which octane do I use?

I would run premium gas in it, but the rule of thumb is, use whatever the lowest is that doesn't cause it to ping under load (accelerating up an on-ramp, etc.) It depends on how it's tuned, as well.

Do you know if the engine was rebuilt with hardened valve seats? If so, you won't need lead substitute. If not, which would be odd these days, you do.

I don't mean to besmirch the regulars here, but there is a better group in which to ask questions about this car - alt.hi-po.big-block-ford-mercury. Post this question there. You'll get advice from a group of seasoned pros.

My '71 351C 4V M-code runs great on 91 octane which is roughly equivalent to 99 octane of yesteryear. The timing is set about

12 BTDC and the car never fails to start on the first try even below freezing. The engine is very responsive. This is California so the gas may even contain alcohol. In Yosemite last year I paid $3.84/gallon of high test. But with a car like yours, you don't worry about the price of gas.

mike

Ya Right, that group of pathetic drunken unemployed alcoholic welfare queens? Please! Visit alt.hi-po.big-block-ford-mercury only if you want to argue with a bunch of self absorbed drunks, who's combined hands on automotive knowledge couldn't get them a job at Jiffy Lube.

You would be much better off leaving Jason O'brien and his group of Butt Buddies to play with themselves...

With regard to the unleaded fuel issue, the chief concern has been for the possibility of exhaust valve recession (sink or seat wear) when engines designed to run on leaded gasoline are operated on unleaded gasoline. If your engine was completely overhauled recently it may have had hardened valve seats installed therefore making this is a moot issue.

Typically, most passenger are subjected to service too mild for this issue to cause problems. The problems typically only occur in older engines when operating under *sustained* high speeds and loads.

With regard to the octane issue, the easiest answer to your question regarding the proper octane fuel to use is to use the octane rating as close as possible to what the manufacturer suggests for the engine. This mindset assumes you have not made major modifications to the engine (compression ratio/timing). Since your engine was completely overhauled recently it may also have had a reduction of the compression ratio from the original engine design therefore making it more "friendly' to current premium fuels (91-93 octane).

I am not sure what version of the 351C engine you have since the 351C was not offered from the factory in the 1969 Mustang model year.

FYI:

2-barrel H-code 351C featured a 9.5:1 compression ratio and a recommended a minimum fuel octane level of 94 4-barrel M-code 351C featured a 11.0:1 compression ratio and a minimum recommended fuel octane level of 99.8

My book says the 290/300 HP 351 was offered as an option in '69. That has to be a Cleveland.

Another way to tell what engine type is to stand on the driver's side of the engine accross from the distributor and near the upper left corner of the valve cover there is a number stamped, either 2 or 4, and in later years, M.

FYI: 8-cylinder engines available for the 1969 Mustang are as follows:

302 2V F-code Boss 302 4V G-code 351W 2V H-code 351W 4V M-code 428 4V CJ Q-code 428 4V CJ Ram Air R-code 390 4V S-code Boss 429 4V Z-code

Production of the 351C began with the 1970 model year and ended with the

1974 model year.

More information on the Ford 335 series engines (including the 351C) is @

Yet another $.02 worth from a proud owner of a 1970 Mach 1 351C 4V M-code @

So the M code is simply a 351 4V. Could be Cleveland or Windsor. Then the only way to tell is to glance at the engine since there are obvious physical differences between the two. My '71 is an M code and is clearly a Cleveland.

mike

The M-code referred to 351 4V CID engines as follows:

1969 - Windsor 1970 - Cleveland 1971 - Cleveland

Quick ways to differentiate the 351W and 351C engines are as follows:

- Coolant passes through the intake manifold of a Windsor engine, not in a Cleveland (features dry manifold)

- The two bolts holding the fuel pump to the block on a Cleveland engine are positioned vertical in relationship to each other - on the Windsor the bolts are positioned horizontally

Yet another $.02 worth from a proud owner of a 1970 Mach 1 351C 4V M-code @

And the distributor is tilted towards the passenger on the Cleveland along with large valve covers using 8 bolts and not six as on the Windsor :)

The '69 M-code is definitely a Windsor. The '70-'71 M-code is a Cleveland. I didn't think of this on the first read-through. Whoops.

The compression ratio for the 1969 2v H-code 351W is 9.5:1, and the 4v M-code is 10.7:1. I'd use premium gas in the M and maybe mid-grade in the H, listening for ping, and keeping in mind the advance curve. The Cleveland's block is obviously visually bigger, although its deck height is a smidgen shorter than the Windsor's. There are five versions of the Cleveland, in all, but you'll only commonly see two. I've only seen a few Boss 351s and one 351CJ.

There was also a 302 Cleveland, but it was only produced in Australia.

To clarify - 351 - five versions of this, in addition to the aussie 302C.