Compression Ratio

[Matt Wilson]

What are you guys running for compression ratios in your flatheads these days, with today's fuels? What issues have you had with pinging/detonation/spark-knock. Also at what altitudes are you running at, and what fuels do you use (octane and ethanol content), and fuel additives? What are your outside summertime temperatures? How hard do you push your engines in terms of amount of throttle (not necessarily rpm)? Also, what's your carburetion setup - stock carb, two-barrel, dual carbs, etc?

I'm interested in what you have to say for any of the flatheads - 218's, 230's, 236's, 251's, 265's and whatever others you may have.

Reason I ask is because I've spoken with a gentleman who I believe is very familiar and experienced with our flatheads, and he says that with today's fuels (specifically E10 87 or even 93 octane), at my altitude (less than 1000 feet here in the DFW, Texas area) and my summertime temperatures (100 deg F or more), I run a great risk of pinging/detonation if I run more than about 7:1 CR unless I go to 100 octane gas.

We happened to be talking mostly about the 25" long engines (251, 265), but I got the impression that he thought it would be very similar with a 230. And yet I've seen a number of posts by various people over the years (many of them not that long ago), who say they've operated with 7.5:1, 8:1, 9:1, sometimes more, so either this gentleman is mistaken (but he's very reputable, it seems), or the folks who posted these CR's are using high-grade fuel, additives, or they are operating in even lower altitudes (can't be much lower) and/or lower temperatures or compensating in some other way...or maybe they posted what they THOUGHT were their compression ratios, but they were mistaken.

I was rather blown away when he told me to limit my CR so much. He strongly encouraged me to use 100 octane fuel to avoid these problems.

I'd like your input, as it will guide me in an upcoming rebuild.

Thanks.

[Matthew Welcher PWA]

I would really have to think about 100 octane fuel being necessary in a Flathead with that compression ratio. I understand the heat and elevation aspect of the engines performance but that sounds like a stretch to me.

[Matt Wilson]

Thanks for the input, Matt. I tend to agree. If detonation was really a problem with these engines, I think we'd hear more about it.

One thing I'll say is that I am planning to add more carburetion to the engine, either in the form of a two-barrel carb, dual single-barrel carbs, or something, so more fuel will find its way into the engine, and this gentleman knows this, so that mofht adding to his concern, but even so, I just have a hard time believing it would be a problem. I think it will be difficult for me to keep the CR below 7:1 on the engine I'm getting, so I'll probably find out how much of a problem it is sooner or later. If it does end up causing a problem, then I'll adjust accordingly, by finding higher octane fuel, retarding the timing, adding octane additive, or if necessary, by finding a low-compression head from an early engine.

I was hoping maybe I could find someone who has done this kind of thing in an operating environment similar to mine, who could tell me what their experience has been.

Thanks!

[DrPepper]

Agree with Matthew...

It really doesn't matter what engine you have, they all require fuel, air, compression and spark at the correct ratio to operate. Pre-ignition or spark knock is when the fuel mixture ignites prior to the timed spark. If the head is cooling properly, the engine is running at the correct temp and mixture it shouldn't knock. Lack of lube for valve guides without the lead seems more of a concern to me.
Also, adding a 2BBL carb has no affect on the engines ability to run without knock. The engine will pass the same quantity of air/fuel mix at 1000 RPM with or without the 2BBL carb. The 2BBL will allow the engine to run (breath) at a higher RPM and maintain control of the mixture. Note that there are progressive 2BBL carbs, like half of a 4BBL, (Holly made a bunch, google it) where the secondary opens after the primary so there is no lag in the off idle which happens in "over carburated" engines. Take a 318 with a 750+ CFM 4BBL with large primaries on a street engine, change to a 600 CFM with proper primary size and it will give much better off idle response and still red line it.
IMHO
DrPepper

[Jason Mills]

It seems to me that the Static Compression ratio has less to do with octane requirements than the Dynamic Compression ratio...

And I do not know what the tip over point is between 87 and 91 octane. The DCR has more of an influence, and I believe 160-175 psi cranking pressure may be about the beginning of needing higher octane fuel.. The slant six forum would have some depth on this as they seem to explore the limits of high performance and street driven at the same time.. granted it is a totally different engine, but the depth over there in tuning has impressed me.

I've got 140 PSI in my flathead. Have not driven it enough yet to know if it has any ping/knock issues. Hope to have more info in a month or so when the roads dry out.

[Jason Mills]

The 1958 230's went to 8:1 compression ratio from 7:1, so that ought to be safe. and according to All-Par the 251 went to 8:1 in '67

http://www.allpar.com/mopar/flat.html

Not suggesting that all the info on that site is correct, but I have read somewhere else that the 230's went to 8:1 in 1958.

[Matt Wilson]

Thanks for the info, Dr. Pepper. I agree with everything you said, except that I could see that more carburetion could have an impact because more air and fuel would get crammed into the cylinders, so it seems that there could be more tendency for pre-ignition to occur.

Thanks for the suggestion, Jason. I may have to go visit the slant six page. Also, I have read that 8:1 was the final compression ratio for the 230, so I agree that it should not be a problem on today's fuel either, which makes the info I got from my other source all the more questionable.

[Desoto61]

Thanks for the info, Dr. Pepper. I agree with everything you said, except that I could see that more carburetion could have an impact because more air and fuel would get crammed into the cylinders, so it seems that there could be more tendency for pre-ignition to occur.

If the carburetors are properly tuned then the amount of air and fuel are the same in both cases. Without turbo or superchargers the amount of air in the cylinder is mostly dependent on intake/exhaust/cam design than it is carburation. More carburation makes it easier to run rich, but it's like having a bigger hose, it's only important if the hose you have can't carry water fast enough.

I'd think the cylinder head design would have the most to do with limiting compression ratio. I wouldn't think you can get as high due to the side valve configuration necessitating a bigger head area. OHV engines can get around this by being interference engines, you don't have that luxury. My Desoto with a 413 is supposed to be around 10:1 and runs fine on 93 with an additive to protect the valves. So the gas can handle it, if the engine can't then there's something else causing problems.

[Matt Wilson]

If the carburetors are properly tuned then the amount of air and fuel are the same in both cases. Without turbo or superchargers the amount of air in the cylinder is mostly dependent on intake/exhaust/cam design than it is carburation. More carburation makes it easier to run rich, but it's like having a bigger hose, it's only important if the hose you have can't carry water fast enough.

I'd think the cylinder head design would have the most to do with limiting compression ratio. I wouldn't think you can get as high due to the side valve configuration necessitating a bigger head area. OHV engines can get around this by being interference engines, you don't have that luxury. My Desoto with a 413 is supposed to be around 10:1 and runs fine on 93 with an additive to protect the valves. So the gas can handle it, if the engine can't then there's something else causing problems.

Well, what I'm thinking of is a dual or even triple carb setup. Surely that gets more air and fuel into the cylinders?

As for the cylinder head, yes, I would think the shape would have a big influence on the pre-ignition characteristics. The flatheads may be more prone to this than some other engines, but other types, but I still have a hard time believing they would have issues with 7.5 or 8:1, even in the heat of the Texas summertime.

[Desoto61]

Well, what I'm thinking of is a dual or even triple carb setup. Surely that gets more air and fuel into the cylinders?

Sure in total, but that only matters when you need it which assumes the engine can demand it. E.g. if the motor moves 500 cfm of air at redline under ideal conditions, then a 650 cfm carb will handle everything that engine can use and more. Swaping that for a 1000 cfm carb means the carb can flow more air, but the engine still only moves half that. It will not help since you're only ever using half the capability of the carb. You're using a fire hose to fill a bucket.

That's why they started going to multiple barrel/carb setups. The main barrels/carb is sized to normal operation of the engine, with the extras coming in only above the limits of the main barrel/carb, but same principle applies, too much is not better, it's just too much.

[DrPepper]

Desoto's got it...

but there is one more factor to the oversize carb. When you look at the throttle pedal movement to carb butterfly opening ratio the matched carb is going to win the test every time. Looking at the throttle opening as 25, 50, 75 and 100% open vs. cfm at engine speed (steady running not accelerating)
Percent Throttle --- 650 cfm --- 1000 cfm
25 --- 163 --- 250
50 --- 325 --- 500
75 --- 488 --- 750 exceeds requirement
100 --- 650 --- useless
So, what it does during mild acceleration is drop your manifold vacuum much sooner and give you a "touchy throttle". Stomp it and it will fall on it's face.
Carter AFB's had a whole series of vacuum secondary 4 bbl's that worked great. You could stomp it all you wanted, but the secondary butterfly's were actually opened by manifold vacuum. So, as the engine accelerated off idle it allowed the manifold vacuum to be stable and smoothed out the acceleration.
The flat head Ford with (3) 97's (and so did Mopar 6 packs and others) had progressive linkage to run off the center carb on the road and only bring on 1 and 3 when it was stomped.
Just my $.02
DrPepper

[Jason Mills]

I've read somewhere that mixture quality (too lean) will effect the propensity to detonate... which is common knowledge. I understand that such can be an issue within the cylinder chamber as well, where the air-fuel ratio is lean in spots and rich in others.. Supposedly the flat-head design is somewhat good at limiting this issue because of the turbulence involved in the air-fuel flowing into the chamber and then tumbling over into the cylinder...

I've often wondered if the flat head ford can provide insight into tuning the 230/251 Dodge engine.

Also if you are building a fresh engine, I've read that putting a coating on the intake valve (stem side) and on the exhaust valve face, as well as coating the combustion chamber can make a noticeable difference in detonation sensitivity.

[Desoto61]

We're getting far down the rabbit hole here, there are scientists and engineers that make whole careers out of combustion science.

You are correct in that detonation can be helped and hurt by all of those things. Coatings on valves I would think have limited benefit since the body of the valve is only exposed before and after the combustion stroke occurs (maybe the valve face since it's inside the combustion chamber) but any coating would have to be applied in some sort of deposition process to survive inside a combustion chamber, and that's not something your local machinist could do. Honestly it would probably be like buffing the paint on your seat springs, lots of work for little gain compared to other ways to spend the same time and money.

In the end there are lots of basics from engines in general and other flatheads specifically that do apply. The trick is to define what you want the engine to do as specifically as you can. Like most things building an engine for the low end torque our trucks like is not the same as building them for high end horsepower which most hot-rodders are going for.