main things to look at. Though must keep track of any clean-up cuts made on the block and head surfaces.

 

[RRSweden]

Been reading the three hotrod articles about head-porting. Very interesting reading.
I thought the 30 degree back-cutting of the valves sounds pretty easy achieved http://www.hotrod.com/articles/valve-angle/
I think it sounds like something you could do by yourself and a hand-drill.. Am I way wrong?

Edit: Now I get it... first I thought they were talking about the "stem" of the valve... (I blame it on the language-barrier ;)

 

[RRSweden]

Been reading the three hotrod articles about head-porting. Very interesting reading.
I thought the 30 degree back-cutting of the valves sounds pretty easy achieved http://www.hotrod.com/articles/valve-angle/
I think it sounds like something you could do by yourself and a hand-drill.. Am I way wrong?

I find it strange that if there's, as the articles states, like 15% of flow to gain by cutting the back of the valves, then why are they designed that way? Even the aftermarket valves from Manley, Milodon etc. needs to be modified. Seems a little dumb to me..

 

[Garys1969RR]

The flow can be improved by back cutting the intake valves. The Eddys I have came that way. Passenger car valves need to go 100 K miles or more, so the factory builds for longevity, mainly. There are lots of small things you can do to improve flow, and those articles will steer you in the right direction. My 915s worked great after I did the porting, following those guidelines. My 3600 pound RR ran consistent low 12s at 5600 feet. Thats like mid 11s at sea level.

 

[Garys1969RR]

I have back cut valves with a valve grinder or lathe. Spinning the valve in a drill works good for cleaning and polishing the valve with emery cloth.

 

[RRSweden]

I have located a set of 915 heads for sale now.
The valves have been replaced with bigger ones. 2.14 int, 1,80 exh.

He asks 6000SEK for them (about 700 dollars).

So now I have to consider werther to buy them or not... If they measure 78cc in the chambers they would raise my CR to 9.91:1

So now I need some advise... What difference would this do in terms of cam selection and what other parameters would this change affect?

 

[dvw]

Bolt on a set of Edelbrock heads, 484 Purple shaft, performer RPM, 600 Eddy carb, 3.91, headers. Run 12s with the stock converter and 255 street tires. Dead stock unmachined smog short block. drive and race anywhere on 86 octane.
Doug

 

[RRSweden]

Bolt on a set of Edelbrock heads, 484 Purple shaft, performer RPM, 600 Eddy carb, 3.91, headers. Run 12s with the stock converter and 255 street tires. Dead stock unmachined smog short block. drive and race anywhere on 86 octane.
Doug

Thanks, but that don't really answer my question ;)

 

[RRSweden]

This is what I'm offered..... The guy says it's surface rust that probably will buff off...

 

[Garys1969RR]

OK so you're at .028" below deck, .020" head gasket, 915 heads at 79 cc, and your C Ratio will be 9.9 to 1. The 906 heads are 88 cc and will get you 8.9 to 1. The 915 heads will give you better squish/quench, so your'e better off there. A cam of .480 to .510 lift should work well. Duration around 284 is good. You may want to see if he will be interested in trading parts to lower the cost. I got my 915s like that.

 

[Garys1969RR]

By the time you get the heads, clean them up, valve job, porting etc, you may have more in them than you care to spend. How is the current valve job? I bought some 906s once, lapped in the valves, threw them on a 451 and it ran great! You just never know till you try it. It's all a learning experience. After this one, you will have a better idea of how to do the next one.

 

[RRSweden]

So now I made up my mind, and decided to go for the 915 heads. Will get them the day after tomorrow!
I can only tell from the pics he sent me, and that he claims they ran really good the few miles he had them on before upgrading to alu.heads. He also made a compression test with really poor piston rings, and they did good on that test. So I'm hoping theres not too much work left on them.
I'll probably try to carefully port them a little bit...

...I'll post pics when I get them!

 

[Banzaiii67]

I have a real similar build on my 383.. Here's a quick video of what mine sounds like.

 

[RRSweden]

I have a real similar build on my 383.. Here's a quick video of what mine sounds like.

Sounds great!
Tell us about the setup! carb, cam etc.
Did you have it dyno-tested?

 

[Banzaiii67]

Sounds great!
Tell us about the setup! carb, cam etc.
Did you have it dyno-tested?

'68 383
9.1:1 compression (KB162 Pistons down in the hole .024, steel shim head gasket, entire short block was balanced just not decked..ARGH!)
516 Heads w/ 2.08-1.74 valves with stock non-adj rocker arms
Comp XE274H cam
Doug Thorley 1 3/4" Headers
FBO A688 Electronic Ignition
Edelbrock Performer RPM Intake
Quick Fuel SS 680 VS
Stock 5/16" fuel line with mechanical fuel pump
Jones Max Flow 2 1/2" mufflers, turn-downs at rear axle.

727 Auto with Transgo TF2Shift Kit
11" Turbo Action 2800 TC

8 3/4" 489 powerloc with 3.73 gears

Runs on 89 octane all day with no pinging, although i usually run 92 non-ethanol My timing is 19*initial/35* total.

No dyno testing or time slips.. it's a street car mostly, hope to make it to the track this year.

If i could make one change, I would have had Engle make me a K56/58 Cam..

 

[RRSweden]

'68 383
9.1:1 compression (KB162 Pistons down in the hole .024, steel shim head gasket, entire short block was balanced just not decked..ARGH!)
516 Heads w/ 2.08-1.74 valves with stock non-adj rocker arms
Comp XE274H cam
Doug Thorley 1 3/4" Headers
FBO A688 Electronic Ignition
Edelbrock Performer RPM Intake
Quick Fuel SS 680 VS
Stock 5/16" fuel line with mechanical fuel pump
Jones Max Flow 2 1/2" mufflers, turn-downs at rear axle.

727 Auto with Transgo TF2Shift Kit
11" Turbo Action 2800 TC

8 3/4" 489 powerloc with 3.73 gears

Runs on 89 octane all day with no pinging, although i usually run 92 non-ethanol My timing is 19*initial/35* total.

No dyno testing or time slips.. it's a street car mostly, hope to make it to the track this year.

If i c

Sounds similar to my build!
Good to hear you have no pinging issues
The standard fuel in sweden is 95 octane, so I hope I'll be fine.

 

[Banzaiii67]

Sounds similar to my build!
Good to hear you have no pinging issues
The standard fuel in sweden is 95 octane, so I hope I'll be fine.

Here's a combo that will end up very similar to your. It's from Brian Haflinger from IMM Engines. He's a pretty well known Mopar Engine builder on the west coast.

KB 162 pistons
440 source heads. I shim the springs up some.
Mill them so you have 80cc (about .020 off).
Compression will be 9.2 with their gasket and no decking. If you want more you can deck the block some.
Comp XE268 cam, can use stock rockers.
RPM intake
750 holley DP
1 3/4 hooker headers
Balancing is highly recommended!
Use a nice 9 keyway timing set so you can advance the cam 6*. This really helps the engine come alive and is very important to this build!!!!
Use a good dist. that you can lock out the timing so it's fully advanced even at idle.

On my dyno, this engine will crank out a little over 400HP and with head porting about 430HP. In the car, it rips!!

Either way, with the 383, you use a small cam and advance it more than normal and it will reward you with a strong punch!

Brian

 

[RRSweden]

Here's a combo that will end up very similar to your. It's from Brian Haflinger from IMM Engines. He's a pretty well known Mopar Engine builder on the west coast.

KB 162 pistons
440 source heads. I shim the springs up some.
Mill them so you have 80cc (about .020 off).
Compression will be 9.2 with their gasket and no decking. If you want more you can deck the block some.
Comp XE268 cam, can use stock rockers.
RPM intake
750 holley DP
1 3/4 hooker headers
Balancing is highly recommended!
Use a nice 9 keyway timing set so you can advance the cam 6*. This really helps the engine come alive and is very important to this build!!!!
Use a good dist. that you can lock out the timing so it's fully advanced even at idle.

On my dyno, this engine will crank out a little over 400HP and with head porting about 430HP. In the car, it rips!!

Either way, with the 383, you use a small cam and advance it more than normal and it will reward you with a strong punch!

Brian

Souds promising!
One thing that worries me a bit is that I have flat top pistons with no valve reliefs, and on my new (today!) 915 heads ther's bigger valves than my old 906's. I'm also planning to buy the Xe268 cam, wich will replace my old 270/270 magnum cam. The Xe-cam har a couple of thousands more lift as well as 10* longer duration on the exh than the magnum cam...

 

[RRSweden]

Is Brian Haflinger on this forum? Maybe he could tell if my piston/cam-combo will give clearance enough..

 

[RRSweden]

When I disasseblied the new 915 heads THIS shows up under one spring...

What to do? Can I use it as it is, and just prey, or can I fix it in any way do I start to look for another hobby.... ;)