Rev Limit of 3.2 Engine

[chris_seven]

We have a 3.2 Carrera engine which is pretty much stock apart for having had carbs fitted to replace the Injection system.

What do you think the safe rev limit is for this motor ?

It is currently being used to around 6000rpm and has, in the past had a failure when used to 6500rpm.

What do we need to change to make it rev to 7500? Pistons, Rods?

We know we will have to change the cams and I guess rods with a smaller diameter pin may help but what else needs doing?

[Trackrash]

I think the rods would be the big question even with ARP bolts. Obviously the pistons won't take it and I'm guessing you will be using race valve springs.

The 74mm stroke should allow 7500 rpm with out a problem, I would think.

I'm guessing the experts will be chiming in....

[gtc]

After 7000 rpm I would start to think about oil pump upgrades.

[Tippy]

Cross-drilled crank I've heard is mandatory and aftermarket rods and lighter pistons over Mahle's weight.

[Walt Fricke]

7,500 should need only ARP or equivalent rod bolts. Stock rods of this era are very stout, will work well over 8,000 RPM. Stock springs will work up to 7,500 - mine with all outers broken (I was shifting at 8K) ran to 7,600 before valve float, which felt like a soft rev limiter. Same when all the inners on another set broke.

The 3.2 pump is plenty good - my 2.8 motor came with one, and it seems to do as good a job as my turbo pump.

Cross drilling is not essential either, though on a motor I expect to rev over 8K I did it on general principals. Though my experience is with 66mm and 70.4 cranks, not the 74.4.

I don't know why the stock piston wouldn't take it, but chances are you'll want more squeeze, and why not larger valves, and so on if not really counting cost.

Of course, in a race motor having a cushion against overrevs is not a bad idea.

[m42racer]

Chris PM sent.

[m42racer]

On a more general note, the very first decision has to be the application. Then you can figure out what sort of performance you need and then the budget.

[safe]

I've set my soft rev limiter to 6800, bone stock internals. Its been hitting that on regular basis and sometimes a bit above that on the hard limiter at 7000 for 6-7 years now.
I probably should upgrade to ARP rod-bolts.

[chris_seven]

This engine is fitted to a hillclimb car used in the UK where climbs are quite short.

It is currently not used above 6000rpm because 'It's gone bang once before at 6300 ish, with massive damage to the engine'

We don't do much work on 3.2 engines and this seems to be unusual.

The car does run quite well and is competitive but it has stock '915' Gearing and I think some work on the rev limit and much shorter gearing would be a good place to look for an improvement but I can't unjam the 6000rpm issue.

UK Hill climbs are typically between 1000 and 1500 yards long and require 5-6 gear shifts and the 915 can't be helping.

If it could rev longer and avoid a couple of gearshift I believe the improvement could be quite significant.

[MBEngineering]

HI chris_seven


"but it has stock '915' Gearing"

the gearing in the 915 gearbox is not STD

I have worked out from the photos of the engine what went wrong with the engine when it went pop.

there was a offer on the table but was not taken up on and any questions I asked are ignored, parts have been offer and deliverd and not used/sent back, with i will stick with the basic bits

regards mike

[Black_Hat]

reduce the weight of the reciprocating mass.

then valve train will be limiter

[Tippy]

What went wrong, Mike?

Curious to know as 6500 is not a lot of RPM on a such a short stroke motor.