Another connecting rod question

[Mike R.]

I am going to pull my heads to have them ported and install the correct shim type gasket. All the talk about connecting rods has me thinking of going further and replacing the rods as an insurance measure.

I called Eagle and they recommended their H beam rods for an engine that might see 7000rpm. If I buy these I will have to convert the pistons to floating pins. I could also go to a 6.125" rod with a 350 piston designed for a 6" rod and improve the rod/stroke ratio which would improve the top end. If I do this I will have to take the whole engine apart. At a minimium I think I would have to rebalance the crankshaft(???)

The engine seems to be in great shape but I don't know how many miles are on it or what was done to the rods when it was apart. I am not particularly anxious to spend a lot of money for nothing.

I have read the various posts on the subject. The question is whether it is worth taking the engine apart just to change to rods. I do reach 6500 rpm on the tach now and then and I am not sure of the accuracy of the tach. With the heads pocket ported, it seems the engine will be much more willing to reach high rpm and it will be hard not to indulge.

So I have a few questions

Should I get the H beam type rods (which are only availible with floating pins?

If I get the H beam rods, is there enough benefit to go to the longer rods?

How much risk of a rod failure is there using the stock redline on the stock tach?

Should I just leave the bottom end alone for now?

[Domenic T.]

Re: Another connecting rod question

Mike,
I would leave the bottom end alone unless you want to do the complete engine, pistons, probable bore,balance, all bearings etc.

If your rings are good just do the heads if that is what you want to do.
I would test the play in the timing chain or just replace it also.

DOM

[John H.]

Re: Another connecting rod question

I called Eagle and they recommended their H beam rods for an engine that might see 7000rpm. If I buy these I will have to convert the pistons to floating pins. I could also go to a 6.125" rod with a 350 piston designed for a 6" rod and improve the rod/stroke ratio which would improve the top end. If I do this I will have to take the whole engine apart. At a minimium I think I would have to rebalance the crankshaft(???)

Mike -

There are other strong rods out there that don't require the use of floating pins, and the "long rod theory" (6" or 6.125", for more dwell at TDC) is VASTLY overrated, and makes about as much sense as flushing $1,000 down the toilet. Any rod change will require re-balancing, unless you're lucky enough to find rods that weigh exactly the same as the stock rods - overall, big end only, and small end only, and that's not going to happen.

What year is your car?

[Joe L.]

Re: Another connecting rod question

I am going to pull my heads to have them ported and install the correct shim type gasket. All the talk about connecting rods has me thinking of going further and replacing the rods as an insurance measure.

I called Eagle and they recommended their H beam rods for an engine that might see 7000rpm. If I buy these I will have to convert the pistons to floating pins. I could also go to a 6.125" rod with a 350 piston designed for a 6" rod and improve the rod/stroke ratio which would improve the top end. If I do this I will have to take the whole engine apart. At a minimium I think I would have to rebalance the crankshaft(???)

The engine seems to be in great shape but I don't know how many miles are on it or what was done to the rods when it was apart. I am not particularly anxious to spend a lot of money for nothing.

I have read the various posts on the subject. The question is whether it is worth taking the engine apart just to change to rods. I do reach 6500 rpm on the tach now and then and I am not sure of the accuracy of the tach. With the heads pocket ported, it seems the engine will be much more willing to reach high rpm and it will be hard not to indulge.

So I have a few questions

Should I get the H beam type rods (which are only availible with floating pins?

If I get the H beam rods, is there enough benefit to go to the longer rods?

How much risk of a rod failure is there using the stock redline on the stock tach?

Should I just leave the bottom end alone for now?

Mike------

Most (but not all) 1965 Corvette small blocks used connecting rod GM #3864881. If you have these rods, they should be adequate for a street 327, especially if you have them magnaflux inspected and fitted with ARP-2000 rod bolts. So, how can you tell? Well, if you have a very late car, it's almost certain you have these. Otherwise, you probably need to check them to be sure.

If you have earlier rods, I would not be comfortable with them. No doubt, many 1962-E65 SHP small blocks lived very well with them, but I would not be comfortable with them.

So, what do the 3864881 look like? See the attached:

[Mike R.]

Re: Another connecting rod question

Thanks everyone.

My car is a 65 FI convertible built in Dec 1964.

I see the pictures of the 881 rods but I what is the difference between these and the earlier ones?

Mike------

Most (but not all) 1965 Corvette small blocks used connecting rod GM #3864881. If you have these rods, they should be adequate for a street 327, especially if you have them magnaflux inspected and fitted with ARP-2000 rod bolts. So, how can you tell? Well, if you have a very late car, it's almost certain you have these. Otherwise, you probably need to check them to be sure.

If you have earlier rods, I would not be comfortable with them. No doubt, many 1962-E65 SHP small blocks lived very well with them, but I would not be comfortable with them.

So, what do the 3864881 look like? See the attached:

[Joe C.]

Re: Another connecting rod question

Thanks everyone.

My car is a 65 FI convertible built in Dec 1964.

I see the pictures of the 881 rods but I what is the difference between these and the earlier ones?

More beef........in these areas:

[Tracy C.]

Re: Another connecting rod question

............

So, what do the 3864881 look like? See the attached:

I'll take a wild guess that the rod in the picture has at least seven NOS brothers all of which are stashed safely away in a California rat hole. And lest anyone should wonder...they would NOT be for sale..

tc

[Mike R.]

Re: Another connecting rod question

Thanks I will take a look when I pull the engine. I am trying to get some information on the prior engine work but it is unlikely that the information still exists. I would feel much better with some modern design rods. My goal is to have the best possible performance and reliability with minimal deviation from stock.

[Duke W.]

Re: Another connecting rod question

You have a lot of conflicting criteria. You are considering many expensive "upgrades", but don't want to spend a lot of money. You need to do some serious thinking.

The Eagle SIR 5700SP small bearing pressed pin rods are more than adequate for a 7000 rev 327, and they are only marginally heavier that OE.

I don't know why Eagle would recommend H-beam rods. They are very heavy relative to OE.

As John said the rod length issue is WAAAAAAAAAAAAAAAAY overblown.

And you still haven't told us what year/engine you have.

Duke

[Mike R.]

Re: Another connecting rod question

Hi Duke, it always good to hear your counsel

I did previously post that the car is a 1965 375hp FI convertible. It is stock as far as I know with the exception of a thicker head gasket (I can get a .040 feeler between the block and head. I am pulling the heads to send them to Gromm in San Jose for porting, I was happy with their work on my big block heads in the past.

My goal is to have a bullet proof bottom end within the rev limitations of the 30-30 cam and the (ported) original heads.

I would also value your opinion on how much difference I would expect to see if I converted (back) to a undercar exhause system. I currently have Vettepacks 2.25" mufflers in my sidepipes. I like the look of the sidepipes but they are quite loud. Also, how much power difference is there between headers and stock manifolds with the combination I will have?

I also posted another question of AF mixture that generated no responses. The only thing that I can think of that would explain the data in the post is an intemitent ignition miss at higher rpm. When I look at the rpm on the graph it doesn't look like this is happening and it seem to run fine. The post is here:

https://www.forums.ncrs.org/showthre...11941&uid=5914

Thanks

Mike

You have a lot of conflicting criteria. You are considering many expensive "upgrades", but don't want to spend a lot of money. You need to do some serious thinking.

The Eagle SIR 5700SP small bearing pressed pin rods are more than adequate for a 7000 rev 327, and they are only marginally heavier that OE.

I don't know why Eagle would recommend H-beam rods. They are very heavy relative to OE.

As John said the rod length issue is WAAAAAAAAAAAAAAAAY overblown.

And you still haven't told us what year/engine you have.

Duke

[Duke W.]

Re: Another connecting rod question

Search the archives "327 LT-1", and that thread has links to dyno tests and other info on the Corvette Forum including headers versus manifolds with open exhaust. With the on-car exhaust the difference would be even less. Use your own judgement.

OE manifolds, under the car exhaust are the way to go. I have never recommended anything else. You can keep it totally original appearing and still make about 300 SAE corrected RWHP with a useable power bandwidth to 7200 and decent low end torque with the LT-1 cam.

Back when these projects were done, the Eagle SIR 5700SP wasn't available. They were released in 2007 and are half the price of the Crower Sportsmans.

A WOT A/F ratio in the range of 12.5-13.5:1 from off idle to max revs is perfectly acceptable.

Duke

[Jim L.]

Re: Another connecting rod question

I am pulling the heads to send them to Gromm in San Jose for porting, I was happy with their work on my big block heads in the past.

Gromm. Wow.

A friend of mine used Gromm when he was building up a 283 with which to go vintage racing.

Starting with real-deal, single pyramid, power-pak heads, Gromm did work that let that 283 dyno at 394 HP.

Gromm.... good choice.

Jim

[Mike R.]

Re: Another connecting rod question

THanks Duke,

I have read the various posts and intend to go with the LT-1 cam and SIR rods. The 30-30 cam that I have now seems to have marginal vacuum. Sometimes it idles great at 900 but other times it wants to die and needs 1050 to have a reliable idle. I am going to measure the backpressure of my current exhause system but I plan to go to the undercar system for noise, originality and power reasons.

Of course all this might change if I get a big surprise after opening up the engine!

Mike

Search the archives "327 LT-1", and that thread has links to dyno tests and other info on the Corvette Forum including headers versus manifolds with open exhaust. With the on-car exhaust the difference would be even less. Use your own judgement.

OE manifolds, under the car exhaust are the way to go. I have never recommended anything else. You can keep it totally original appearing and still make about 300 SAE corrected RWHP with a useable power bandwidth to 7200 and decent low end torque with the LT-1 cam.

Back when these projects were done, the Eagle SIR 5700SP wasn't available. They were released in 2007 and are half the price of the Crower Sportsmans.

A WOT A/F ratio in the range of 12.5-13.5:1 from off idle to max revs is perfectly acceptable.

Duke

[Duke W.]

Re: Another connecting rod question

That sounds like a good plan. Better than average head massaging should yield 230+/190 CFM at 0.5" lift, 28" H20 test depression. At that point on a carbureted SHP engine the manifold becomes the limitation, but the FI manifold has higher flow efficiency, so I think you can achieve/exceed 300 SAE corrected RWHP on a Dynojet.

The LT-1 cam should idle at 900 @ 12", which is a couple of inches more than the 30-30 cam, and the LT-1 cam should yield about 80 percent peak torque at 2000.

Under the car exhaust with the repro off road mufflers is the best exhaust system.

Duke

[George J.]

Re: Another connecting rod question

Mike------

Most (but not all) 1965 Corvette small blocks used connecting rod GM #3864881. If you have these rods, they should be adequate for a street 327, especially if you have them magnaflux inspected and fitted with ARP-2000 rod bolts. So, how can you tell? Well, if you have a very late car, it's almost certain you have these. Otherwise, you probably need to check them to be sure.

If you have earlier rods, I would not be comfortable with them. No doubt, many 1962-E65 SHP small blocks lived very well with them, but I would not be comfortable with them.

So, what do the 3864881 look like? See the attached:

Joe,
you state that most '65's have these rods. I thought that I had read in a prior thread that they changed in '66. Can you confirm that it was '65? I am pretty sure mine were what you show and my car is an April '65 build date.

George