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Q&A > General Tech Advice > 351C Engine Questions-Comp Ratio and Induction ETC > Community Forums > Mustang Forum Australia - Mustang Tech

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351C Engine Questions-Comp Ratio and Induction ETC Reply to topic

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shaun071
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Location: Goombungee QLD

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Hi All, I've decided to go with a 351C for the 69 Coupe. I have a 351C I built up about 12 years ago to run straight LPG and want to give it a freshen up and convert it back to running on premium 98 fuel specs are:

stock bottom end with .020" flat top pistons
302C closed chamber heads 2v intake 4v exhaust valves hardened seats etc
Crow 214/224 @ 050 cam
Redline street torker single plane intake
XD Bosch electronic distributor recurved for LPG and cam

I can't remember if the block was zero decked or just skimmed to true it up but the heads just got a skim to get them straight. So I can't give an accurate compression ratio- it could be as low as 10.5 to as high as 11.3:1 static.
I'm going to replace the pistons but would like to know whether to go to a dished top or whether I can still run a flat top and run 98 fuel?

What compression would be fine with 98 fuel?

Induction setup- Stick with the single plane or change to a dual plane?
(I was thinking an Edelbrock performer rpm Air Gap)

Carb size? (and brand edelbrock, holley Demon etc?)
(I was thinking around 650 cfm)

This engine was in an XD with AOD/3.25 rear and was a great daily driver on LPG. Started to pick up from about 1500rpm then power would die off at 5500rpm. So an induction setup that will work in that power band would suit me fine- I'm happy to sacrifice some top end for drivability.
The coupe will be an almost daily cruiser so I'm not after a billion hp or something thats a pig to drive. It also has to be semi economical. The rest of the drive train will be AOD with a higher stall convertor (undecided on RPM, probably 2400) and 3.5 rear.

Cheers Shaun.


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hybrid
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IMO, a 351 with flat tops and closed chamber heads will have too much compression for a street car.
My dad was using the same setup on his 351C and it was just too high - we recently switched to open chamber heads when we did his new cam and it behaves much nicer.
Before we put the injection on, it would run on all the time - a real pain in the ass.

Why not change to open chamber heads and leave the pistons alone?

I would switch to dual plane - and yes the air gap manifolds are popular for good reason.


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Shaunp
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buy some thick head gaskets you can get them that are about 60 thou crushed, if you don't want to change the pistons

Put an RPM on it, I've got a new 670 Holley Avenger here in the box if you want it.


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boofhead
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I have a 670 Holley Avenger here NOT in a box Laughing


Any way, I plug it into my system here and with a 62.8 cc Closed chamber head your 351 if decked would be 10.7 : 1 compression or if not then just over 10:1 .

My experience is your on the edge at 10.7 with Iron heads, smallish Cam for that static compression. It could be OK though your pushing it a little. I suggest you open the chambers a little around the valves as they get a little shrouded and that will also drop your compression just enough.

I agree the RPM or RPM Airgap would work well. A 650 approximately would also be fine. Some good headers and she will be a nice runner.


I will someday think of something clever to say.

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Shaunp
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Here you go you can get Clevo head gaskets up to 80 thou compressed, so its no trouble to drop the compression with no work at all.

www.summitracing.com/d...4294924325


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shaun071
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Thanks for the link Shaunp and I'm definitely interested in the carb but it might be a couple of weeks before i've got the cash. If you could PM the price that'd be great.

Boof, I was thinking of opening up the chambers a bit for the same reasons. The chambers are 58cc so it bumps the comp up a little more. A couple of people on cleveland forums suggest opening the chamber to the same size/shape as the US 4V closed chamber. I won't know whether its zero deck until I pull the heads. The plan was to pull it down and re-ring and new gaskets at a minimum. It's done about 150,000km since the last rebuild and has been on an engine stand for about 6 years.

The pistons in it were just stock cast, nothing special. I was also wanting to replace the rod bolts as I'd just reused the factory bolts. It was a budget build so if parts were in good nick they just got reused. If I do decide to put new pistons in, could I get away with the next oversize, 030" or go to 040"?


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boofhead
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The two main weakness of the Cleveland were the stock cast pistons and particularly the stock two piece valves. I would definitely replace the valves to be a one piece item. I would also keep increasing the bore to a minimum (as they are known to have thin walls) - you will not know how far is necessary until the out of round is checked though with 150k your likely to get away with the 10 thou (so 30 max).


I will someday think of something clever to say.

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shaun071
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When the heads were done all the valves were replaced with 1 piece, 2V inlets and 4V exhausts with hardened exhaust inserts and bronze guides. I didn't spend a lot on the bottom end as I was a broke apprentice when the engine was built. It all seemed to work without any issues though. No rattles, misses or smoking, never used oil etc and the gas carb I was using limited it to around 5500rpm anyway.


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shaun071
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boofhead wrote
I have a 670 Holley Avenger here NOT in a box Laughing


Any way, I plug it into my system here and with a 62.8 cc Closed chamber head your 351 if decked would be 10.7 : 1 compression or if not then just over 10:1 .

My experience is your on the edge at 10.7 with Iron heads, smallish Cam for that static compression. It could be OK though your pushing it a little. I suggest you open the chambers a little around the valves as they get a little shrouded and that will also drop your compression just enough.

I agree the RPM or RPM Airgap would work well. A 650 approximately would also be fine. Some good headers and she will be a nice runner.


I found a pic of chamber mods on a cleveland forum. If I use this as a guide am I heading in the right direction?

Another suggestion was to just take about 1/4 inch off the quench pad which should increase chamber enough.


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boofhead
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Indeed.


I will someday think of something clever to say.

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