1975 HEI distributor on 1972 engine - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 12-30-2017, 09:40 AM Thread Starter
 
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1975 HEI distributor on 1972 engine

Engine/rebuilt transmission are back in (after car sat for 14 years) and all is well, but am lacking power.

History: I swapped the original distributor on other engine (428, about 10.25:1 CR) with HEI from a 75 GP. Could never get it running right. If set to 6 BTC (stock for 69 GTO) the car was a dog off the line. If advanced for power off line, the engine pinged BAD on the highway (I learned the hard way what detonation does to an engine...).

Now this distributor is on a 1972 ZX 400 block (California 2-bbl) with 670 72cc heads (I'll swap these with 75 cc code 13s later) and I assume the stock camshaft (kick myself for not pulling it/looking for ID # while engine was apart, but lobes are about 3/8" high) - calculated static CR is 9.55:1. Running results are the same as 428... at 6 BTC (idling w/vac adv line pulled), it has little power off line or while accelerating. Set to 10 BTC and is much better but still not good. My dad had a 1973 Catalina 400 2-bbl with single exhaust and I figure this car should run at least that good (has 4-bbl, dual exhaust and is 500 lbs lighter - though with 2.56 rear axle, I probably have longer gears in this car). I've never heard a single "ping" and don't want to experiment with this engine (rod/main bearings are fine).

First thing I noticed with HEI is no change in idle speed when pulling hose from vac advance to set timing (I think this because I'm using ported vacuum now, and original was manifold, but that doesn't depend on distributor...). Just estimating, there may be 6 degrees mechanical advance with the HEI, and 20 vac advance. I don't know what original distributor specs/curve was but can look it up (I charted a 67 400 long ago and think it had more centrifugal/mechanical advance).

Any info would be appreciated. This engine is basically stock, but at 9.5:1, CR is between years 1969 (10:1) and 1975 (est 9:1) so not sure how to approach this. I'd like to keep the vacuum advance - ported will advance it on the hwy. So far it has no problem starting when hot.
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post #2 of 4 (permalink) Old 12-31-2017, 11:35 AM
 
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You can't "estimate" your timing. 9.5 compression will require premium 93 octane or maybe better so you can advance your timing to where it should be and not ping. Retarding the timing and not having the correct distributor curve will kill engine power and torque as you have found out. So get this right first.

The original cam may also be the reason your engine has no power. Your engine should have the "066" cam which is rather mild, so I think you may be looking for too much out of your engine. Compression is too high for a street engine and the cam is too small for any real power.
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post #3 of 4 (permalink) Old 01-02-2018, 12:40 PM Thread Starter
 
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I 'estimated' using a light, but not a dial back (and just learned my HF unit is off 10%) without isolating vacuum/mech advance - and eyeballed it as the balancer mark moved past ticks on cover. Ended up I was way off... instead of 6/20 mech/vac advance, it's 12/16.

I compared timing specs from 67 and 69 manuals (damaged engine was 69 428 with 62 heads) to measurements with this 75 HEI. Books didn't give maximum centrifugal advance for 67 and 69 (only up to 2300, but maybe that was supposed to be max) - I got 3 more degrees after 2000 rpm with the 75 dist.

BIG difference is 10 degrees max vac advance with 67 and 69 points distributors, and 16 max with the 75 HEI. Curves don't look much different though 67 starts steeper.

The 428 with '75 HEI would detonate steadily when I'd accelerate lightly on the highway at 70 mph (2200 RPM in this car) - it had little load but 16 degrees of vac adv would kick in. I didn't accelerate hard from 70 because it rattled like crazy and power dropped off. Looking at the diagram (I think I got it right...), the 428 had too much vac advance with 75 HEI - a vac adv limiter might have fixed the detonation problem. I knew nothing about detonation damage before. Attached are photos of what it did to bearings (maybe you can tell... is all main bearing damage from detonation? I was thinking the front bearing was out of alignment - engine ran strong, but the shop did a bad job rebuilding that engine).

Back to this car... Set init timing to 10 BTDC spec and hooked to manifold vacuum (as was in those days). Car seemed "lighter" driving low speed through neighborhood (than with ported vac), but not much different with highway cruising. I'd set it to 12 (may have heard a couple knocks at WOT on hwy), so set back to 10 - it was also harder to start at 12 when hot. At 10 with manifold vac, I was surprised after 20 mi drive, getting off highway, then a few blocks to the house, the engine was still about 140F (had tested thermostat in boiling water - it opened, then closed), with no pressure in cooling system. Is good in a way, but engine not at proper operating temp (has no AC, 4-row copper radiator).

Got sidetracked when master cylinder finally gave out.. was having to pump brakes, so got more confident with good brakes. Takes off pretty good (can break tire loose now) and chirps into 2nd gear - haven't tried to do a "power braking burnout". On highway, punched it from 70 mph, it kicked down and took off pretty good - let off gas a bit so it'd shift into 3rd (close to 95/4500rpm) and felt the torque pulling... got scared and let off (I'm older now). No detonation. I'd filled up with 91 octane gas. Figure it's running about like it should, but it's not hot summer so there will likely be problems with detonation later. Should note this engine has "8-eyebrow" KB silv-o-lites, so want to avoid detonation (I know these are much less than ideal but I just wanted to get this car running again). Doesn't mean so much, but I think this is running at least as strong as the 73 Catalina 400 - that car weighed 4190 lbs, this car is 3553 (69 GTO hdtp is 3080), and I have lower rear axle ratio. Is good enough for now. I checked butterfly on Qjet (is 7040263 so okay for this) and spring is 5/8 turn from zero (okay) - there's a little lag when secondaries open (not really a bog.. feels like linkage sticks/pops loose).

You're right about the cam. Wallace showed 066 in this ZX engine (was a CA A.I.R 2-BBL), but that cam was also used in 67 and 69 10.5:1 325 and 350 HP 400 engines (I'm confused about this). I'm running closed chamber 670s now, and not sure if I'll buy much with the 13 heads (75cc OC) - static CR will drop to 9.29 from 9.55. I have a Melling SPC-7 cam (068 clone?) and probably should have put it in (was going to keep cam/lifters/matched to bores, but 3 lifters seemed stuck hard so I put in new set of Summits), but wanted to see how this would run with a stock 400 - and to get this distributor issue figured out.

That 428 was a mind-blower - with points distributor, 10.25:1, Crane HMV-272-2-NC cam, timing at 9 BTC, 62 heads, 2" (small) exhaust pipes, stock int/exh manifolds, Holley DP (don't know CFM), TH400, and premium pump gas 20 years ago, it'd burn tire (no posi) from 35 mph punch, shift out of first at 60 mph, twist the car a bit, shift out of 2nd at 95, while still increasing acceleration - I backed off when speedo showed 90 (just calculated that this was 130mph) and it was still accelerating/pulling strongly (I thought if this convertible flips, I'm dead. Suspension was tight/good, but still...).

This engine is tight. No oil blowing when I run it like this. Both the 428 and a 67 GP 400 (with Edelbrock upgrades) had too much crankcase pressure and would blow oil from breathers with kickdowns (~4500-5000 rpm). The rings never seated in those engines. Did a leakdown test on this block/heads and it was fine (5%?). The only problem I've seen was just did compression test and all cylinders are 155-160 except #4 at 180. Not sure what to make of this - I've got one lifter intermittent ticks in cyl 2-4 area so need to adjust while running as you wrote in recent post (not sure if loose valve adj might increase compression???). Put in a fresh set of R43S Delcos gapped at 0.040" - was using old plugs and think I had a miss on right bank - it's running smoother.

I just read a thread where a guy swapped 455 #64 87cc heads with #12 72cc heads on a '70 400 so he can run "full advance curve" on pump gas and it runs better (CR Drop from about 9.5 to 8.4:1) - I've much to learn about all this.
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Chris

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post #4 of 4 (permalink) Old 01-02-2018, 05:44 PM
 
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Detonation will hammer the bearings and can wipe them out within just a few thousand miles if it is not cured.

Setting the advance curve/timing has been well covered in other posts. Makes a big difference on engine performance when it is optimized for your particular engine. Here are a couple articles on the Pontiac HEI: Changing the Advance Curve on HEI Distributors & How to Optimize the Centrifugal Advance in Pontiacs - High Performance Pontiac - Hot Rod Network & this is Chevy based, but applies and can be downloaded if you ever need it in the future: http://www.gmcmi.com/wp-content/uplo...stributors.pdf
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