[ER]

I have been beating my head against the wall trying to figure out where my horsepower went. '67 GTO 670 heads w/'71 block .030 over, performer intake w/1407 edelbrock carb, and 280 ah Comp cam with stock springs and rocker arms. Th400 trans. The motor was rebuilt in Dec 2004. I noticed what seemed to be a lack of low-end power immediatly. Mechanic suggested getting dist. recurved did that now I'm having to run almost 75* total timing in order to keep engine temp down. The shop who did the dist. said he gave my 30* mechanical. He said to go 10-12 initial, if I do the temp goes up instantly. Have asked mechanic if cam was degreed correctly and he is 100% sure it is.
I left the "original" torque conv in the trans and we didn't touch the rear end (2.93 gears). Before the rebuild it would light up the tires from a dead stop and catch it going into second. Now, only if on the right kind of pavement or if it's wet will it spin the tires.
I am thinking about roller tip rockers. Will this make any difference ?

Sorry for the long post but I have spent the last 7 or 8 weeks reading through the archives looking for answers and finally had to ask.
Thanks,

ER

[ER]

I have been beating my head against the wall trying to figure out where my horsepower went. '67 GTO 670 heads w/'71 block .030 over, performer intake w/1407 edelbrock carb, and 280 ah Comp cam with stock springs and rocker arms. Th400 trans. The motor was rebuilt in Dec 2004. I noticed what seemed to be a lack of low-end power immediatly. Mechanic suggested getting dist. recurved did that now I'm having to run almost 75* total timing in order to keep engine temp down. The shop who did the dist. said he gave my 30* mechanical. He said to go 10-12 initial, if I do the temp goes up instantly. Have asked mechanic if cam was degreed correctly and he is 100% sure it is.
I left the "original" torque conv in the trans and we didn't touch the rear end (2.93 gears). Before the rebuild it would light up the tires from a dead stop and catch it going into second. Now, only if on the right kind of pavement or if it's wet will it spin the tires.
I am thinking about roller tip rockers. Will this make any difference ?

Sorry for the long post but I have spent the last 7 or 8 weeks reading through the archives looking for answers and finally had to ask.
Thanks,

ER

[67cruiser]

My opinion, loose the gears and go at least 352 range, 373,s better yet, man those gears are killing you!!

[ER]

I've been thinking gears but I have to remember that they were okay before the rebuild. I did put new tires all around 215/60/15 bfg. When I bought the car all four tires were a diferent size all in the 205/70/15 range.

I may have to change either gears or convertor.

ER

[gjn]

Your cam, converter and gears do not match. I had a similar situation, I rebuild my 400 a 5 or 6 years ago (stroked it last year) and went from lighting the tires up with the orginal motor (160k/blow by and all) at will to not being able to spin the tires without major power breaking. My issue was installing a cam that required a min 9.5 compression with 8.5 compression, and stock converter with 3.08's. The cam need more gear, converter and compression. It was like a two stroke motor, ran like %$#% until 2.8k and then it was to about 5.5k. I am not an expert, but I don't believe you should ever run that much advance. I run 33 total on my 461 and ran 36 on my 400. You may check to see if your balancer has slid if it is the original this might be the issue.

[ER]

I have checked the mark on the balancer to tdc and it is correct. I don't know what else could be causing it to demand this much timing.

ER

[gjn]

Just thought of something, one way you can tell if you balancer is way off is the following. Take out the number one spark plug and put your finger over the spark plug whole. Barely, bump the motor over until you finger pops off due to compression. Get a flashlight and see how far down the #1 piston is. Play around a few times until you get the piston as close as you can to the top. Now, Look at the balancer and see if it is near TDC or 0 degrees. If not you balancer has slid. This is the way I line up the distributor when I put it back in after pulling it. I put the #1 piston at TDC and make sure the balancer is close to TDC. Then I turn on the key and turn the distributor to get spark while grounding it against the cylinder head. Be careful it hurts like $%#& if you get shocked. I then advance the distibutor a little bit and start the car and set the intial timing. Hope this helps.

[gjn]

What's your cranking compression? My was only 135 to 140 when I rebuilt my 400, now it is 175 with my 461.

[69convt]

To require 75 degrees of advance suggests that there is something wrong possibly with cam timing. Might want to take the left valve cover off and rotate the crank by hand. See if the intake valve on the number one cylinder starts to open at the correct degree mark that is specified on your cam card

[ER]

What I did to get tdc was took out the #1 plug and "bumped" the key to get close and then turned the motor by hand and watched the #1 piston to the top just before it began to go back down. It took a while to catch it at the right time. Once it was at the top of the stroke, I checked the timing mark on the balancer and the rotor on the dist. and everything lined up. I just don't know what else to check.
I have not done a compression test yet. I would like to think that it should be fine on a fresh rebuild. I guess I'll have to anyway.

I asked the mechanic just the other day about the cam and if it was degreed correctly and he was sure beyond a shadow of a doubt that is was correct. But I guess I can check that too.

ER

[kyle_blake]

Plug the vacuum advance. Then check your total timing at 3000 rpm.

What happened is the mechanic left in your old vacuum advance can in the distributor.

Or the guy who re-curved it.

Probably being stock they can produce a lot of advance, maybe even up to 30 +.

So your 12 base + 30 mecahnica + 30 from the vac pod = 75 or so.

With that cam shaft you will need an adjustable vacuum pod and will have to limit the advance.

Right now, plug the advance and the check the total timing.

Drive the thing too.check for ping

That comp cam really needs a stall of 2400 or more I'd imagine.

Check their website, it's recommended and better gears too.


If you take 12 base timing and then 30 total timing you get 42, which is a bit much.

You'd have to run some heavy springs in there perhaps.

Get a good timing light...with a dial and start testing !

[ER]

The dist is the summit gm hei. He showed me the numbers where it was stock and said he set it to 30 mech allowing for 10 vacuum and 10-12 initial. Right now it's at 40 inital. Timing was set with vacuum disconnected and plugged. The motor seems to like it though. It's running cool with no pinging.

ER

[buwalda]

The Comp Cams 280 will need alot of initial timing, which the engine likes, & that is what you have found with the 40 degrees initial. Some of the aftermarket vac adv units add 30 crank degrees of advance. If the mechanical advance of 30 degrees comes in too early because of too light springs controlling the weights, then you could see around 75 degrees at idle, which would be higher than stock with that 280 cam. My suggestion would be to advance the cam 4 crank degrees, which will improve the low end a little. It may be enough to give you the low end improvement you are after. Rhoads lifters will also improve low end, probably more than advancing the cam. You should check intake valve-to-piston clearance at TDC & just after TDC because advancing the cam reduces this clearance, although you should be ok with 670 heads. I would also check out the rate of advance of your mechanical advance to see if it is coming too quickly.

[uneasyrider]

E.R., kyle_blake, GJN, and Geoff have very good suggestions. All of those above things can add up to a car that ran better over all stock than after the hop up. It is a VERY common problem. The thing is it is recoverable but it means working back through your combo with the above advice and sorting it out. When it's sorted out it will be faster than the original combo I'm sure. My first car was a 1964 Chevy 3/4 ton PU with a cammed up and over bored Corvette 283. My dad left the truck tranny in it with the "granny" first gear and the stock rear gears. If you only used 2nd, 3rd, and 4th to race someone it did ok. 1st was worthless because it would over rev so quick and the throw and ratio spred from 1st to 2nd was NOT set up for drag racing. The rest of the gears were wide ratio so the engine would fall below the cam for a bit after each shift. The tranny was not set up to shift quickly and the rear end gears were so low it would be turning about 5k at 90mph. The 283 was wicked but the truck was way to heavy for the torque/hp curve and the tranny wasn't matched to the engine at all. It was faster than stock but if that engine had been put in a Camaro, 'Vette, Chevell, etc. with a T-10, M-20, etc. and some 4.11's it would have worked GREAT! As it was the rig had a lot of giggle factor and it was fun to cruise down the boulevard and cam out but it wasn't nearly as fast as the engine should have made it you might say because the overall combo was poorly matched. The factory spent LOTS of money testing there combos and plotting tractive effort curves to come up the the cam, tranny, rear gear, and weight senerio that worked well. It will just take a little more time to get yours right now. You've just pushed outside the envelope of the factory combo and a little matching will need to be done now is all. It's all part of the hot rodding sport. Have fun with it and I guarantee you are going to know a BUNCH more when this is done than when you started.

[74t/a]

The cam is slightly large for your combo, but it really sounds like there is a cam timing issue. Did the person that built the engine degree the cam? If not, then it would be a good idea to do a cranking compression test. If the readings are in the low 100's, then it just might be the culprit. A good running street engine will have at least 150lbs/cyl......with about 175lbs. being pretty optimum.

You really need to run a larger cam with high compression that the 670 head will give you. I am not a fan of edelbrock carbs, but that should be something you address after you determine the problem with low power. Go back to the Q-jet carb.

[ER]

What would be the next step up in gears ? I do a lot of cruising and don't want 4:10 s or something that is going to be winding out on the highway. One day I hope all this comes together. The motor seems to be running really well, I just want to get power to the rear wheels. Thanks for all your help so far !

ER

[PunchT37]

I`d re-check the timing chain again first. Then the dist. index next. That overheat problem sounds like late timing and 75 degrees sounds like too much to run. That car should run like a bat out of hell with that cam. High gear and all. A buddy of mine swore his chain was on right but the car wouldn`t start. Guess what?

[taalltheway]

What are the specs of that cam @o.o50? Sounds like you have overcammed it....Put in a smaller cam and you will be OK.

[69fbconv]

Are you using the same intake and carb as before the rebuild? If not, put the old carb back on and try it out. If it's the same - with a different cam you will most likely need to recalibrate that 1407. Is it using the stock calibration? Do one thing at a time. Put the pump shot linkage in the lower hole. If no good, try to top hole. Then proceed to change rods and springs to get it dialed in correctly. I also found that on these carbs with mechanical secondaries, you might experience a bog on acceleration because the secondaries are coming in too quick. You can bend the linkage rod and have them come in later. Made a big difference on mine.

[ER]

T/A- cam @0.05: 232 int & 237 exh

69fbconv- not the same intake or carb. The intake that was on it before was a gm intake I was told it was not native to '67 gto's but a higher performance intake. I still have it and have been tempted to get it cleaned up/painted and put back on. The carb that was on it was an edelbrock/carter 650.
I'll go see if I can get some #s off the intake and see if someone can ID it for me. It may be better than the performer that's on it now.

ER