[Boiler]

I am helping rebuild a 1970 RA IV. The engine is for the restoration of a Judge with a manual transmission and 4:33 gears. It is desired to keep the engine as original as possible. The original RA IV cam will be retained.

The question is what static compression ratio can we run with this combination on 93 octane pump premium?

[blykins]

9-9.5:1 is a safe window.

When dealing with pump gas, you never know what you're gonna get the next time you fill up, so I always advise my customers to stay on the safe side of things. The difference in horsepower from 9:1 to 10:1 is going to be negligible at these power levels. I'd rather have the fuel tolerance than the 10-12 extra horsepower...

[tom s]

X2 what Brent said.Tom

[b-man]

X3

This is why I built my '64 421 HO with only 8.8:1.

If I should ever be forced to run 87 or 89 octane while out on a road trip the engine can tolerate it, and in the hot summer with the A/C running it should be good on our 91 octane which is the best we have available here in So Cal.

[Cliff R]

Good question and you need over 10 to 1 compression to make that cam work in the 400 RAIV engine.

It will be fine on pump gas, the real 041 cam has over 80 degrees overlap, and poor cylinder filling at lower rpms. Dynamic compression will be pretty low and it will easily manage currently available pump fuel, done a few and worked with quite a few customers with the real RAIV engines and all have been fine with higher compression.

If you lower the SCR to 9.5 to 1, it will be a complete TURD with the 041 cam in it.

A few years ago one of our customers sent his real RAIV 400 to one of the "big" Pontiac engine builders for complete rebuilding. We discussed this very same topic. He told me his engine had been just fine on pump gas for decades, but the engine builder insisted on dishing a set of TRW forged pistons to get the compression down to 9.5 to 1 and replacing the RAIV cam with the Comp XE274 cam. I told him not to allow that move and that he wouldn't be happy with it.

He did not heed my advice and got the engine back and said it was a complete TURD in comparison to the original set-up. It also pinged easily on pump gas if he didn't keep the timing back some and really watch what fuel he used in it.

He ended up getting the engine rebuilt again with flat top pistons and real 041 camshaft. Power was restored and not the first hint of detonation or any other issues on a steady diet of pump fuel........FWIW.......Cliff

[steve25]

X5 , it will be fine with 9.5 comp as the original comp was never above 10.5 anyway regardless of the stated 10.75 that the owners manule's and such stated !

Head wise be sure to get the Exh valves back cut with a 29 to 32 degree angle to get the Exh moving early and make sure that the port entrance in the Exh manifolds is ground larger the the Exh port ext area in the head around its circumference by at least 1/16" with 1/8" being preferred and this will cut low end power robbing reversion by a bunch and the motor will pull onto the Cam at a lower rpm.
If you have the extra funds then heat coating the intake and Exh valves will let you sneak up to 9.75 comp without question !

Once you get the motor in the car and running try out dufferent brands of 93 as they are all different formulations despite having the same octane rating, some will tend to ping in cold weather at full throttle and others in hot weather, learn which brands serve you the best during different seasons.

I would also pay up for a chassis dyno session and set the Carbs full throttle jetting up on the rich side and not care about the 3 to 5 hp loss!

[Skip Fix]

Steve alluded to some valve changes-first ditch the factory 2 piece if you have them! They can come apart and give you a real bad day. Second just for grins flow the heads with a flat D port type valve in the intake besides the tulip RAIV shape-probably about a 5% increase even on a stock port.

Also as Steve said these exhaust ports are bad about reversion as they flow good-backwards as well as forward. You need a good velocity exhaust system to help scavenge.

CR-anyone telling you you can get by on factory CR has never dynoed a flat top zero deck RAIV 400 with pump gas then race gas! They may say they don't hear detonation but the pull with race gas tells the real story! I have. I also ran a RAIV headed 455 then a RAIV 400 In NMCA's Top Stock classes that had to stay fairly stock so some personal experience fro several years with RAIV motors-its sitting on an engine stand to make it in the 65 GTO project one day.

[Cliff R]

Anyone telling you to use a RAIV cam in a real RAIV headed 400 engine with dished pistons at 9.5 to 1 compression has never built an engine like that or had any real experience with one. I have, and you don't want to go there........IMHO......Cliff

[PAUL K]

Originally Posted by Cliff R

Anyone telling you to use a RAIV cam in a real RAIV headed 400 engine with dished pistons at 9.5 to 1 compression has never built an engine like that or had any real experience with one. I have, and you don't want to go there........IMHO......Cliff

X 2 !

[Skip Fix]

Might be a turd with a real RAIV cam, I never tried a "real one", but even with 76 degrees of SEAT overlap at .004 on 108 LS UD cam similar with 231/239 @ 0.050 in mine, it made 8 more HP on race gas over 93 octane pump gas-dyno proven. 69cc chambers(no tulips), 0.030 TRW 2262s, zero decked, 4.300 Felpro head gasket. If it did not need more octane it would not have made more HP!

How many of those RAIVs built did back to backs dyno or track with pump gas and race gas to see if they really needed more octane? If they did not or use a knock sensor we're only guessing they were OK on 93 octane.

[PAUL K]

Skip, if degreed that Utradyne according to the cam card, you were closing the intake valve atleast ten degrees sooner than the RAIV cam. That would create considerably more cylinder pressure and raise your octane requirements.

[blykins]

What about if you ran the cam timing way ahead on the factory stick?

[rexs73gto]

I've said it before & I'll say it again , I like the Crane Blueprint RAIV cam. I know cliff has told me it's not an exact copy of the real RAIV cam , but I've used that in 3 400's & now 3 455 462, now it going it to the 469. I have the E-heads round port style & it has right at 10. CR. For me they run good & in town there good I won't say great , but once you get past about 1500-1800 rpms baby it comes alive. Instead of going to an exact RAIV cam 041 go with the Crane version of it. You'll love the power & still be able to use your IV heads with it & be able to say it is a RAM AIR IV. I don't remember what Cliff told me why the Crane does work so good but I've lived for the last 35 years & am going with my new build in the 469 back to the Crane Blue Print RA IV cam. If you will please explain for them Cliff. Thanks Rex.

[Boiler]

Thanks everyone, this is the type of discussion I was looking for when I posted. From other sources, I had been given generic recommendations for static compression ratios of the low 9s without consideration of the camshaft characteristics. Knowing the cam selection has a major impact on the dynamic compression, I was worried about performance of the engine with the recommended lower static compression. A couple of posters in this thread confirm that fear.

Cliff, you recommended at least 10. Where would you recommend setting the static 10 or something a bit more? I assume you would recommend zero deck.

I just ran the CR calculator on the Summit site. Using a .030 over bore, zero deck, Icon 10.8 CC pistons and a Fel-Pro performance gasket at .039 yields a SCR of 10.19:1

One side question, the top of the bore has a pretty significant chamfer. Is this typical of Pontiac blocks? I know we will lose some of it when the block is decked but it seems strange. My previous engine experience has been on Oldsmobiles and they have nothing like the lead chamfer this RA IV block has.

[blykins]

Originally Posted by Boiler

Thanks everyone, this is the type of discussion I was looking for when I posted. From other sources, I had been given generic recommendations for static compression ratios of the low 9s without consideration of the camshaft characteristics. Knowing the cam selection has a major impact on the dynamic compression, I was worried about performance of the engine with the recommended lower static compression. A couple of posters in this thread confirm that fear.

Cliff, you recommended at least 10. Where would you recommend setting the static 10 or something a bit more? I assume you would recommend zero deck.

I just ran the CR calculator on the Summit site. Using a .030 over bore, zero deck, Icon 10.8 CC pistons and a Fel-Pro performance gasket at .039 yields a SCR of 10.19:1

One side question, the top of the bore has a pretty significant chamfer. Is this typical of Pontiac blocks? I know we will lose some of it when the block is decked but it seems strange. My previous engine experience has been on Oldsmobiles and they have nothing like the lead chamfer this RA IV block has.

Keep in mind that the DCR is very important for eyeballing what you can get away with. However, it doesn't work in the way that you can keep adding compression as long as you keep adding camshaft duration.....you can't say, "I'll run 13.5:1 and I'll just run a monster cam to keep the DCR in check." At some point, an engine approaches or becomes 100% volumetrically efficient and then DCR isn't really a good indicator of what will work and what won't.

[steve25]

All 400 , 428 and 455 4bbl motors have the intake air flow improving chamfer at the top if the bore , and even some 2 bbl blocks slipped off the assembly line with them, and yes the earlier blocks had them but not at the bore center.

In general add 2 CCs to your compression tally and when you do you will not be at 10.18 to one, and to top that off you can add in yet another 1.5 CCs for the total first and second compression ring land back volume that few seem to ever add into the compression math !

[Cliff R]

The Crane 041 replacement cam works because it has a lot of off seat timing and similar lobe positions as the 041. Same reason the Wolverine 5059 cam was a great cam to replace the 041 grind.

Once cam grinders started going to short seat timing cams and tighter LSA we immediately noticed that these engines were no longer able to manage pump fuel at higher compression ratios, and the power curves were narrower, and not nearly as much upper mid-range and top end power.

I have been at this for a while, and tried the "modern" lobe profiles a couple of times and got m ars handed to me, so never went that direction again.

Anyhow, no need to turn this thread into another LSA/cam discussion like they did with my 455 Super Duty thread.

My comments above are accurate, IF you lower the compression clear down to 9.5 to 1, do NOT use the real 041 cam in it. I wouldn't use the XE274 cam either.

A very good choice at that point would be the Crower 60243 cam with high ratio rockers on it. It will make over 1hp/cid, 12" vacuum at idle speed, and plenty of power for what you are doing here. I know because did the carb for one of these engines and witnessed the dyno runs with it.......Cliff

[Half-Inch Stud]

Originally Posted by Cliff R

The Crane 041 replacement cam works because it has a lot of off seat timing and similar lobe positions as the 041. Same reason the Wolverine 5059 cam was a great cam to replace the 041 grind.

Once cam grinders started going to short seat timing cams and tighter LSA we immediately noticed that these engines were no longer able to manage pump fuel at higher compression ratios, and the power curves were narrower, and not nearly as much upper mid-range and top end power.

I have been at this for a while, and tried the "modern" lobe profiles a couple of times and got m ars handed to me, so never went that direction again.

Anyhow, no need to turn this thread into another LSA/cam discussion like they did with my 455 Super Duty thread.

My comments above are accurate, IF you lower the compression clear down to 9.5 to 1, do NOT use the real 041 cam in it. I wouldn't use the XE274 cam either.

A very good choice at that point would be the Crower 60243 cam with high ratio rockers on it. It will make over 1hp/cid, 12" vacuum at idle speed, and plenty of power for what you are doing here. I know because did the carb for one of these engines and witnessed the dyno runs with it.......Cliff

Yes indeed; Ping-free performance is paramount to a good build.

[Boiler]

I just added two CCs for the chamfer and used the icon piston with the 4.5 cc relief. This yields a 10.54 SCR. From the discussions above, I assume this is a workable combination using the Crane Blueprint RA IV cam. Anything I'm missing? Other suggestions?

The Crower 60243 with 9.5:1 also looks like an interesting combination.

[SD455DJ]

I have a complete '70 RAIV SR motor (.030 over) with cast rodes, uncut NOS RAIV heads (11/74 casting dates) that has 9.75 to 1 SCR. It's on an engine stand now and is the motor I used at the Pure Stock Drags in the late '90's and early '00's. It has the Crane RAIV blueprint cam as well and in a 4000 (+) lb Judge with a TH400 trans and stock 3.90 gears ran great on 93 unleaded, never pinged or detonated. It would slow down if I added race gas at the track. It was slow off the line (2.0 60 fts), but ran 13.50/60's at over 105 mph repeatedly.

With a real WW Factory RAIV engine with 11.0 to 1 SCR (Eagle forged rods, Ross forged pistons, Crane RAIV cam) it ran 12.80's at 109 mph. These motors need compression and race gas at the race track. The heads measured at 66 cc's, but were cut by the previous owner.

Dennis