Small Block RPM Potential

[Jack Zupan]

I would like to hear about others experience with small block power. I have a Windsor block with 2V Cleveland heads. This seems to be a nice combination and the car runs pretty good. I think it would run mid 12s. One thing that has surprised me since I owned the car is that it tops out at relatively low rpm, around 5500. I expected a short stroke small block to turn 6500 but maybe this is not realistic. My big block Mopars used to turn between 6000-6200 and that is a lot more rotating mass. I also just put a new MSD distributor in the car and it had no effect on maximum rpm.

So my questions are: What are other peoples experience with 351 Ford engines. Is 5500 normal? What determines rpm potential in an engine? Cam, stroke, weight of rotating assembly? Finally, would a Cleveland block have more rpm potential with the smaller main journal diameters than a similar Windor block.

Jack Z

[cobra281]

Jack Z,

I just had a 351W stroker (408) built. Told the builder I wanted an engine that would easily hit 6000 rpm & 500hp. We used a steel crank, forged rods and pistons, all balanced and blueprinted. A crower hydraulic cam, roller rockers, Victor Jr heads and a stealth dual plane manifold with a custom holley 750 vac. On the engine dyno it made peak hp (470) at 5600 rpm. A little disapointing but it makes me wonder what it really takes parts and money wise ( I spent 12K on the motor) to get more out of one of these motors. Anyway a little info for comparisons sake.

Mile High Ed

[CHRIS POTTER]

I have a 351-c, stock stroke, lunati bracketmasterII cam, performer intake, 750 vac secondary Holley, 4 barrel heads, port plates intake and exhaust side. Pulls hard to 6000, before I shift, would probably keep going, but I have stock reconditioned rods, don't want to ventilate the block. Can't imagine why yours doesn't keep pulling, maybe something in the induction system is not quite matched. I would think your engine would do better than mine. Email me if you have any further questions. Chris

[RACER X #99]

Jack, I have a stock 351 crate motor wlth over 5000 track miles and I consistantly shift at 6500 just as I hit the rev limiter. Your engine should easily rev to 6000+ rpm's.Rpm's are usually limited by valve spring float or cam profile, maybe your engine needs more fuel. BTW the Nascar guys spin these motors to 8000+ all day long.
Keep on checking.
RD

[JAM1775]

I run a crate FMS 351W with ported heads, Weaind Stealth with 3/4" spacer. I peak 378 RWHP (450 flywheel) at 6000 rpm .
My rev limiter is set, at 6300 and my shift light at 6100. My RWTQ is 360 @ 4600 rpm. I could be wrong but I thought the Clevelands produce more torque than the Windsors and should hit the TQ peak a bit sooner. Parts are easier for the windsor.

[Jack Zupan]

Thanks for the input guys. You have given a lot of information. I mentioned the MSD distributor because I thought the old Mallory dual point might have been the limiting factor but it's not. I'm now thinking cam as the next best possiblity. I don't hear a miss or valve float the engine just runs out as they do when they stop making power. I imagine you would hear it if the valve springs weren't keeping up right?

Jack Z

[Richard Hudgins]

Hi Jack,

Valve float at low rpm's (6000 is low) will be felt as a flattening out of the power.

Much like a soft rev limiter.

the best indication is the sound of the exhaust. It will start to go flat sounding and a little out of sequence. In other words, it will lose the crisp sound and drop into the flat bass area.

If you could see the top of your carb, you would also see "fuel standoff". This is the best indication of float, but you really need to be on a dyno to see the standing wave produced.

I would check your seat pressures as well as rates per your cam manufacturer specs.

[Rick Parker]

Jack:

You stated that you have 2V Cleveland heads on 351 Windsor block. Has the camshaft ever been changed? Is it the OEM stock cam? If it is, 5500 RPM is about the top, regardless of ignition
or cylinder head configuration. I'm asuming the timing is set properly. To move horsepower & torque up the RPM scale you will need too consult with a camshaft manufacturer to get a camshaft with timing specs that will allow the engine to reach
it's RPM potential. Should be able to spin 6000-6500 with a stock
rod balanced bottom end. Regarding one block vs another. The block is just the foundation, it provides the rigidity to support the crank and rod rotating assembly and creates the shortblock assembly; it will along with the quality of crank and rods ultimatly determine the rpm limit of the engine before the crank begins to flex and ultimatly fail.

Rick

[Jack Zupan]

Richard,

Thanks for your post. When I ran my 440 Mopar 20 years ago, it would miss when it was overreved. I interpreted this as "valve float", i.e. the hydralic lifters couldn't pump up fast enough for the engine speed. Maybe that wasn't a correct conclusion.

Rick,

The camshaft is a "Crane 270" with Crane valvesprings and Comp Cams roller rockers. I don't have anymore information than that since I bought the car used and that was on the build sheet. It's probably correct as other parts of the build sheet were honest. I have timing set to 12 degrees advanced and the distributor is set to advance to full at aroung 2800 rpm.

Jack Z

[Rick Parker]

Jack:

A lot of good information being shared on this thread. You might want to consider checking the port matching between the intake manifold and cylinder head face. If there is any significant mismatch, turbulance could be created causing a roadblock to further performance gains. When you begin to combine parts such as this careful attention to detail is critical. Refer to the success of the Boss 302 which was an effort to overcome the failed tunnelport 302 effort in early Trans Am racing.

Just a thought

Rick

[Jack21]

Jack,

I'm thinking valvesprings more than anything else, particularly with a hydraulic roller and mix & match parts. The roller lifters tend to be heavier than regular hydraulic lifters, so a stiffer spring is needed. Talk to the tech guys at Crane & Comp and see which spring you should be using.

Almost any aftermarket performance cam, even the milder grinds, should be able to rev to 6000 - 6200.