How much overbore safely?
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Joined: Jul 2005
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From: Auburn, CA
How much overbore safely?
I'm thinking of doing a quick rebuild on my 1991 4.9 with 100k miles. The engine runs very well for it's miles but I need a bit more torque for towing. My thought is go .80" overbore, do a multi-angle valve job and maybe do some simple port matching. I might also have it balanced while I have it apart.
I tow an open car trailer and my total weight fully loaded with a full tank of gas and two passangers is 9400 Lbs. It would be far more enjoyable driving across the USA going over slight hills without downshifting out of fifth gear. The way the truck is now I can maintain 75MPH most of the time but I really have to pay attention to the hills.
TIA,
Vince
I tow an open car trailer and my total weight fully loaded with a full tank of gas and two passangers is 9400 Lbs. It would be far more enjoyable driving across the USA going over slight hills without downshifting out of fifth gear. The way the truck is now I can maintain 75MPH most of the time but I really have to pay attention to the hills.
TIA,
Vince
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Joined: May 2002
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From: Broken Arrow, OK
Overboring to increase performance is a waste of time and money. On a late block I doubt 0.080 is doable, maybe on an early block, but have it sonic checked first.
You really shouldn't be pulling that load in OD anyway.
You really shouldn't be pulling that load in OD anyway.
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Joined: Jul 2005
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From: Auburn, CA
I'm too lazy to do the math but just by eye it seems with such a long stroke engine that .80" would give twenty to thirty FT. Lbs. of torque. I mean if the block had enough wall thickness to handle it.
Between that and having everything else freshen up would probably get me over where I am now (just needing a RCH more power).
As far as gearing goes I haven't heard of too many problems blowing up fifth gear with a six.
I will take your information into account though and thanks!
Vince
Between that and having everything else freshen up would probably get me over where I am now (just needing a RCH more power).
As far as gearing goes I haven't heard of too many problems blowing up fifth gear with a six.
I will take your information into account though and thanks!
Vince
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From: Broken Arrow, OK
It isn't .8", it's .08". That will give you around 312 cubic inches and maybe 5 more ft-lbs. Extra displacement does nothing if you can't give it an equivalent amount more air. The equivalent amount more air would do about the same if it was used by the engine with original displacement. If you can't get at least 10% more displacement out of a bore/stroke job it isn't worth it from a power production standpoint. Even then there are really only one or two practical reasons to do it.
Chances are that 5th gear won't blow up. It will probably be the main shaft from lack of lubrication when the fluid over heats. I tow in 5th all the time, but if the engine is struggling even a little bit it gets downshifted to 4th.
Chances are that 5th gear won't blow up. It will probably be the main shaft from lack of lubrication when the fluid over heats. I tow in 5th all the time, but if the engine is struggling even a little bit it gets downshifted to 4th.
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Joined: Jul 2005
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From: Auburn, CA
.08 of course is correct and it does work out to 2CI per twenty thou.. I guess I should do a compression check and see where I am now before getting too ahead of myself.
Breathing shoudn't be much of a problem at 2500 RPM I don't think.
Maybe dropping .060 off the cylinder head and going .060 overbore might do what I want it to do. That, a multi-angle valve job and just being freshed up could do it you think?
I'm trying to stay away from going to a stand alone engine managment system (I have a spare TEC3 that I would like to sell to pay for the rebuild).
Sounds like you are in the same boat as me...
Thanks,
Vince
Breathing shoudn't be much of a problem at 2500 RPM I don't think.
Maybe dropping .060 off the cylinder head and going .060 overbore might do what I want it to do. That, a multi-angle valve job and just being freshed up could do it you think?
I'm trying to stay away from going to a stand alone engine managment system (I have a spare TEC3 that I would like to sell to pay for the rebuild).
Sounds like you are in the same boat as me...
Thanks,
Vince
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Joined: May 2002
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From: Broken Arrow, OK
Chances are that the shortblock doesn't have anything wrong with it and you will just be throwing money away to go through it. Mine has 306k miles on it and still runs as strong as it did 100k miles ago.
You are on the right track with working the head over and breathing is a problem at 2500 rpm. You are trying to feed 50 cubic inches per cylinder with valves that have trouble feeding 35-40 cubic inches. Milling the head 0.060 will significantly raise the compression, possibly to the point of having to run premium fuel. It will also require you to check the valvetrain geometry.
You shouldn't need to do much tuning unless you change the cam to something with a significant amount of overlap. It won't run to its full potential, but it will be pretty close.
You are on the right track with working the head over and breathing is a problem at 2500 rpm. You are trying to feed 50 cubic inches per cylinder with valves that have trouble feeding 35-40 cubic inches. Milling the head 0.060 will significantly raise the compression, possibly to the point of having to run premium fuel. It will also require you to check the valvetrain geometry.
You shouldn't need to do much tuning unless you change the cam to something with a significant amount of overlap. It won't run to its full potential, but it will be pretty close.
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Joined: Jul 2005
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From: Auburn, CA
So do somthing like the Clifford Research blueprinted head with like .030 milled might do what I want?
http://cliffordperformance.net/Merch...tegory_Code=F3
Regular gas is something I would call a requirement.
A head swap would be far less of a job! I could just run the head to my local speed shop and them do the work and save a few bucks while I'm at it.
Do you really think that just head work will do the trick?
Thanks,
Vince
http://cliffordperformance.net/Merch...tegory_Code=F3
Regular gas is something I would call a requirement.
A head swap would be far less of a job! I could just run the head to my local speed shop and them do the work and save a few bucks while I'm at it.
Do you really think that just head work will do the trick?
Thanks,
Vince
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From: Broken Arrow, OK
If the compression and leakdown numbers check out, yes just play with the head and call it good.
Don't do the Clifford blueprinted head. It's for a carbed engine, so the chamber is too big, and it won't do anything near what they claim.
Don't do the Clifford blueprinted head. It's for a carbed engine, so the chamber is too big, and it won't do anything near what they claim.
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Joined: Jul 2005
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From: Auburn, CA
I built up a Chev* 250 using CR stuff back in the day and they make good stuff.. But their numbers are a bit on the high side.
Back on track. What should I have done to the head and have you done it?
Thanks,
Vince
Back on track. What should I have done to the head and have you done it?
Thanks,
Vince
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Joined: Jul 2005
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From: Auburn, CA
Silver Streak:
Have you gone to oversize valves as well as different rockers? I've been reading some of your old posts and can't figure out what you have done to get your torque numbers as high as they are.
Thanks again,
Vince
Have you gone to oversize valves as well as different rockers? I've been reading some of your old posts and can't figure out what you have done to get your torque numbers as high as they are.
Thanks again,
Vince
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Joined: May 2002
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From: Broken Arrow, OK
The CR stuff is now outdated and way overpriced compared to the stuff you can order from Summit or PAW. Most of their stuff is just reboxed from other manufacturers.
My engine is stock from the oilpan to the intake manifold except for 3.8 V6 rocker arms, which didn't do much in a usable rpm range. The only mod I have that really made a big difference was the headers and exhaust. Everything else was insignificant as far as power and torque production. I have 19lb injectors and an adjustable regulator. They help throttle response a lot, which is important on the track but won't show up on the dyno. They also allow me to disconnect the battery without the truck running like crap for a week while the computer relearns everything. The MSD had the same affect as the injectors; no real power increase, but slightly better response. The MSD probably is not worth $200 without a heavily modded engine.
On your head I would just blend all the rough edges in the port and get a good valvejob done. That should help a lot. I have not done this on mine because of the mileage on the engine. I'm just going to build a new engine to do everything to.
My engine is stock from the oilpan to the intake manifold except for 3.8 V6 rocker arms, which didn't do much in a usable rpm range. The only mod I have that really made a big difference was the headers and exhaust. Everything else was insignificant as far as power and torque production. I have 19lb injectors and an adjustable regulator. They help throttle response a lot, which is important on the track but won't show up on the dyno. They also allow me to disconnect the battery without the truck running like crap for a week while the computer relearns everything. The MSD had the same affect as the injectors; no real power increase, but slightly better response. The MSD probably is not worth $200 without a heavily modded engine.
On your head I would just blend all the rough edges in the port and get a good valvejob done. That should help a lot. I have not done this on mine because of the mileage on the engine. I'm just going to build a new engine to do everything to.
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Joined: May 2002
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From: Broken Arrow, OK
I think my 300 is about done. It still runs strong, but I've noticed funny noises lately and every once in a while I see a bit of smoke. I'll sleep better on road trips knowing I've got a new bullet in the chamber.
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From: Sussex County, DE
Mine is spoiled, I guess. It makes funny clickity clack noises when it's cold unless I use Mobil 1. I can't even use another brand of synthetic! It goes away when the engine gets warm, though, but with Mobil 1, it never does it, cold or hot.