Decreasing Pinto CR

[Flighter]

Hi folks.

I may need to decrease the compression ratio of my Pinto in order to run a milder cam on a road driven car (changing from FR32 to BF134). I've yet to pull the head off to see what the chamber volume is etc., and the engine is reputably quite fresh so I've zero interest in a rebuild with lower comp pistons. If it comes to it, I'm hoping to get away with using a thicker gasket, but in the event that the Reinz one isn't thick enough at 1.3mm, are there any other thicker ones of decent quality?

Thanks.

[snapper1]

Well the standard one is 1.65mm
Cometic & Athena do multi layer ones so any thickness is possible

[Graham]

to be honest i dont see you having a big issue its not like your going from an HT1 to a 134.

you need to take it to bits and see whats in there and what the actual compression ratio is first

[Dyno]

to be honest i dont see you having a big issue its not like your going from an HT1 to a 134.

you need to take it to bits and see whats in there and what the actual compression ratio is first

Correct, these cams are very close to each other. Different setup but both are mild cams (not by profile but duration) need approx the same ratio. I should ask first, what's the ratio right now?

[Graham]

Correct, these cams are very close to each other. Different setup but both are mild cams (not by profile but duration) need approx the same ratio. I should ask first, what's the ratio right now?

as suggested, the odds are if its on an FR32 it has cast pistons to cr is likely to be 10.5:1 or less. Dirk will probably say that 10.5 is too much for 134, dont think id want to try that much on a big pair of carbs, but experience tells me on a pair of 40's or a twin choke which both limit breathing especially if the head is nothing special 10.5 will work ok assuming your not feeding it low octane fuel. so if its only got 10.1 i would forget all about changing the compression and just fit the cam

[Dyno]

I use closer to 10 than 10,5/1 in Belgium. Special with BP134. But also depend how good the head is, the better the head at low them more air it will take and the faster it is pinking.

[Flighter]

The head is currently unknown, as the car is a fairly recent purchase. I'm told it has been fitted with unleaded seats, so there's a very good chance it has been skimmed at least once already. The block has supposedly been bored by 0.040" too, which, all other things being equal, increases the compression ratio by 0.15 - 0.2 points as well. I'll be sticking with the 32/36 carb, as that's plenty good enough for a daily driver. I too think that 10.5:1 might be pushing it with a BF134, hence the interest in thicker gaskets to get it closer to 10:1 if need be. It looks I can get Cometic gaskets locally without too much hassle, although they sure are expensive compared to a Fel-pro, which is what I use on my other Pinto. Cheaper than pulling the engine apart and fitting lower compression pistons etc., so I mustn't complain.

[Dyno]

With standard carb you can not go any better as the Piper BP134. FR32 is for sure the WRONG cam. Only use FR32 if you fit twin Webers. And this will not need any compression higher as 10/1. Plenty of filling at very low revs. The rest will not add extra power, usual only trouble.

[Forest_rallying]

You need to cc your head and work the compression ratio out before decreasing anything. Chances are you are no higher than 10.1 anyway unless a lot has been skimmed off the head. Don't forget to allow for the volume between the piston crown and the deck of the block. Many are led to believe that their compression is higher than it really is.

[Flighter]

I will absolutely cc the head, and have already prepared a CR calculator that allows for gasket volume, ring land volume, deck height etc. It determines both static and dynamic ratios, which I'm aiming to put close to 10:1 and 8:1 respectively so I can run pump fuel. It was while playing around with the numbers I measured on my other engine that I realised I might need to decrease the static CR to run the BF134 on the new one. As you say, it all depends on what I find after I pull the head off. That I can tailor the thickness of a Cometic gasket to suit my exact needs is a great relief.

[Dyno]

You have probably measured the end compression of you engine with a compression meter? And now you are trying to calculate what the CC inside is with the measurement you got before taking of the head? Or do I have it wrong? Because you are always talking about decreasing the compression ratio but the engine is still closed. Can't see and other reason why you are worried in advance?

[Flighter]

No, I haven't made any measurements yet. I will rely on CC of head etc. after I pull it off the engine. Instead, I was preparing for the possibility that I need to decrease CR by asking whether quality head gaskets are available that are thicker than the Victor Reinz ones (which are 1.3mm). Cometic do, and they are not too hard to find where i live, so I should be able to get the 10:1 static ratio I want after I've CC'd the head. I also checked my CR calculator works correctly using known measurements from my other Pinto. If, for example, I wanted to put a BF134 in that engine, I definitely need a thicker head gasket to lower my CR. That in turn got me thinking that I might need a thicker one in the 'unknown' engine..

[Dyno]

Ha, ok, I understand. You can also take a few cc's out of the head by grinding. 2 or 3 cc max.

[hawk1903]

Also, ideally, would not it be a good idea to gain some volume by machining valve pockets ( also machining the chamber as Onyd mentioned above ) but trying to keep the squish as low as possible with the rather correct gasket ?

[snapper1]

What is considered a desirable squish distance?

[hawk1903]

I would say 0.85-0.90 mm if possible. I have heard that people do use less than 0.85 mm but probably not less than 0.80 mm.

[Graham]

depends on what you rev it to! as well as how strong the rods are, ive been as low as 0.75mm yb rods and forged pistons on an engine revved to 8, when the head came off you could just make out the chamber shape on the piston due to lack of carbon build up in the squish area

[Flighter]

Valve pockets would be useful also to avoid valve strike if a belt breaks (due to high lift cam), but tearing the engine down to do that isn't on the cards.