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gemini86 10-16-2004 02:15 PM

max compression ratio on 89 octane
Ok, I've done a bunch of searching and haven't found a real answer to my question.

I want to put a bigger cam into my engine (ford 400m or whatever you want to call it) but the cam I want needs a higher compression ratio. Here's the cam specs:

Cam Style Hydraulic flat tappet
Basic Operating RPM Range 2,000-5,000 RPM
Intake Duration 050 inch Lift 216
Exhaust Duration 050 inch Lift 216
Advertised Intake Duration 272
Advertised Exhaust Duration 272
Intake Valve Lift with Factory Rocker Arm Ratio 0.524
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.524
Lobe Separation (degrees) 110

I don't want to buy 93 octane gas to cruis around town, so my question is, how high of a compression ratio can I run on 89 octane gas? Also, how high will my compression ratio be with said cam and flat top pistons installed in my motor rated at 8.5:1 CR stock?

oldschoolrods 10-16-2004 02:22 PM go there, it can calculate your compression ratio for you, but you need basics like bore, stroke, deck height, gasket thickness, and Head Volume. Also you didn't mention what year that 400 was, 400s are notorious for having valve troubles around 50k or so. I know a guy that has a 400m, pulled his heads at 50k had .011 shaved off them and a valve job done, put an edelbrock intake with an 850cfm holley on top of it all and man that thing runs.

lluciano77 10-16-2004 04:40 PM

You are not going to find a real answer. It is all an educated guess. The valve timing events, ignition flame front, sharp spots in the combustion chamber, and other things that can't be calculated easily will limit the accuracy of your guess. You can estimate and try to get in the ballpark. If you get detonation at low speeds you will need to drop your timing a couple degress, or change cams.

With only 216o duration at .050", I would aim for 10:1. With this ratio, cam, and octane, you should be pretty safe with up to 34o total timing.

gemini86 10-16-2004 10:24 PM

Ok thanks guys... I was really only looking for a ballpark guess anyway. I've been reading about all this a lot lately and there just seems to be a lot of different opinions over the matter. I'm planning on smoothing everything out in there (no sharp edges) and I'm also wondering... Is smothing the edges of the falve face bad for the valves? I thought the margin needed to be a certain thickness to keep from overheating and burning up the valve?

66GMC 06-22-2005 11:57 PM

TSB from Ford -- 400 Truck 80-82

I was working as a partsman in a Ford Dealership between 1979 and 1992, and have a somewhat fuzzy recollection about a TSB (or even recall?) that Ford had issued on (80-82?) 400 truck engines.

They were equipped with a distributor that was not calibrated properly, and causing detonation problems. The fix was a brand new distributor that had been specifically calibrated to address this.

I can't recall the exact tag #, but I think it was E2TE-12127-??. It also had a paper label on the vac advance with "WIN" on it.

The reason that I remember this is that I was having detonation / timing problems with my 351C. The stock single-point, a Mallory dual-point, and a 1974 model Duraspark distributor ALL had timing/detonation issues. Adjusting them "by ear" in order to avoid ping resulted in a "labored" idle and overheating ... even at -30 C frigid Alberta winter temperatures!

I found a guy that had a distributor curving machine, and was getting set to have him overhaul / calibrate it. I was talking to our service manager about my plans, when he mentioned this 400 dist, and offered to let me try one.

It fixed all of my problems!

Here is a post on a Ford Enthusiast's site that also mentions this:

I have a 1979 50 state F-250 4X4 with a 400/C6 combo that I'm the 2nd owner of. It has the original emissions sticker on the hood but the timing spec has been altered to read 4 deg BTDC. Since I know the original owner of this truck and have all the service records my guess is that there was a factory TSB which instructed the technician to change the timing and update the sticker. The reason I think this is because the original part number for the sticker has been similarly altered indicating an updated revision.

On Edit: There were 2 TSBs relating to "Engine Detonation" which came out in 1980 for the 1979 400 motors 80310 and 80310S as well as one in '81 for "Engine - Surging/Stalling/Hard Restart" 811022 and another in '83 for "Distributor - Low Power/Poor fuel economy" 83524. I don't have any other info on them but I would guess they may have had some instructions for adjusting the engine timing.


k-star 06-23-2005 06:08 AM

Me personally i would not go over a 9:1 static on an engine you are going to run 89 octane in.... But like the others pointed out there are so many factors that can make or break this combo.

To add to what is allready mentioned.

Make sure you can keep it cool ,,,180 deg max... this will keep detonation down..

Make sure you have the correct heat range plug, and the correct timing curve.

Deburr everything in the combustion chamber,,,,,

The bad thing about running close to the edge is if you get a bad load or 2 bad loads of fuel and your not paying attention to whats going on you will have problems....

Learn to read the plugs, You might not hear the detonation.....


Rick WI 06-23-2005 07:28 AM

To add to Keith's post, do not modify the valve face, it's not going to be the problem source if you run too high of a compression ratio.

454C10 06-23-2005 07:49 AM

With 216 degrees duration and 110 lobe centers I would go for 9.25:1 to 9.5:1 cr with 91 octane and 9.0:1 to 9.25:1 cr with 89 octane.

SpankyNuts 06-23-2005 08:54 AM

my buddy has 10:1 with iron heads. when he shuts his car off it keeps running a little.

78 monte 06-23-2005 07:45 PM

On 89 octane I'd keep it to 8.5:1 - 9.1:1 max.

Max Keith 06-23-2005 08:20 PM

maximum compression
9:1 with steel heads and 10:1 with aluminum.

brainsboy 06-24-2005 06:36 AM

IMO I dont see why that cam wouldnt work with 8.5:1 compression. The numbers they give you with cams are just guildlines anyhow. That cam is fairly small for a 400ci motor.

I agree with most of the posts here. I would'nt go over 9.5:1


pasadenahotrod 06-24-2005 06:45 AM

Compression ratio
The weight of the car you've got that engine in makes a differnce too. My old T bucket roadster had a PowerPack 283 punched to 292 with a solid lifter cam and 12.5:1 compression. I always used straight regular cheap gas and never had any problems with heating or detonation or dieseling, but that same engine put in a 57 Chevy wouldn't perform well without high-test.

brainsboy 06-24-2005 11:51 AM


Originally Posted by pasadenahotrod
The weight of the car you've got that engine in makes a differnce too. My old T bucket roadster had a PowerPack 283 punched to 292 with a solid lifter cam and 12.5:1 compression. I always used straight regular cheap gas and never had any problems with heating or detonation or dieseling, but that same engine put in a 57 Chevy wouldn't perform well without high-test.

Are you sure you didnt miscalculated your compression ratio? If not, Im guessing your cam probably had enough overlap to bleed off the compression that made it somewhat workable, still 85 octane and 12.5:1 is hard to believe. I have 10.68:1 with aluminum heads and right around 2000 lbs and have heard pinging at times.

pmeisel 06-25-2005 02:48 AM

My 2005 Lincoln LS V6 has 10.5:1, the factory recommends 91 octane but I run it on 89 all the time with no problems....

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