Engine Compression... HELP - Team Camaro Tech
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post #1 of 11 (permalink) Old Feb 8th, 10, 10:53 PM Thread Starter
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Devon R
 
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Location: Kansas
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Engine Compression... HELP

I have a Chevy Small Block .305, and it is a bear to start.

It has been modestly bored out to be a .307,
has 46cc heads on it,
flat top pistons (neither domed or dished),
.040 gasket,
3.48 stroke,
I ran a compression tester in one of the cylinders, it hit 200+ psi, but I couldn't read it as it blew outta my hands, I'll grab a helper tomorrow so I can get an accurate reading on that.
I have plugged these stats into a online compression ratio and it gave me a static compression ratio of 11.81:1, which seems too high for pump gas, which would explain why it is having trouble starting....

Is this ratio correct? (I can give you more stats on it, those are just off the top of my head.) And if it is correct, is there anything I can do to help it run on pump gas(87, 91, and (I think) 97 octane are available in my area.), such as cooler spark plugs, or a longer duration camshaft....this motor does need to be streetable....

Any help would be greatly appreciated.
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post #2 of 11 (permalink) Old Feb 9th, 10, 12:13 AM
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Steiner
 
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Re: Engine Compression... HELP

Just my opinion, but if you're wanting to stay streetable the only way to do it would be to put on a set of more commonly sized 305 heads with a 58cc chamber. That'll bring you down to 10:1 or so. That's just based what you provided and if you are indeed at 11.8:1 currently.

To get the dynamic ratio down to where it would run on pump gas would require a massive cam (for a 307)....something like a 305 advertised duration I would think. That would put the power band waaaay up there and most likely wouldn't be very much fun on the street.

You can find Vortec 305 heads all over the place, even new and cheap from GM (you'll need a Vortec intake)....you just have to make sure they're '96 and up and the correct casting number. Some earlier engines were designated Vortec but had the old swirl port heads that pooped out around 4500 RPM.

'69 Camaro
Dart 400-AFR 195-224/224 HR-Powerjection III TB with F.A.S.T. Sportsman XFI
TKO 600-Moser 3.42-Detroit Truetrac
500hp/538lbft

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post #3 of 11 (permalink) Old Feb 9th, 10, 05:12 AM
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Bill
 
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Re: Engine Compression... HELP

Devon,
If it does indeed have 46cc heads on it, the compression you came up with is about right. I really do not think it is going to run on any type of pump gas that I know of unless maybe you have E85 available ?

What heads do you have on it that are 46cc ?? I think the stock 305 heads are around 58cc ?

Even so ,,,,, you should not have a problem with starting. There are plenty of higher compression engines out there and a decent mini starter should turn it over without any issues at all.

Bill Koustenis
Owner
Advanced Automotive Machine
Waldorf Md
1971 Chevelle "Heavy Chevy" original owner


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post #4 of 11 (permalink) Old Feb 9th, 10, 05:51 AM
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Steiner
 
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Re: Engine Compression... HELP

He didn't say, but I think the issue given his user name may be that he's trying to run 11.8:1 with an '87 factory duration cam. Since those motors weren't exactly revvers I can't begin to imagine how low the duration might be if so.

'69 Camaro
Dart 400-AFR 195-224/224 HR-Powerjection III TB with F.A.S.T. Sportsman XFI
TKO 600-Moser 3.42-Detroit Truetrac
500hp/538lbft

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'69 Camaro Beater-SFT 327-M20-Moser 4.10-sold
'02 Z/28 vert-stock-sold and totaled
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post #5 of 11 (permalink) Old Feb 9th, 10, 09:29 AM
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Charlie
 
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Re: Engine Compression... HELP

When you say it's a bear to start, can you be a little more specific? Does the starter have a hard time turning over or does it just crank and crank before it fires? When you took the compression test, did you do every cylinder? Just taking a reading on 1 cylinder will not tell you what the other 7 are making. It may have an issue in another cylinder. Does the hard starting issue occur all the time or only when hot? There are other factors we need to know.

Charlie Knudsen 67 Camaro RS/SS 350 TKO 600, 4.10's, AFR 195 Street Heads, Comp 270H, Rhoads Lifters, Crane Ignition, Pertronix Dizzy, Performer RPM Q-Jet, Heavily Modified Q-Jet, Hooker Comp Headers, MagnaFlow 2 1/2", 144 Decibles(Alpine CDA-7998 Head Unit, RF X7 and RF 750X Amps, MB Quart Mid's & High's, Kicker L7 12" Solo Barics)
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post #6 of 11 (permalink) Old Feb 9th, 10, 11:33 AM
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AJ
 
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Re: Engine Compression... HELP

Did you guys factor in the stock deck height?
Stock type 4 valve relief pistons relief's are like 2cc's a piece
I got 10.4:1 using a calculator.

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'14 5.0 Mustang
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post #7 of 11 (permalink) Old Feb 9th, 10, 12:08 PM
kt
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kelly
 
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Re: Engine Compression... HELP

Cant see cylinder pressure(to high) cause a starting issue, please correct me if i am wrong...

Kelly
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post #8 of 11 (permalink) Old Feb 9th, 10, 12:21 PM
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AJ
 
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Re: Engine Compression... HELP

Quote:
Originally Posted by kt View Post
Cant see cylinder pressure(to high) cause a starting issue, please correct me if i am wrong...

Kelly
I was thinking the same, if your cylinder pressure is too low, it will actually take longer to start to build the pressure it needs.
Isn't cylinder the product or good ring seal and cam more so than STATIC compression?
Can someone clarify this, thats what i'm thinking, could be wrong though.

The only Mustang I'd ever own is a Fender.

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'14 5.0 Mustang
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post #9 of 11 (permalink) Old Feb 9th, 10, 02:00 PM Thread Starter
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Devon R
 
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Re: Engine Compression... HELP

Thanks for all the input guys,
to clear things up it is a .305 that has been rebuilt, and it indeed has 46cc heads on it,I bought them on craigslist never used. They have been milled (thus the smaller chambers). The ECM for the motor was ripped out of the car before I bought it, so it is not computerized.... To make matters a bit more confusing I am using a 76' smallblock... So the motor is set up with a holley carb, vacuum advance distributor etc... My cam is not stock it is a mildly warmed up cam on the lower end so I would get better throttle response off the line (.305s can be dogs in that sense). I can check and give the actual specs on the cam to clear up the duration issues.

I installed a 90's S-10 starter for the v6 model so it would fit below the headers, so it is not a mini starter but it cranks fine. When I try and start it it cranks and cranks and will sometimes kick but it never starts. I know the timing is set right, so that isnt a problem.
I have just dropped the motor in the Camaro so I have not drove the car with this motor before...

I am actually at school at the current moment so I can't get those readings on the other 7 cylinders but I can get those readings tonight when I get home..
I have read an Engine masters article recently that took a low compression .305 and got 372 hp out of it by replacing the heads among other things. Those heads were 58cc if I remember correctly. I can grab the article when I get home and see if I'm correct on that. Some one mentioned that 58cc heads would lower my ratio to am more acceptable 10:1.

If anyone needs more info just lemme know...
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post #10 of 11 (permalink) Old Feb 9th, 10, 02:23 PM
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AJ
 
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Re: Engine Compression... HELP

I don't think your compression is over 11:1
Dropping to a smaller cc head will raise the compression, but you need to factor in piston in the hole, head gasket, valve relief cc's, stroke and bore, it's not just based on head chamber size. What did you plug in for the deck height, gasket, stroke, bore, and valve relief cc's online?
Cranking and kicking back could be washing down the cylinders, what is your initial timing set at?

The only Mustang I'd ever own is a Fender.

'68 333" Camaro
'14 5.0 Mustang
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post #11 of 11 (permalink) Old Feb 10th, 10, 06:00 PM Thread Starter
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Devon R
 
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Re: Engine Compression... HELP

That is good to hear!
Well the motor has the 46cc heads on it, the pistons do not have valve reliefs, it has the stock stroke forthe .305, ...it was bored .030 over from the original bore. I used a
.040 gasket, the block hasn't been decked so it is the stock .305 deck height. I did not factor in the deck height as I do not remember what it is, I believe it is .025 however.
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