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Old 03-02-2003, 09:43 AM   #1 (permalink)
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What compression ratio with pump gas?

What's the highest compression ratio you can run on pump gas (i.e. 93 octane)? With the pistons I have on my 347 I can get 10.9:1 with a 58cc combustion chamber, 10:3:1 with 61cc and 9.9:1 with 66cc.

I was going to go with an AFR 185 61cc head but can I go with the 58cc head and still run pump gas?
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Old 03-02-2003, 01:59 PM   #2 (permalink)
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What compression ratio with pump gas?

I don't think 10-1 or 11-1 is going to make a big deal on your motors output. High "static" compression is over rated. I've seen many 10-1 motors in the 400ci range make 600 hp on pump gas.
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Old 03-02-2003, 02:26 PM   #3 (permalink)
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What compression ratio with pump gas?

It all depends on the cam you choose. With a lot of overlap, your dynamic compression will be lower especially at low speeds. This will prevent pinging even with a 11:1 or 12:1 engine if the cam is wild enough. If you go with the 10.9 heads use a cam with about 240+ duration @ .050. 230+ duration for the other head choices. Maybe even a 220 duration cam with the 9.9 heads
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Old 03-02-2003, 05:29 PM   #4 (permalink)
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What compression ratio with pump gas?

The cam I have has 224 duration a @0.50. Is this going to be a problem with 10.3:1 compression? I want to get the AFR heads and they only have a 58cc or a 61cc combustion chamber.
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Old 03-02-2003, 06:00 PM   #5 (permalink)
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What compression ratio with pump gas?

Hey! blkfrd!

Hows thwe weather over there in corona? Clear and blue over here in Hemet! Did you go to the first day of racin at Carlsbad? I had to work but next time im there. Can't wait for the New banning track to going eithier...................

Ed
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Old 03-02-2003, 06:15 PM   #6 (permalink)
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What compression ratio with pump gas?

Quote:
On 2003-03-02 16:29, mstnggt66 wrote:
The cam I have has 224 duration a @0.50. Is this going to be a problem with 10.3:1 compression? I want to get the AFR heads and they only have a 58cc or a 61cc combustion chamber.
What is the overlap of the cam? Maybe you just tell me what the cam is and i'll look up the specs. You may have some pinging with that cam. Cams such as Comp extreme energy cams tend to close the exhaust valve early to maintain good dynamic compression ratio. Their not the best choice for AFR heads since the exhaust ports flow very well. A single pattern cam would be better. Sorry to get off on dual pattern cams.
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'65 Mustang, 331 custom built with 289 H beam rods and 383W piston, 282S cam, self ported '70 351w heads, T5z, 9" w/3.50s.
430 HP @6500
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Old 03-02-2003, 06:22 PM   #7 (permalink)
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What compression ratio with pump gas?

Quote:
On 2003-03-02 17:00, BFXcomet wrote:
Hey! blkfrd!

Hows thwe weather over there in corona? Clear and blue over here in Hemet! Did you go to the first day of racin at Carlsbad? I had to work but next time im there. Can't wait for the New banning track to going eithier...................

Ed
I'm enjoying the beautiful weather outside as I rip my old balcony off the side of my house. One of these "excuse me" cantilevered balconies that came with the house. Only 13' x 4'. New one will be 14' x 14'.

My car is not road worthy yet. I'm about 2/3 complete on the restoration. I put the drivetrain in and told my wife that i'd stop working on it until the balcony is done. I'm very motivated to get it up and back to the mustang!

Are you talking about Drag City in Banning? I'll be there when I have something to drive.


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Tracy Blackford: Anaheim Hills, Ca
'65 Mustang, 331 custom built with 289 H beam rods and 383W piston, 282S cam, self ported '70 351w heads, T5z, 9" w/3.50s.
430 HP @6500
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Old 03-02-2003, 06:25 PM   #8 (permalink)
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What compression ratio with pump gas?

The cam is a Comp Cams Magnum 280HR.

What heads would you recommend?
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Old 03-02-2003, 06:26 PM   #9 (permalink)
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What compression ratio with pump gas?

It takes some goniometric skills but you can calculate the effective compression ratio by calculating the cylinder volume at intake valve closing and using that in the compression ratio calculation. 8:1 effective seems to be a good rule of thumb, even for race engines. The difference is of course that their volumetric efficiency is much higher than of street engines, so they will have high cylinder pressures (and octane requirements) anyway.

You can calculate it by using the following for the stroke

0.5 x stroke x (1+ cosinus(IVC))

With IVC being the angle after bottom dead centre where the intake valve closes.
For example a camshaft of 280 deg and a 106 deg intake centre line would give an IVC of 66 deg ABDC. The effective compression stroke for a 347 becomes then: .5x3.40x(1+cos(66))=2.39". Calculate the cylinder displacement with this stroke and you can calculate the effective compression ratio.

Hope this helps.
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Old 03-02-2003, 06:30 PM   #10 (permalink)
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What compression ratio with pump gas?

All I really need to know is if I can run pump gas or what I need to do to run pump gas.

I really appreciate everyones help though.
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Old 03-02-2003, 06:46 PM   #11 (permalink)
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What compression ratio with pump gas?

Quote:
On 2003-03-02 17:25, mstnggt66 wrote:
The cam is a Comp Cams Magnum 280HR.

What heads would you recommend?
You might be OK with 10.3:1 and that cam. Hard to say for sure. Maybe someone else on this forum who has a similiar setup can chime in and give you some first hand experience. Nice cam. I thought of doing a roller retrofit but went with a Comp 282S solid lifter cam instead. I'm running 9.9:1 compression with it. Another big factor will be the distributor advance curve. Be sure to get the distributor curved for your setup.

AFR 165s or 185s would kick. 165's for more torque down low and mid. 185's for HP up high. I'm running fully ported '70 351W heads (61 cc heads in case your wondering). Call 'um TCB 160's...LOL. Flow is 220 / 180 @ 0.500. They'll give me torque between 2500 and 6500 of well over 300 ft-lbs with peak torque of about 400 @ 4500.

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[ This Message was edited by: blkfrd on 3/3/03 5:46am ]
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Old 03-04-2003, 10:29 AM   #12 (permalink)
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What compression ratio with pump gas?

I'm 99% sure I'm going to run AFR 185s with the 58cc combustion chamber. I've talked with someone that has the exact same setup with a cam that has more duration but less lift and he runs pump gas fine. I've done some rough calculations and this should give me 10.6:1 compression. Correct me if I'm wrong but 11:1 is the highest you can run on pump gas?!?!
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Old 03-04-2003, 10:53 AM   #13 (permalink)
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What compression ratio with pump gas?

effective compression is what will cause the pinging if you have too much. alot of the early muscle cars had high static compression ratios like 11.5:1 - 12.5:1 in some cases, but they had low effective compression ratios. that and the fact that they ran leaded gas is what allowed them to run without detonation. so to answere your question about "11:1 being the max" its hard to say cause every different engine set up is going to be different. i long blocked a 355 chevy with a 10.75:1 static compression ratio, but i ran a 107 deg lobe seperation angle which helped to bleed off cylinder pressure. the motor has never pinged. running a compression test will give you a good idea of how much cylinder pressure you are generating.
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Old 03-04-2003, 11:53 AM   #14 (permalink)
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What compression ratio with pump gas?

With that combination you should be fine with 93 octane. You will probably not need as much timing as you ran before, maybe only 33-35* total as the chambers on those heads are pretty efficient espectially with a flat top piston.
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Old 03-04-2003, 12:58 PM   #15 (permalink)
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What compression ratio with pump gas?

Sorry, this is not totally pertinent to your topic, but I am running 12-13 lbs. of boost with a 306, not sure what the static compression ratio is under boost. I would think well over 12:1, but I'm not sure. I have had no problems on CA's crappy 91 octane pump gas to date.
I would think you can definitely go to the highest you mentioned. The Camaro's run 10.25:1, I believe and don't have any problems. Plus, like the one guy mentioned, you can get away with a little bit more with aluminum heads. My two cents...

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[ This Message was edited by: jcm3 on 3/4/03 11:59pm ]
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