There have been several discussions about rebuilding the 327/300, and several opinions about cam choice. Regardless of cam choice, is it still a good idea to do the pocket porting, port matching, and angle cutting on the valves ?
-
-
Re: Low end torque
Yes, it is, Steve. These mods improve the port efficiency without regard to an rpm range. As long as you're not "porting" the head's entire runner, you'll feel the improvement all the way from off-idle to redline. -
Re: Low end torque
Yes, it is, Steve. These mods improve the port efficiency without regard to an rpm range. As long as you're not "porting" the head's entire runner, you'll feel the improvement all the way from off-idle to redline.Comment
-
Re: Low end torque
Deliberate mis-matching on the exhaust side can aid in anti-reversion of exhaust gases. This procedure is a "shade tree" dirt track racers "trick".
Dale.Comment
-
Re: Low end torque
Deliberate mis-matching on the exhaust side can aid in anti-reversion of exhaust gases. This procedure is a "shade tree" dirt track racers "trick".
Dale.Comment
-
Re: Low end torque
Hmmmm, if you want some real grunt, why not put a 350 crank in the thing. If you are rebuilding, you are buying pistons anyway and they are cheaper for a 350 most of the time. Find a stock 350 crank, cast or steel, whittle down the mains and rods and your there. The .250 longer stoke and the increase in cubic inches will give it plenty of grunt and cam selection gets a little easier.
Don't go crazy with the griner in the heads and if you have never done a set don't pratice on the heads you want to use.
Concentrate on the area from the valve seat down 1 inch, the most gain for the least effort. Same in the runners, gasket match and let it go.Comment
-
Re: Low end torque
Hmmmm, if you want some real grunt, why not put a 350 crank in the thing. If you are rebuilding, you are buying pistons anyway and they are cheaper for a 350 most of the time. Find a stock 350 crank, cast or steel, whittle down the mains and rods and your there. The .250 longer stoke and the increase in cubic inches will give it plenty of grunt and cam selection gets a little easier.
Don't go crazy with the griner in the heads and if you have never done a set don't pratice on the heads you want to use.
Concentrate on the area from the valve seat down 1 inch, the most gain for the least effort. Same in the runners, gasket match and let it go.Comment
-
Re: Low end torque
Yes. You just need to smooth out the ridge above the valve where the first tool cut was made to rough form the valve pocket. Remove any casting flash from inside the port, but do not enlarge them. Match the manifold head interface using a GM gasket as a template. The ports do not need to be "polished". If you go with multiangle valve seat cuts the minimum seat thickness for the inlet should be .040" and .060" on the exhaust. Books like "How to Hot Rod a Small Block Chevy", and the old Chevrolet Power Manuals from the seventies and eighties have all the details.
The improved flow efficiency from these mods will cause the engine to loose just a bit of low end torque, but the top end should be 5 to 10 percent stronger and the engine should rev nicely to 5500.
The anti-reversion feature as mentioned by Dale consists of making the header opening larger than the exhaust port, however, because there is no effective wave action with the production manifolds my recommendation is to match the exhaust port outlet to the manifold inlet to provide the smoothest flow.
Doing this work on the heads/manifolds is beneficial to any engine. The cam should be selected based on low end torque and the heads reworked to make as much top end power as possible. By reworking the heads on a 300 HP engine with the OEM cam, you'll maintain the nice idle quality and strong low end torque, but you'll have nearly as much top end power as a production L-79.
DukeComment
-
Re: Low end torque
Yes. You just need to smooth out the ridge above the valve where the first tool cut was made to rough form the valve pocket. Remove any casting flash from inside the port, but do not enlarge them. Match the manifold head interface using a GM gasket as a template. The ports do not need to be "polished". If you go with multiangle valve seat cuts the minimum seat thickness for the inlet should be .040" and .060" on the exhaust. Books like "How to Hot Rod a Small Block Chevy", and the old Chevrolet Power Manuals from the seventies and eighties have all the details.
The improved flow efficiency from these mods will cause the engine to loose just a bit of low end torque, but the top end should be 5 to 10 percent stronger and the engine should rev nicely to 5500.
The anti-reversion feature as mentioned by Dale consists of making the header opening larger than the exhaust port, however, because there is no effective wave action with the production manifolds my recommendation is to match the exhaust port outlet to the manifold inlet to provide the smoothest flow.
Doing this work on the heads/manifolds is beneficial to any engine. The cam should be selected based on low end torque and the heads reworked to make as much top end power as possible. By reworking the heads on a 300 HP engine with the OEM cam, you'll maintain the nice idle quality and strong low end torque, but you'll have nearly as much top end power as a production L-79.
DukeComment
-
another question
Wally, Just so I'm sure I understand,
1 350 pistons are same OD as for 327, so they are interchangeable ?
2 The whittling you mention is on the crank journals ?
3 No other changes required to accommodate the longer stroke ?
SteveComment
-
another question
Wally, Just so I'm sure I understand,
1 350 pistons are same OD as for 327, so they are interchangeable ?
2 The whittling you mention is on the crank journals ?
3 No other changes required to accommodate the longer stroke ?
SteveComment
-
Re: another question
No, Steve. 350 and 327 pistons only share the same bore size at 4.0". The pin height is in a different location to account for the stroke difference. So, you can't use a 350 piston in a 327 unless you can live with the piston being more than .10" down in the hole at TDC (you don't want to do this). What he means by "whittling" the crank journals is that all but the latest 327s had smaller crank and rod journals so if your block is an early small-journal block, you would need to grind the 350 crank to the 327-sized main journals to use it in your 327 block (you would also need to buy large-journal rods to match the 350 crank). This would allow you to stroke your engine from the 327s 3.25" stroke to the 350s 3.48" stoke making your engine a 327 on the outside, but a 350 on the inside.
Just my opinion here (and considering it's free, take it for what it's worth) but I don't recommend stroking a small-journal 327 with a large-journal 350 crank -especially a cast crank. If there was no other way...possibly, but not this way.Comment
-
Re: another question
No, Steve. 350 and 327 pistons only share the same bore size at 4.0". The pin height is in a different location to account for the stroke difference. So, you can't use a 350 piston in a 327 unless you can live with the piston being more than .10" down in the hole at TDC (you don't want to do this). What he means by "whittling" the crank journals is that all but the latest 327s had smaller crank and rod journals so if your block is an early small-journal block, you would need to grind the 350 crank to the 327-sized main journals to use it in your 327 block (you would also need to buy large-journal rods to match the 350 crank). This would allow you to stroke your engine from the 327s 3.25" stroke to the 350s 3.48" stoke making your engine a 327 on the outside, but a 350 on the inside.
Just my opinion here (and considering it's free, take it for what it's worth) but I don't recommend stroking a small-journal 327 with a large-journal 350 crank -especially a cast crank. If there was no other way...possibly, but not this way.Comment
-
I'm trying
Let's see if I have this right yet;
If I keep the OEM cam, and do the head modifications, I lose a little on the low end, but gain some on the high end ?
If I do the head modifications, is it possible to choose a cam that would maintain (or increase) the low end torque of the original set up ? It's ok if the idle roughens up a bit; I wouldn't mine having a little "bloopity bloop".
SteveComment
-
I'm trying
Let's see if I have this right yet;
If I keep the OEM cam, and do the head modifications, I lose a little on the low end, but gain some on the high end ?
If I do the head modifications, is it possible to choose a cam that would maintain (or increase) the low end torque of the original set up ? It's ok if the idle roughens up a bit; I wouldn't mine having a little "bloopity bloop".
SteveComment
Comment