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Compression ratio question

June 17 2017 at 7:07 AM
ArnieO  (Login ArnieO)
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I am building a 390 for a 67 cougar I have a set of C8AE-H heads that have been pocket ported and have CJ valves . My question is that with the gas of today would 10:1 pistons be to high with cast iron heads ? I don't want to have to add octane booster or kick the timing way back. It will be mostly a street machine , but would be run at the strip on occasion . the gears for the rear are as of yet undecided I found a set of 4:10s which is lower than I wanted but the price was right and that should be fun in a light car. Anyway I am figuring out if the power increase from 9:1 to 10:1 would worth it or if I'm looking for spark knock troubles . Thanks for all input in advance .....ArnieO

 
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WerbyFord
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10.0 iron ok w 4.11 light car

June 17 2017, 10:52 AM 

Probably getting near the edge but assuming you have some cam in there say 220-230 duration to go with the 4.11 gears, aluminum intake, blocked heat riser, forged pistons if you can, keep it below 190F when you hammer on it, and keep it clean (which means get on it a lot, and 4000 wont get it done), you should be fine.

If you're getting 10.0 with a good tight quench distance say .050" deck+gasket or less, that's even safer.

 
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Ross
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The key is knowing what you have

June 17 2017, 11:13 AM 

Specific parts matter, and measuring those parts matters more.

10:1 with a 280-ish adv cam on 106 is fine, especially if everything is measured and a tight deck

If you are going to run a mild cam and don't know exactly which pistons, likely that you are less than 10:1 but not knowing means it's hard to predict

Bottom line though, if you cam for 4.11s in a 390, up to 10:1 and maybe even 1.5:1 "should" be good
[linked image]
---------------------------------
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed

 
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ArnieO
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Ross & Werbyford

June 17 2017, 12:58 PM 

Thanks , I should have included the cam . It's from Oregon a562/566 lift with230/236 duration , I also have the old reliable street master intake and found the article on here on porting them . It's a 3 pedal car I plan to run a regular off the shelf head gasket set not a thin set . Like I said it's not a pure 1/4 mile car just a fun driver for weekends and take to the strip once in a while . I guess if I do end up with a small ping I could back the timing down then kick it up at the strip with some good gas. I just didn't want to over kill on the compression and have it rattle -n- knock , but still want to get all I can out of it (trying to walk that thin line) Thanks again ArnieO


    
This message has been edited by ArnieO on Jun 17, 2017 1:02 PM


 
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Ross
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Other questions

June 17 2017, 1:46 PM 

The biggest questions are:

1 - Which pistons are you running?
2 - Did you deck the block and how much?
3 - Do you know the advertised duration on the cam and is it hydraulic or solid?

Here's the truth of the matter, if those are stock pistons and you run the Blue Felpro out of the box, you are likely closer to 9:1 than 10:1

Also, the thicker gasket may not hurt or help, or it may only hurt due to lack of quench, but it depends on the pistons.

[linked image]
---------------------------------
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed

 
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ArnieO
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Ross

June 17 2017, 2:54 PM 

Ross I am getting the block down to the machine shop either this week or next to have the machining done , then I'll find out if the block needs to be decked the heads have been surfaced 10 . I was trying to decide so as soon as I find out how much over bore I will have to go to which pistons to go with the flat tops ( which should be 10:1 correct?) or the dished either way they will be after markets The cam is a hydraulic Oregon cams just told me the duration was 230 and 236 I figure that is at .0050 lift. According to what I have found the C8AE-H heads have from 67-70 cc chambers so with a .0010 cut they should be around 65.5 -68.5 cc from what I understand Thanks ArnieO

 
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Ross
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Parts choice

June 18 2017, 5:09 AM 

Arnie, the first thing I would recommend is driving the machine work based on a parts choice, not the other way around.

An uncut FE block has a deck height on 10.170 and is likely not squared to the mains, so quench and compression can differ from cyl to cyl. I would recommend cutting the block, you get a fresh sealing surface on both sides of the gasket and you tell them you want it squared to the mains. Most any shop does that nowadays. The question is, to what deck height?

You can almost be certain that it will like a .010 (10.160) cut to square things up on an old seasoned block, and depending if it was cut before, you may have to go with an even lower deck. Regardless, the nice thing is, if it's 10.170 and you want to go 10.160, but the machine shop says it's already cut, you can use the thicker and cheaper Felpro blue gasket and get your .010 back.

So, lets assume a 10.160 deck with a Felpro 1020 head gasket. About .040 of head gasket, and you'd want the piston to be in the hole .010 or so (at most) to get a decent quench to help against detonation. Not all pistons are the same, in fact they are all over the place from company and even within a company. For a budget build that can boogie now and then, the L2291F30 is a good piston. It's reliable, but not light and doesn't have a modern ring pack, but won't let you down for the level of power you need.

The L2291F30 (.030 over) at a 10.160 would put you at .044 quench and 10.0-10.2 compression depending on where you ended up with the heads using a 1020 gasket. If you had to cut the block to 10.150 because it was cut already and needed a fresh cut or was very crooked, you'd run an 8554 Felpro blue gasket and end up the same, (actually just slightly lower and a quech about .003-.004 more), but the pistons would be slightly above deck, not an issue

Now, if you want to spend a little more money, I'd talk to Brent Lykins about some Racetechs, there are off the shelf and custom options, but, your money gets you a better set of rings (more power and easier break in), potentially the exact pin height you need, so you cut the block to exactly where you want you don't have to be concerned with head gasket choices as much, and a lighter piston with more valve relief room. I don't think your description drives that, but it would make a very nice build if this is a car worth the extra money.

I have done a few of the L2291 combos, that combo at 10.2:1 compression with the L2291s or a custom set of pistons, and a cam with a .006 duration of 276 or bigger on a common 106 ICL (both are likely what you have with 230 @ .050) will run well on pump gas. That engine will run great with a set of headers and a good intake. However, keep in mind, a thicker head gasket will not make it run better on pump gas beyond about .060 in quench (head gasket + deck clearance) after that, you lose the benefit of everything churning around in side from the tight clearances.

I hope I didn't confuse things, but when you choose a piston, you need to look at pin height, also called compression height and know your deck clearance. Lots of us can help, but if it fits your budget, and if my numbers match where you end up, that L2291 can be a good match for a durable budget build



[linked image]
---------------------------------
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed

 
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Ross
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Arnie, one last thing

June 18 2017, 5:26 AM 

Those heads won't help you make big power, but they can run really well on the street with the right gear and the intake should work really well. In fact i am doing one right now that is very similar, but a long rod motor with extra parts I am collecting. It's really a "bench motor" using a bunch of leftover stuff that is coming together to a pretty cool little engine. However, the parts choice is almost identical, including cam (although its a Comp), intake and heads, the big difference is a set of Diamond pistons and a longer connecting rod, but at this power level I wouldn't call that a big deal

The rub though is you'll really want headers to make that thing run well if you haven't thought of that. We are talking probably a 40-50 hp difference from the 390GT manifolds and big numbers across the whole curve, it is significant

The problem is, the only "affordable" headers for that combo are FPAs and you have to specific the GT exhaust flange. Hooker 6114s fit the car well, but the exhaust port on your head is about .250 lower when compared to the bolts, which makes it very tough for headers to seal. Now, people have run them, but I can tell you, changing header gaskets once the engine is in is not where you want to be on a Cougar/Mustang
[linked image]
---------------------------------
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed

 
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ArnieO
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Ross

June 18 2017, 4:10 PM 

I have a set of Dougs D-626-Rs 1-7/8" pipes into 3" collectors . These heads weren't the originals I got them of E-bay a few years back ( this has been a long project lol) had them checked out and they were good and what they were suppose to be , pocket ported and CJ valves( new stainless steel) with new dual springs and retainers , ect. So they have the top bolts all in a row I found a article on here I'm not sure if it was Jay or who tested and over the stock GT mustang/ Cougar manifolds the headers gave 47.8 HP increases so I'll take a easy 50 ponies lol. And yes I don't think I want to change gaskets on that very often and I'm hoping that I won't have to drop the right side to change starters if the need should arise . Thanks Ross and all who chimed in for the advice and help ...ArnieO

 
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Ross
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Headers

June 20 2017, 4:59 AM 

It is likely that your headers are for CJ/early exhaust ports not your GT/unibody heads. I know you have the bolts in a line, but it is more than that when you convert a bolt pattern

Get a header gasket and lay it on the head and the header, it needs to match both. Not the bolts, the bolts are easy, but the location of the exhaust port.

If it does, awesome, but most headers are built for the CJ/early port which is .250 or so higher. What happens is the lower C8AE-H (same for the other unibody heads other than CK) is that the sealing ring of the header is unsupported and doesn't compress the gasket.

If they don't line up, then the easiest route is a set of Edelbrock or CJ heads OR FPA headers to match your heads.

Check it, so far what you are saying makes me think it may leak. One way to get lucky is if the Dougs use a full flange for sealing and not a welded ring, but that adds a machining step that most do not do.
[linked image]
---------------------------------
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed

 
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ArnieO
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Re headers

June 20 2017, 9:52 AM 

I called Summit before ordering them and they told me that they fit all with the top bolts in a row (which is what I have) except the CJ heads because of the port size and location. They also said they wouldn't fit the factory GT heads because of the bolt pattern . With that said I will check the port location and size as you suggested just to make sure . Thanks again for the help and input Ross..... ArnieO

 
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Ross
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No problem

June 20 2017, 5:09 PM 

Check them though

A C8AE-H out of a truck and a C4AE out of a Galaxie have all 4 bolts across the top and they have very different exhaust ports.

I hope it works for both, but there are few parts counter guys that understand the difference

Here is are pics from Bob Sprowl, bolts are the same and all 4 across the top, but the difference in the calipers is the difference in port location

[linked image]

[linked image]
[linked image]
---------------------------------
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed

 
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ArnieO
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Headers

June 21 2017, 10:59 AM 

Ross I checked using my calipers to measure distance from upper bolt holes to exhaust port celling then checked from the lower bolt holes to the exhaust port floor then checked the gaskets and it will work . Just to double check I e mailed Dougs and they said no problem on the C8 heads , so it looks like a go. Thanks ArnieO btw by checking every thing I found one of the gaskets were damaged in shipping

 
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Ross
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Good to hear

June 22 2017, 4:03 AM 

I am surprised they made them for the C8AE-H and not the C8OE-N, but hey, no need to look a gift horse in the mouth. Now you know
[linked image]
---------------------------------
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed

 
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Jackie
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Spark knock issues

June 17 2017, 9:17 PM 

After boreing my 390 block out .030 and installing scat 445 stroker kit and using C4AE-G iron heads, I figured my compression to be somewhere around 10.0-1 or maybe 10.5-1 not really sure, after doing a compression check and every hole had a little over 200psi compression, and by making the engine happy setting the timing @ 20*, the 1st problem I had was run on after shut down, and ofcourse heavy detonation on acceleration, after doing some controversial research, I topped my tank off with E85. The detonation stopped, and no more run on after shutdown, another advantage of E85 is the engine is running cooler, I did rejet my carb, I know there may be some negative issues with using E85, but I think the advantages out weighs the disadvantages


    
This message has been edited by 64fdgal on Jun 17, 2017 9:19 PM


 
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Bill Ballinger
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I had mine milled, the pistons are .002 proud of the deck

June 18 2017, 6:08 AM 

and the C4AE-Gs have been angle milled a degree and a CNC LS7 port ground in as best as they can fit. The stock mirroring of the ports requires some creativity, LOL. Iron is also very hard on bits, they were done in 2002, have had the face milled to fit a street Dominator and is 10.25:1. 2.055-1.58 valves, it has a very lively quiet port, and does 274 intake at my cam lift and about 285 at .600 with 185-190 on the exhausts at cam lift and .600. it never pings with the L2291s and revs really quick to 6500-7000. I want to do better on my BBMs and build a 410.

 
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Bill Ballinger
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I found that a time slip diid exist from when it was in the truck

June 20 2017, 6:22 AM 

The 65 F250 4X4 with a NP435 and 4.56s with 33" tall tires. Weight was 5100 with driver. It was an 1/8th mile and it pulled a stout 7.96 at 88 mph. I don't know how that converts to HP and 1/4 mile time. But folks thought it GD fast for a high-boy, I used 2nd to launch and didn't use the clutch, shifted at 6800. Its still alive and that was 2003 I believe.

I would really like to see what it will do in the Galaxie. People swore it had nitrous.


    
This message has been edited by 65billgal on Jun 20, 2017 6:25 AM


 
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Randy
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About..

June 20 2017, 10:35 AM 

12.4 in the 1/4 mile. Pretty fast for that weight and trans.

 
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ArnieO
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Thats pretty damn good Bill

June 20 2017, 11:26 AM 

For the weight and a truck gear box . I have to wait and see what this frankenstine I'm putting together does , I call it a frankenstine not because it's a monster but because it's being made up of so different parts lol. But on a budget I am hoping it will be fairly quick and a fun weekend car , but as the old saying goes the proof will be in the pudding ....ArnieO


    
This message has been edited by ArnieO on Jun 20, 2017 11:33 AM


 
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