I was reading chad_xf tread about bestway to spend $5000 on a cleavland and it was mentioned a cheap way to boost cubes in a 302 was to run a 351 crank with the standard 302 rod. I thought that buy doing that you would be push the pistons into the head. is this so? Im after hight compresion as Im running strait gas.
Kieta don't worry about the long rod, I don't think its worth the axtra cost when running on LPG, what mixer setup do you have?
Having longer rod is better for higher RPM, a place not usually seen by LPG fuelled engines, if you are going straight gas build the combo around that.
I have to suggest you are propagating another myth , the fues is NOT relelvent to revs and planty of lpg motor get reved ahrd a lot , ask any cab driver on fixed payin working nights.. ( I have an EL on gas which often sees 6000rpm.)
Most people who are selling a 351 crank will let the rods go with it for the same price, ie 200 to 250$, the additional costs for the special pistons to use the 302 rod make the build a bit more expensive.
I asked what mixer setup kieta is going to use because an Impco 425 will not supply a 351 past 5K, a twin GRA maybe 6-6200rpm, the whole engine package if using a 425 may not benefit from the long rod. The GRA engine combo might benefit from the longer rod. At the end of the day you probably wouldn't notice the difference between a long or short rod, the long rod combo just adds more cost to the build, and the effect it has on the behaviour of the combo has been debated since time began, but its generally thought that long rod gives gains in the higher rpms, I have never seen an LPG fuelled engine combo that does "high" rpm's without leanout. I've owned both 425 and twin GRA setups and neither will fuel what I would consider high RPM's, so to me the possible benefit of the longer rod is negated by the fact kieta is going to run on LPG, so I would say the fuel type does have an effect on RPM's but it's more due to fuel delivery restrictions rather than fuel type.
If the engine is going to see over 6Krpm then you might consider the longer rod, but with LPG its really hard to get fuel in above 6000rpm, so do you really need the longer rod? I'd rather have more initial airspeed with the shorter rod(at least thats what the theory is), hence an earlier TQ production. At least thats my understanding of the whole situation, LPG is drawn in by the vacuum.
Having an EL on gas means nothing, and doing 6000rpm with a 4.0 is a whole lot different than a 5.8, the 5.8 would still be consuming more air. just because you can spin 6000rpm doesn't mean its making power up there, I'd imagine on a dyno it would peak much much lower, generally there's no point revving much past the peak power RPM of the combo unless you are dropping below the TQ peak on shifts. You aim to spend the most time in the area under both curves, revving past that you will only go slower, and make more noise, which acheives nothing.
Also I've heard a few bad stories about the special ACL piston? never used one on my engines. Down the coast in the shop I worked at they always used std length 351 rods, we did one long rod combo but that engine saw 7000+rpm frequently, had 4V's, and solid, you typically don't build LPG engines like that. LPG just isn't a high PPM high HP fuel, unless its forced, then its another story, even then RPM isn't a requirement.
In the end its only a few hundred dollars more to go the long rod, having owned 351's with both common LPG systems, I would rather spend that 2-300? dollars elsewhere, probably on ignition, commonly under-powered on many LPG vehicles.
Impco with hydraulic cam, use std rod length.
GRA twin with solid cam, maybe use longer rod.
Just my opinion based on my time spent with LPG and 351's.
ACL make a piston specifically for this conversion, but Brenx referred to a few bad experiences from other users about this brand, so you may want to wait for someone who can recommend another brand.
I know of some one who had an acl pistons/rings,6"rod and 351 crank set up and he had nothing but dramas with oil consumption.. it got to the stage where the motor after six months was pulled down and fitted with a custom piston ($495 piston/rings)that "pro mod cyl heads" in preston-melbourne had made up, to suit the 6" rod 351 clevo's.What a relief that was,no more oil consumption..!!!motor is still going great 6 years later..!!
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