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Discussion Starter · #1 ·
I have a 79 that started life with a 301, then had a 400 installed. I just finished my first ever attempt at a build - a 455 bored 60 over, summit 2802 cam, Torker 2 intake, Holley Street Avenger 770 (maybe too small?),6X-4 heads, summit long tube headers with purple hornies bolted straight to the end of the headers.

Thanks to books by Jim Hand and Rocky Rotella, a few calls to Butler performance and some youtube videos, once I got it dropped in it actually started right up.

My neighbors want me to rethink the exhaust.

Keep in mind I am a rookie, so be gentle if the question is a dumb one.

I am curious as to what my compression ratio might be. According to what I have read, it appears that standard 6X-4 heads bolted to a 455 will produce over 9:1 compression. These appear to have been cut down, because I had to get the intake cut down the equivalent of the intake manifold gaskets to bolt it on. My understanding is that this combined with the over bore will increase the compression ratio.

Any input/thoughts/advice appreciated.
 

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To be exact you would need to have your heads CC to determine actual volume, also need distance from deck to piston, head gasket thickness, cc of valve relief head gasket bore diam, etc.. Just rough guess if they milled .04 off the head you would have dropped from 94 cc to 90 cc. with this assuming .015 in the hole for pistons and standard felpro head gasket I come up around 9.8:1. you really need to know a little more info to be closer, and with you CR being close to 10:1 you need to be very cautious of detonation (run the highest octane fuel you have available and keep your timing adv bellow 34 deg) until you confirm no detonation. Your cam 2802 should bleed off some cylinder pressure keeping your dynamic CR down a little and if your piston height to deck is closer to 0 deck then the tight quench will help prevent detonation.

Congratulations on your first build, hope it works out well and you have many years of enjoyment.
 

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Discussion Starter · #3 ·
Thanks for the input. I know I have too many blanks in the equation to get an exact CR, should have documented better. Will on the next one. You confirmed my suspicion though that I am a lot closer (or at) 10:1 than maybe I wanted to be.

Currently running 93 in it.

Other than detonation, is there a risk of damaging the heads or other components because of the CR? I have a stock starter, is it going to have a shorter lifespan?

I've only run it a few minutes so far, and haven't driven yet, but that should happen this weekend. Can't wait.
 

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I see you've posted the same topic for a 3rd time. I had to delete the first duplicate.

Please post a topic only once. When you have multiple threads on the same topic. people may be making an effort to provide an answer that has already been provided in the duplicate thread. Really gets confusing. We've found you get the best results with a single thread.

Let me know if you want to delete this thread, or the one on Second Generation. I can merge the responses into one thread if desired. Just pick which one you want to keep active.
 

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Discussion Starter · #5 ·
Sorry about that. I'd like to keep this one active, seems a more appropriate location.

I hit the back button on my tablet when posting in the second gen forum and it posted twice for some reason.

I'll be more careful moving forward. I knew better than that.....
 

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"...Other than detonation, is there a risk of damaging the heads or other components because of the CR? I have a stock starter, is it going to have a shorter lifespan?..."

The detonation IS what causes the damage. It can break pistons and rings, as well as beating up on the the rods bearings. :(

As mentioned, you must know all the specs on your engine, in order to figure your CR. This link will show you exactly what specs must be known.

http://www.wallaceracing.com/cr_test2.php

If you hear it "pinging", try reducing the timing slightly, til the pinging stops. It may also help to unhook and plug the vac to the vac advance on the dist. Some stock type vac advance cans add way too much advance to your timing. They sell adjustable vac advance, and mechanical advance kits, which will allow you to dial in your timing curve and max total timing. About the only other solution is either to mix in a little racing gas with your 93 pump gas, or switch to E-85, if it is available in your area.

I fought those old stock type starters for a long time. Now, I wouldn't use anything less than a Summit mini starter. :smile22:

http://www.summitracing.com/parts/sum-820311/overview/make/pontiac
 

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Discussion Starter · #8 ·
Ponyakr - I almost bought that starter. I have the luxury of living about 20 minutes away from the Georgia branch of Summit Racing. Sometimes that can be a problem.

I didn't hear any detonation when I started it, just sounds like I need to be careful.

I'm a little frustrated because I wasn't able to do anything with it for the past few days. Went out today to start it up and it turns over just fine but won't start. Got fuel and air, and with a very small shot of starter fluid it kicks over but won't stay running. I didn't change anything.

I may have just hijacked my own thread, but any suggestions would be appreciated.
Tjs - I'll give that a shot
 

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Just for grins do a pumping comp test.If it pumps over 170-175 it may have a detonation issue.Tom
No wonder my velle wouldnt run on pump gas....210-218psi LOL

It sure loved Sunoco 116 though. I shoulda ran alky on it. It had a 460/480 lift comp nitrous cam in it with a high LSA so hardly any bleed off...crappy combination but used 1.6 rockers so bumped up the lift a tad.

(.350" dome, 70cc head chamber, .34" head gasket, .000/.001" deck, .4.036" bore, 3.48" stroke...iron heads)
 

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Bleed off is BS!We put large cams in for a reason!To make more HP which comes from cyc pressure.It may bleed off comp at idle but get it up to where the engine operates and the pressure goes up.Even if you dont hear it does not mean its not there.The silent killer.Tom
 

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Found that out when I discovered my 455ho with 87cc Edelbrock round ports wanted a bit more initial timing, but could not tolerate a bunch of total when the Comp roller hydraulic cam I put in along with a bump in compression really started to pack the cylinders in the midrange. I do think that the more you know about your distributor so you can tune both mechanical and vacuum characteristics, the more you can push the limit on octane. Also the weight of the car, and gearing have an effect on the load the engine feels.
 
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