[ForceFed86]

So I threw together this 4 bolt main 85 SBC. Running vortec heads and a pretty mean cam. Hyd. Flat Tappet Cam, Duration: 303/313
Duration @ .050": 234/244, lift: .488/.510, 106 LC. 600 edelbrock carb

I've done all the "mods" necessary to the heads to allow a high lift cam. Threw it in an 86 mazda rx-7. Car weighs in at 2700lbs. Th350 trans with a 2500 stall.

So my problem is the car starts and idles fine. Pretty "lopey" and I get some popping out of the exhaust. With the base timing @ 19 and total around 36. The engine revs very slowly and runs like a total dog. If I advance base timing to 26* or so it's a whole new animal. And has a very snappy rev. I'm afraid to drive it like this because I don't want the timing to go through the roof when the mechanical adv. kicks in. Any suggestions? Should I just weld the fly weights and run 34-36* base and call it good? Ign system is just an HEI accel distrib. Will this cause hard starting?

I think part of my issues are due to the low compression and some what aggressive cam setup. I believe the engine was an 85 SBC out of a truck of some sort. Originally the motor had 458642 head castings 72 cc. 1.94 int 1.50exh. So with the vortec heads I should be at about 9:1 com...or so I'm told.

What would U all do in my situation?





U can see some of the build her eif your interested.

http://www.v8rx7forum.com/v8-rx-7-bu...get-build.html

[ap72]

I'd get a cam that is a much better match.

26º may not be too much initial timing though.

[oldbogie]

Originally Posted by ForceFed86

So I threw together this 4 bolt main 85 SBC. Running vortec heads and a pretty mean cam. Hyd. Flat Tappet Cam, Duration: 303/313
Duration @ .050": 234/244, lift: .488/.510, 106 LC. 600 edelbrock carb

I've done all the "mods" necessary to the heads to allow a high lift cam. Threw it in an 86 mazda rx-7. Car weighs in at 2700lbs. Th350 trans with a 2500 stall.

So my problem is the car starts and idles fine. Pretty "lopey" and I get some popping out of the exhaust. With the base timing @ 19 and total around 36. The engine revs very slowly and runs like a total dog. If I advance base timing to 26* or so it's a whole new animal. And has a very snappy rev. I'm afraid to drive it like this because I don't want the timing to go through the roof when the mechanical adv. kicks in. Any suggestions? Should I just weld the fly weights and run 34-36* base and call it good? Ign system is just an HEI accel distrib. Will this cause hard starting?

I think part of my issues are due to the low compression and some what aggressive cam setup. I believe the engine was an 85 SBC out of a truck of some sort. Originally the motor had 458642 head castings 72 cc. 1.94 int 1.50exh. So with the vortec heads I should be at about 9:1 com...or so I'm told.

What would U all do in my situation?





U can see some of the build her eif your interested.

http://www.v8rx7forum.com/v8-rx-7-bu...get-build.html

Cam's not timed correctly.

Bogie

[ForceFed86]

Cam was degree'd in at 3* adv... ? Could U explain a little why U think this may be the problem? How can I confirm this? I had an engine builder friend come by and check it for me. I just lined up the timing marks and he said it was 3 degrees adv and not to mess with it.

Thanks!

[oldbogie]

Originally Posted by ForceFed86

Cam was degree'd in at 3* adv... ? Could U explain a little why U think this may be the problem? How can I confirm this? I had an engine builder friend come by and check it for me. I just lined up the timing marks and he said it was 3 degrees adv and not to mess with it.

Thanks!

Sure doesn't sound like it, the symptoms are classic for a retarded cam. there can be other things such as ignition strength, timing and carb mixtures that have close symptoms, but the great idle, will not rev, has no power, and needs a lot of base advance are just a perfect fit for a late cam. Radical cams are very intolerant of near misses

Being "degreed in" means a degree wheel, positive stop, and dial indicator were used.

In order the drill goes:

1) using the degree wheel and positive stop find true TDC. Lots of instructions on how to do that on the web so I won't be-labor that
here.

2) with the degree wheel adjusted to TDC, a dial indicator mounted to a part of the lifter not affected by a hydraulic plunger or to a solid lifter, with the cam's degree card, the crankshaft is walked around in the normal direction and the cam events at number measured for action against the cam card.


You mentioned low compression, that may be playing a part. But low compression with a Vortec head would lead me to think that you're running dished pistons as the Vortec on a 350 with flat tops gets some good compression. Radical cams are 100% dependent upon high speeds to operate properly. At low speeds the large amount of overlap and the late closing intake lets a lot mixture out of the cylinder. This reduces the density of the charge and causes a huge loss of bottom end power. The recovery process is with a lot of advance and a lot of compression. Starting backwards, the high compression recovers some of the low speed mixture density so the stuff will burn. The "extreme" advance provides the low density (even at high compression) time to get a burn across the chamber before the exhaust valve opens. These things restore some idle quality and build up bottom end torque. If you actually achieved a 3 degree advance on the cam this would also help some.

There is nothing wrong with running a base timing of 26 degrees but then the total curve needs to be reduced in the amount that variable can put in so the top end timing isn't getting to be 44-50 degrees, unless you're running alcohol than that much and more is needed. As the revs come up the ram effect will fix the in-cylinder density problems making extreme amounts of advance unnecessary at high speed. Some guys lock out the variable timing and run the distributor fully advanced all the time with radical cams, this is especially easy to do with race cars since idle and low speed running qualities aren't too important. A trick if you do this is to use a cut out switch on the ignition so the starter is free to spin the engine up before early firing ignition is dumped on it. Another good thing is a multi-spark box, the multiple strikes improve the chances of getting a light at idle and low speeds, this really settles the bottom end down by minimizing miss fires.

Bogie

[ForceFed86]

Originally Posted by oldbogie

Sure doesn't sound like it, the symptoms are classic for a retarded cam. there can be other things such as ignition strength, timing and carb mixtures that have close symptoms, but the great idle, will not rev, has no power, and needs a lot of base advance are just a perfect fit for a late cam. Radical cams are very intolerant of near misses

Being "degreed in" means a degree wheel, positive stop, and dial indicator were used.

In order the drill goes:

1) using the degree wheel and positive stop find true TDC. Lots of instructions on how to do that on the web so I won't be-labor that
here.

2) with the degree wheel adjusted to TDC, a dial indicator mounted to a part of the lifter not affected by a hydraulic plunger or to a solid lifter, with the cam's degree card, the crankshaft is walked around in the normal direction and the cam events at number measured for action against the cam card.


You mentioned low compression, that may be playing a part. But low compression with a Vortec head would lead me to think that you're running dished pistons as the Vortec on a 350 with flat tops gets some good compression. Radical cams are 100% dependent upon high speeds to operate properly. At low speeds the large amount of overlap and the late closing intake lets a lot mixture out of the cylinder. This reduces the density of the charge and causes a huge loss of bottom end power. The recovery process is with a lot of advance and a lot of compression. Starting backwards, the high compression recovers some of the low speed mixture density so the stuff will burn. The "extreme" advance provides the low density (even at high compression) time to get a burn across the chamber before the exhaust valve opens. These things restore some idle quality and build up bottom end torque. If you actually achieved a 3 degree advance on the cam this would also help some.

There is nothing wrong with running a base timing of 26 degrees but then the total curve needs to be reduced in the amount that variable can put in so the top end timing isn't getting to be 44-50 degrees, unless you're running alcohol than that much and more is needed. As the revs come up the ram effect will fix the in-cylinder density problems making extreme amounts of advance unnecessary at high speed. Some guys lock out the variable timing and run the distributor fully advanced all the time with radical cams, this is especially easy to do with race cars since idle and low speed running qualities aren't too important. A trick if you do this is to use a cut out switch on the ignition so the starter is free to spin the engine up before early firing ignition is dumped on it. Another good thing is a multi-spark box, the multiple strikes improve the chances of getting a light at idle and low speeds, this really settles the bottom end down by minimizing miss fires.

Bogie

Wow, thanks for the detailed reply. Yes, when the cam was degree'd in it was done with all the tools U mentioned. The pistons are the fed mog. OEM replacement .030 over cast dished pistons.

As far as limiting the mechanical adv. Could I just pull the distrib apart and weld up the "window" or travel limits of the fly weights? This would limit the total adv and allow me to run a reasonable 24-26 base timing? Or would that not be worth my time? When I can just tack weld the weights wide open and run base at 34-36*? I know I don't have an especially loose converter (2500) but with the extreme light weight of the car do you think I could get by?

Thanks again!

Heres a vid clip of the idle...

[ap72]

Are you revving a car in a closed garage?

And yes, welding in the amount of travel is what you need to do to limit it. Looks like you knew the answer to your problem before you even asked it

[ericnova72]

Weld the travel limit slot, not the weights!

[ap72]

Originally Posted by ericnova72

Weld the travel limit slot, not the weights!

Yea, I should have clarified that. If you google it I'm sure there are plenty of examples as tis a very common procedure.

[oldbogie]

Originally Posted by ForceFed86

Wow, thanks for the detailed reply. Yes, when the cam was degree'd in it was done with all the tools U mentioned. The pistons are the fed mog. OEM replacement .030 over cast dished pistons.

As far as limiting the mechanical adv. Could I just pull the distrib apart and weld up the "window" or travel limits of the fly weights? This would limit the total adv and allow me to run a reasonable 24-26 base timing? Or would that not be worth my time? When I can just tack weld the weights wide open and run base at 34-36*? I know I don't have an especially loose converter (2500) but with the extreme light weight of the car do you think I could get by?

Thanks

You don't work at the Lazy B, do ya? I ask 'cause there is or was a guy in Everett that built one of these monsters some years back.

Look at the Summit or Jegs catalog, there are bushings you can put in there, that'll let you try the idea out without making the kind of commitment you get with weld. Or use a sheet metal screw to lock it up and then try it out and see how it likes it.

Wow, how I wish you'd got here before selecting pistons. The D dish relief is better at keeping a high squish/quench function than those circular dish pistons. That lets you push up on compression ratio without the engine trying to detonate the heads off. But CHP got over 400 hp out of a 350 with the OEM style piston, a Comp 268 cam, and Vortec heads, so once you get it dialed in this thing will scream and then some.


Bogie

[cranky1]

What's your cranking psi? Did you cc everything to find your true compression ratio? I know with Mopars, what the book says the head cc should be is never the same as actual. Low compression engines generally like more timing and the lower it is, the more they like it seems. I've had the pleasure (displeasure is more like it lol) of tuning on two different cars that had extremely low compression. One liked 50 degrees total timing! and idled best with over 20 initial and this thing was degreed in and the the timing marks were checked when the cam was installed. It was a 440 with 8-1. Another was a 318 Dodge Demon and it ran it's best number with 52 total! 3 different timing lights were used for 'good measure'. This little turkey ran 13.50 and we didn't see any evidence of burning up plugs etc. The compression on this thing was a whopping 7.8-1. How it ran that hard is beyond me. The 318 had a cam similar to yours with .480 lift and the 440 had a Purple Shaft 284/484. The 318 should have had a much smaller cam but that's what it got....

[ForceFed86]

Originally Posted by ap72

Are you revving a car in a closed garage?

And yes, welding in the amount of travel is what you need to do to limit it. Looks like you knew the answer to your problem before you even asked it

Nope, I had the first bay open halfway and the back door open. It does make sense. I'm use to crank sensors and "tricking" the ECM on my fuel injection. This is all new stuff to me. (buddy suggested the welding the "window" not me...

Originally Posted by oldbogie

You don't work at the Lazy B, do ya? I ask 'cause there is or was a guy in Everett that built one of these monsters some years back.

Look at the Summit or Jegs catalog, there are bushings you can put in there, that'll let you try the idea out without making the kind of commitment you get with weld. Or use a sheet metal screw to lock it up and then try it out and see how it likes it.

Wow, how I wish you'd got here before selecting pistons. The D dish relief is better at keeping a high squish/quench function than those circular dish pistons. That lets you push up on compression ratio without the engine trying to detonate the heads off. But CHP got over 400 hp out of a 350 with the OEM style piston, a Comp 268 cam, and Vortec heads, so once you get it dialed in this thing will scream and then some.


Bogie

Nope, this is my second rx-7 build. Put a Grand National motor/trans in my first.

Like I said this was a budget build. I bought the entire engine and trans running in an old wrecked station wagon for $230!



Compression tested 155 across the board and had a reman tag on it. I didn't select those pistons. Whom ever did the rebuild on it did. I've literally got 2100 invested in this project so far. I figure if It runs mid to low 12's I'll be thrilled!

Thanks for the help all. I just finished limiting the mech adv. I'll toss it in tomorrow and start over. :cheers:

[morcey2]

You said that you're running an Accel HEI distributor? Are you running a vac advance can on it? If you're running just a mechanical advance distributor w/ no vac can, that can make it a dog down low with off-idle and part throttle performance. Also, if you have a can that's expecting lots more vacuum than you'll get from the cam you're running, it won't do any good either. Being an Accel, it's probably adjustable and would just require a little work to get it tweaked.

Aside from that, my opinion would be to swap out the cam for something w/ a little less overlap/duration and go w/ better matching valve springs. If you don't want to replace the cam, swap out the lifters for Rhoades or some other "variable duration" lifter.

That or swap out the pistons for flat-tops, but that's lots o' work.

Matt

[ForceFed86]

Originally Posted by morcey2

You said that you're running an Accel HEI distributor? Are you running a vac advance can on it? If you're running just a mechanical advance distributor w/ no vac can, that can make it a dog down low with off-idle and part throttle performance. Also, if you have a can that's expecting lots more vacuum than you'll get from the cam you're running, it won't do any good either. Being an Accel, it's probably adjustable and would just require a little work to get it tweaked.

Aside from that, my opinion would be to swap out the cam for something w/ a little less overlap/duration and go w/ better matching valve springs. If you don't want to replace the cam, swap out the lifters for Rhoades or some other "variable duration" lifter.

That or swap out the pistons for flat-tops, but that's lots o' work.

Matt

My mistake it's just an AC delco HEI. It does have the vac adv, but I don't have it attached. I'm just going to lock out timing at 34-36* and see how it reacts. I have matching springs for this cam. I haven't installed them yet. I was wanting to break in the cam on the weak OEM vortec springs. I'll swap out the stronger springs as soon as I get it running decent.

I did manage to lock out the adv and run 36*. The car starts better and cruises much nicer. Tried advancing it a bit more and the starter groaned a bit. Still lacking power down low. But I believe this could be due to the "questionable at best" carb installed.

And to make things worse I'm chasing a nasty shimmy in the front end. Starts shaking like hell at about 30mph. Just had the drive shaft balanced today, that wasn't it. Replaced all 4 rims/tires with the rim/tire setup on my daily....no change there either. I tired spinning up the front wheels with an old electric motor they look to be true. So I guess I'm biting the DIY bullet and taking it in to the alignment shop tomorrow. There goes the budget!

I'll keep U all posted on the timing issues.

[barnym17]

Vortecs Generally don't like more than 32 degrees total mechanical might want to experiment by backing up a little.