Where are we redlining these motors?

The one I just put in has been recently rebuilt, so I assume the valve springs are up to spec. So at what point am I revving on borrowed time? I need to tell my co-drivers what point they should back off the throttle. With 3.73 in there now I think we will be redline limited more than drag limited on top.

Less than 2 weeks till race day

If its a factory cam and springs 5500 if your lucky more likely 5000.

There wouldn't be much point of reving it more than 200 RPM or so over were the cam makes it's max power.

Depends on the build. Lot of cam & carburetion 6500, milder cam, but with plenty of carburation 5700-5800.
Stock camshaft 4500-4800.
Running decent valve springs with whats recommend by the camshaft maufacturer is the key.
The real test is what gives the best drag strip ET's.
Sometimes going past the power peak in first gear will get you in the strong point of the torque curve when upshifting into 2nd.
The ET does not lie.

Not drag racing it, but road racing it. I'm more worried about the point at which the motor is going to start breaking stuff than anything else. From the sound of it, with the 1bbl and a stock cam, its almost impossible to rev it so high you break it? We are likely to do 1000 miles around a road course with it in 2 weeks.

The 1 bbl carb will be the biggest restrictor. My 250 had an Isky cam, forged pistons and the rods were de-burred and shot blasted so I could rev it to the moon, but the 1 bbl carb I had on it stopped making power at 4200 rpm. Even when I ran a 1 bbl to 2 bbl adaptor the power level went to 5000 rpm.

Where you are comfortable.

Personally, I have my rev limited set around 5k RPM, even with my stuff. I don't race and don't want to cause premature death. I have not scientific data to back up that number.

Tell your guys past 4200 rpm, your wasting time. 4500 rpm change ups, and no more than 4800 rpm. Stock ring and crank clearances need to be 1.5 and 3 thou up on what the Ford piston and crank bearing max figures suggest to be able to ensure they can take 7000 rpm, so unless you have removed each slug, and plastigaged each bearing and measured against Fords factory blue prints, the above figures stand.

Info from Aussie Ford sedan and F5000 racer Kevin Bartlett, and Dave Benett from Perfect Tune, and Ian Tait from Tait Automtive. Each have won more car races than anyone else, and have built and tuned race engines, Dave the all conquering 1972 Holden Torana XU1 GTR cylinder head.

Ford I6's in the 1965 to 1983 cars were 1,5:1 rod ratio engines with cams no bigger than 256 degrees, so past the power peak, your just waisting time. But in sedan racing, there has to be an over rev capability to tide bettwn gears, so no one stitches you up. If there is power at 3600 rpm, then you need a 16% over rev capability, esepcially when between gears. That means past 4200 rpm, your wasting time.

The other restriction is that without a good cam, a six will be coarse, rough and vibrating at the 4200 to 4800 rpm area, and you don't want the stock cast rods, piston rings and cast pistons to see any more stress than the have to. Funny thing is, when you improve carburation with a stock cam, a six cylinder smoothens out, and as you cam, carb and extract the exhast, the whole engine becomes smoother, making high rpms less of an issue.

put a pigger jet in there, that's an extra 1000rpm right there.

MPGmustang":1jtlxi0o said:

put a pigger jet in there, that's an extra 1000rpm right there.

Click to expand...

... did you mean piggier jet? ( 8) )

the 5 main 170 feels nervous above 4000, balanced 250 redline set at 4500 climbs over 5k quickly at WOT, rev limiter is on my list..

Redline = Parts on the road minus @100 RPM ...

have fub

Most 6 cyl have a issue around 4K inherant to there design. Ford of Austraila set us one of the first OHC 6s because of it. Had to tweek the fuel map a bit to fix it. Some other stuff might have been done I wasnt told about.

If I'm shifting for max acceleration, I shift at 4300-4400. I wouldn't take a straight six over 4600.

I've heard that seven main bearing 200 ci engines can be revved to 7000 rpms reliably, but I wouldn't even attempt to test this. I was having difficulty getting mine up to 4000 rpms before bolting on the carb and improving the ignition.

I havent driven my 250 enough to know were to red line it but have pulled over 7K real quick and easy. It even has the old log head ported. The one with the second from smallest combustion chambers milled .062. Even the log is hogged out.

The issue with rev limit is actually cam timing related mostly. The earlier four bearing cranks, which were actually heavier than the seven bearing cranks, were unable to rev without a torsional period that shook everything loose. By the time of the 1978 200 crank, 3M casting I think, the period was shifted up far enough for you not to run into it. The log head best crank for best rev range is the Aussie 1993 EF crank, a 12 counter weight item, similar to Holdens 12 counterweight 3.3 crank. Ford Australia then downgraded that crank to another type when they sorted the main bearings and oiling system.

In a similar way, the cam timing on the 1985 Aussie 3.3 122 hp Holden created vibration at 4500 to 5500 rpm, but when the breathing and induction and extractors were added for 142 hp, it reved like a top to 5500 rpm and beyond.So if you cam it up, carb it up, and improve the exhast, the same thing happens. This is especially true of earlier Holden engines with there poorer Red motor crank...the 202 was always a vibrstion ridden un reving hulk, but when a set of triple carbs, small chamber head, headers and wild cam were added, it took off, with the two slight harshness periods at 4600 to 5300 not noticable when the engine reved so quickly. When the steel crank or later blue motor 12 counter weight crank was added, the vibration period was at 7500 rpm, so it was okay for racing to 7000 rpm.

The Australian and Argentine 221 and the US and Australian 250 were bigger engines, with the same 8 counterweight cranks as the 200, just longer stroke and bigger main bearings. The tests from Bill Santicecine for Ford in the late sixties said that the crank was okay to 7500 rpm, with forged rods, only the bolts were and issue, and noted that the oil pump might need uprating.

The Aussie cross flow 250 OHV six, and the canted valve Hemi 3939 cc 239 cubic inch OHC, which both have the same crank, is that there is a torsional vibration period exists at 4600 to 5300 depending on the cam timing. The British Prodrive/Tickford (Aston Martin) engineers who did the first 1992 216 hp at 4600 rpm XR6 said the stock 250 Oz crank was marginal at 5000 rpm, and Ford Australia had to upgrade the crank when the 3984 cc 20 thou over 4 litre six came out.

The point is the canted valve engines (1976-1992 x-flow, and 1988-1992 OHC'S) 'created' a vibration problem that didn't apparently exist on the earlier non cross flow engines. This was also noted with the canted valve Clevelands verses Windsor V8's...changing the heads to flatter angles suddenly created more power from less shroading and larger vlaves and a shorter flame path, but more detonation and vibration. When an engine is detonation limited, this is a reason why the canted valve engines like our Aussie X-flow are not always best.